Review

Technologies in Marine Antifouling and Anti-Corrosion

Coatings: A Comprehensive Review
Hua Liang 1,2 , Xiaolong Shi 1,2 and Yanzhou Li 3, *

1 College of Mechanical Engineering, Yancheng Institute of Technology, Yancheng 224051, China;

lianghua@ycit.edu.cn (H.L.); shixiaolongycit@163.com (X.S.)
2 College of Automotive Engineering, Yancheng Institute of Technology, Yancheng 224051, China
3 School of Mechanical and Vehicle Engineering, West Anhui University, Yueliangdao Road, No. 1,
Lu’an 237010, China
* Correspondence: liyanzhou9336@163.com; Tel.: +86-150-4421-1299

Abstract: With the rapid development of marine engineering, effective antifouling and anti-corrosion
technologies are essential for ensuring the safety and longevity of marine facilities. This review
synthesizes current research on various coating technologies designed to combat marine biological
fouling and corrosion. It analyzes the causes of marine biological fouling and corrosion, discusses
their potential impacts on the safety of ships and marine structures, and emphasizes the need for
effective protective systems. The review covers current antifouling coating technologies, including the
preparation of low-surface-energy coatings, conductive coatings, biomimetic coatings, polysiloxane
coatings, polyurea coatings, epoxy coatings, polyurethane coatings, and high-entropy alloy coatings.
Anti-corrosion coatings are also discussed, with a focus on the characteristics of epoxy, polyurethane,
and polyurea coatings, as well as metal-based coatings, alongside their corrosion resistance in
marine environments. Based on existing research, the review summarizes ongoing challenges in
marine antifouling and anti-corrosion coating technologies, and offers perspectives on future research
directions and technological developments.

Keywords: marine environment; biofouling protection; antifouling coating; anti-corrosion coating

Citation: Liang, H.; Shi, X.; Li, Y.

Technologies in Marine Antifouling
and Anti-Corrosion Coatings: A 1. Introduction
Comprehensive Review. Coatings Marine biofouling refers to the process in which aquatic organisms attach to and
2024, 14, 1487. https://doi.org/ grow on wet surfaces of objects or engineering equipment, forming biofilms or biological
10.3390/coatings14121487 communities [1–4]. Common biofouling organisms include crustaceans such as barnacles
Academic Editor: Ludmila
and goose barnacles, bivalves like mussels and oysters, cnidarians such as hydroids and
B. Boinovich anemones, bryozoans, algae, and tube worms, etc. [5–10]. Due to differences in climate
and hydrological conditions across regions, the types of fouling organisms are highly
Received: 1 October 2024 diverse, and their distribution and reproduction exhibit significant spatial and temporal
Revised: 19 November 2024
variations [10,11].
Accepted: 22 November 2024
Marine biofouling causes several issues, including increased drag and fuel consump-
Published: 26 November 2024
tion [9,12,13]. Organisms attached to ship hulls make the surface rougher and heavier,
reducing speed and maneuverability. This increases fuel use, costs, and carbon emis-
sions [14,15]. Also, biofouling can introduce invasive species, as organisms attached to
Copyright: © 2024 by the authors. vessels are transported to foreign waters, disrupting local ecosystems and threatening bio-
Licensee MDPI, Basel, Switzerland. diversity [16]. A classic example is the comb jelly, introduced to the Black Sea in the 1980s
This article is an open access article through ship ballast water, causing a sharp decline in native species like anchovies [17].
distributed under the terms and Biofouling also impacts aquaculture by reducing oxygen and nutrients available to farmed
conditions of the Creative Commons fish when species such as mussels and algae attach to aquaculture nets. In some cases, this
Attribution (CC BY) license (https:// has caused oxygen depletion and large-scale fish die-offs, resulting in significant financial
creativecommons.org/licenses/by/ losses for fish farms.
4.0/).

Coatings 2024, 14, 1487. https://doi.org/10.3390/coatings14121487 https://www.mdpi.com/journal/coatings

Coatings 2024, 14, 1487 2 of 39

Additionally, fouling organisms like barnacles and mollusks can clog seawater pipes,
reduce equipment efficiency, and lead to system failures. In nuclear power plants, bio-
fouling has even compromised cooling systems [18]. And biofouling accelerates material
corrosion, particularly in high-salinity seawater environments, where the metabolic prod-
ucts of microorganisms can exacerbate corrosion, leading to microbial-induced corrosion
(MIC). This form of corrosion is hazardous in critical infrastructure like bridges and offshore
platforms, where it can compromise the integrity and safety of metal structures, shortening
their lifespan and increasing maintenance costs. Since the beginning of maritime activities,
bio-fouling has been a constant challenge.
Corrosion refers to the degradation and loss of material properties in metals or non-
metals due to exposure to surrounding media such as water, air, salts, or acids [19–21].
Based on the environment, corrosion can be classified into natural forms like atmospheric,
soil, seawater, and microbial corrosion, and artificial forms such as high-temperature gases,
radiation, acids, bases, and molten salts. In marine environments, metal corrosion is
prevalent [22–24]. Due to the constant exposure of marine equipment to seawater, metal
corrosion is commonplace in aquatic environments. This corrosion significantly increases
maintenance and repair costs and can lead to equipment failure or destruction. In China,
metal corrosion is estimated to cause annual economic losses of up to CNY 150 billion.
Another example is the “Deepwater Horizon” offshore rig managed by BP in the Gulf of
Mexico, where the failure of a corroded subsea valve caused an explosion, leading to one
of the most catastrophic environmental disasters in history [25–27].
In response to the issues above, researchers have studied several biofouling and corro-
sion protection technologies, including physical removal, seawater electrolysis, chemical
biocides, antifouling coatings, and anti-corrosion coatings. Physical removal involves
manually or mechanically eliminating fouling organisms and corroded material from ma-
rine surfaces through scrubbing or scraping. This approach is simple and cost-effective,
particularly for removing larger organisms like barnacles, mussels, and surface corrosion.
However, it is ineffective against microorganisms like bacteria and algae and does not
provide long-term corrosion protection, requiring frequent maintenance. Seawater electrol-
ysis generates oxidizing chemicals like chlorine and hypochlorite, which damage the cell
walls of fouling organisms and reduce corrosion by creating a protective layer on metal
surfaces. This method is ideal for large-scale biofouling and corrosion prevention, and is
labor-saving, but its high energy consumption limits its application to areas with ample
energy supply. Chemical biocides, including liquid chlorine, sodium hypochlorite, and
chlorine dioxide, are added to seawater to kill or inhibit the growth of fouling organisms
quickly and reduce corrosion. Although relatively low in cost and fast-acting, the increasing
focus on environmental protection has restricted the use of these chemicals due to their
potential negative impact on surrounding ecosystems and marine life. Therefore, the re-
search on marine antifouling and anti-corrosion technologies mainly focuses on preparing
coatings on substrate surfaces.
Compared to other approaches, the advantage of coating methods lies in their ability
to preserve the original properties of the substrate while enhancing surface performance
through the application of a coating, achieving a complementary balance between the
substrate and surface properties. Therefore, developing high-performance marine coatings
and exploring their service behavior in marine environments is crucial for protecting
marine equipment and extending the service life of components. In this context, this paper
reviews antifouling and anti-corrosion coating systems in marine environments. Based on
existing research, it analyzes the problems of anti-corrosion and antifouling coatings. It puts
forward suggestions and prospects to provide ideas for the future innovative development
of marine antifouling and anti-corrosion coatings.

2. Marine Fouling and Corrosion Mechanisms

Marine fouling and corrosion are significant processes affecting surfaces immersed or
exposed in marine environments. Fouling occurs when marine organisms attach to surfaces,
Coatings 2024, 14, x FOR PEER REVIEW 3 of 39
2. Marine Fouling and Corrosion Mechanisms
Coatings 2024, 14, 1487 Marine fouling and corrosion are significant processes affecting surfaces immersed
3 of 39
or exposed in marine environments.
2. Marine Fouling and Corrosion Mechanisms Fouling occurs when marine organisms attach to sur
faces, whilefouling
Marine corrosion involves are
and corrosion thesignificant
electrochemical
processesreactions
affectingbetween the material and
surfaces immersed
while corrosion involves the electrochemical reactions between thethe
material and seawater,
or exposed in marine environments. Fouling occurs when marine organisms attach toof
seawater, leading to substrate degradation. Understanding mechanisms these pro
sur-
leading to
cesseswhile substrate
is crucial degradation.
for developing Understanding the mechanisms of these processes is
faces, corrosion involves theeffective protective
electrochemical strategies.
reactions between the material and
crucial for developing effective protective strategies.
seawater, leading to substrate degradation. Understanding the mechanisms of these pro-
2.1.Fouling
2.1. Fouling
cesses Mechanism
is crucial for developing effective protective strategies.
Mechanism
Inmarine
In marine environments,
environments, the the surfaces
surfaces of submerged
of submerged substrates
substrates areaffected
are often often affected
by by
2.1. Fouling Mechanism
biofouling,with
biofouling, with fouling
fouling varying
varying based
based onsurface
on the the surface properties.
properties. Figure Figure
1 shows1vessels
shows vessel
fouledInby
fouled marine
bymarineenvironments,
marineorganisms.
organisms. the
Figuresurfaces
Figure 1a of submerged
1a illustrates fouling
illustrates substrates
by theby
fouling greenare
the often
alga,
greenand affected
Figure
alga, by
and1b
Figure 1b
biofouling,
shows with
fouling byfouling varying
barnacles [28]. based on the surface properties. Figure 1 shows vessels
shows fouling by barnacles [28].
fouled by marine organisms. Figure 1a illustrates fouling by the green alga, and Figure 1b
shows fouling by barnacles [28].

(a)(a) (b) (b)

Figure1.1.Ships
Figure Ships impacted
impacted by by marine
marine organism
organism fouling.
fouling. The images
The images depict
depict (a) (a) caused
fouling foulingbycaused
the by th
greenalga
green algaand
alga and
and (b)
(b) fouling
fouling by by barnacles
barnacles [28].
(b) fouling by barnacles [28]. [28].

Biofouling
Biofouling typically
typically progresses
progresses through several
through stages,stages,
several including conditioning
including film,
conditioning film
biofilm,
biofilm, microbial
microbial fouling,
fouling, and
and macrofouling,
macrofouling, as illustrated
as illustrated in Figure
in Figure 2
biofilm, microbial fouling, and macrofouling, as illustrated in Figure 2 [29]. When an ob [29].
2 When
[29]. When an object
an ob-
is immersed in seawater,
ject a conditioning film film
composed of polysaccharides, proteins, and
jectisisimmersed
immersed ininseawater,
seawater, a conditioning
a conditioning composed
film composed of polysaccharides, proteins,
of polysaccharides, proteins
organic/inorganic
and organic/inorganic macromolecules
macromolecules rapidly
rapidlyforms
forms onon itsits
surface
surface[30].
[30].These
These substances
substances
and organic/inorganic
originate macromolecules rapidly forms on itsofsurface [30]. Theseand substance
originate from
fromplanktonic
planktonicorganic
organic particles,
particles,metabolic
metabolic byproducts
byproducts marine organisms,
of marine organisms,
originate
decayed
and decayed
from
organic planktonic
matter
organic matter
organic
in theinseawater. particles,
Due to
the seawater. Due
metabolic
their
to hydrophilic
byproducts
their hydrophilicfunctional of marine
groups,
functional
organisms
these
groups,
and decayed
macromolecules organic matter
preferentially in the
adhere seawater.
to the Due
substrate, to their
forming hydrophilic
the
these macromolecules preferentially adhere to the substrate, forming the conditioning conditioning functional
film [31]. groups
these
This filmmacromolecules
alters the surface’spreferentially
chemical adhere
properties, to
makingthe itsubstrate,
more forming
attractive
film [31]. This film alters the surface’s chemical properties, making it more attractive for for the conditioning
planktonic
organisms
film [31]. to
planktonic attach.
This film alters
organisms the surface’s chemical properties, making it more attractive fo
to attach.
planktonic organisms to attach.

Figure 2. The process of biofouling formation [29].

Figure 2. The process of biofouling formation [29].

Figure 2. The process of biofouling formation [29].

Coatings 2024, 14, 1487 4 of 39

Within minutes to hours, bacteria and diatoms accumulate on the conditioning film.
This occurs because the film provides a suitable attachment matrix and creates a microenvi-
ronment that helps bacteria and diatoms resist external forces such as water flow and other
pressures. External forces can still reversibly remove the bacteria and diatoms at this stage.
However, once these organisms become established, they secrete chemicals that attract pro-
tozoa and invertebrates [32]. Additionally, the extracellular polymeric substances secreted
by bacteria and diatoms promote their growth and offer resistance to harmful substances,
eventually forming a dense biofilm. Over the following days or weeks, macro-organisms
such as large algae, barnacles, and mollusks further attach to the biofilm, creating a mature
macrofouling community.
The specific progression of biofouling can vary depending on environmental factors
such as location, season, temperature, salinity, and light availability [33]. For example, in
coastal areas, abundant nutrients and microbial populations accelerate the formation of
biofilms, while in offshore regions, stronger water currents may delay initial attachment [34].
Seasonal changes affect water temperature and light availability, with the warmth and
high light intensity of spring and summer promoting the rapid growth of microorganisms
and algae [34,35]. Temperature is particularly crucial for biofilm maturation; higher tem-
peratures accelerate the attachment of bacteria and diatoms, whereas lower temperatures
slow this process [36,37]. Light intensity impacts algae, with strong light in shallow waters
promoting algal growth, which further accelerates macrofouling, while limited light in
deeper waters slows these processes [38,39].
Table 1 summarizes the attachment processes and strategies of various marine organ-
isms. Each species has its unique attachment method, making it difficult to use a single
approach to prevent marine biofouling comprehensively. However, research shows that
there are two main strategies for effective biofouling control—the first is the antifouling
strategy, which prevents organisms from attaching to surfaces before adhesion occurs. This
is typically achieved using specialized chemical coatings, such as organotin antifouling
coatings, photocatalytic antifouling coatings, and zwitterionic polymer antifouling coatings.
The second is the fouling-release strategy, which aims to weaken the adhesion of organisms,
making them easier to remove. Examples include conductive antifouling coatings and
silicate-based antifouling coatings. These antifouling coating technologies and related
studies will be thoroughly discussed in Section 3.

Table 1. Overview of common biofouling species’ various attachment processes and strategies.

Biofouling Type Process Strategies

Initial binding through electrostatic and
Pyrrolo [1,2-a]pyrazine-1,4-dione coating [44];
hydrophobic forces; stronger attachment via
Bacteria [40–43] Nitric oxide-releasing copolymer brush
capsules, fimbriae, and slime. Dependent on
coating [45].
surface properties.
Proteins adhere noncovalently via van der Waals
Lignin-based antifouling coating [48];
forces, hydrophobicity, and electrostatic
Proteins [46,47] tin-free self-polishing coating with
interactions, with irreversible adhesion following
indole-derivative resin [49].
conformational changes and denaturation.
Temporary adhesion using extracellular Bioinspired micro-textured surfaces based on Brill
substances; permanent attachment via pads, stalks, fish scales [51];
Diatoms [50]
or tubes. A significant component of biogenic antimicrobial coating with Houttuynia
microbial slimes. and Scutellarin additives [52].
Temporary attachment with acidic
mucopolysaccharides; permanent adhesion using Catechol and trifluoromethyl methacrylate
Bryozoa [50,53–56]
proteins and mucopolysaccharides. Common in copolymer coating [57].
marine biofouling.
Coatings 2024, 14, 1487 5 of 39

Table 1. Cont.

Biofouling Type Process Strategies

They temporarily grip with cyprid cement.
Permanent bonding uses adult cement, which is
Barnacles [58–63] Albofungin-based antifouling coating [64].
rich in amino acids. Barnacles can attach to most
underwater surfaces.
They adhere temporarily to glycoproteins. Zwitterionic polydopamine-based antifouling
Mussels [65–74] Permanent bonding uses strong scleroproteins. coating [75]; fluorinated polysiloxane and
Mussel adhesion is highly durable. perfluoropolyether-based lubricant coating [76].
They initially attach via spore flagella and Carbon fiber-reinforced lubricant-infused slippery
Algae [77] mucilage. Permanent bonding occurs through coating [78]; degradable acrylate polymer coating
secreted adhesives, mostly glycoproteins. with polylactic acid and natural rosin [79].

2.2. Corrosion Mechanism

Marine corrosion is a complex process influenced by multiple factors, including
electrolytes in seawater, biofouling, and physical forces [80,81]. Seawater is rich in corrosive
ions, such as chloride ions (Cl− ), hydroxide ions (OH− ), and hydrogen ions (H+ ). These
ions accelerate electrochemical reactions or disrupt the bond between coatings and metal,
making metals more susceptible to oxidation-reduction reactions, where metal atoms are
converted into metal ions. Today, ships, yachts, and other vessels are made of various
materials, such as wood, steel, fiber-glass, aluminum, and others. However, the most
widely used materials are steel and aluminum.
The corrosion mechanisms of commonly used ship materials like steel and aluminum
can be explained as follows.
Steel, an alloy of iron and carbon, is a multi-phase material with different electrode
potentials, forming galvanic cells. When steel is immersed in seawater, an electrolyte
solution, numerous corrosion micro-cells form on its surface, initiating the electrochemical
reaction of iron and causing electron loss. The corrosion process involves several steps [82].

Anodic reaction: Fe → Fe2+ + 2e− (1)

Iron undergoes oxidation at the anode, losing electrons to form Fe2+ ions.

Cathodic reaction: O2 + 2H2 O + 4e− → 4OH− (2)

At the cathode, oxygen reacts with water, producing hydroxide ions.

Formation of iron hydroxide: Fe2+ + 2OH− → Fe(OH)2 (3)

Fe2+ combines with OH− to form iron hydroxide.

Further oxidation of Fe2+ in the presence of oxygen:
(4)
4Fe(OH)2 + O2 + 2H2 O → 4Fe(OH)3
Fe(OH)2 is further oxidized to Fe(OH)3 , facilitated by oxygen.

Formation of rust: 4Fe2+ + O2 + (4 + x)H2 O → 2Fe2 O3 · xH2 O + 8H+ (5)

Finally, Fe2+ reacts with oxygen and water to form loose iron oxide, which cannot
provide an effective protective layer, leading to continued metal corrosion.
Relevant studies have also utilized the mechanism of pitting corrosion in ionic solu-
tions to explain the localized corrosion of steel in seawater [83,84]. The pitting corrosion of
steel under the attack of chloride ions involves the following stages: First, chloride ions
Coatings 2024, 14, 1487 6 of 39

disrupt the protective oxide film (passive film) on the steel surface, exposing the metal to
seawater and initiating dissolution, where iron forms iron ions and iron chlorides [83],

Fe + nCl → FeCln (6)

Subsequently, the dissolved iron ions undergo hydrolysis with water, releasing hy-
drogen ions (H+ ), which lowers the local pH in the pit cavity, thereby accelerating the
corrosion process,
Fen+ + H2 O → Fe(OH)n−1 Cln + H+ (7)
To maintain charge balance within the cavity, additional chloride ions (Cl− ) migrate
from the bulk solution into the pit cavity, further increasing the local chloride ion concen-
tration and enhancing the corrosive environment. The continuous expansion of pitting
ultimately results in localized corrosion.
The surface of the aluminum oxide film is covered with hydroxyl groups (-OH), which
determine the surface charge of the oxide film in solution [85,86]. In neutral solutions,
aluminum oxide films are typically positively charged [87]. In chloride-containing solutions,
due to the hydroxyl properties and surface charge of the oxide film, the aluminum oxide
film can attract Cl− from the solution, creating conditions for the formation of corrosion
pits, a process referred to as the adsorption of chloride [87]. Furthermore, Cl− can enter
the aluminum oxide film through adsorption and migration, a behavior known as the
penetration of the oxide film by Cl− [87].
The mechanisms of chloride penetration may include the following: (i) transport
of chloride ions through the oxide film through oxygen vacancies [88]; (ii) transport of
chloride ions through the oxide film through water channels [89,90], and (iii) localized film
dissolution or thinning [91].
When chloride ions penetrate the oxide film and reach the metal substrate, the initia-
tion of pitting occurs through chloride-assisted dissolution reactions at the metal/oxide
interface beneath the oxide film [87]. Water molecules provide the electrolyte necessary for
localized dissolution, a process referred to as localized dissolution and water in oxide films.
The following sequence of reactions outlines the process of pitting initiation, incorporating
the adsorption of chloride ions, the transport of chloride through the oxide film via oxygen
vacancies, and the localized dissolution of aluminum atoms at the metal/oxide interface,
driven by three consecutive one-electron transfer reactions [87], as follows:

1
Al(oxide)OH + H+ ⇌ Al(oxide)OH2+ (8)
−1

2
Al(oxide)OH2+ + nCl− ⇌ Al(oxide)OH2+ Cl−
n
n
(9)
−2
3
Al(oxide)OH2+ Cl− n +
n + nVO·· ⇌ Al[ n (ClO∗ )(oxide)]OH2 (10)
−3
4
Al[(nClO· )(oxide)]OH2+ ⇌ Al+ [(nClO· )(oxide)]OH2+ + e− (11)
−4
5
Al+ [(nClO∗ )(oxide)]OH2+ ⇌ Al++ [(nClO∗ )(oxide)]OH2+ + e− (12)
−5
6
Al++ [(nClO· )(oxide)]OH2+ → Al+++ [(mClO· )(oxide)]OH2+ + (n − m)Cl− + e− (13)
In the notation, Al(oxide)OH, Al refers to a substrate aluminum atom beneath the
oxide film, while oxide represents the covering film subject to localized dissolution. OH
indicates surface hydroxyl groups. VO denotes oxygen vacancies in the film, and ClO
refers to a chloride ion in an oxygen lattice site. Al[(nClO)(oxide)]OH2 + describes the
oxide film enabling chloride ion transport, and represents zero-valent aluminum beneath
a chloride-containing film before pitting. At the pitting site, Al+ [(nClO)(oxide)]OH2 + ,
Coatings 2024, 14, 1487 7 of 39

Al++ [(nClO)(oxide)]OH2 + , and Al+++ [(mClO)(oxide)]OH2 + represent mono-, di-, and triva-
lent aluminum ions at the metal/oxide interface. Equation (9) indicates that chloride
clusters can develop on the oxide-coated surface, and the chloride ion concentration within
the oxide film diminishes as pitting begins. Consequently, the condition 0 < m < n in
Equation (13) suggests that some chloride ions are expelled from the film near the pit
during the initiation of pitting.
An alternative pathway for chloride transport through passive films on aluminum
involves localized film thinning or dissolution, as mentioned earlier. This process continues
with the reactions outlined in (9), followed by [87]
′
n 3
Al(oxide)OH2+ Cl− Al nCl− (oxide) OH2+


n ⇌ (14)
−3′

4′
Al nCl− (oxide) OH2+ ⇌ Al+ nCl− (oxide) OH2+ + e−

 

(15)
−4′

5′
Al+ nCl− (oxide) OH2+ ⇌ Al++ nCl− (oxide) OH2+ + e−

 

(16)
−5′
 i
6′
h
Al++ nCl− (oxide)]OH2+ → Al+++ mCl− (oxide)]OH2+ (n − m)Cl− + e−



(17)

The notation Al[(nCl)(oxide)]OH2 + represents the oxide film on aluminum undergo-

ing thinning. The symbols Al+ [(nCl)(oxide)]OH2 + , Al++ [(nCl)(oxide)]OH2 + , and Al+++
[(mCl)(oxide)]OH2 + denote mono-, di-, and trivalent aluminum ions, respectively, located
at the metal/oxide interface, specifically at the site where pitting begins.
The propagation stage of aluminum pitting involves the formation of blisters beneath
the oxide film. As corrosion progresses, hydrogen accumulates under the oxide layer,
leading to blister formation. When the hydrogen pressure reaches a critical value, the oxide
film ruptures, exposing the corrosion pit to the external electrolyte and further accelerating
corrosion. Over time, this process ultimately leads to the occurrence of corrosion.

3. Progress in Marine Anti-Fouling Coatings

The development of antifouling technology spans several centuries, with its origins
tracing back to around 200 BC when wooden ship hulls were coated with hot pitch, tar, and
grease to reduce biological fouling. These methods were later replaced by more effective
lead sheathing [92]. During the Age of Exploration, the accumulation of organisms like
barnacles and algae significantly impacted ship speed, leading to using copper or brass
to coat the hulls. The brass sheathing on the Cutty Sark is a notable example, effectively
reducing hull fouling [92]. By the late 1700s, harmful substances like arsenic (As), sulfur
(S), and mercury (Hg) were introduced for hull preservation [93]. In 1926, the U.S. Navy
created a rosin-based antifouling coating with copper and mercury oxides as additives,
preventing biological fouling for up to 18 months [94–96]. With the advancement of
technology, antifouling coating technologies have continuously evolved toward adopting
various mechanisms to meet specific application needs. Currently, research on antifouling
coatings includes several types, such as organotin antifouling coatings, low-surface-energy
antifouling coatings, biomimetic coatings, conductive antifouling coatings, photocatalytic
antifouling coatings, silicate-based coatings, and rubber anti-corrosion coatings.

3.1. Organotin Antifouling Coatings

Organotin antifouling coatings are a class of coatings formulated with organotin
compounds primarily used to prevent marine organisms from adhering to the surfaces of
ships and other marine structures. During the 1970s and 1980s, self-polishing copolymer
paints containing organotin compounds were widely applied. These coatings utilized
hydrolyzable tributyltin methacrylate copolymer as the film-forming agent, with cuprous
oxide as a biocide and zinc oxide as a filler. The antifouling mechanism of these coatings
Coatings 2024, 14, 1487 8 of 39

relied on the gradual dissolution of active ingredients in water, releasing toxic substances.
The coatings demonstrate significant antifouling properties and retain their effectiveness
over a prolonged period [97,98].
However, organotin-based antifouling coatings have presented significant environ-
mental hazards. Tributyltin causes irreversible damage to marine ecosystems, with its
primary issue being its ability to disrupt the endocrine systems of marine organisms,
affecting reproductive functions and causing deformities. Humans can face significant
health risks once marine organisms contaminated with TBT enter the food chain [98].
The European Union has progressively strengthened restrictions on the use of organotin
compounds [99,100]. Between 1989 and 2002, the EU introduced Directive 89/677/EEC to
restrict the use of organotin compounds as biocides in free-association paints, banning the
sale of such coatings. This was followed by Directives 1999/51/EC and 2002/61/EC, which
further tightened and expanded the scope of restrictions. On 28 May 2009, the EU adopted
Decision 2009/425/EC to impose additional restrictions on the use of dibutyltin (DBT),
dioctyltin (DOT), and tributyltin (TBT) compounds, serving as a technical supplement to
Directive 76/769/EEC. On 1 June 2009, Annex XVII of the REACH Regulation officially
replaced Annex I of Directive 76/769/EEC, continuing the restrictions. Subsequently, on
4 June 2009, the decision explicitly prohibited the use of DBT, DOT, and TBT compounds.
In 2010, the decision was incorporated into Annex XVII of the REACH Regulation through
Regulation (EU) No. 276/2010, which, from 1 July 2010, imposed a maximum tin content
limit of 0.1 wt. % for products containing tributyltin and triphenyltin compounds. In 2008,
the International Maritime Organization (IMO) imposed a complete ban on using organotin
compounds in marine coatings [101].

3.2. Low-Surface-Energy Antifouling Coatings

Low-surface-energy antifouling coatings primarily work by reducing the adhesion
force between fouling organisms and the coating surface, thereby preventing the attach-
ment of marine organisms. These coatings exhibit low surface energy, viscosity, and
contact angles, making it difficult for marine fouling organisms to adhere effectively to the
surface [102–105].
Marine low-surface-energy antifouling coatings are divided into silicone-based and
fluorine-based systems, including polysiloxanes, fluorinated polyurethanes, silicone
polyurethanes, and fluorinated silicones. Due to the presence of Si-O bonds in their
main chain, polysiloxanes exhibit hydrophobicity and weather resistance, making them a
typical low-surface-energy coating. However, polysiloxanes have limitations in terms of
mechanical properties and adhesion to substrates, often requiring copolymerization or
grafting with other resins for modification. For example, polyurethane modification can
improve mechanical properties, while epoxy resin modification can enhance heat resistance
and adhesion [106].
In addition to silicone-based coatings, fluorinated coatings are widely used due to their
excellent hydrophobic properties and low surface energy. Polytetrafluoroethylene (PTFE)
has a high water contact angle of 114◦ , and its surface microporosity makes it vulnerable to
microbial attachment. As a result, it is commonly combined with other resins. Incorporating
fluorinated polymers can improve the coating’s antifouling performance and durability.
For instance, acrylic resins modified with perfluoroalkyl acrylate exhibit excellent hardness,
impact resistance, and superior low surface energy properties. Fluorinated resins’ high
costs and molding temperatures limit their practical application [107].
Researchers have also been exploring combining silicone- and fluorine-based systems
to improve the performance of low-surface-energy coatings. Fluorinated polysiloxane
merges the high elasticity of Si–O bonds with fluorinated side chains that reduce surface
energy, effectively integrating the antifouling properties of both silicone and fluorinated
resins. For example, Sun et al. [108] introduced silicone and fluorine into acrylic resin,
producing a coating with superior overall performance. To improve the efficacy of low-
surface-energy coatings against bacteria and other fouling organisms, researchers have
 ane merges the high elasticity of Si–O bonds with fluorinated side chains that reduce sur-
face energy, effectively integrating the antifouling properties of both silicone and fluori-
nated resins. For example, Sun et al. [108] introduced silicone and fluorine into acrylic
resin, producing a coating with superior overall performance. To improve the efficacy of
Coatings 2024, 14, 1487 low-surface-energy coatings against bacteria and other fouling organisms,9 of researchers
39
have explored composite modifications by incorporating antimicrobial agents. For in-
stance, Oktay et al. [109] introduced nanosilver into polysiloxane coatings, enhancing the
explored composite
antimicrobial modifications
properties by incorporating
and maintaining the lowantimicrobial agents.
surface energy For instance,This ap-
characteristics.
Oktay et al. [109] introduced nanosilver into polysiloxane coatings, enhancing
proach resulted in composite antifouling coatings with excellent antifouling the antimi-
and antimi-
crobial properties and maintaining the low surface energy characteristics. This approach
crobial performance.
resulted in composite antifouling coatings with excellent antifouling and antimicrobial
performance.
3.3. Biomimetic Antifouling Coatings
3.3. Biomimetic
Biomimetic Antifouling Coatings
antifouling coatings replicate the antifouling mechanisms found in na-
ture,Biomimetic antifouling such
utilizing principles coatings
as replicate the antifouling mechanisms
superhydrophobicity, found in nature,
micro/nanostructure replication,
utilizing principles such as superhydrophobicity, micro/nanostructure replication,
and chemical mimicry to inhibit the attachment of fouling organisms. A significant break- and
chemical mimicry to inhibit the attachment of fouling organisms. A significant break-
through in this field is the creation of “shark skin” surfaces that replicate the microscopic
through in this field is the creation of “shark skin” surfaces that replicate the microscopic
structure of natural shark skin. The surface of shark skin is highly intricate, consisting of
structure of natural shark skin. The surface of shark skin is highly intricate, consisting of
small arrayscalled
small arrays calledplacoid
placoid scales
scales or dermal
or dermal denticles.
denticles. TheseThese denticles
denticles exhibit exhibit riblet-like
riblet-like
morphology,
morphology, with with nanostructured
nanostructured protuberances
protuberances on theofsurfaces
on the surfaces of their
their grooves, grooves, as
as shown
shown
in Figure 3 [110,111]. These surfaces achieve antifouling through physical surface modi- surface
in Figure 3 [110,111]. These surfaces achieve antifouling through physical
modifications
fications withoutwithout
chemicalchemical
additives,additives,
providingproviding an experimental
an experimental example of biomi-
example of biomimetic
surfacesurface
metic design. design.

Figure Morphology
Figure 3.3.Morphology of of shark
shark skin
skin andand shark
shark skin-patterned
skin-patterned surfaces.
surfaces. A. Detailed
A. Detailed images showing
images showing
the dermaldenticles
the dermal denticles from
from different
different partsparts
of theofshark’s
the shark’s body.
body. The The
scale scale
bars are bars
1 mmare
for1images
mm for images A-
A-1,
1, A-2,
A-2, andand
A-4,A-4,
andand 500for
500 µm µmimage
for image A-3 [111].
A-3 [111].

Research on other biomimetic antifouling coatings has been initiated, inspired by

shark skin biomimetic coatings. Scardino et al. [112] explored the potential of biomimetic
surfaces by studying a variety of natural surfaces, including gorgonians, marine mammal
skin, and pumpkin shells. Their work highlights the diverse possibilities of nature-inspired
antifouling strategies. Yin et al. [113] used femtosecond laser direct writing to create
superhydrophobic biomimetic surfaces on polytetrafluoroethylene (PTFE), resulting in
excellent self-cleaning and mechanical stability. Liu et al. [114,115] designed an intelligent
 Research on other biomimetic antifouling coatings has been initiated, inspired by
shark skin biomimetic coatings. Scardino et al. [112] explored the potential of biomimetic
surfaces by studying a variety of natural surfaces, including gorgonians, marine mammal
skin, and pumpkin shells. Their work highlights the diverse possibilities of nature-in-
Coatings 2024, 14, 1487 spired antifouling strategies. Yin et al. [113] used femtosecond laser direct writing to cre-
10 of 39
ate superhydrophobic biomimetic surfaces on polytetrafluoroethylene (PTFE), resulting
in excellent self-cleaning and mechanical stability. Liu et al. [114,115] designed an intelli-
gent superhydrophobic
superhydrophobic surface surface that dynamically
that dynamically adjustsadjusts its wettability
its wettability based on based on external
external stimuli
like pressure or light, providing adaptive antifouling functionality. Schultz et al. [116]etcon-
stimuli like pressure or light, providing adaptive antifouling functionality. Schultz al.
[116] contributed by developing a novel antifouling coating inspired
tributed by developing a novel antifouling coating inspired by the fibrous microstructure by the fibrous micro-
structure
of dolphinofskin.dolphin
Using skin. Using
a Laser a LaserVelocimeter,
Doppler Doppler Velocimeter,
the impact ofthebiofilm
impactcomposition
of biofilm com- and
thickness on surface friction in turbulent boundary layer flows was measured. Xuwas
position and thickness on surface friction in turbulent boundary layer flows et al.meas-
[117]
ured. Xudemonstrated
further et al. [117] further demonstrated
that surfaces that surfaces
with specific with specific
microstructures, suchmicrostructures,
as 3 µm pillarssuch and
as 3 µm pillars and 12 µm grooves, exhibit strong resistance to algae
12 µm grooves, exhibit strong resistance to algae attachment, validating the effectiveness of attachment, validat-
ing the effectiveness
biomimetic of biomimetic
surface structures surface structures in antifouling.
in antifouling.
The Nepenthes (pitcher
The Nepenthes (pitcher plant) has plant) has unique
unique insect-catching
insect-catching capabilities,
capabilities, and
and itsits leaf
leaf
surface micro/nano-structure increases roughness, providing
surface micro/nano-structure increases roughness, providing excellent hydrophobicity excellent hydrophobicity
and anti-fouling properties, making it difficult difficult forfor contaminants
contaminants to to adhere
adhere toto the
the surface.
surface.
These features inspire the design of slippery liquid-infused porous porous surfaces
surfaces (SLIPSs)
(SLIPSs) coat-
coat-
ings. Li et al. [118]
[118] showed
showed that that SLIPS
SLIPS exhibit
exhibit significantly
significantly better anti-fouling
anti-fouling performance
compared to regular hydrophobic surfaces. Li Li et
et al.
al. [119]
[119] proposed
proposed aa method
method for for fabricat-
fabricat-
ing SLIPS with dual protection by using mold-based lithography to create a pyramidal
microstructure on
microstructure onaapolydimethylsiloxane
polydimethylsiloxane(PDMS) (PDMS)coating,
coating, followed
followed byby infusion
infusion withwith a
a lu-
lubricant
bricant to to complete
complete the the
SLIPS.SLIPS. Figure
Figure 4a,c 4a,c
showshow the contact
the contact angle angle
changes changes
on smoothon smooth
PDMS
PDMS (s-PDMS)
(s-PDMS) and rough and rough
PDMS PDMS (r-PDMS) (r-PDMS)
surfaces. surfaces. The r-PDMS
The r-PDMS surfacesurface
has ahas a micro-
microstruc-
ture that forms
structure a thinaair
that forms thinlayer, resulting
air layer, in a larger
resulting contactcontact
in a larger angle compared
angle comparedto s-PDMS.
to s-
Figure
PDMS.4b,dFigure compare the contact
4b,d compare the angle
contact changes after lubricant
angle changes infusion infusion
after lubricant on different surfaces.
on different
On the s-PDMS
surfaces. On thesurface,
s-PDMSthe lubricant
surface, thereplaces
lubricantthe water–PDMS
replaces interface, decreasing
the water–PDMS interface, de- the
contact
creasingangle and making
the contact angle and the surface
makingmore hydrophobic.
the surface On the r-PDMS
more hydrophobic. On thesurface,
r-PDMS the
lubricant
surface, thefillslubricant
the original
fillsair
thelayer, making
original the surface
air layer, making more hydrophilic.
the surface morePerformance
hydrophilic. testsPer-
show
formancethat,tests
in a show
static that,
marine in aenvironment,
static marineSLIPSs exhibit SLIPSs
environment, very low algaevery
exhibit coverage,
low algaeand
after being immersed in a 3.5 wt. % NaCl solution for 400 days,
coverage, and after being immersed in a 3.5 wt. % NaCl solution for 400 days, SLIPS main-SLIPS maintain excellent
electrochemical stability.
tain excellent electrochemical stability.

(a) (b)

(c) (d)
4. Schematic
Figure 4.
Figure Schematic diagrams
diagrams of
of contact
contact interfaces
interfaces between
between the
the water
water droplets
droplets and
and surfaces:
surfaces: ((a)
a) s-
s-
PDMS, (b) s-PDMS-L, (c) r-PDMS, and (d) r-PDMS-L [119].
PDMS, (b) s-PDMS-L, (c) r-PDMS, and (d) r-PDMS-L [119].

Additionally, biomimetic
Additionally, biomimetic antifouling
antifouling coatings
coatings cancan utilize
utilize active
active small
small molecules
molecules syn-
syn-
thesized or extracted from marine organisms. For instance, bioactive
thesized or extracted from marine organisms. For instance, bioactive compoundscompounds likelike
al-
kaloids extracted from red algae or peptides isolated from sponges have demonstrated
alkaloids extracted from red algae or peptides isolated from sponges have demonstrated
significant antifouling
significant antifouling efficacy.
efficacy. Darya
Darya et
et al.
al. [120]
[120] found
found that extracts from
that extracts different sea
from different sea
cucumber organs
cucumber organs inhibited
inhibited the
the growth
growth of barnacle larvae
of barnacle larvae and
and other
other fouling
fouling organisms,
organisms,
though their effectiveness in coatings requires further research. To improve antifouling
under a static conditions, Zhu et al. [121] the organic phosphorus scale inhibitor DTPMPA
was introduced into a superhydrophobic coating, creating the DTPMPA superhydropho-
bic anodized aluminum (DSAA) coating. This coating combines physical and chemical
antifouling mechanisms, proving effective in dynamic and static environments.
 bic anodized aluminum (DSAA) coating. This coating combines physical and chemical
antifouling mechanisms, proving effective in dynamic and static environments.

3.4. Conductive Anti-Fouling Coating

Coatings 2024, 14, 1487 11 of 39
The anti-fouling mechanism of conductive coatings is based on their electrical con-
ductivity. When a specific current is applied in a seawater environment, the coating trig-
gers
3.4. an electrolytic
Conductive reaction,
Anti-Fouling producing compounds with anti-fouling properties. One
Coating
such compound is NaClO, a potent
The anti-fouling mechanism of conductive biocide that oxidizes
coatings andon
is based disrupts the cellconduc-
their electrical membranes
oftivity.
microorganisms
When a specific andcurrent
fouling is organisms,
applied in a leading
seawaterto the leakagethe
environment, of cellular contents and
coating triggers
interference with metabolic processes. Simultaneously, it
an electrolytic reaction, producing compounds with anti-fouling properties. One disrupts enzymes andsuchproteins
within
compoundthe organisms,
is NaClO, afurther
potent inhibiting
biocide thatmetabolism.
oxidizes and It also degrades
disrupts microbial biofilms,
the cell membranes of
microorganisms
reducing and fouling
the likelihood organisms,
of fouling leading
organisms to the leakage
attaching of cellular
to surfaces contents
like ship hulls and
or pipe-
interference with metabolic processes. Simultaneously, it disrupts
lines, inhibiting their growth and reproduction to achieve an anti-fouling effect. enzymes and proteins
within
Thisthetechnology
organisms, further
was first inhibiting metabolism.
introduced It also degrades
by Mitsubishi Heavymicrobial
Industriesbiofilms,
in the re-1990s,
ducing the likelihood of fouling organisms attaching to surfaces like ship hulls or pipelines,
incorporating conductive agents such as copper, aluminum, and molybdenum into coat-
inhibiting their growth and reproduction to achieve an anti-fouling effect.
ings [122]. By applying a small electric current in seawater environments, the coatings
This technology was first introduced by Mitsubishi Heavy Industries in the 1990s,
initiate an electrochemical
incorporating conductive agents reaction,
such generating hypochlorite
as copper, aluminum, ions and other
and molybdenum intoactive
coat- sub-
stances thatBy
ings [122]. prevent
applying thea adhesion andcurrent
small electric fouling of marine
in seawater organisms. the
environments, In recent
coatingsyears,
initi- inno-
vations in conductive antifouling
ate an electrochemical coatingshypochlorite
reaction, generating have advanced ions andthe other
materialsactiveand mechanisms
substances
that prevent
used. the adhesion
For instance and fouling
[123], carbon of marine
nanotubes organisms.
(CNT) In recent years,
and graphene have innovations
been introducedin as
conductive
fillers antifouling
to enhance coatings have
the coatings’ advancedand
conductivity the materials
stability. and mechanisms
Research showsused.that For
applying
instance [123],
low-voltage carbon
pulses to nanotubes (CNT) and
these conductive graphene
coatings have abeen
creates introduced
hydrogen as fillers
bubble layertoon the
enhance the coatings’ conductivity and stability. Research shows that applying low-voltage
surface, acting as a physical barrier that significantly reduces bacterial adhesion by up to
pulses to these conductive coatings creates a hydrogen bubble layer on the surface, acting
99.5%, offering substantial antibacterial and antifouling effects.
as a physical barrier that significantly reduces bacterial adhesion by up to 99.5%, offering
substantial et
Zhang al. [124] developed
antibacterial and antifouling a porous,
effects. non-locking cathode structure using CNT-
PVDF Zhangthat generates negative
et al. [124] developed charges through
a porous, capacitive
non-locking charging.
cathode Figure
structure 5 illustrates
using CNT- the
impact of ionic strength and cathode potential on membrane
PVDF that generates negative charges through capacitive charging. Figure 5 illustratespermeability in thethecapaci-
impact
tive of ionic strength
antifouling system.and Figurecathode potential
5a shows onas
that membrane permeability increases,
NaCl concentration in the capacitive
membrane
antifouling system.
permeability Figure
decreases, 5a shows that
indicating that as NaCl ionic
higher concentration
strengthincreases,
weakensmembraneelectrostaticper-repul-
meability decreases, indicating that higher ionic strength weakens
sion, allowing organic matter to adhere more easily. Figure 5b demonstrates that mem- electrostatic repulsion,
allowing organic matter to adhere more easily. Figure 5b demonstrates that membrane
brane permeability improves as the cathode potential becomes more negative, suggesting
permeability improves as the cathode potential becomes more negative, suggesting en-
enhanced antifouling performance. This porous, non-locking cathode structure effectively
hanced antifouling performance. This porous, non-locking cathode structure effectively
prevents
preventsorganic
organic substance attachment.Compared
substance attachment. Compared to traditional
to traditional copper-based
copper-based coatings,
coatings,
these conductive antifouling coatings offer better environmental compatibility,
these conductive antifouling coatings offer better environmental compatibility, long-lasting long-last-
ing antifouling
antifouling performance,
performance, andreduced
and the the reduced
releaserelease
of harmfulof harmful
chemicals. chemicals.

Effectofofionic
Figure5.5.Effect
Figure ionicstrength
strength ((a) and cathode
a) and cathode potential
potential(b)
(b)ononmembrane
membranepermeability in ain a ca-
permeability
capacitive antifouling system
pacitive antifouling system [124]. [124].

Huang et al. [125] proposed a biomimetic antifouling interface coating that mimics the
antifouling properties of biofilms and improves the coating’s conductivity. This coating
uses polyethylene glycol gel as the support structure and electron transfer layer, with
a hydration layer made of phospholipids and ampholytes. This coating shows low ab-
sorption in biological matrices, opening new possibilities for the practical application of
conductive coatings.
Coatings 2024, 14, 1487 12 of 39

3.5. Photocatalytic Antifouling Coatings

Photocatalytic antifouling coatings are based on TiO2 particles, a semiconductor
material [126,127]. Upon exposure to ultraviolet light, electrons in TiO2 are elevated from a
lower energy state to a higher one, creating negatively charged electrons and positively
charged holes. These electrons and holes react with water or oxygen on the surface,
producing highly oxidative free radicals and other reactive species that break down organic
substances adhered to the coating, achieving antifouling effects. TiO2 is a harmless natural
mineral, and the oxidizing agents it produces have a short lifespan, leaving no long-term
environmental impact [128]. Studies have demonstrated that TiO2 -treated surfaces can
effectively prevent the adhesion of diatoms and bryozoan larvae when exposed to light. In
a 24 h cycle, over 80% of the larvae died before attaching. After 48 h, the remaining larvae
failed to develop into active organisms, further proving the strong inhibitory effect of this
coating in the early stages of biofouling [129,130].
This coating faces challenges in low-UV areas, such as shaded spots and the underside
of ships, where the photocatalytic process is not activated. Researchers are modifying
TiO2 to work with visible light to improve its performance in low-light conditions. Se-
lim et al. [131] developed an environmentally friendly, UV-visible, silicone-rich, spherical
TiO2 nanocomposite material for ship applications. Using a polymer solution injection
method, various concentrations of nanofillers were mixed into the silicone nanocompos-
ite. The study found that the optical films of single-crystal TiO2 combined with a vinyl
polydimethylsiloxane matrix enhanced photocatalytic activity. Researchers have also
conducted experiments to explore the commercial applications of photocatalytic coating
technology [132,133]. The photocatalyst company developed a commercial marine antifoul-
ing coating based on this photocatalytic principle called Ecotio2 [134]. Ecotio2 utilizes
nano-sized titanium dioxide, which is activated when exposed to sunlight or artificial
light sources to generate electrons and holes. These further react with oxygen and water
molecules in the air, producing highly oxidative free radicals. These free radicals effectively
break down organic pollutants such as oils, bacteria, and biofilms, preventing marine
fouling organisms like barnacles and algae from attaching to the ship’s surface.

3.6. Other Antifouling Coatings

In addition to the aforementioned antifouling methods, several new types of coat-
ings have also gained attention in marine antifouling research. Silicate-based antifouling
coatings, which use silicates as their main component, form a highly alkaline protective
layer in marine environments [135–137]. This protective layer alters the environmental
conditions on the surfaces of ships or structures, making them less favorable for the attach-
ment of marine organisms. Additionally, silicate coatings exhibit excellent hydrophilicity
and self-cleaning properties, and compared to traditional toxic coatings, they are more
environmentally friendly.
Biodegradable antifouling coatings have also become a research hotspot. Due to
numerous microorganisms and enzyme-secreting marine organisms, polymer coatings
composed of biodegradable polymers gradually degrade in seawater through microbial
activity. This continuous degradation leads to a renewed smooth surface, causing the
detachment of fouling organisms and achieving antifouling effects [138]. Yang et al. [139]
synthesized copolymers of caprolactone and acrylic acid using phosphazene base catalysis,
adjusting crystallinity and hydrophilic/hydrophobic properties to control the degradation
and mechanical performance of the copolymers. Abid Ali et al. [140] synthesized biodegrad-
able polyurethanes using ε-caprolactone, cyclohexyl isocyanate, and 1,4-butanediol, and
modified them by incorporating clay and antifouling agents. Polarized light microscopy
showed that the clay effectively controlled the size of polycaprolactone spherulites, regulat-
ing the degradation rate and significantly inhibiting the adhesion of E. coli and diatoms.
Zwitterionic polymer antifouling coatings are hydrophilic and prevent the attachment
of fouling organisms by balancing anionic and cationic groups [141–144]. Recent studies
have explored adding zwitterionic-functionalized nanoparticles to these coatings to en-
Coatings 2024, 14, 1487 13 of 39

hance their strength and antifouling properties. For example [145–147], nanoparticles like
sulfobetaine methacrylate-functionalized silica have shown good results in preventing the
adhesion of bacteria and fungi. Although these coatings hold promise, challenges like
nanoparticle agglomeration must be addressed. Further research is needed to optimize
their performance for marine applications.
Ling et al. [148] compared the adhesion of Chlorella vulgaris on titanium dioxide
surfaces with different microstructures and experimentally studied the antifouling perfor-
mances of these surfaces. The study used the Ostwald ripening method to fabricate nano-
needle and hierarchical sisal-like titanium dioxide surfaces. The results of the Chlorella
vulgaris adhesion tests show that all titanium-based structures exhibited antifouling perfor-
mances an order of magnitude higher than that of clean glass slides. Significant differences
were observed between Ti-Zr samples and flat Ti samples, with the coverage of flat Ti
being comparable to that of the Ti-needle structure, while the bulk metallic glass sisal-like
structure consistently exhibited the lowest biofouling values. Compared to smooth glass,
the sisal-like nanostructure reduced biofouling by approximately 330 times over one month,
while the reduction for smooth titanium surfaces was only 10 times. The contact area and
Wenzel roughness of the coatings significantly influenced the level of biofouling on the
surface, but their mechanisms of action differed. When the contact area was low, such as
0.5 µm2 for Ti-needle and Ti-Zr samples, biofouling levels did not significantly decrease.
However, for bulk metallic glass samples with extremely low contact areas, biofouling was
significantly reduced. Furthermore, only very high Wenzel roughness, as in bulk metallic
glass sisal-like structures, significantly reduced biofouling, whereas samples with Wenzel
roughness values ranging from 1 to 11.8, such as flat Ti surfaces, did not show significant
effects. Semi-field tests further validated the superior performance of the bulk metallic
glass structure, which had a total attachment rate of only 2.5 percent, demonstrating the
most effective antifouling performance. These results indicate that photocatalytic self-
cleaning properties and structure-based antifouling characteristics play a significant role in
combating biofouling.
Another study by Ling et al. [149], based on the properties of low elastic modulus and
fouling release mechanisms, proposed the use of calcium alginate (Ca-Alg) self-polishing
coatings as an anti-biofouling material. The research revealed that the physicochemical
properties of divalent copper and divalent calcium alginate bulk films significantly affect
their anti-biofouling performance. These physicochemical properties are highly dependent
on the composition ratio, which directly leads to substantial variations in the anti-biofouling
effectiveness of alginate films against Chlorella vulgaris. Notably, in the case of copper
alginate (Cu-Alg) coatings, the continuous release of copper ions and the self-polishing
effect significantly enhanced the anti-biofouling capability, reducing biofouling by up to
100 times compared to glass slides. Further testing in a natural river water environment
showed a decrease in the number of Chlorella vulgaris, though the total biofouling eventu-
ally reached saturation. The coverage rates for copper alginate and calcium alginate were
25% and 35%, respectively, which are comparable to glass. These findings indicate that by
appropriately adjusting the composition ratios of alginate materials, their anti-biofouling
performances can be effectively optimized.
Ubong Eduok et al.’s study [150] investigated the enhancement of sol-gel coatings for
the corrosion and antifouling protection of steel by encapsulating thermophilic Bacillus
licheniformis endospores. The hybrid sol-gel coating is also doped with zinc molybdate
(MOLY) and zinc aluminum polyphosphate (ZAPP) pigments. Figure 6 shows the ap-
pearance of three different samples—bare steel, abiotic coating (A), and biotic coating
(B)—after immersion for 10 weeks at various time intervals. The yellow circles and arrows
further highlight the key areas in the experimental results. The yellow circles specifically
indicate the coating edges, which serve as attachment points for hard-shelled barnacles.
These barnacles are not only adhered to the coating surface but also extend onto the black
insulating tape covering the edges. The bare steel panels immediately showed signs of
corrosion upon immersion in seawater, whereas the coated samples remained corrosion-
 ance of three different samples—bare steel, abiotic coating (A), and biotic coating (B)—
after immersion for 10 weeks at various time intervals. The yellow circles and arrows fur-
ther highlight the key areas in the experimental results. The yellow circles specifically in-
dicate the coating edges, which serve as attachment points for hard-shelled barnacles.
Coatings 2024, 14, 1487
These barnacles are not only adhered to the coating surface but also extend onto the14black of 39
insulating tape covering the edges. The bare steel panels immediately showed signs of
corrosion upon immersion in seawater, whereas the coated samples remained corrosion-
free. Over time,
free. Over time, all
all coatings
coatings gradually faded and
gradually faded and appeared
appeared dull. For the
dull. For the abiotic
abiotic coating
coating
(A), surface pitting was observed at the 2-week mark, with increasing delamination
(A), surface pitting was observed at the 2-week mark, with increasing delamination as the as
the exposure time lengthened. Among all the abiotic coatings, the one containing
exposure time lengthened. Among all the abiotic coatings, the one containing ZAPP pig- ZAPP
pigments
ments (SZ) (SZ) retained
retained a relatively
a relatively clear
clear surface
surface throughoutthe
throughout thestudy,
study,but
butfailed
failedby
by week
week 8,
8,
while inhibitor-modified coatings (SZ and SM) demonstrated better corrosion
while inhibitor-modified coatings (SZ and SM) demonstrated better corrosion resistance. resistance.

(a) (b)
6. Field
Figure 6.
Figure trial samples
Field trial samples inin duplicates:
duplicates: ((a)
a) abiotic-
abiotic- and
and (b)
(b) biotic-coated
biotic-coated steel
steel panels
panels compared
compared
with bare/uncoated steels [150].
with bare/uncoated steels [150].

For the
For the biotic
biotic coating,
coating,encapsulated
encapsulatedBacillus
Bacilluslicheniformis
licheniformisendospores
endospores exhibited
exhibited ex-
excel-
cellent antifouling and anticorrosive properties. While surface delamination
lent antifouling and anticorrosive properties. While surface delamination began around began
around
week week
4 for the4biotic
for the biotic the
coating, coating, theremained
coating coating remained
intact by intact
week by
10, week
showing10, showing
superior
superior performance
performance compared compared to steel,
to the bare the bare steel,
which which experienced
experienced significant
significant biological
biological fouling,
fouling, including
including the attachment
the attachment of barnacles
of barnacles and otherand other organisms.
organisms. The
The biotic biotic coatings,
coatings, particu-
particularly
larly those
those with thewith the bacterial
bacterial endospores,endospores, showed significant
showed significant antifoulingantifouling
effects andeffects
better
and betterresistance
corrosion corrosion than
resistance than the
the abiotic abioticThis
coatings. coatings.
study This study demonstrates
demonstrates that in-
that incorporating
corporating
bacterial bacterial endospores
endospores into sol-gel
into sol-gel coatings, coatings,
along along with
with doping withdoping
MOLYwithandMOLY
ZAPP andpig-
ZAPP pigments,
ments, significantlysignificantly improves
improves their their antifouling
antifouling and anticorrosion
and anticorrosion performance performance
in marine
environments.
in marine environments.
High-entropy alloy coatings have also become a research focus in metal-based antifoul-
ing coatings. Unlike traditional alloys composed of one or a few elements, high-entropy
alloys are characterized by their multi-element solid solution structure. These alloys can
leverage the “cocktail effect” to enhance specific material properties through the random
distribution of multiple elements, making them promising for marine applications. Verma
et al. [140] applied laser cladding technology to prepare a CoCrCuFeNi HEA coating on
DMR 249A ship steel, and the results show an 88% reduction in E. coli growth, demonstrat-
ing excellent antibacterial performance. Gao et al. [151] used selective laser cladding to
create a CoCrFeCuNi HEA coating, which exhibited a nearly 90% inhibition rate against
Coatings 2024, 14, 1487 15 of 39

E. coli and over 75% inhibition against S. aureus, further validating its antifouling potential.
Zhou et al. [152] prepared Al0.4CoCrCuFeNi HEA using vacuum induction melting, and
their study showed an inhibition rate of nearly 100% against Pseudomonas aeruginosa and
99.99% against Bacillus subtilis. The release of high concentrations of Cu ions from the
HEA effectively inhibited bacterial growth and biofilm formation.
These new antifouling coatings show great potential in terms of environmental friend-
liness and antifouling performance, but further research is needed to assess their long-term
performance and applicability in natural marine environments.

4. Progress in Marine Anti-Corrosion Coatings

Research on marine anti-corrosion systems primarily focuses on polymer, metal-based
alloy, and ceramic-based coating systems [153–157]. Polymer anti-corrosion coatings are
the most widely used. Their anti-corrosion mechanism mainly involves physical barriers,
corrosion inhibition, and sacrificial anode cathodic protection. Metal-based coatings use
metals as the primary component, providing both anti-corrosion protection and a degree of
wear resistance. These coatings form a physical barrier to block corrosive substances and
offer cathodic protection through the sacrificial consumption of metal elements, preventing
substrate corrosion. Such coatings are suitable for marine environments and high-salinity
areas, and are commonly used for critical marine equipment components, offering wear
and corrosion resistance. Ceramic-based coatings are made from ceramic materials and
provide strong corrosion resistance and high-temperature stability. Typically composed
of materials like aluminum oxide, silicon carbide, and zirconium oxide, these coatings are
known for their high hardness, wear resistance, and chemical corrosion resistance. They
primarily protect equipment in high-temperature environments and under extreme marine
corrosion conditions.

4.1. Polymer Coatings

Polymer coatings are extensively used in marine anti-corrosion applications. Various
types, such as epoxy, polyurethane, and acrylic, have been developed to meet the demands
of protecting marine structures and equipment. The following sections will discuss key
types of polymer-based anti-corrosion coatings and their latest advancements.

4.1.1. Epoxy Anti-Corrosion Coating

Epoxy anti-corrosion coatings use epoxy resin as the primary film-forming material,
with molecular chains containing two or more epoxy groups. Commonly used epoxy
resins include bisphenol-A and phenolic epoxy resins. Due to their excellent mechanical
properties, acid and alkali resistance, reliable corrosion protection, and strong adhesion
to metal surfaces, epoxy coatings have become widely used anti-corrosion materials. The
epoxy and hydroxyl groups in epoxy resins can undergo ring-opening reactions with
amino and carboxyl groups and condensation reactions with hydroxyl and isocyanate
groups, facilitating modification. Researchers have recently developed various modified
epoxy resins to enhance corrosion resistance through chemical modification. For example,
introducing organosilicon resins can reduce surface energy and form hydrophobic layers,
improving corrosion resistance [158]. The interpenetrating polymer network structure
of acrylic resins increases the crosslink density of the coating, enhancing its water and
chemical resistance [159]. Adding polyaniline improves the conductivity of the coating and
provides a passivating effect on the substrate, further enhancing corrosion protection [160].
Epoxy coatings, known for their high adhesion, strength, chemical resistance, and durability,
are extensively applied in marine heavy-duty anti-corrosion fields.
In light of these characteristics, researchers have been exploring new ways to enhance
corrosion resistance through chemical modification. Narula et al. [161] studied two novel
epoxy resins, ERH and ERO, as corrosion inhibitors for carbon steel in acidic media. ERH,
hexaglycidyl N-phenylaminopropoxy-N,N,N-triphenyl propane-1,3-diamine, contains six
epoxy functional groups. At the same time, ERO, octaglycidyl N,N,N,N-tetraphenylamino-
 epoxy resins, ERH and ERO, as corrosion inhibitors for carbon steel in acidic media. ERH,
hexaglycidyl N-phenylaminopropoxy-N,N,N-triphenyl propane-1,3-diamine, contains
six epoxy functional groups. At the same time, ERO, octaglycidyl N,N,N,N-tetraphenyla-
mino-N,N,N,N-tetrapropoxy methylene dianiline, has eight epoxy groups. The heteroa-
Coatings 2024, 14, 1487 16 rings
of 39 can
toms in these resin molecules, such as nitrogen and oxygen, and the aromatic
form coordination bonds with the metal surface, providing corrosion protection through
electron donor mechanisms. The corrosion inhibition mechanism of ERH and ERO is
N,N,N,N-tetrapropoxy methylene dianiline, has eight epoxy groups. The heteroatoms
based on their chemical adsorption onto the carbon steel surface, forming a protective film
in these resin molecules, such as nitrogen and oxygen, and the aromatic rings can form
that
coordination the
prevents bondspenetration
with the metalof corrosive species. corrosion
surface, providing The oxygen and nitrogen
protection atoms in the
through electron
epoxy
donorgroups shareThe
mechanisms. electrons with
corrosion the d-orbitals
inhibition mechanismofofiron, forming
ERH and ERO isstable
based coordination
on their
bonds, thereby
chemical effectively
adsorption reducing
onto the carbon corrosion.
steel surface, forming a protective film that prevents
theFigure
penetration of corrosive
7 presents species. The
the molecular oxygen and
dynamics nitrogen results
calculation atoms inbased
the epoxy groups func-
on density
sharetheory
tional electrons withillustrating
(DFT), the d-orbitals of optimized
the iron, forming stable coordination
molecular structures bonds,
of RH thereby
and ERO in
effectively reducing corrosion.
their protonated states, along with the distribution of the highest occupied molecular or-
Figure 7 presents the molecular dynamics calculation results based on density func-
bital (HOMO), lowest unoccupied molecular orbital (LUMO), and electrostatic potential
tional theory (DFT), illustrating the optimized molecular structures of RH and ERO in their
(ESP). The distribution of HOMO and LUMO shows that protonated ERO has a lower
protonated states, along with the distribution of the highest occupied molecular orbital
HOMO-LUMO
(HOMO), lowest gap, indicating
unoccupied that its molecules
molecular can more
orbital (LUMO), easily transfer
and electrostatic electrons
potential (ESP).to the
metal
The surface, enhancing
distribution of HOMO adsorption
and LUMO onto the that
shows metal. This electron
protonated ERO hastransfer
a lowerallows
HOMO- it to pro-
tect
LUMO gap, indicating that its molecules can more easily transfer electrons to the metalmolec-
the metal more effectively from corrosion. The ESP map reveals changes in the
surface,
ular chargeenhancing adsorption
distribution afteronto the metal. This
protonation, electron
showing transfer
higher allows itcharge
negative to protect density
the metal
around the more effectively
aromatic ringsfrom
and corrosion.
epoxy groupsThe ESP map reveals
of ERO. changes inits
This increases theinteraction
molecular with
thecharge
metaldistribution after protonation,
surface, forming showing
a more stable higher negative
adsorbed charge density
layer through arounddonor–ac-
the electron the
aromatic rings and epoxy groups of ERO. This increases its interaction with the metal
ceptor mechanism, which blocks the penetration of corrosive media. Therefore, the opti-
surface, forming a more stable adsorbed layer through the electron donor–acceptor mecha-
mized molecular structure of ERO, with additional functional groups and improved
nism, which blocks the penetration of corrosive media. Therefore, the optimized molecular
charge distribution,
structure of ERO, with significantly
additional increases
functional electron
groups and density
improvedandcharge
enhances its corrosion
distribution,
protection performance, providing higher inhibition efficiency compared
significantly increases electron density and enhances its corrosion protection performance, to traditional
coatings andhigher
providing ERH.inhibition efficiency compared to traditional coatings and ERH.

Figure Optimized
7. 7.
Figure Optimizedstructures, HOMO,LUMO,
structures, HOMO, LUMO, and
and ESP
ESP of ERH
of ERH and and
EROERO
[161].[161].

Otherresearchers
Other researchershave
havealso
alsoexpanded
expanded onon epoxy
epoxy resin
resin modification.
modification. Hsissou
Hsissou R R et al.
et al. [162] synthesized an aromatic heteroatom-derived epoxy resin for carbon steel
[162] synthesized an aromatic heteroatom-derived epoxy resin for carbon steel protection, protec-
tion, introducing heteroatoms to enhance the molecular structure’s stability and chemical
introducing heteroatoms to enhance the molecular structure’s stability and chemical ac-
activity. The experimental results show that this epoxy resin improves adhesion and
tivity. The experimental results show that this epoxy resin improves adhesion and exhibits
exhibits superior corrosion resistance in acidic and salt spray environments compared
superior corrosion
to traditional resistance
epoxy resins. Caiin et
acidic andshowed
al. [163] salt spray
that environments
combining micacompared to tradi-
with organic
tional epoxyinresins.
molecules Cai et
epoxy resin al. [163]
creates showed
a modified that combining
mica-epoxy coating. mica with organic
The incorporation of molecules
mica
enhances the barrier effect of the coating, preventing corrosive agents from penetrating
the metal substrate and significantly improving corrosion resistance compared to standard
epoxy resins. Chonkaew et al. [164] enhanced the corrosion resistance of epoxy resin by
incorporating organic clay and nitrile rubber, making it suitable for protecting structural
steel in splash zones. Kathalewar et al. [165] modified epoxy resin with cashew nut shell
oil, increasing toughness threefold. Zhang et al. developed a water-based epoxy–styrene–
acrylate composite emulsion with excellent stability and corrosion resistance using a new
synthesis approach. They first modified epoxy resin with methacrylic acid, then formed
the emulsion with the modified epoxy–acrylate polymer as the core, polystyrene ester as
the intermediate layer, and carboxyl acrylate polymer as the shell. The findings indicate
 intermediate layer, and carboxyl acrylate polymer as the shell. The findings ind
the emulsion exhibited outstanding anti-corrosion properties. These modificati
significantly improved the anti-corrosion properties of epoxy resins, expanding
Coatings 2024, 14, 1487 plications in marine environments. The performances of these coatings
17 of 39 are de
mainly by the quality of the resin and curing agents, making the developmen
modifiers and curing agents a key focus for improving epoxy coatings.
that the emulsion exhibited outstanding anti-corrosion properties. These modifications
have significantly improved the anti-corrosion properties of epoxy resins, expanding their
4.1.2. Polyurethane
applications Anti-Corrosion
in marine environments. Coating of these coatings are determined
The performances
mainly Polyurethane
by the quality ofanti-corrosion
the resin and curing agents,
coatings making
consist the development
mainly of resin with i
of polyurethane,
modifiers and curing agents a key focus for improving epoxy coatings.
ular chains formed by polyols and isocyanates linked through urethane bonds.
thane
4.1.2. is often used
Polyurethane as a topcoat
Anti-Corrosion in protective coatings in marine environments. R
Coating
ersPolyurethane
have modified anti-corrosion coatingsto
polyurethane enhance
consist mainlyitsofanti-corrosion
polyurethane, with properties
its molecu- by incor
larepoxy
chains resins,
formed by polyolsresins,
silicone and isocyanates
fluorine linked through
resins, urethane bonds.
nano-silica, graphene,Polyurethane
and boron ni
is often Cai
usedetas al.
a topcoat
[166] developed a composite coating by grafting CeO2have
in protective coatings in marine environments. Researchers nanoparti
modified polyurethane to enhance its anti-corrosion properties by incorporating epoxy
carbon nanotubes and dispersing them into polyurethane. The test results show
resins, silicone resins, fluorine resins, nano-silica, graphene, and boron nitride.
coating
Cai et exhibited superior
al. [166] developed corrosioncoating
a composite and permeability
by grafting CeO resistance compared to ot
2 nanoparticles onto
urethane-based
carbon nanotubes and coatings.
dispersingYuthemet al.into
[167] investigated
polyurethane. Thethe effects
test resultsofshow
different
that diiso
this coating exhibited superior corrosion and permeability resistance
such as isophorone diisocyanate, 4,4′-dicyclohexylmethane diisocyanate, and h compared to other
polyurethane-based coatings. Yu et al. [167] investigated the effects of different diiso-
ylene diisocyanate, on polyurethane’s properties. Cao et al. [168] synthesized b
cyanates, such as isophorone diisocyanate, 4,4′ -dicyclohexylmethane diisocyanate, and
lignin polyoldiisocyanate,
hexamethylene (OH-EL) containing primary
on polyurethane’s aliphatic
properties. Caohydroxyl
et al. [168] groups
synthesized through a
nary reaction
bio-based without
lignin polyol a catalyst.
(OH-EL) containing They
primarythenaliphatic
polymerized
hydroxylOH-EL with HDI
groups through a to p
preliminary reaction without a catalyst. They then polymerized
thermosetting lignin-based polyurethane (LPU) coating with high lignin conten OH-EL with HDI to pro-
duce a thermosetting
in Figure 8. The lignin-based polyurethane that
study demonstrated (LPU)thecoating with high
coating lignin content,
exhibited excellent as anti-c
seen in Figure 8. The study demonstrated that the coating exhibited excellent anti-corrosion
properties.
properties. When When
the OH-ELthe content
OH-EL wascontent
54.8 wt. %, wasthe 54.8
coatingwt. %, the
achieved coating
a tensile strengthachieved
strength of 81.6 MPa. After immersion in a 3.5% NaCl solution
of 81.6 MPa. After immersion in a 3.5% NaCl solution for 0.5 h with a carbon steel substrate, for 0.5 h with a car
substrate,
the the coating
coating showed showed amodulus
a high impedance high impedance
(|Z| 0.01 Hz) modulus 10 Ω0.01
of 8.3 × (|Z| 10 2
·cm Hz), whichof 8.3 × 10
remained at 3.9 × 10 10 Ω ·cm2 after 40 days of immersion.
which remained at 3.9 × 10 Ω·cm after 40 days of immersion.
10 2

Figure
Figure 8. The
8. The synthesis
synthesis pathway
pathway ofand
of OH-EL OH-EL and arepresentation
a schematic schematic representation of the LPU anti
of the LPU anti-corrosion
coating
coating [166].
[166].

Xu et al. [169] investigated a functionalized multi-walled carbon nanotubes

(FMWCNTs)-reinforced multifunctional self-healing polyurethane (PU) composite
coating. Figure 9 illustrates the synthesis process of the PU-FM coating, where appropriate
polyurethane soft and hard segments were selected as the main chain, and crosslinked
through a Diels–Alder (DA) reaction to form side chains, with multi-walled carbon
nanotubes (FMWCNTs) incorporated into the polyurethane. The self-healing mechanism
of the coating primarily relies on the photo-thermal self-healing process. In the PU
matrix (PU-FM3), the DA reaction serves as the fundamental mechanism for self-healing,
where DA bonds break and regenerate at crack sites to repair damage. During the
 polyurethane soft and hard segments were selected as the main chain, and crosslinked
through a Diels–Alder (DA) reaction to form side chains, with multi-walled carbon nano-
tubes (FMWCNTs) incorporated into the polyurethane. The self-healing mechanism of the
coating primarily relies on the photo-thermal self-healing process. In the PU matrix (PU-
Coatings 2024, 14, 1487 FM3), the DA reaction serves as the fundamental mechanism for self-healing, 18 where
of 39 DA
bonds break and regenerate at crack sites to repair damage. During the photo-thermal
repair process, carbon nanotubes absorb infrared light energy and convert it into heat,
photo-thermal
promoting repair process,
the breakage of DA carbon
bonds nanotubes
and theabsorb infrared
formation light energy
of furan rings andandconvert
maleimide.
it into heat, promoting the breakage of DA bonds and the formation of furan rings and
After removing the light, the crack temperature rapidly drops, prompting the recombina-
maleimide. After removing the light, the crack temperature rapidly drops, prompting
tion
theofrecombination
the furan ringofand themaleimide,
furan ring and thereby completing
maleimide, therebythe self-healing
completing the repair. Self-heal-
self-healing
ingrepair.
performance tests
Self-healing showed that
performance testslow-power
showed that (0.25 W·cm )(0.25
low-power
2 infrared
W·cmlight
2 led tolight
) infrared poor re-
pair
ledresults,
to poorwhile
repair high power
results, while(1 W·cm
high 2) quickly raised
power (1 W·cm2 ) quicklytheraised
temperature but could
the temperature butcause
localized overheating.
could cause localized However,
overheating. using 1 W·cm
However, 2 power
using 1 W·cm 2 power
for 90 s successfully repaired the
for 90 s successfully
repaired
coating the coating
cracks. Undercracks.
these Under these conditions,
conditions, the tensilethe tensile of
strength strength of the coating
the coating was
was maintained
maintained at its original level, with a repair efficiency
at its original level, with a repair efficiency of up to 70%. of up to 70%.

Figure 9. Schematic illustration of synthesizing PU-FM coating [169].

Figure 9. Schematic illustration of synthesizing PU-FM coating [169].

4.1.3. Acrylic
4.1.3. Anti-Corrosion
Acrylic Anti-Corrosion Coating
Coating
Acrylic
Acrylic anti-corrosion coatings
anti-corrosion coatings use
useacrylic
acrylicresin
resinasas
thethe
primary
primaryfilm-forming
film-formingmaterial,
material,
offering excellent water resistance, corrosion protection, environmental
offering excellent water resistance, corrosion protection, environmental friendliness, and friendliness, and
fast curing properties. Rahman et al. [170] developed a modified acrylic coating by incorpo-
fast curing properties. Rahman et al. [170] developed a modified acrylic coating by incor-
rating iron oxide, epoxy resin, and ammonium sulfate with acrylic resin. The findings show
porating iron oxide, epoxy resin, and ammonium sulfate with acrylic resin. The findings
that the modified coating significantly enhanced the barrier against corrosive substances
show that the modified
and exhibited improvedcoating significantly
impermeability enhanced
and corrosion the barrier against corrosive sub-
resistance.
stancesHuang
and exhibited
et al. [171]improved
developed impermeability and corrosion
an acrylic-based coating combiningresistance.
an acrylate phosphate
Huang and
monomer et al. [171] developed
graphene oxide (GO).an acrylic-based
First, GO was modifiedcoating combining
using an acrylateanphosphate
acrylate phos-
phate monomer and graphene oxide (GO). First, GO was modified using an graphene
monomer through a thiol-Michael addition click reaction to produce modified acrylate phos-
oxide
phate (PGO), improving
monomer through its compatibility with
a thiol-Michael acrylicclick
addition resin. The PGO
reaction to was then uniformly
produce modified gra-
dispersed in a waterborne epoxy resin based on acrylic, forming a PGO/acrylic
phene oxide (PGO), improving its compatibility with acrylic resin. The PGO was then uni- composite
coating. The composite coating was applied evenly to the surface of carbon steel substrates
formly dispersed in a waterborne epoxy resin based on acrylic, forming a PGO/acrylic
using casting or spraying methods, as shown in Figure 10. After 40 days of immersion
composite coating. The composite coating was applied evenly to the surface of carbon
testing, the impedance modulus (|Z| 0.01 Hz) of the PGO/acrylic coating remained at a
steel substrates
high level. GOusing
formed casting or spraying
an effective methods,
barrier within as shown
the coating, in Figure
preventing the10. After 40 days
penetration
of of
immersion
corrosive testing,
agents suchthe as
impedance
water andmodulus
ions. The(|Z|uniform0.01distribution
Hz) of the of PGO/acrylic
PGO not onlycoating
remained at a high level. GO formed an effective barrier within the
enhanced the physical barrier properties of the coating, but also improved its adhesion coating, preventing
thethrough
penetration of corrosive
interaction agentsresin.
with the acrylic such as water and ions. The uniform distribution of
PGO not only enhanced the physical barrier properties of the coating, but also improved
its adhesion through interaction with the acrylic resin.
Coatings 2024, 14,14,
Coatings x FOR PEER REVIEW 19 of 39
2024, 14,1487
Coatings2024, x FOR PEER REVIEW 19 39
19 of 39

Figure 10. Preparation process of acrylic-based coating.

Figure 10. Preparation process of acrylic-based coating.
Figure 10. Preparation process of acrylic-based coating.
4.1.4. Graphene-Based
4.1.4. Graphene-Based Anti-Corrosion
Anti-Corrosion Coatings
Coatings
4.1.4. Graphene-Based Anti-Corrosion Coatings
Graphene-based anti-corrosion
Graphene-based anti-corrosion coatings,
anti-corrosion coatings, coatings, which
which useuse graphene,
graphene, graphene
graphene oxide,
oxide, and and
Graphene-based which use graphene, graphene oxide, and
their modified
their modified forms
forms as primary
as primary fillers,
fillers,provide
provide corrosion
corrosion resistance,
resistance, weatherability,
weatherability, and and
their modified forms as primary fillers, provide corrosion resistance, weatherability, and
wear
wearresistance.
resistance. These
These coatings also exhibit thermal and electrical conductivity, with gra-
wear resistance. Thesecoatings
coatingsalso also exhibit
exhibit thermal
thermal and andelectrical
electricalconductivity,
conductivity, with gra-
with
phene’s atomic-scale
phene’s atomic-scale thickness,
thickness, mechanical
mechanical properties,
properties, chemical
chemical stability,
stability, and andability
ability to to
graphene’s atomic-scale thickness, mechanical properties, chemical stability, and ability
actact
as as
a barrier
a barrier against
against gasesgases and andionsions contributing
contributing to toitsitspotential as asa useful anti-corro-
to act as a barrier against gases and ions contributing topotential
its potential a useful anti-corro-
as a useful anti-
sion nanofiller.
sion nanofiller.
corrosion nanofiller.
Wu Wu et et
al.al.
[172][172]synthesized
synthesized graphene
graphene oxide/polyaniline
oxide/polyaniline (GO/PANI)
(GO/PANI) composite
composite parti-
parti-
Wu et al. [172] synthesized graphene oxide/polyaniline (GO/PANI)
cles using
cles usinga chemical
a chemical oxidative
oxidative polymerization
polymerization method
method and and dispersed
dispersed them them in phenolic
in phenolic
cles using a chemical oxidative polymerization method and dispersed them in phenolic
resin with
resin withcyclohexanone
cyclohexanone and and tetrahydrofuran
tetrahydrofuran as assolvents
solvents to tocreate
create a composite coating.
resin with cyclohexanone and tetrahydrofuran as solvents to create aa composite
composite coating.
coating.
The electrochemical
Theelectrochemical
electrochemicaltest test results
testresults show
resultsshow that
showthat the
that impedance
the impedance modulus
modulus of the
of composite
the composite coat-
coat-
The the impedance modulus of the composite coating
inging
reached 4.4 × 10 10 Ω·cm2 at 0.01 Hz, significantly higher than the 1.28 × 107 Ω·cm2 of
reached 4.4 ×4.4
reached 10
10 × 10 Ω·cm10 2
Ω·cm 2
at 0.01at 0.01 Hz, significantly
Hz, significantly higherhigher than the than1.28the×1.2810 Ω7 × ·10 2
cm Ω·cm
7 of pure2 of
pure phenolic
pure phenolic
phenolic resin.
resin. resin. Mechanical
MechanicalMechanical performance
performanceperformance tests revealed
tests revealed
tests revealed that incorporating
that incorporating
that incorporating AOFGs
AOFGsAOFGs increasedin- in-
creased
the the
glass glass
transition transition
temperature, temperature,
storage storage
modulus, modulus,
creased the glass transition temperature, storage modulus, tensile strength, and yield
tensile tensile
strength, strength,
and yield and
strengthyield of
strength
the
strength of
phenolic the
of the phenolic
resin. resin.
The tensile
phenolic The
resin. The tensile
strengthtensilestrength
reached
strength reached
11.19 11.19
MPa,11.19
reached MPa,
compared compared
to only 1.82
MPa, compared to only
toMPa
only
1.82
for MPa
1.82pure
MPa for pure
phenolic
for pure phenolic
resin.
phenolicTheresin.
presence
resin.The The presence
of graphene
presence of oxide
graphene
of layersoxide
graphene layers
restricted
oxide layers restricted
the movement
restricted thethe
of
movement
the phenolic
movement of of
the
resin’s
thephenolic
molecular
phenolic resin’s molecular
chains,
resin’s thereby
molecular chains,
enhancing
chains, thereby the enhancing
thereby mechanical
enhancing the mechanical
properties
the mechanicalof the
properties
composite
properties ofcoating.
thethe
of composite coating.
Additionally,
composite coating. Additionally,
the aniline oligomers
Additionally, thethe aniline
in theoligomers
aniline composite
oligomers inparticles
thethe
in compositeformed
composite
particles
organic formed organic
interconnections interconnections
and hydrogen and
bonds hydrogen
with
particles formed organic interconnections and hydrogen bonds with the phenolic the bonds
phenolic with the
resin, phenolic
which greatlyresin, im-
resin,
which
proved greatly
interfaceimproved
interactions,interface interactions,
significantly significantly
enhancing the
which greatly improved interface interactions, significantly enhancing the coating’s cor- enhancing
coating’s the
corrosion coating’s
resistance cor-
and
rosion
rosionresistance
mechanical and
performance.
resistance andmechanical
mechanical performance.
performance.
Bai
Baiet al.
Baietetal. [173]
al.[173] modified
[173]modified graphene
modifiedgraphene graphene oxide
oxide
oxide (GO)
(GO)
(GO) using
using
using isophorone
isophorone
isophorone diisocyanate
diisocyanate
diisocyanate (IPDI)
(IPDI)
(IPDI)to
toobtain
obtain
to obtainmodified
modified
modified graphene
graphene
graphene oxide
oxide (MGO),
(MGO),
oxide (MGO), then
thenthen combined
combined
combined MGOMGO with
withwith
MGO polytetrafluoroeth-
polytetrafluoroethylene
polytetrafluoroeth-
ylene (PTFE)
(PTFE)
ylene (PTFE) to to
to modify modify WPU,
modify WPU, as as
as shown
WPU, shown in in
in Figure
shown Figure 11.11.
11. They
Figure They prepared
prepared
They MGO/WPU
MGO/WPU
prepared MGO/WPU coatings
coatings and
coatings
and MGO-PTFE/WPU
MGO-PTFE/WPU composite
composite coatings
coatings with
with varying
varying
and MGO-PTFE/WPU composite coatings with varying concentrations. The results show concentrations.
concentrations. The The results
results show show that
that
the the
that combined
combined
the combined useuse ofuseof of
MGO MGO andand
MGO PTFE
andPTFE significantly
significantly
PTFE improved
improved
significantly improved thethecorrosion
the corrosion resistance
resistance
corrosion of the
resistance
ofcoatings.
the coatings.
MGO MGO acted acted
as a as a
barrier barrier
to to
block block
the the diffusion
diffusion
of the coatings. MGO acted as a barrier to block the diffusion of corrosive agents. At of of corrosive
corrosive agents. agents.
At theAt thethe
same
same
time,time,
PTFE PTFE
enhancedenhanced the the dispersion
dispersion of MGOof MGO
and and
filled
same time, PTFE enhanced the dispersion of MGO and filled the gaps between particles, filled
the the
gaps gaps
between between particles,
particles, further
further
furtherextending
extending extending thethe
the diffusion diffusion
path of
diffusion path
the
pathof thethe
corrosive
of corrosive
medium.
corrosive medium.
medium.

Figure 11. Cont.

Coatings 2024, 14, x1487
FOR PEER REVIEW 2020ofof 39
39

Figure 11. The

Figure 11. The anti-corrosion
anti-corrosion mechanisms
mechanisms of
of the
the worn
worn pure WPU coating
pure WPU coating (a),
(a), the
the worn
worn MGO/WPU
MGO/WPU
composite coating ( b ), and the worn MGO-PTFE/WPU composite coating (c
composite coating (b), and the worn MGO-PTFE/WPU composite coating (c) [173].) [173].

4.1.5.
4.1.5. Polyurea
Polyurea Anti-Corrosion
Anti-Corrosion Coatings
Coatings
Polyurea
Polyurea is is an
anessential
essentialmaterial
materialfor for marine
marine corrosion
corrosion protection,
protection, typically
typically catego-
categorized
into three
rized types:types:
into three aromatic polyurea,
aromatic aliphatic
polyurea, polyurea,
aliphatic and PAE
polyurea, andpolyurea. Among
PAE polyurea. them,
Among
PAE polyurea,
them, an aliphatic
PAE polyurea, secondary
an aliphatic amine amine
secondary chain extender, reacts with
chain extender, reactsisocyanates slower
with isocyanates
and hasand
slower higher
has chemical inertness
higher chemical than traditional
inertness hydroxylhydroxyl
than traditional polyethers, offering superior
polyethers, offering
corrosioncorrosion
superior resistance. PolyureaPolyurea
resistance. coatings coatings
exhibit high tensile
exhibit highstrength, elongation
tensile strength, at break,
elongation
and
at excellent
break, adhesionadhesion
and excellent in variousinapplications.
various applications.
In related research,
In related research, Kanwal et al. [174]
et al. [174] developed
developed aa microencapsulated
microencapsulated coating coating using
using
an epoxy–polyamine matrix, with the hexamethylene diisocyanate monomer (HDMI)the
an epoxy–polyamine matrix, with the hexamethylene diisocyanate monomer (HDMI) as as
shell
the material
shell andand
material polythiourea formaldehyde
polythiourea formaldehyde (PTF)(PTF)
as theascore
the material. The electrochemi-
core material. The electro-
cal corrosion
chemical protection
corrosion of this coating
protection applied applied
of this coating to 304 stainless steel wassteel
to 304 stainless investigated.
was investi- The
results The
gated. show that temperature
results and pH significantly
show that temperature and pH affect microcapsule
significantly synthesis, and syn-
affect microcapsule elec-
trochemical
thesis, tests have indicated
and electrochemical teststhat
havethe PTF coating
indicated that exhibited good corrosion
the PTF coating exhibited resistance
good cor- in
simulated seawater environments. Research by Arunkumar et al. [175]
rosion resistance in simulated seawater environments. Research by Arunkumar et al. [175] demonstrated that
polyurea coatings
demonstrated that on low-carbon
polyurea steelon
coatings IS2062 and aluminum
low-carbon steel IS20625052-H32 substrates
and aluminum showed
5052-H32
notable corrosion resistance, achieving over 90% protection efficiency
substrates showed notable corrosion resistance, achieving over 90% protection efficiency in KOH, H 2 SO 4 , and
NaCl media, with strong adhesion to the metal substrates.
in KOH, H2SO4, and NaCl media, with strong adhesion to the metal substrates.
Beyond microencapsulation
Beyond microencapsulation techniques,
techniques, ZhangZhang etet al.
al. have
haveexplored
exploredfurther
furtheradvance-
advance-
ments in polyurea composites. They [176] synthesized sulfonated
ments in polyurea composites. They [176] synthesized sulfonated graphene (SG) by graphene (SG) bycova-
cova-
lently grafting sulfonic acid groups onto the edges of graphite nanosheets, incorporatingit
lently grafting sulfonic acid groups onto the edges of graphite nanosheets, incorporating
into a waterborne polyurea (WPUA) matrix to form a composite coating. Electrochemical
it into a waterborne polyurea (WPUA) matrix to form a composite coating. Electrochemi-
impedance spectroscopy (EIS) tests revealed that the 0.3% SG composite coating exhibited
cal impedance spectroscopy (EIS) tests revealed that the 0.3% SG composite coating ex-
the best corrosion resistance. Chen et al. [177] studied the properties of sulfonated elec-
hibited the best corrosion resistance. Chen et al. [177] studied the properties of sulfonated
troactive polyurea (S-EPU), finding that S-EPU coatings, compared to non-electroactive
electroactive polyurea (S-EPU), finding that S-EPU coatings, compared to non-electroac-
polyurea (N-EPU), demonstrated enhanced electrocatalytic activity, forming a denser passi-
tive polyurea (N-EPU), demonstrated enhanced electrocatalytic activity, forming a denser
vation layer on metal substrates, providing superior corrosion protection. Lyu et al. [178]
passivation layer on metal substrates, providing superior corrosion protection. Lyu et al.
investigated the durability of the QF-162 polyurea coating in seawater environments. After
[178] investigated the durability of the QF-162 polyurea coating in seawater environments.
120 days of immersion, the mechanical properties of the QF-162 coating showed only minor
After 120 days of immersion, the mechanical properties of the QF-162 coating showed
changes, with its chemical bond structure remaining stable, indicating excellent resistance
only minor changes, with its chemical bond structure remaining stable, indicating excel-
to seawater corrosion.
lent resistance to seawater corrosion.
 Coatings 2024, 14, x FOR PEER REVIEW 21 of 39
Coatings 2024, 14, 1487 21 of 39

4.1.6.Silicone
4.1.6. SiliconeResins
Resins
Silicone resins
Silicone resins are
are aa class
class ofofpolymers
polymerswith witha abackbone
backbone ofof
alternating
alternating silicon andand
silicon ox-
ygen atoms connected to organic groups. Polydimethylsiloxane
oxygen atoms connected to organic groups. Polydimethylsiloxane (PDMS) is a typical (PDMS) is a typical exam-
ple, characterized
example, characterized by itsbylow
its surface
low surface energy, highhigh
energy, flexibility, andand
flexibility, smoothness.
smoothness.
Silicone resins are used in anti-corrosion coatings,
Silicone resins are used in anti-corrosion coatings, primarily primarily asastopcoats,
topcoats, offering ex-
offering
cellent corrosion resistance, weatherability, and insulation properties.
excellent corrosion resistance, weatherability, and insulation properties. These coatings These coatings
formaahighly
form highlycross-linked
cross-linkedSi-O Si-Onetwork
networkstructure
structureafter
aftercuring,
curing,providing
providinggood goodheatheatand
and
weatherresistance,
weather resistance,similar
similartotofluoropolymer-based
fluoropolymer-basedcoatings. coatings.Yang
Yangetetal. al.[179]
[179]developed
developedaa
UV-curablefluoro-silicone
UV-curable fluoro-silicone coating
coating withwith excellent
excellent hydrophobicity
hydrophobicity and corrosion
and corrosion re-
resistance.
sistance. Experiments confirmed its effectiveness in protecting
Experiments confirmed its effectiveness in protecting tinplate, achieving the expected tinplate, achieving the ex-
pected anti-corrosion
anti-corrosion performance.
performance. In another In another
study, Yaostudy,
et al.Yao et created
[180] al. [180]acreated a corrosion-
corrosion-resistant
resistant composite
composite coating using coating using quaternary
quaternary ammonium-modified
ammonium-modified graphene oxide graphene oxide and
and organosili-
organosilicon
con polymer, with polymer, with showing
test results test results showingprotection
a corrosion a corrosion protection
efficiency efficiency
of 99.97%. Addi-of
99.97%. Zhang
tionally, Additionally, Zhang
et al. [181] et al. [181]
designed designed an epoxy-modified
an epoxy-modified silicone coating silicone
embeddedcoatingwith
em-
bedded withself-healing
polysiloxane polysiloxane self-healingThe
microcapsules. microcapsules. The coating
coating demonstrated demonstrated
self-healing self-
properties
healing
under UVproperties
and moisture underexposure,
UV and moisture exposure,
significantly significantly
improving improving
its mechanical its mechan-
strength and
corrosion resistance.
ical strength and corrosion resistance.
Another
Anotherinvestigation
investigationby byLiLietetal.
al.[182]
[182]produced
producedaabiomimetic
biomimeticanti-corrosion
anti-corrosioncoating coating
by
bycombining
combiningfunctionalized
functionalized MXene
MXene withwithan organic polyurethane
an organic polyurethanematrix.matrix.
Figure Figure
12 shows 12
that
showsa bionic
that aanti-corrosion coatingcoating
bionic anti-corrosion was prepared by firstbyfunctionalizing
was prepared first functionalizing MXene’s two-
MXene’s
dimensional
two-dimensional layered structure
layered with methionine,
structure with methionine, then creating an organic
then creating an organicpolyurethane
polyure-
coating with disulfide bonds. Finally, the functionalized MXene
thane coating with disulfide bonds. Finally, the functionalized MXene was combined was combined withwith
the
polyurethane
the polyurethane coating to form
coating to formthe the
composite
composite bionic
bioniccoating.
coating.The Theexperimental
experimentalresults results
demonstrate
demonstratethat thatthethecoating’s
coating’striple
tripleanti-corrosion
anti-corrosionmechanism—MXene’s
mechanism—MXene’s“maze “mazeeffect”,
effect”,
methionine’s
methionine’scorrosion
corrosioninhibition,
inhibition,and andthe theself-healing
self-healingproperties
propertiesof ofthe
the disulfide
disulfide bonds—
bonds—
led to a remarkable 140% self-healing efficiency and excellent
led to a remarkable 140% self-healing efficiency and excellent corrosion resistance. corrosion resistance.

Figure 12. Anti-corrosion process of bionic coating [182].

Figure 12. Anti-corrosion process of bionic coating [182].

4.1.7.Nanocomposites
4.1.7. Nanocomposites
Incorporatingnanoparticles
Incorporating nanoparticlesinto intopolymer
polymer matrices
matrices significantly
significantly improves
improves thethe physi-
physical
cal and
and chemical
chemical properties
properties of coatings,
of the the coatings, providing
providing enhanced
enhanced barrier
barrier effects
effects and and corro-
corrosion
sion resistance.
resistance. Li[183]
Li et al. et al.developed
[183] developed
a tannica acid
tannic acid (TA)-functionalized
(TA)-functionalized Ce-MMT Ce-MMT nano-
nanocompos-
composite and investigated its impact on the anti-corrosion properties of WPU
ite and investigated its impact on the anti-corrosion properties of WPU coatings. The results coatings.
The results
indicate that indicate that incorporating
incorporating TA@Ce-MMTTA@Ce-MMT
enhanced theenhanced
long-termthe long-term
corrosion corrosion
resistance of
resistance
the of the WPU
WPU coatings coatings
due to improveddue physical
to improved physical
barrier effectsbarrier
and theeffects and theaction
synergistic synergis-
be-
tic action
tween and Ce3+TA
TA between andAs
ions. 3+ ions. As
Cedepicted indepicted in Figure
Figure 13, 13, including
including MMT helps MMT helps
repair repair
internal
internalindefects
defects in theand
the coating coating and the
extends extends the diffusion
diffusion path of corrosive
path of corrosive substances.
substances. In anodic In
Coatings 2024, 14, x FOR PEER REVIEW 22 of 39
Coatings 2024, 14, 1487 22 of 39

anodic areas,
areas, when thewhen the pHdue
pH decreases decreases
to metaldue to metal ion
ion hydrolysis, TA hydrolysis,
is released in TA is released
response, and in re-
sponse,
3+ and Ce3+ ions, along with MMT, form insoluble precipitates at the cathodic sites,
Ce ions, along with MMT, form insoluble precipitates at the cathodic sites, preventing
preventing
oxygen ingress. oxygen
These ingress. These
components components
are then are
transformed intothen
moretransformed
stable Ce(OH)into more
4 or CeO 2 stable
Ce(OH) or CeOoff
particles,4 sealing 2 particles, sealing
the cathodic off the cathodic regions.
regions.

Figure Theanti-corrosion
13.The
Figure 13. anti-corrosion mechanism
mechanism of TA@Ce-MMT
of the the TA@Ce-MMT composite
composite in WPUincoatings
WPU coatings
[183]. [183].

4.2. Metal-Based Alloy Coatings

4.2. Metal-Based Alloy Coatings
Metal-based alloy coatings protect marine engineering equipment by forming a barrier
Metal-based alloy coatings protect marine engineering equipment by forming a bar-
layer. Due to the similar physical and chemical properties between the metal coatings
rier
and marineDue
layer. tosubstrates,
metal the similarthey
physical and chemical
form relatively strongproperties between the
adhesion, preventing metal coatings
premature
and marine during
detachment metal substrates, they
service caused byform relatively
property strongAdditionally,
differences. adhesion, preventing premature
metal coatings
detachment
typically meet the requirements for hardness and wear resistance. Various types of metalcoatings
during service caused by property differences. Additionally, metal
typically meetforthe
coatings used requirements
corrosion control, for hardness
including and wear resistance.
corrosion-resistant elementsVarious
such astypes
Ni, Cr,of metal
Ti, Zr, Mo,
coatings Nb,for
used Sb,corrosion
Cu, Al, Mn, Au, and
control, Ag, were
including studied in the early
corrosion-resistant years [184–186].
elements such as Ni, Cr,
TheZr,
Ti, primary focus
Mo, Nb, ofCu,
Sb, current research
Al, Mn, Au,onandmetal-based
Ag, were corrosion-resistant coatings
studied in the early yearsincludes
[184–186]. The
zinc–aluminum alloys, nickel-based alloys, and iron-based alloys, with relevant studies
primary focus of current research on metal-based corrosion-resistant coatings includes
also being conducted on novel materials such as high-entropy alloys.
zinc–aluminum alloys, nickel-based alloys, and iron-based alloys, with relevant studies
also
4.2.1.being conductedAlloy
Zinc-Aluminum on novel materials such as high-entropy alloys.
Coatings
As regards metal-based coatings, zinc–aluminum alloy coatings are relatively widely
4.2.1.
used inZinc-Aluminum Alloy
marine anti-corrosion Coatings Alloys combine the high electrochemical activity
applications.
of ZnAs with the chemical
regards stability
metal-based of Al. The
coatings, zinc–aluminumalloy
zinc–aluminum alloys used forare
coatings arc-spraying
relatively widely
typically contain 5% to 25% aluminum by mass. High-velocity arc-spraying
used in marine anti-corrosion applications. Alloys combine the high electrochemical technology can ac-
also produce pseudo-alloy coatings with even higher aluminum content. The commonly
tivity of Zn with the chemical stability of Al. The zinc–aluminum alloys used for arc-spray-
used material is a Zn-15% Al alloy coating, which consists primarily of Zn-rich and Al-rich
ing typically contain 5% to 25% aluminum by mass. High-velocity arc-spraying technol-
phases [187]. Li et al. [188] developed Zn-Al alloy coatings with varying aluminum contents
ogy can also produce
on low-carbon steel usingpseudo-alloy coatings method.
a hot-dip galvanizing with even Thehigher
resultsaluminum
show that as content.
the The
commonly used material is a Zn-15% Al alloy coating, which consists
aluminum content increased to 10% by mass, the adhesion and microhardness of the Zn-Al primarily of Zn-rich
and Al-rich
coatings phasessignificantly.
improved [187]. Li etAtal.the
[188] developed
same Zn-Al alloy
time, the coating’s coatings
impedance with varying
increased, the alu-
corrosioncontents
minum current density decreased,steel
on low-carbon and using
the overall corrosion
a hot-dip resistance method.
galvanizing improved,The as results
shownthat
show in Figure 14.aluminum
as the Furthermore, studiesincreased
content have confirmed
to 10% that
byby changing
mass, the aluminum
the adhesion and micro-
content, the properties of the alloy can be regulated.
hardness of the Zn-Al coatings improved significantly. At the same time, the coating’s
However, even though zinc–aluminum alloys can resist some corrosion, they are
impedance increased, the corrosion current density decreased, and the overall corrosion
still affected by pitting corrosion, which makes them less reliable in extreme conditions.
resistance improved, as shown in Figure 14. Furthermore, studies have confirmed that by
To address the corrosion issues in zinc and aluminum alloy coatings, researchers have
changing
found that the aluminum
adding elementscontent, the properties
such as magnesium, of the
silicon, andalloy can be
rare earth regulated.
metals significantly
enhances the corrosion resistance of the coatings. Su et al. [189] studied the corrosion
resistance of hot-dip aluminum–zinc alloy coatings with added silicon, rare earth elements
(RE), and magnesium in a sodium chloride environment. By comparing different coatings
such as Al-Zn-Si, Al-Zn-Si-RE, and Al-Zn-Si-RE-Mg, they found that adding magnesium
 Coatings 2024, 14, 1487 23 of 39

and rare earth elements improved the microstructure of the coatings and reduced defects.
In salt spray corrosion tests, after 168 h, the Al-Zn-Si coating showed extensive red rust,
while the Al-Zn-Si-RE and Al-Zn-Si-RE-Mg coatings had only minor rust. After 360 h,
the Al-Zn-Si coating was almost completely covered in red rust, whereas the other two
coatings still showed only minor rust. Electrochemical tests revealed that the Al-Zn-Si-
RE-Mg coating had the highest polarization resistance (Rp) at 2429.7 Ω/cm2 , compared to 23 of 39
Coatings 2024, 14, x FOR PEER REVIEW
843.5 Ω/cm2 for the Al-Zn-Si coating, indicating a significantly higher level of corrosion
resistance, particularly in marine-like environments.

(a) (b)
Figure14.
Figure (a)Polarization
14.(a)Polarization curves
curves andand (b) impedance
(b) impedance spectra
spectra of alloy
of Zn Al Zn Alcoatings
alloy coatings with differen
with different
Al contents.
Al contents.

4.2.2. However,
Nickel-Based Alloy
even Coatings
though zinc–aluminum alloys can resist some corrosion, they are stil
Nickel-based
affected by pitting coatings have which
corrosion, continuously
makesbeen them studied as materials
less reliable for marine
in extreme cor-
conditions. To
rosion resistance. Initial research focused on nickel-based coatings for
address the corrosion issues in zinc and aluminum alloy coatings, researchers have found marine corrosion
resistance, which later expanded to include corrosion-resistant elements and surface treat-
that adding elements such as magnesium, silicon, and rare earth metals significantly en
ments to enhance their protective properties further [190–193]. Among the notable studies,
hances
Li the corrosion
et al. [194] fabricated aresistance
Ni coatingof theexcellent
with coatings. Su et al. [189]
hydrophobic studied
properties thesurface
on the corrosion re
sistance
of of hot-dip
304 stainless steel,aluminum–zinc
achieving a water alloy coatings
contact anglewith added
of 149.4 silicon,
◦ . This rareprevents
coating earth elements
the substrate from contacting corrosive solutions and exhibits strong corrosion resistance.coatings
(RE), and magnesium in a sodium chloride environment. By comparing different
such as chromium
Adding Al-Zn-Si, Al-Zn-Si-RE,
and molybdenum and toAl-Zn-Si-RE-Mg,
nickel-based alloys they found that
enhances theiradding
pitting magnesium
and
crevice corrosion resistance.
and rare earth elements improved the microstructure of the coatings and reduced defects
Shao
In salt et al.
spray [195] investigated
corrosion tests, afterthe168
effect of Fe/Mn
h, the Al-Zn-Si content
coating on showed
copper–nickel alloyred rust
extensive
coatings’ wear and corrosion resistance. The results show that as the
while the Al-Zn-Si-RE and Al-Zn-Si-RE-Mg coatings had only minor rust. After 360 h, theFe content increased,
the hardness of the coating gradually improved, while the polarization resistance initially
Al-Zn-Si coating was almost completely covered in red rust, whereas the other two coat-
increased and then decreased, reaching a maximum of 19.81 kΩ·cm2 . Figure 15 illustrates
ingsmicrostructure
the still showed only minor rust.
of corrosion Electrochemical
products and the corrosiontests mechanism
revealed that the30Al-Zn-Si-RE-Mg
after days of
coating had the highest polarization resistance (Rp) at 2429.7
exposure to simulated seawater for copper-nickel alloys with different Fe contents. The Ω/cm 2, compared to 843.5

Ω/cm2 for
addition of the
Fe ledAl-Zn-Si
to densercoating, indicating
protective films of aCu significantly
2 (OH)3 Cl andhigherCu2 O level
on theofcoating
corrosion re
sistance,
surface, particularly
preventing in marine-like
the further penetration environments.
of corrosive substances and enhancing corrosion
resistance. In the early stages of corrosion product formation, Fe dissolves much faster than
Cu in seawater,
4.2.2. Nickel-Basedand the dissolved
Alloy Fe exists as hydrated compounds and γ-FeOOH, which
Coatings
gradually accumulate to form a corrosion product layer. This layer effectively inhibits the
Nickel-based coatings have continuously been studied as materials for marine corro
formation of metal ions such as Cu2+ and Ni2+ . It maintains the amorphous structure of
sioncorrosion
the resistance.
film,Initial
reducingresearch
sensitivefocused
points inonthe
nickel-based
microstructure coatings for marine
and preventing corrosion re
localized
damage to the film, thereby improving the corrosion resistance of the copper-nickel alloy. treat
sistance, which later expanded to include corrosion-resistant elements and surface
ments to enhance
However, due to thetheir
wide protective propertiesalloys
variety of nickel-based further and[190 193]. Among
the–numerous thethat
factors notable
affect studies
Li et corrosion
their al. [194] fabricated
resistance,aitNi coating with
is essential excellent
to study hydrophobic
the impacts properties
of different on the surface
heat treatment
processes on theirsteel,
of 304 stainless performance
achieving anda select
watermaterials based on
contact angle the corrosive
of 149.4°. mediumprevents
This coating and the
service conditions.
substrate from contacting corrosive solutions and exhibits strong corrosion resistance
Adding chromium and molybdenum to nickel-based alloys enhances their pitting and
crevice corrosion resistance.
Shao et al. [195] investigated the effect of Fe/Mn content on copper–nickel alloy coat-
ings’ wear and corrosion resistance. The results show that as the Fe content increased, the
hardness of the coating gradually improved, while the polarization resistance initially in
creased and then decreased, reaching a maximum of 19.81 kΩ·cm2. Figure 15 illustrates
the microstructure of corrosion products and the corrosion mechanism after 30 days o
exposure to simulated seawater for copper-nickel alloys with different Fe contents. The
 nickel alloy. However, due to the wide variety of nickel-based alloys and the numerous
factors that affect their corrosion resistance, it is essential to study the impacts of different
Coatings 2024, 14, 1487 24 of 39
heat treatment processes on their performance and select materials based on the corrosive
medium and service conditions.

Figure 15.
Figure Surface microstructure
15. Surface microstructure ofof corrosion
corrosion products
products and
and the
the corrosion
corrosion mechanism
mechanism of
of copper–
copper–
nickel alloys with different iron contents after 30 days of corrosion in simulated seawater [195].
nickel alloys with different iron contents after 30 days of corrosion in simulated seawater [195].

4.2.3. High
4.2.3. High Entropy
Entropy Alloy
Alloy Coatings
Coatings
High-entropyalloys,
High-entropy alloys,aanewly
newlydeveloped
developed class
class of of alloys
alloys in materials
in materials science,
science, can can ex-
exhibit
hibit corrosion
corrosion resistance
resistance when when the element
the element composition
composition is appropriately
is appropriately balanced. balanced. They
They show
potential for applications
show potential in marine
for applications engineering
in marine engineeringcorrosion-resistant
corrosion-resistant environments.
environments. Wu
et al. [196] used laser cladding technology to fabricate
Wu et al. [196] used laser cladding technology to fabricate FeNiCoCrMo FeNiCoCrMo x high-entropy alloy
x high-entropy
coatings on the on
alloy coatings surface of 316 stainless
the surface steel and
of 316 stainless studied
steel their electrochemical
and studied properties
their electrochemical in a
prop-
3.5%
ertiesNaCl solution.
in a 3.5% NaCl The results show
solution. that the
The results polarization
show curves of the curves
that the polarization high-entropy alloy
of the high-
coating
entropyexhibited
alloy coating no significant
exhibited no hysteresis loops,
significant indicating
hysteresis loops,that the coating
indicating thathas
theacoating
strong
resistance
has a strong to pitting corrosion
resistance in thecorrosion
to pitting 3.5% NaCl insolution.
the 3.5%The NaClcoating had aThe
solution. wide passivation
coating had a
zone and a passive film formed on the surface, demonstrating excellent
wide passivation zone and a passive film formed on the surface, demonstrating excellent corrosion resistance.
Wang et al.resistance.
corrosion [197] successfully
Wang etprepared CoCrFeNiTixprepared
al. [197] successfully high-entropy alloy coatings
CoCrFeNiTi on the
x high-entropy
surface of Q235 steel using laser cladding technology. When x = 0.1,
alloy coatings on the surface of Q235 steel using laser cladding technology. When x = 0.1, the coating exhibited an
FCC phase. exhibited
the coating As the x valuean FCC increased,
phase. As tetragonal
the x valueFeCr, rhombohedral
increased, tetragonal NiTi, andrhombohe-
FeCr, hexagonal
CoTi phasesand
dral NiTi, appeared.
hexagonal TheCoTi
alloyphases
structure was primarily
appeared. The alloycharacterized
structure by wastypical dendrites,
primarily char-
with Ni and Ti enriched in the interdendritic regions and Cr and
acterized by typical dendrites, with Ni and Ti enriched in the interdendritic regions and Fe concentrated within the
dendrites. The hardness of the cladding layer increased with the
Cr and Fe concentrated within the dendrites. The hardness of the cladding layer increased Ti content. In a 3.5% NaCl
solution,
with the pitting
Ti content.corrosion was identified
In a 3.5% NaCl solution,as thepitting
main form of corrosion
corrosion for the CoCrFeNiTi
was identified as the main x
high-entropy alloy coatings, and the corrosion resistance improved
form of corrosion for the CoCrFeNiTix high-entropy alloy coatings, and the corrosion re- with increasing Ti
content. Huang et al.
sistance improved with[198] demonstrated
increasing thatHuang
Ti content. addingetaluminum altered the passive
al. [198] demonstrated film
that adding
structure of
aluminum altered Al x CoCrFeNi high-entropy alloys, with x values of
the passive film structure of AlxCoCrFeNi high-entropy alloys, with 0, 0.1, and 1.0. As thex
aluminum
values of 0,content
0.1, andincreased,
1.0. As the the passivecontent
aluminum film transitioned
increased, the from a dual-layer
passive structure
film transitioned
consisting of an outer layer of iron chromium oxide and an inner layer of chromium oxide
from a dual-layer structure consisting of an outer layer of iron chromium oxide and an
to a three-layer structure with an outer layer of iron chromium oxide, a middle layer
inner layer of chromium oxide to a three-layer structure with an outer layer of iron chro-
of chromium oxide, and an inner layer of aluminum oxide. This change enhanced the
mium oxide, a middle layer of chromium oxide, and an inner layer of aluminum oxide.
protective properties of the passive film, resulting in the improved corrosion resistance of
This change enhanced the protective properties of the passive film, resulting in the im-
the alloy in supercritical water.
proved corrosion resistance of the alloy in supercritical water.
4.2.4. Other Metal-Based Alloy Coatings
In addition to the alloys above, research in marine corrosion protection has also
explored materials such as copper-based alloys, magnesium alloys, zirconium-based al-
loys, and molybdenum-based alloys. Copper-based alloy coatings enhance the passive
Coatings 2024, 14, 1487 25 of 39

film on component surfaces, making them crucial for extending the service life of marine
pipelines [199]. Zirconium-based alloys offer excellent pitting and crevice corrosion resis-
tance, making them suitable for harsh marine environments [200]. Generally, the main
advantages of metal coatings include high strength and hardness and good toughness
when subjected to external impacts [201,202]. Given their good conductivity, metal coatings
can also be employed for the cathodic protection of pipelines, further improving corrosion
resistance. However, metal coatings also have drawbacks, such as lower resistance to
electrochemical corrosion, the high cost of certain metals, difficulty in repairing the damage,
and limited oxidation resistance.

4.3. Ceramic Coating

Due to ceramic materials’ ionic and covalent bonding structure, they demonstrate
better corrosion resistance in harsh environments such as seawater, acids, and atmospheric
conditions. Ceramic coatings have better ageing resistance than polymer coatings. They
are resistant to degradation from UV exposure, allowing them to maintain their corrosion
resistance over more extended periods in both atmospheric and marine environments.
Ceramic coatings offer better wear and corrosion resistance advantages compared to metal
coatings. Standard ceramic coatings include nitrides, oxides, carbides, and borides of
transition metals. Nitride coatings are the most widely used and extensively studied
ceramic coatings for marine protection. In addition, oxide and carbide ceramic coatings are
also vital areas of research and development.

4.3.1. Titanium Nitride Ceramic Coatings

TiN coatings offer solid strength, wear resistance, and chemical stability, making
them suitable for protecting marine metal pipelines. TiN has a face-centered cubic crystal
structure, combining metallic and covalent bonds. This gives it both good strength and
hardness and stable chemical properties, as it is insoluble in water and resistant to dilute
acids [203]. Zhao et al. [204] found that the excellent corrosion resistance of TiN coatings
is primarily due to their insolubility in water and most acids, combined with their high
chemical stability. Additionally, advanced coating preparation techniques enhance the
coatings’ density, improving their corrosion resistance.
To further enhance the performance of TiN coatings, introducing new elements has
been proven to be an effective approach. Adding aluminum to TiN coatings results in the
formation of TiAlN coatings, which improve adhesion to the substrate, as well as wear
resistance and corrosion resistance. Studies have shown that when aluminum content
is controlled within a specific range, it can effectively enhance the coating’s mechanical
properties, oxidation, friction, and corrosion resistance. In research by Li et al. [205] and
Chinchanikar et al. [206], TiAlN coatings demonstrated superior hardness, wear resistance,
and corrosion resistance compared to TiN coatings, although the performance of single-
layer TiAlN coatings reached its limit.
To solve this, researchers developed multilayer structures to improve titanium nitride-
based ceramic coatings. For example, in Bonu et al. [207], unbalanced magnetron sputtering
technology created ultra-thin multilayer coatings on Ti/TiN and TiAl/TiAlN coatings on
Ti6 Al4 V substrates. The results show that these coatings exhibited significantly better wear
resistance than the Ti6 Al4 V substrate, and their corrosion resistance in simulated seawater
improved by nearly an order of magnitude, as seen in Figure 16.
Coatings 2024,
Coatings 14,14,
2024, x FOR
1487PEER REVIEW 26 of 39
26 of 39

Figure
Figure 16.16. Corrosion
Corrosion development
development in thick
in thick multilayers
multilayers with
with bi-layer
bi-layer thicknesses
thicknesses of 1–5
of 1–5 µmµm and
and in in
ultra-thin multilayers
ultra-thin with
multilayers bi-layer
with thicknesses
bi-layer of of
thicknesses 10–20 nmnm
10–20 [207].
[207].

4.3.2.
4.3.2. Chromium
Chromium Nitrogen-Based
Nitrogen-Based Ceramic
Ceramic Coating
Coating
CrNCrN coatings
coatings have
have highhardness,
high hardness,goodgoodwear
wearresistance,
resistance,and and strong
strong corrosion
corrosion re-
resis-
tance, making them suitable for coating components that require
sistance, making them suitable for coating components that require mechanical perfor- mechanical performance
and corrosion resistance in underwater environments [208]. CrN coatings are typically
mance and corrosion resistance in underwater environments [208]. CrN coatings are typ-
produced using physical vapor deposition, chemical vapor deposition, or gas nitriding.
ically produced using physical vapor deposition, chemical vapor deposition, or gas nitrid-
Shan et al. [209] applied CrN coatings to a 316L stainless steel substrate using multi-arc ion
ing. Shan et al. [209] applied CrN coatings to a 316L stainless steel substrate using multi-
plating, and the results demonstrate a significant improvement in both wear and corrosion
arc ion plating, and the results demonstrate a significant improvement in both wear and
resistance in seawater environments. Building on the CrN coating, researchers have en-
corrosion resistance in seawater environments. Building on the CrN coating, researchers
hanced its overall performance by incorporating aluminum. For example, CrAlN coatings,
have enhanced its overall performance by incorporating aluminum. For example, CrAlN
formed by adding a certain amount of aluminum to CrN, showed notable improvements
coatings, formed by adding a certain amount of aluminum to CrN, showed notable im-
in hardness and protective properties [210,211].
provements in hardness and protective properties [210,211].
Since single-layer coatings are less effective at preventing the penetration of corrosive
Since single-layer coatings are less effective at preventing the penetration of corrosive
agents, researchers have optimized CrAlN coatings by developing multi-layer structures.
agents,
Xia etresearchers have optimized
al. [212] deposited multi-layer CrAlN coatings
Cr/CrAlN by developing
coatings on a steel multi-layer
substrate using structures.
multi-arc
Xiaion
et technology,
al. [212] deposited multi-layer Cr/CrAlN coatings on a steel substrate
finding that multi-layer coatings exhibited superior wear and corrosion using multi-
arcresistance
ion technology,
compared finding that multi-layer
to single-layer coatings
coatings. exhibited
Further research superior
indicated wear
thatand corro-
multi-layer
sion resistance compared to single-layer coatings. Further research indicated
structures not only effectively relieve internal stress, but also inhibit the growth of defects, that multi-
layer structures
preventing not onlysubstances
corrosive effectively fromrelieve internal stress,
penetrating but also As
the coating. inhibit the growth
a result, of
multi-layer
defects, preventing corrosive substances from penetrating the coating.
Cr/CrAlN coatings present enhanced protective performance and hold promising potential As a result, multi-
layer
for Cr/CrAlN
applicationcoatings
in marine present
pipeline enhanced protective
protection. The mainperformance
characteristicsand of hold
CrAlNpromising
coatings
potential
includeforhighapplication
hardness,inhigh marine pipeline
melting protection.
point, excellentThewearmainandcharacteristics of CrAlN
corrosion resistance, and
coatings include and
strong thermal highoxidation
hardness,stability.
high melting point, excellent wear and corrosion re-
sistance, andTiAlN
Like strongcoatings,
thermal CrAlNand oxidation
coatingsstability.
employ multiple protective mechanisms, such
Like TiAlN coatings, CrAlN coatings
as corrosion barriers, oxide protection layers, employ multiple
uniform protective
material mechanisms,
consumption, andsuch
multi-
as layered
corrosion barrier effects. CrAlN coatings exhibit superior corrosion resistance inmulti-
barriers, oxide protection layers, uniform material consumption, and marine
layered barrier effects.
environments compared CrAlN to coatings exhibit superior
TiAlN coatings, especially corrosion
when exposedresistance in marineagents
to corrosive en-
vironments compared
like chloride ions. to TiAlN coatings, especially when exposed to corrosive agents like
chloride ions.
4.3.3. Other Ceramic Coatings
4.3.3. Other Ceramic
In addition to Coatings
the previously mentioned ceramic coatings, oxide and carbide ceramic
coatings haveto
In addition also
the shown
previouslypotential in protecting
mentioned marine equipment.
ceramic coatings, For instance,
oxide and carbide ceramicre-
searchers
coatings havehave
alsodeveloped multi-component
shown potential in protecting nanoceramic coatings by
marine equipment. Foradding alloying
instance, re-
searchers have developed multi-component nanoceramic coatings by adding alloying el-de-
elements, such as Cr-Si-N, Ti-Nb-N, Ti-Al-Si-N, and others. Additionally, alternating the
position
ements, of different
such as Cr-Si-N,materials,
Ti-Nb-N, forming multilayer
Ti-Al-Si-N, andorothers.
gradient coatings likealternating
Additionally, Cr/CrN/CrAlN,the
has also been shown to enhance corrosion resistance [213–215].
deposition of different materials, forming multilayer or gradient coatings like
In the field
Cr/CrN/CrAlN, of also
has oxidebeen
ceramics,
shownmaterials
to enhancesuch as Al2 O3resistance
corrosion , Cr2 O3 , SiO 2 , TiO2 , and ZrO2 are
[213–215].
widely used as protective coatings for metal substrates due to their excellent passivation,
Coatings 2024, 14, 1487 27 of 39

insulation, corrosion resistance, and wear resistance properties [215]. For example, Khosravi
et al. [216] successfully developed TiO2 -SiO2 multilayer coatings using the sol-gel method,
creating physical and electrochemical barriers that significantly reduce the corrosion of
marine equipment substrates caused by seawater.
While ceramic coatings generally outperform metal coatings in terms of hardness and
resistance to electrochemical corrosion, they suffer from weaker adhesion to substrates and
lower toughness. These limitations lead to a higher risk of delamination in highly corrosive
environments, restricting their overall effectiveness.

5. Discussion
The previous sections have analyzed the characteristics, advantages, and typical case
studies of various marine antifouling and anti-corrosion coatings. This section will focus on
the challenges that antifouling and anti-corrosion coatings still face, and potential strategies
for improvement to provide insights for future research directions in this field.

5.1. Antifouling Coatings

Marine antifouling coatings have been developed over centuries, from simple bar-
riers like asphalt and tar to advanced coatings like low-surface-energy, conductive, and
biomimetic types. Despite progress, challenges remain.
Organotin coatings, though effective, have harmed marine ecosystems, with trib-
utyltin causing reproductive issues in marine life. As a result, the International Maritime
Organization has banned organotin compounds in marine coatings. Low-surface-energy
coatings, such as silicones and fluoropolymers, reduce marine organism adhesion by lower-
ing surface energy. However, they encounter issues with mechanical strength and substrate
adhesion. Silicone coatings exhibit good hydrophobicity and weather resistance due to Si–O
bonds, yet have weak mechanical properties and poor adhesion, requiring blending with
other materials to improve. Fluorinated coatings provide excellent antifouling performance
but are limited by high production costs and molding temperatures. Inspired by natural
antifouling mechanisms like shark skin, biomimetic coatings reduce bacterial adhesion, but
long-term durability and mechanical performance still need further validation.
Conductive antifouling coatings generate antifouling effects by applying an electric
current, but the reliance on electricity limits their widespread application. For instance,
Mitsubishi Heavy Industries developed a conductive coating using copper and aluminum,
which produces sodium hypochlorite upon applying an electric current to inhibit bio-
fouling [122]. However, the complexity of the coating, material costs, and the need for
additional power are significant barriers to large-scale adoption [199]. The requirement for
electric power also limits the coating’s application scenarios. Zwitterionic polymer coatings
exhibit good antifouling properties, but their long-term stability in marine environments
needs further research, particularly in balancing hydrolysis and degradation. Controlling
degradation rates while maintaining antifouling effectiveness remains a crucial challenge
for biodegradable antifouling coatings. Ensuring that the coating degrades while retaining
antifouling functionality requires further optimization.
Photocatalytic coatings face activation challenges under light, requiring UV exposure
to work effectively. However, in shaded areas or on the underside of ships, the UV intensity
is too low, which limits their antifouling performance [129]. High-entropy alloy coatings
have shown excellent antibacterial and antifouling capabilities in laboratory experiments,
but their economic cost remains high.
Here are the following recommendations set to address the challenges faced by various
antifouling coatings in the current research.
(i) Low-Surface-Energy Coatings
The focus can be on improving mechanical performance and substrate adhesion. Com-
bining silicones with fluorinated compounds has developed more durable and practical
antifouling coatings. Sun et al. [108] demonstrated that the combination of silicones and
fluorinated compounds significantly enhanced the overall performance of these coatings.
Coatings 2024, 14, 1487 28 of 39

Further improvements can be made by adopting a multilayer structure, combining a me-

chanically robust base layer with a low-surface-energy outer layer to ensure durability and
substrate adhesion. Additionally, introducing elastomers or copolymerizing with reinforc-
ing materials can enhance abrasion resistance and impact strength while maintaining low
surface energy.
(ii) Zwitterionic Polymer Coatings
Nanoparticle reinforcement has shown potential to improve both mechanical strength
and antifouling effectiveness. For example, sulfobetaine methacrylate-functionalized silica
nanoparticles have demonstrated excellent antifouling properties, effectively inhibiting
the adhesion of E. coli and diatoms [145–147]. Optimizing the ratio and modification of
nanoparticles can further improve coating uniformity and long-term durability.
(iii) Photocatalytic Coatings
Enhancing performance under low-light conditions is critical in this field. By chem-
ically modifying TiO2 , it can be activated under visible light. For example, silicon-rich
spherical TiO2 nanocomposites can maintain high photocatalytic activity even in low-light
environments [131]. Further strategies include developing multifunctional photocatalytic
materials, such as combining TiO2 with other visible light-sensitive sensitizers or metal
nanoparticles to improve photocatalytic efficiency. Co-catalysts can also optimize light
absorption, enhancing antifouling performance in visible and low-light conditions.
(iv) High-Entropy Alloy Coatings
For high-entropy alloy coatings, the main challenge is cost. Optimizing alloy com-
positions by replacing or reducing expensive elements like Co and Ni with lower-cost,
high-performance alternatives such as Mn and Fe can help reduce costs while maintaining
antifouling efficacy. This approach expands the application potential of HEA coatings.

5.2. Anti-Corrosion Coatings

Marine anti-corrosion coatings, like antifouling coatings, face similar technical chal-
lenges. Epoxy coatings provide a certain level of corrosion resistance, but their high
brittleness makes them prone to cracking under mechanical stress, compromising their
protective function [217]. Polyurethane coatings exhibit limited self-healing capabilities,
making restoring their original protective properties difficult after mechanical damage [218].
Acrylic coatings are environmentally friendly and offer fast curing properties, but their
barrier performance is limited, making them susceptible to the influence of corrosive agents
in long-term marine environments [170]. Polyurea coatings offer excellent corrosion re-
sistance, but due to the lower strength of low-carbon steel and aluminum, ceramic films
generated by micro-arc oxidation may develop microcracks and micropores, reducing their
corrosion resistance [219].
In metal-based alloy coatings, zinc–aluminum coatings provide good corrosion re-
sistance but can experience mechanical damage and pitting, impacting their long-term
stability and protective ability. Nickel-based alloy coatings exhibit strong resistance to
seawater corrosion. Their performance varies due to their complex composition and their
sensitivity to environmental conditions and heat treatment processes, making them a
subject of further research for diverse corrosive environments.
Ceramic coatings excel in wear resistance and corrosion protection, but they also have
certain limitations. For example, the single-layer structure of TiN coatings is less effective in
preventing the penetration of corrosive agents, and their corrosion resistance may gradually
degrade over time [203]. CrN coatings, while hard and wear-resistant, are vulnerable to
localized corrosion in the presence of chloride ions in marine environments [208]. Oxide
and carbide coatings, despite their strong passivation and corrosion resistance, often exhibit
weaker adhesion to substrates and limited toughness, leading to delamination in highly
corrosive conditions [216].
Coatings 2024, 14, 1487 29 of 39

The following recommendations are made to address the challenges faced by various
anti-corrosion coatings in current research:
(i) Epoxy Coatings
For epoxy coatings, adding organosilicon or multilayer copolymer structures can
enhance their mechanical properties, reducing the occurrence of cracks and improving their
long-term corrosion resistance [162];
(ii) Polyurethane Coatings
Polyurethane coatings can be improved by introducing nanomaterials or self-healing
components, thereby enhancing their self-repair capabilities [218]. Incorporating graphene
or graphene oxide for acrylic coatings can improve their barrier properties and mechanical
strength [171];
(iii) Metal-Based Alloy Coatings
More advanced surface coating methods and modified elemental compositions can
be developed in metal-based alloy coatings to improve coating performance. For zinc–
aluminum alloy coatings, adding magnesium, rare earth elements, and silicon can enhance
the microstructure of the coating, thereby improving its corrosion resistance and pitting
protection [189]. Optimizing heat treatment processes and controlling alloy composition
for nickel-based alloy coatings can significantly enhance their performance in different
corrosive environments [195]. A way to lower the costs of high entropy alloys is to substitute
expensive elements with more affordable ones without sacrificing performance.
It is worth noting that while metal-based coatings can provide excellent wear resis-
tance, their potential environmental impact due to heavy metals cannot be overlooked. For
instance, Cu, as a bio-toxic element, exhibits significant toxicity to marine organisms at
high concentrations (24 h median lethal concentration, LC50: 4.5–8.7 mg/L) [220], leading
to reduced growth and reproductive capacity. Cr(VI) is a highly toxic substance with strong
carcinogenic and mutagenic properties, characterized by high bioavailability in organisms.
It has been designated as a priority pollutant by many countries. The World Health Organi-
zation recommends that the concentration of Cr(VI) in drinking water should not exceed
0.05 mg/L, underscoring its hazards in environmental and health contexts [221]. Zinc, an
essential nutrient for biological growth and development, plays a critical role in enzyme
functions, particularly in DNA synthesis and protein metabolism. However, excessive
concentrations of zinc (24 h LC50: 36.7–80.1 mg/L) can also harm marine organisms [222],
potentially causing slowed growth and reproduction. Therefore, the development of metal-
based coatings must strike a balance between environmental friendliness and functionality;
(iv) Ceramic Coatings
For ceramic coatings, new strategies can be adopted to improve their adhesion and
long-term durability. For example, titanium nitride (TiN) coatings can be enhanced by
adopting multilayer structures to prevent the penetration of corrosive agents, thus im-
proving the coating’s overall durability [207]. Adding aluminum to TiN coatings can also
improve adhesion, wear resistance, and corrosion protection. Chromium nitride (CrN)
coatings can be optimized by using multilayer Cr/CrAlN structures, which not only reduce
internal stress, but also prevent the infiltration of chloride ions, thus addressing the issue of
localized corrosion in marine environments [212].
In the field of marine antifouling and anticorrosion, the economic feasibility of coating
products is a critical factor for their large-scale application. Currently, organic-based
coatings such as epoxy zinc-rich primer, acrylic polyurethane paint, and self-polishing
antifouling coatings are widely used in commercial markets, priced at approximately USD
0.15/kg, USD 0.19/kg, and USD 0.45/kg, respectively (based on pricing from Shandong
Gangshan Special Coatings Company, Weifang, China). The significantly higher cost of
self-polishing antifouling coatings is attributed to the use of high-performance resins and
functional additives, which provide superior acid, alkali, and seawater resistance. These
Coatings 2024, 14, 1487 30 of 39

coatings are typically applied on the large surfaces of ship hulls, decks, and other marine
engineering structures.
In contrast, the cost of metal-based coatings is significantly higher. For example,
nickel-based self-fluxing powders processed using laser cladding technology are priced
at approximately USD 122/kg (data from Beijing Yijin New Materials Technology Co.,
Beijing, China), far exceeding that of organic-based coatings. Such metal-based coatings,
due to their excellent mechanical strength and corrosion resistance, are primarily used on
critical components of ship structures. However, the high material costs and complexity of
laser processing limit their economic feasibility for large-scale applications. Additionally,
photocatalytic coatings, conductive antifouling coatings, and high-entropy alloy coatings
are still in the experimental stages. Their costs largely depend on material composition
designs, such as the inclusion of high-performance metals like chromium (Cr) and cobalt
(Co), further driving up their overall costs.
Actual cost data indicate that there are significant differences in the economic feasibility
and performance of various coatings, necessitating a balance between the two. Future
research should focus on developing coating technologies that achieve a compromise
between cost and performance, paving the way for their large-scale application.

6. Conclusions
Marine antifouling and anti-corrosion coating technologies are crucial for ensuring
the safety and long-term operation of marine facilities. Existing antifouling and anti-
corrosion coating technologies are continuously evolving, with research mainly focusing
on low-surface-energy coatings, biomimetic antifouling coatings, conductive antifouling
coatings, and photocatalytic coatings. Low-surface-energy coatings reduce the adhesion of
marine organisms by lowering surface energy, with polysiloxane coatings being a typical
example due to their hydrophobicity and weather resistance. Biomimetic coatings pre-
vent fouling by mimicking natural surface structures, utilizing superhydrophobicity and
micro/nanostructure replication. Conductive antifouling coatings generate antifouling bio-
cides through electrochemical reactions. Current research aims to enhance the conductivity
and stability of these coatings by adding conductive fillers like carbon nanotubes, thereby
improving their antifouling performance. Photocatalytic coatings use TiO2 photocatalytic
particles, activated by UV light, to generate reactive oxygen species that degrade organic
matter and prevent biological adhesion.
In terms of anti-corrosion, polymer coatings, particularly those enhanced with het-
eroatoms, have made significant progress in improving corrosion resistance and mechanical
strength, and are widely used in marine corrosion protection. Self-healing polyurethane
coatings, incorporating biobased polyols and continuously improving self-healing capa-
bilities, enhance environmental performance and corrosion protection. Polyurea coatings,
combined with graphene, provide high chemical inertness and excellent corrosion resis-
tance, making them suitable for harsh marine environments. Metal coatings, such as
zinc–aluminum and nickel-based alloys, exhibit good pitting resistance and adaptability
to complex aquatic environments. High-entropy alloy coatings, known for their excellent
corrosion and wear resistance, have become a new focus in corrosion protection research.
Additionally, ceramic coatings are being studied for their potential to improve both wear
resistance and overall corrosion performance.
Future research should focus more on corrosion behavior in environments combining
seawater, hydrogen sulfide, and other factors, further exploring the corrosion character-
istics and mechanisms in these environments. The green environmental properties and
biocompatibility of coating materials will become an important direction in the future
design of anti-corrosion coatings. Research into how to improve the mechanical properties
and stability of coatings while minimizing their environmental impact will be key to the
development of future coating technologies.
It should be noted that although new coatings exhibit excellent performance under
laboratory conditions, their high costs significantly limit their adoption on an industrial
Coatings 2024, 14, 1487 31 of 39

scale. The coating area for large ship surfaces typically spans thousands of square meters,
and the production of new coatings involves expensive raw materials and complex man-
ufacturing processes. For instance, metal-based coatings often require costly metals and
energy-intensive processing techniques, resulting in a per-square-meter cost much higher
than that of traditional coatings. The shipbuilding industry is highly cost-sensitive, and
operators tend to prefer cost-effective and well-established technologies over the substan-
tial upfront investment required for new technologies. This economic reality poses a major
challenge to the industrial application of innovative coatings.

Author Contributions: Conceptualization, H.L. and Y.L.; methodology, Y.L.; validation, Y.L.; for-
mal analysis, Y.L.; investigation, X.S.; resources, H.L.; data curation, H.L.; writing—original draft
preparation, H.L.; writing—review and editing, H.L.; visualization, H.L.; supervision, H.L.; project
administration, H.L.; funding acquisition, H.L. All authors have read and agreed to the published
version of the manuscript.
Funding: 2024 Anhui Provincial University Scientific Research Project (Natural Science Category,
Key Project, No. 2024AH052003); 2024 Provincial Department of Education Science and Engineering
Teachers’ Internship Program in Enterprises (No. 2024jsqygz76).
Conflicts of Interest: The authors declare no conflicts of interest.

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