molecules

Review
Advances in the Modification of Silane-Based Sol-Gel Coating
to Improve the Corrosion Resistance of Magnesium Alloys
Jiao Li 1 , Huanhuan Bai 2 and Zhiyuan Feng 1, *

1 School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China;
lijiao35@mail2.sysu.edu.cn
2 Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA;
huanhuan.bai@uky.edu
* Correspondence: fengzhy25@mail.sysu.edu.cn

Abstract: As the lightest structural materials, magnesium (Mg) alloys play a significant role in vehicle
weight reduction, aerospace, military equipment, energy saving, and emission reduction. However,
the poor corrosion resistance of Mg alloys has become a bottleneck restricting its wide application.
Developing a good surface protective coating can effectively improve the corrosion resistance of Mg
alloys. The silane-based sol-gel coating technology has been widely used in the corrosion protection
of Mg alloys in recent years due to its advantages of simple process, accessible tailoring of film
composition and structure, and excellent corrosion resistance. Whereas the synthesis of sol-gel
coatings includes the hydrolysis and dehydration process, which may inherently contain micron or
nano defects in the coatings, thereby making it detrimental to the anti-corrosion effect. Therefore, in
order to enhance their protection against corrosion, the appropriate modification of sol-gel coatings
has become a current research hotspot. This review is based on the modification methods of silane-
based sol-gels on the surface of Mg alloys, which are divided into four categories: bare sol-gel,
nanoparticles, corrosion inhibitors, and sol-gel-based composite coatings. The modification methods
and corrosion protection mechanism are discussed respectively, and the application, development,
and research strategies of silane-based sol-gel coatings are included.

Keywords: sol-gel; corrosion; nanomaterials; Mg alloys; coating

Citation: Li, J.; Bai, H.; Feng, Z.
Advances in the Modification of
Silane-Based Sol-Gel Coating to
Improve the Corrosion Resistance of
1. Introduction
Magnesium Alloys. Molecules 2023,
28, 2563. https://doi.org/10.3390/
Mg alloys are known as the lightest structural materials [1,2]. Compared with other
molecules28062563 metals, Mg and its alloys have the advantages of low density (lightweight), excellent
biocompatibility, being environmentally friendly, good recyclability, having high specific
Academic Editors: Amany M. Fekry
strength, and having a specific stiffness and good damping performance [3–5]. Due to the
and Franco Bisceglie
above advantages, Mg alloys have broad application prospects in the fields of aerospace,
Received: 30 January 2023 the automobile industry, medical materials, national defense, and military equipment and
Revised: 18 February 2023 communications [6–9].
Accepted: 8 March 2023 The standard electrode potential of Mg is very low, which is −2.37 VSHE , and it has ex-
Published: 11 March 2023 tremely high chemical and electrochemical activity [10,11]. Therefore, in contact with other
metal materials, Mg alloys often act as anodes and are subjected to electrochemical corro-
Molecular volume o f oxide
sion [12,13]. Additionally, the Pilling–Bedworth (P–B) ratio ( Molecular volume o f metal ) of MgO
Copyright: © 2023 by the authors.
is only about 0.81 < 1, which results in the formation of a porous and unprotective oxide film
Licensee MDPI, Basel, Switzerland. on Mg substrate and easily leads to pitting corrosion, stress corrosion cracking, galvanic
This article is an open access article corrosion, and other forms of corrosion [13,14]. Therefore, poor corrosion resistance is the
distributed under the terms and main disadvantage that hinders the application of Mg and its alloys [13,15,16].
conditions of the Creative Commons There are several approaches to increase the corrosion resistance of Mg and its alloys,
Attribution (CC BY) license (https:// including internal methods such as the modification of alloy composition/microstructure
creativecommons.org/licenses/by/ (fabrication of high-purity alloys, refinement of microstructure, etc.) [12,17–20]. The ex-
4.0/). ternal methods include surface treatment and coating processes (electroplating, chemical

Molecules 2023, 28, 2563. https://doi.org/10.3390/molecules28062563 https://www.mdpi.com/journal/molecules

 There are several approaches to increase the corrosion resistance of Mg and its alloys,
including internal methods such as the modification of alloy composition/microstructure
(fabrication of high-purity alloys, refinement of microstructure, etc.) [12, 17-20]. The ex-
Molecules 2023, 28, 2563
ternal methods include surface treatment and coating processes (electroplating, chemical 2 of 30
conversion coating, electrochemical conversion coating, laser/ion or electron beam treat-
ment, sol-gel coating, and organic coating, etc.) [6, 21-29]. Among them, the surface coat-
ing method may be the most simple and direct method. The sol-gel coating is widely used
conversion coating, electrochemical conversion coating, laser/ion or electron beam treat-
to protect Mg alloys against corrosion due to the following advantages:
ment, sol-gel coating, and organic coating, etc.) [6,21–29]. Among them, the surface coating
1. method
No complicated
may be the equipment
most simple is needed.
and direct method. The sol-gel coating is widely used to
2. protect
It can Mg
be synthesized at room
alloys against corrosion temperature
due to the without
following vacuum conditions.
advantages:
3. Large-area thin films can be prepared on the surface of Mg.
1. No complicated equipment is needed.
4. Modification of the coating composition and microstructure is simple. It is easy to
2. It can be synthesized at room temperature without vacuum conditions.
prepare a uniform multi-component composite coating, and the thickness can be ad-
3. Large-area thin films can be prepared on the surface of Mg.
justed at the micron level.
4. Modification of the coating composition and microstructure is simple. It is easy to
5. It can be used alone, as a pretreatment layer or as a sealing layer [30-34].
prepare a uniform multi-component composite coating, and the thickness can be
Typically,
adjusted theatsol-gel
the microncoatings
level.are synthesized by using polymer gel methods. Sols
are 5.
obtained through the hydrolysis and condensation
It can be used alone, as a pretreatment layer orreactions of compounds
as a sealing layer [30–34]. containing
highly chemically
Typically, theactive precursors,
sol-gel coatingsorganic solutionsby
are synthesized (such
using aspolymer
ethanol),gel and catalystsSols
methods.
(such as weak acids and bases). The sol is aged for a period of
are obtained through the hydrolysis and condensation reactions of compounds containingtime until the colloidal
particles
highlyare aggregated,
chemically active resulting in reduced
precursors, organic fluidity.
solutionsWhen (such the sol turnsand
as ethanol), intocatalysts
a gel, the
(such
linear structure becomes a network structure, thus forming a polymer
as weak acids and bases). The sol is aged for a period of time until the colloidal particles network. Then, are
through curing resulting
aggregated, methods such as drying
in reduced and heat
fluidity. When treatment, it will
the sol turns eventually
into a gel, thebe deposited
linear structure
on the surfacea of
becomes the alloy
network [34-36]. In
structure, the forming
thus synthesisaof silane-based
polymer sol-gel
network. coating,
Then, the most
through curing
commonly adopted precursors are alkoxysilanes, such as tetraethoxysilane
methods such as drying and heat treatment, it will eventually be deposited on the surface (TEOS) due to
its relatively high thermal stability and moderate reactivity over a wide
of the alloy [34–36]. In the synthesis of silane-based sol-gel coating, the most commonly range of temper-
atures [37, 38].
adopted These precursors
precursors help the such
are alkoxysilanes, production of silane-based
as tetraethoxysilane sol-gel
(TEOS) dueachieve high-
to its relatively
wearhigh thermal stability and moderate reactivity over a wide range of temperaturestech-
resistance, good adhesion, and corrosion protection [37]. Additionally, sol-gel [37,38].
nology
These can be used tohelp
precursors prepare thin films, powder
the production materials,
of silane-based bulkachieve
sol-gel air-condensing
high-wear materials,
resistance,
etc.,good
and adhesion,
is widely used in the fields
and corrosion of anti-corrosion,
protection biology,sol-gel
[37]. Additionally, electricity, and optics
technology can be[39-
used
41].to prepare thin films, powder materials, bulk air-condensing materials, etc., and is widely
Generally,
used the film
in the fields formation of the
of anti-corrosion, silane-based
biology, electricity,sol-gel film on[39–41].
and optics Mg alloys needs to
go through three stages (hydrolysis, adsorption, and condensation)
Generally, the film formation of the silane-based sol-gel film on Mg [42]. The schematic
alloys needs to
diagram of silane
go through coating
three stages formation process
(hydrolysis, is shownand
adsorption, in Figure 1.
condensation) [42]. The schematic
diagram of silane coating formation process is shown in Figure 1.

Figure 1. Schematic of silane coatings formation process. The first stage, siloxane hydrolysis: (1); In
Figure
the 1. Schematic
second stage,ofdehydration
silane coatings formation process.
condensation between The firstmolecules:
silanol stage, siloxane hydrolysis:
(2); In (1); the
the third stage,
In the second stage, dehydration condensation between silanol molecules: (2); In the third stage,
Si-OH bond is combined with the -OH on the Mg alloy: (3).
the Si-OH bond is combined with the -OH on the Mg alloy: (3).
The silane-based sol-gel is rich in a cross-linked outer silane layer (Si–O–Si bonds), that
The
can silane-based
form sol-gel islayer
a stable Mg–O–Si richatinthe
a cross-linked outer silane
metal oxide/sol-gel layerproviding
interface (Si–O–Si abonds),
corrosion
thatprotection
can form aeffect.
stable[43,44].
Mg–O–Si layer at the metal oxide/sol-gel interface providing
Figure 2 is a schematic diagram of the steps and processes a cor-
rosion protection effect. [43, 44]. Figure 2 is a schematic
commonly used to obtain sol-gel coatings on Mg alloys. diagram of the steps and processes
commonly Whenused
thetosilane-based
obtain sol-gel coatings
sol-gel on Mg
coatings arealloys.
exposed to saline environments, although
they have a good protective effect in the short term, they cannot provide sufficient anti-
corrosion effects in a long run. The reason is that the synthesis of sol-gel coatings includes
the hydrolysis and dehydration process, which may inherently contain micron or nano
defects in the coatings, therefore making them detrimental to the anti-corrosion effect.
They will eventually allow the electrolyte to diffuse to the coating/substrate interface,
resulting in the corrosion of the substrate [39,45–47]. The corrosion protection of sol-gel
coatings can be improved by increasing their thickness. However, if the sol-gel coating
 Molecules 2023, 28, 2563 3 of 30

is too thick, cracking and delamination will occur due to the high residual stress on the
surface during the drying and curing stages. Additionally, it reduces the adhesion between
the sol-gel coating and the metal surface [48]. Several efforts have been made in recent years
to improve the corrosion protection properties of sol-gel coatings. The incorporation of
corrosion inhibitors into sol-gel coatings has emerged as a promising approach to overcome
the above disadvantages [49]; using nanoparticles as a filler is also an effective way to
Molecules 2023, 28, x FOR PEER REVIEW 3 of 30
increase corrosion protection [50,51]. Another advantageous way to achieve this goal is to
use composite coatings, such as sol-gel as a pretreatment layer or as a sealing layer. Details
of all these methods are within the scope of this review.

Figure 2. Schematic of steps and processes used to obtain sol-gel coatings.

Figure 2. Schematic of steps and processes used to obtain sol-gel coatings.
So far, there have been many excellent reviews on related topics such as the history and
When the silane-based sol-gel coatings are exposed to saline environments, although
applications of the sol-gel process. However, these reviews mainly focus on the development
they have a good protective effect in the short term, they cannot provide sufficient anti-
history of sol-gel [30,35,45,47,52] and the basic principles [30,35,38,39,45,53,54], including self-
corrosion effects in abiocompatibility,
healing, improved long run. The reason is thatsuperhydrophobicity,
antifouling, the synthesis of sol-gel etc.coatings includes
[30,35,38,45,52–54].
the hydrolysis and dehydration process, which may inherently contain
However, no attempts have been made with the perspective of silane-based sol-gel micron or coating
nano
defects in the coatings, therefore making them detrimental to the anti-corrosion
modification methods on Mg alloys. Herein, the focus of this article is on the modification effect.
They will eventually
methods, rather thanallow the electrolyte
the chemical synthesistoprocess
diffuse of
tosol-gel.
the coating/substrate
The modification interface,
methodsre- are
sulting in the corrosion of the substrate [39, 45-47]. The corrosion protection of
divided into four categories: (1) Bare sol-gel; (2) Nanoparticles; (3) Corrosion inhibitors; and sol-gel
coatings can be improved
(4) Sol-gel-based composite by coatings.
increasing their
This thickness.
review providesHowever, if the sol-gel
a comprehensive coating
account of is
the
too thick, cracking and delamination will occur due to the
topic, while also offering some perspectives on future developments. high residual stress on the sur-
face during the drying and curing stages. Additionally, it reduces the adhesion between
the
2. sol-gel coatingCoatings
Bare Sol-Gel and the metal surface [48]. Several efforts have been made in recent
years to improve the corrosion
Organic-inorganic hybridprotection properties
(OIH) coatings of sol-gel
prepared coatings.
by the sol-gel The incorporation
method are suitable for
ofcorrosion
corrosionprotection
inhibitors[52,55].
into sol-gel coatings combine
OIH coatings has emerged as a promising
the advantages approach
of organic to over-
polymers (i.e.,
come the above disadvantages [49]; using nanoparticles as a filler is also an effective way
to increase corrosion protection [50, 51]. Another advantageous way to achieve this goal
is to use composite coatings, such as sol-gel as a pretreatment layer or as a sealing layer.
Details of all these methods are within the scope of this review.
So far, there have been many excellent reviews on related topics such as the history
Molecules 2023, 28, 2563 4 of 30

impact resistance, flexibility, and lightweight) with the properties of inorganic materials (i.e.,
high adhesion, chemical resistance, thermal stability, and mechanical strength) [35,52]. For
example, Hu et al. studied the preparation of SiO2 (TV) sol with TEOS and triethoxyvinyl-
silane (VTEO) as the precursors and compared its corrosion resistance with another SiO2
(T) sol prepared with only TEOS as a precursor on AZ91 Mg alloy [56]. The experimental
results showed that the corrosion resistance of SiO2 (TV) sol coating was better than that
of SiO2 (T) coating under immersion in aqueous 3.5 wt.% NaCl. Khramov et al. studied
the synthesis of stable hybrid coatings with phosphonate functional groups through the
sol-gel route of co-condensation of TEOS and diethylphosphonatoethyl-triethoxysilane
(PHS) [57]. The improved corrosion protection of phosphonate-containing coatings, as
compared to pure sol-gel coatings, had been observed and explained by the favorable com-
bination of barrier properties of the organo–silicate matrix with strong chemical bonding of
phosphonate groups to the Mg substrate. At the same time, during the preparation process,
catalysts, solvents, aging time, processing temperature, etc. all have a great influence on
the sol-gel coating. Hernández–Barrios et al. studied the influence of catalyst (acetic acid)
concentration, immersion time, and aging time on the synthesis and deposition process
of TEOS–GPTMS composite sol-gel coating on AZ31 Mg alloy, and analyzed the effect
of these synthesis parameters on corrosion performance [58]. The experimental results
showed that the gel kinetics produced by the catalyst concentration of 10 vol% was more
stable. However, the low pH value promoted the formation of corrosion products and
hydrogen on AZ31 alloy, which affected the morphological characteristics of the coating
in the deposition stage. On the contrary, when the catalyst concentration was less than
5 vol% and the immersion time was equal to or less than 30 s, the obtained coating was
continuous and uniform. Almost no corrosion products were observed at the substrate
interface. Simultaneously, the chemical bonding between the Mg surface and the sol-gel
network was achieved. On the other hand, the aging time was another factor affecting the
quality of sol-gel. The aging time of 3 days and 6 days can get the optimum viscosity, pH
value, and sol-gel reaction. It can fully densify the SiO2 network, reducing the formation of
corrosion products and hydrogen. In conclusion, continuous, uniform, and dense sol-gel
coating can be obtained by proper controlling of the synthesis parameters, which can slow
down the surface corrosion process of AZ31 Mg alloy during sol deposition. Furthermore,
corrosion tests showed that the corrosion current density was about an order of magnitude
lower compared to AZ31 substrates in aqueous 0.1 M NaCl.
The above simple OIH coating can improve the adhesion between the substrate and the
sol-gel coating, but it can only function as a physical barrier. Once the film layer is damaged,
it will no longer have a protective effect until the film layer is completely separated from
the substrate. As shown in Figure 3, the schematic diagram of the anti-corrosion principle
of bare sol-gel coatings is presented. In order to obtain OIH sol-gel coating with better
Molecules 2023, 28, x FOR PEER REVIEW
anti-corrosion performance, other additives must be introduced in the OIH network 5 of 30such

as corrosion inhibitors and nanoparticles.

Figure 3.
Figure Theschematic
3. The schematicdiagram
diagramofof the
the anti-corrosion
anti-corrosion principle
principle of bare
of bare sol-gel
sol-gel coatings.
coatings.

2.1. Bare Sol-Gel with Corrosion Inhibitor

The corrosion inhibitor-loaded sol-gel coating is mainly achieved by incorporating
appropriate inhibitors in the sol preparation process [59, 60]. These corrosion inhibitors
are stored in the prepared sol-gel coating. The stored inhibitors can be released by the
change in the coating status or the external environment, providing corrosion inhibition
and a self-repairing effect on Mg alloys to further improve the corrosion resistance [61].
Currently, corrosion inhibitions mainly include inorganic salts and nitrogen-containing
Molecules 2023, 28, 2563 5 of 30

2.1. Bare Sol-Gel with Corrosion Inhibitor

The corrosion inhibitor-loaded sol-gel coating is mainly achieved by incorporating
appropriate inhibitors in the sol preparation process [59,60]. These corrosion inhibitors
are stored in the prepared sol-gel coating. The stored inhibitors can be released by the
change in the coating status or the external environment, providing corrosion inhibition
and a self-repairing effect on Mg alloys to further improve the corrosion resistance [61].
Currently, corrosion inhibitions mainly include inorganic salts and nitrogen-containing
organic compounds. Cerium salt is one of the most commonly used inorganic corrosion
inhibitors [62]. Zhong et al. studied the effect of cerium concentration on the microstructure,
morphology, and anti-corrosion properties of the cerium-based sol-gel coating on AZ91D
Mg alloy [63]. Five different concentrations of cerium nitrate hexahydrate were added
as dopants to the sol-gel. It was determined that the degree of decomposition of silane
chains in the sol-gel network improved with increasing cerium concentration. Cerium
present in the sol-gel coatings exhibited the ability to deposit on the active sites, leading to
the formation of corrosion products with greater protective features. This phenomenon
can slow down the corrosion process and provide an additional protective effect on the
substrate [1]. EIS results showed that the corrosion resistance of the coating increased
first and then decreased with the increase in cerium concentration. When the cerium
concentration was 0.01 M, corrosion resistance reached its maximum (Rp = 143 kΩ·cm2 ) in
aqueous 3.5 wt.% NaCl. Afterward, Murillo–Gutiérrez et al. and Hernández–Barrios et al.
performed similar research and reached the same conclusion [37,64].
Some organic compounds with special functional groups and structures are promising
for corrosion inhibition. They mainly contain nitrogen, oxygen, sulfur, phosphorus, or
aromatic rings in their structure. Galio et al. synthesized a composite sol-gel coating doped
with 8-hydroxyquinoline (8-HQ) and applied it to Mg alloy AZ31 [61]. The positive effect
of 8-HQ on corrosion protection can be explained by the formation of insoluble and stable
complexes Mg(8-HQ)2 that block the propagation of corrosion and fill the microporous and
micro defects in the sol-gel coating.
Amino acid is an environmentally friendly inhibitor that is relatively cheap and
easy to prepare [65,66]. The corrosion inhibition ability of amino acids can be attributed
to their propensity to form hydrogen bonds with oxides or hydroxides on metal sur-
faces [66]. Li et al. prepared a sol-gel coating containing the corrosion inhibitor paeonol
condensation tyrosine (PCTyr) Schiff base on the surface of ZE21B Mg alloy to improve
corrosion resistance and biocompatibility [67]. The icorr value of PCTyr Schiff base sol-gel
(3.64 × 10−6 A/cm2 ) was about two orders of magnitude lower than that of the substrate
(1.31 × 10−4 A/cm2 ) in simulated body fluid (SBF). These results demonstrate the poten-
tial of Schiff base-loaded sol-gel coatings to enhance corrosion resistance. As the coating
degrades, the PCTyr Schiff base was released slowly. Then, Mg ions enriched around
the interface provided more chance for chelating with PCTyr Schiff base and drove the
deposition of PCTyr Schiff base-Mg on the coating defects. Therefore, a dense protective
layer was formed gradually to slow down further corrosion, which reveals the self-healing
mechanism of the PCTyr Schiff base sol-gel coatings.
Heterocyclic compounds, especially ones that have a lone pair of electrons, were
considered effective inhibitors in the electrolyte or sol-gel coatings through the adsorp-
tion or complexation with metal surfaces [68,69]. Shi et al. prepared a sol-gel coating
with GPTMS and tetraethoxysilane (TMOS) as precursors and added 2-methylpiperidine
during the gel synthesis [70]. The EIS results showed that the corrosion resistance of the
sol-gel coating on AZ91D was significantly improved after adding 2-methylpiperidine.
Wang et al. prepared phytic acid/silane composite coatings on AZ31 Mg alloys to improve
corrosion resistance and biological activity [71]. When the molar ratio of phytic acid-to-
3-aminopropyltrimethoxysilane (γ-APS) was 1:1, the hybrid coating was intact without
noticeable cracks. When tested in a SBF solution, the corrosion impedance of the coated Mg
alloy (13,835 Ω·cm2 ) was about 27 times higher than that of the bare substrate (507 Ω·cm2 ).
The immersion test showed that with the increase of immersion time in SBF, the hybrid coat-
 gel coating on AZ91D was significantly improved after adding 2-methylpiperidine. Wang
et al. prepared phytic acid/silane composite coatings on AZ31 Mg alloys to improve cor-
rosion resistance and biological activity [71]. When the molar ratio of phytic acid-to-3-
aminopropyltrimethoxysilane (γ-APS) was 1:1, the hybrid coating was intact without no-
ticeable cracks. When tested in a SBF solution, the corrosion impedance of the coated Mg
Molecules 2023, 28, 2563 6 of 30
alloy (13,835 Ω·cm2) was about 27 times higher than that of the bare substrate (507 Ω·cm2).
The immersion test showed that with the increase of immersion time in SBF, the hybrid
coating gradually dissolved due to the hydrolysis of the chemical bonds in the hybrid
ing gradually
coating but diddissolved dueIntoaddition,
not peel off. the hydrolysis of the chemical
the dissolution bonds inthe
zone induces thedeposition
hybrid coating
and
but did not peel off. In addition, the dissolution zone induces the deposition
growth of cap mineralized layers, which can provide further corrosion protection for Mg and growth
of capLimineralized
alloy. layers, which
et al. also performed can provide
a comparable further corrosion
investigation and gotprotection for Mg alloy.
a similar conclusion [72].Li
et al. also performed a comparable investigation and got a similar conclusion
Hydroxyapatite (HA) has previously been proven as a corrosion inhibitor because of its [72]. Hydrox-
yapatite (HA)
buffering effecthasandpreviously
reducingbeen provengradients
potential as a corrosion inhibitor
between anodesbecause
and of its buffering
cathodes[73].
effect and reducing potential gradients between anodes and cathodes [73].
Nikbakht et al. added HA nanoparticles to the sol-gel, which helped to optimize the bar- Nikbakht et al.
added
rier HA nanoparticles
properties to the
of the coating sol-gel,
[74]. which helped
The results showedtothat
optimize the barrier
the addition properties
of 500 mg/L toof
the coating [74]. The results showed that the addition of 500 mg/L to 1000 mg/L provided
1000 mg/L provided desirable corrosion resistance. The corrosion resistance of the modi-
desirable corrosion resistance. The corrosion resistance of the modified sol-gel was three
fied sol-gel was three orders higher than that of the bare substrate in the SBF solution.
orders higher than that of the bare substrate in the SBF solution. When a higher amount of
When a higher amount of HA (2000 mg/L) was added in sol-gel, an agglomeration of na-
HA (2000 mg/L) was added in sol-gel, an agglomeration of nanoparticles was observed,
noparticles was observed, resulting in bad corrosion resistance.
resulting in bad corrosion resistance.
Mg2+2+can chelate with many organic groups to form corrosion-resistant products [75,
Mg can chelate with many organic groups to form corrosion-resistant products [75,76].
76]. Therefore, taking advantage of this feature is a good way to design efficient self-heal-
Therefore, taking advantage of this feature is a good way to design efficient self-healing
ing coatings. According to the research of various scholars, when a corrosive medium
coatings. - According to the research of various scholars, when a corrosive medium such
such as Cl reaches the substrate through the macro or nanopores of the sol-gel, Mg2+, 2+, and
as Cl− reaches the substrate through the macro or nanopores of the sol-gel, Mg and
corrosion inhibitors will be released. Then, Mg2+2+can form complexes with the corrosion
corrosion inhibitors will be released. Then, Mg can form complexes with the corrosion
inhibitors to fill the macro or nanopores, resulting in a more compact sol-gel coating.
inhibitors to fill the macro or nanopores, resulting in a more compact sol-gel coating. Hence,
Hence, the barrier
the barrier effect ofeffect of the coating
the coating will improved.
will be well be well improved. The principle
The principle of sol-gelofanticorrosion
sol-gel an-
ticorrosion with corrosion inhibitors is
with corrosion inhibitors is shown in Figure 4. shown in Figure 4.

Figure 4. The schematic diagram of sol-gel coating containing corrosion inhibitors.

Figure 4. The schematic diagram of sol-gel coating containing corrosion inhibitors.
2.2. Bare Sol-Gel with Nanoparticles
2.2. Bare Sol-Gelyears,
In recent with Nanoparticles
carbon nanostructures (i.e., graphene, fullerene, nanodiamond, and
carbonIn recent years,
nanotube) carbon
have beennanostructures
widely used as(i.e., graphene,
nanofillers fullerene,materials
in composite nanodiamond,
due to and
their
carbon nanotube) have been
good functionalization widely
ability, highused as nanofillers
mechanical in composite
strength, materials
good electrical due to their
properties, and
excellent
good chemical inertness
functionalization [77].
ability, It was
high also demonstrated
mechanical strength, that
goodthe additionproperties,
electrical of nanoparticles
and
to the hybrid
excellent matrix
chemical might[77].
inertness reduce thealso
It was crack-forming
demonstrated ability
that and porosityof
the addition ofnanoparti-
the sol-gel
coatings.
cles There are
to the hybrid many
matrix reports
might on the
reduce the addition of carbon
crack-forming nanostructures
ability and porosity as
of fillers
the sol-in
sol-gel
gel coatings.
coatings. ThereFor
areinstance, Nezamdoust
many reports et al. successfully
on the addition deposited various
of carbon nanostructures amounts
as fillers in
of hydroxylated multi-walled carbon nanotube composited into the sol-gel (PTMS/OH-
MWCNT) on AM60B Mg alloy [78]. SEM observed that the micro-cracks on the pure PTMS
sol-gel coating disappeared after the incorporation of OH-MWCNTs. As the nanotube
content increased, the surface roughness of the sol-gel decreased, possibly due to the
formation of denser, low-porosity coatings. Moreover, the corrosion resistance of the
phenyl-trimethoxysilane (PTMS) sol-gel film was significantly enhanced after the addition
of OH-MWCNTs at a concentration of 500 ppm. The main mechanism was due to the
filling of defects and the formation of longer corrosion paths by the added OH-MWCNTs.
At the same time, the water contact angle increased from approximately 86.95◦ to 94.65◦ ,
which indicated a significant improvement in hydrophobicity. Malik et al. prepared a
GPTMS/graphene oxide (GPTMS/GO) coating on AZ91 Mg alloy by the chemical co-
deposition technique [79]. The graphene oxide sheets were grafted with silanol groups. In
aqueous 3.5 wt.% NaCl, the electrochemical tests showed that the corrosion resistance of
Molecules 2023, 28, 2563 7 of 30

AZ91 Mg alloy was improved. Since the graphene oxide-grafted GPTMS forms a passive
layer on Mg alloy, a covalent metal siloxane bond (Mg-O-Si) and a layered structure of
graphene oxide are formed on the substrate, which increases its hydrophobicity to 108◦
and enhances its adhesion and hardness.
Samadianfard et al. added sodium dodecyl sulfate-modified fullerene (F-SDS) and
oxidized fullerene (OF) nanoparticles in sol-gel [80,81]. EIS experiments performed in
3.5 wt.% NaCl solution confirmed that the addition of fullerene nanoparticles significantly
enhanced the corrosion resistance of the sol-gel coating. The mechanism was attributed
to the decrease in the number of defects through chemical interactions. Similarly, after
adding F-SDS nanoparticles, the micro-defects in the sol-gel coating also well decreased.
In addition, the EIS tests revealed that the corrosion protection performance of the sol-gel
coating was significantly improved after the addition of F-SDS nanoparticles (500 ppm).
Nezamdoust et al. synthesized sol-gel coatings containing different amounts of hydroxy-
lated nanodiamonds (HNDs) and deposited them on AM60B Mg alloys [82]. After adding
0.01, 0.02, and 0.05 wt.% of HND nanoparticles, the micro-defects in the sol-gel coatings
were well decreased. AFM analysis showed that the average roughness of the sol-gel film
was about 6.7 nm, which increased to 16.1 and 20.2 nm after adding 0.005 and 0.02 wt.% of
HND, respectively. When the mass fraction of HNDs was 0.01 wt.%, the corrosion protec-
tion effect was the best. The enhanced corrosion resistance was attributed to the denseness
of the coating (due to the chemical interaction with HND), the formation of tortuous paths
for the diffusion of the corrosion solution, and the filling of defects by nanoparticles.
Other nanoparticle fillers (such as silica particles [83,84]) were also able to be incor-
porated with sol-gel coatings. Wang et al. combined the sol-gel system with fluorinated
attapulgite particles (FATP@SiO2 ) to prepare a superhydrophobic surface on the AZ31 Mg
alloy (ATP is an inexpensive magnesium-aluminosilicate-rich clay mineral with nanorod-
like crystal morphology and reactive -OH groups on the surface) [85]. The water contact
angle of the prepared surface was as high as 161◦ with a sliding angle of 4◦ . The icorr value
of coating was 5.519 × 10−8 A/cm2 , decreased by three orders of magnitude compared
to bare AZ31. The results of EIS demonstrated that the corrosion resistance of the coating
decreased gradually with the prolongation of immersion time.
According to the above works, when nanoparticles are added to the silane-based
sol-gel, a denser coating can be formed. The corrosive medium (such as Cl− ) is hard
Molecules 2023, 28, x FOR PEER REVIEW 8 of to
30
reach the substrate, thereby improving the protective effect on Mg alloys. The principle of
sol-gel anticorrosion with nanoparticles is shown in Figure 5.

Figure 5.
Figure The schematic
5. The schematic diagram
diagram of
of sol-gel
sol-gel coatings
coatings containing
containing nanoparticles.
nanoparticles.

2.3. Hybrid (Inhibitors and Nanoparticles)

A possible synergistic effect can be achieved by employing both nanoparticles and
inhibitors in the sol-gel coating. For example, Ashraf et al. studied the improvement of
corrosion protection by introducing a series of amino acids as inhibitors and TiO 2 nano-
particles as surface modifiers in sol-gel coatings [86]. Electrochemical results showed that
Molecules 2023, 28, 2563 8 of 30

2.3. Hybrid (Inhibitors and Nanoparticles)

A possible synergistic effect can be achieved by employing both nanoparticles and
inhibitors in the sol-gel coating. For example, Ashraf et al. studied the improvement
of corrosion protection by introducing a series of amino acids as inhibitors and TiO2
nanoparticles as surface modifiers in sol-gel coatings [86]. Electrochemical results showed
that a sequence of four amino acids used in the sol-gel enhanced the protection performance
of the coating. The sequence of amino acids used was cysteine > serine > alanine > arginine.
When the sol-gel coating containing 0.5 wt.% cysteine and 1.0 wt.% TiO2 nanoparticles had
the best result. It effectively inhibited the corrosion of AZ91 Mg alloy in aqueous NaCl.
After 2 h of immersion, the value of Rct reached to 224.09 kΩ·cm2 .
However, adding corrosion inhibitors directly into the coating matrix may deteriorate
the coating or may reduce the activity of the corrosion inhibitor. Therefore, the encapsula-
tion/intercalation of corrosion inhibitors using micro/nano-containers has proven to be a
better option to provide long-term corrosion protection and self-healing capabilities. The
use of natural organoclay mineral nanotubes provides an efficient and environmentally
friendly method for the encapsulation of corrosion inhibitors. Montemor et al. investigated
the corrosion behavior of AZ31 Mg alloy treated with multi-walled carbon nanotubes
(CNTs) modified water-soluble bisaminosilane [87]. Before applying the silane solution,
the carbon nanotubes were treated with an aqueous rare earth solution containing cerium
nitrate or lanthanum nitrate. Electrochemical studies showed that the activation of carbon
nanotubes with rare earth salts inhibited the corrosion of the Mg AZ31. Analytical and
microscopic studies revealed that carbon nanotubes were uniformly dispersed in the sol-
gel coating, and the carbon nanotubes acted as an inhibitor container. Adsul et al. first
encapsulated the corrosion inhibitor Ce3+ /Zr4+ in kaolin clay nanotubes and aluminum-
pillared montmorillonite clay, and was then dispersed into the sol-gel matrix before finally
being deposited on the AZ91D Mg alloy via the impregnation method [88,89]. After an
immersion in 3.5% NaCl solution for 1–24 h, the self-healing and anti-corrosion abilities of
the coatings were evaluated by weight-loss experiments, potentiodynamic polarization,
and EIS measurements. These results confirmed that this new method had self-healing and
good anticorrosion properties to AZ91D Mg alloy.

2.4. Substrate Pretreatment and Repair Agent

In addition to adding fillers during the preparation of sol-gel, some scholars have
found that the substrate treatment (i.e., acid treatment, alkali treatment, heat treatment)
will also affect the protective effect of sol-gel coatings. Saxena et al. found that specific
alkali treatments of the Mg substrate before sol-gel coating can improve the corrosion
resistance of the coatings because they can promote the sol-gel condensation process [90].
Supplit et al. studied the anticorrosion effect of the sol-gel coating on the AZ31 Mg alloy
after the substrate was pickled with hydrofluoric acid, phosphoric acid, nitric acid, and
acetic acid [91]. Pickling significantly reduced the corrosion of AZ31 Mg alloy, and acetic or
hydrofluoric acid provided the best results. Hydrofluoric acid was preferred because the
optical appearance of the Mg surface was better after hydrofluoric acid treatment. Dalmoro
et al. studied the effect of hydrofluoric acid, acetic acid, and Na3 PO4 /NaOH pretreatment
in the preparation of organophosphine-sol-gel hybrid coatings on AZ91 Mg alloy [92]. The
analysis results showed that Na3 PO4 /NaOH pretreatment can form a good and stable
passivation layer, which was beneficial to the further deposition of the sol-gel coating.
Therefore, the alkaline pretreatment of the AZ91 alloy has advantages over pickling (acetic
acid or hydrofluoric acid). L. Diaz et al. studied the effect of heat treatment (200 ◦ C)
of Mg alloy substrate on the corrosion resistance of organic-inorganic composite sol-gel
coatings [93]. Heat treatment improves the protective performance of the passivation film
on AZ61, thereby inhibiting the dissolution of Mg and the formation of hydrophilic groups
during the coating process.
Some scholars have introduced an interesting perspective to explore sol-gel coatings—
repairing sol-gel coatings through electrolytes. For example, Zhong et al. studied the
Molecules 2023, 28, 2563 9 of 30

addition of zinc nitrate to NaCl solution for the repair of partially damaged sol-gel coatings
on Mg alloys [94]. Zinc nitrate not only prevents the development of the corrosion process
but also may repair partially damaged sol-gel coatings by forming precipitates covering
micron-sized cracks or defects.
The full names and abbreviations of the silane precursors used in the cited documents
in Section 2 are summarized in Table 1.

Table 1. The full names and abbreviations of the silane precursors used in the cited literature in
Section 2.

Chemical Name Abbreviation Ref.

1 Tetraethoxysilane TEOS [37,56–58,63,67,74,80–82,85,86,88,89,91,92,94]
2 Triethoxyvinylsilane VTEO [56]
3 Diethylphosphonatoethyltriethoxy-silane PHS [57]
4 3-Glycidoxypropyltrimethoxysilane GPTMS [37,58,61,67,70,74,79–82,85,88,89,94]
5 Vinyltriethoxysilane VETO [63]
6 3-(Trimethoxysilyl)propylmethacrylate MAP [64]
7 Methyltriethoxysilane MTES [74,91]
8 Tetraethoxysilane TMOS [70,93]
9 3-Aminopropyltrimethoxysilane γ-APS [71,72]
10 Phenyl-trimethoxysilane PTMS [78]
11 Triethoxyvinylsilane TEVS [86]
12 Bis-[triethoxy amino] silane BAS [87]
13 Bis-1,2-(TriethoxySilyl)Ethane BTSE [90]
14 Methylmethoxysilane MTMS [92]
15 γ-Methacryloyloxypropyltrimethoxysilane MAPTMS [93]

3. Composite Sol-Gel Coatings

In recent years, composite films have received extensive attention in the field of
corrosion protection of Mg alloys [95]. Due to the structural limitation of a single coating,
the defects are able to form a path for the corrosive medium to penetrate the coatings. The
composite coating decouples the defects on each individual coating so the formation of
corrosion paths is minimized, thereby improving the protection effect. The sol-gel coating
can be used as both a pretreatment layer and a sealing layer because of its unique properties.

3.1. Sol-Gel Coating as Pretreatment

The pre-treatment layer is crucial to the corrosion protection of the Mg alloys as
it is the layer that is in contact with the substrate that provides immediate protection.
The pretreatment of Mg alloy by silane-based sol-gel coating can effectively improve the
corrosion resistance and the adhesion of the subsequent coating [96].
Lu et al. studied the effect of sol-gel pretreatment on the properties of the Mg-rich
epoxy primer of AZ91D Mg alloy [97]. After the sol-gel pretreatment, the adhesion strength
of the Mg-rich epoxy coating on AZ91D alloy showed an average increase of seven MPa,
which improved the adhesion force significantly. Salt spray and electrochemical tests
showed that sol-gel coating not only prevented electrolytes from penetrating into the sub-
strate but also prevented corrosion products from diffusing outward, effectively improving
the barrier effect of the coating system. Liu et al. reported a two-step procedure to intro-
duce multifunctional anticorrosion coatings on Mg alloys [98]. The first step was to treat
NaOH-activated Mg with bistriethoxysilylethane (BTSE) to immobilize a tightly crosslinked
corrosion-resistant sol-gel coating (Mg-B). The second step was to treat a modified Mg-B
with γ-APS to form a surface with amine function groups (Mg-B-A). According to the polar-
ization curves of Mg-B and Mg-B-A, icorr was reduced by ~68% and ~89% compared with
bare Mg, respectively. Fernández–Hernán et al. used functionalized graphene nanoplatelets
(COOH-GNPs) as nano reinforcers (SG/GNPs) for sol-gel (SG) coating [99]. In the first
24 h of immersion in aqueous 3.5 wt.% NaCl, the SG + SG/GNP bilayer coating structure
showed the highest polarization resistance and the lowest corrosion current density. After
immersion for 168 h, the coated structure showed the lowest hydrogen evolution with an
almost 50% reduction compared to the uncoated substrate. The presence of COOH-GNPs
Molecules 2023, 28, 2563 10 of 30

increased the toughness of the coating, making it more difficult for chloride ions to reach
the substrate, delaying the initiation of the corrosion process, whereas multilayer SG + SG
cannot remain crack-free after heat treatment because it was too thick.
Zhang et al. successfully prepared a corrosion-resistant polymer coating with self-
cleaning properties on AZ31 Mg alloy by poly(3-aminopropyl)trimethoxysilane (PAPTMS)
pretreatment followed by covering with polypropylene (PP) [100]. Results indicated that the
PAPTMS/PP coating surface possessed a micrometer-scaled porous spherical microstruc-
ture and super-hydrophobicity with a high-water contact angle (162 ± 3.4◦ ) and low sliding
angle (5 ± 0.6◦ ) due to the low surface energy (10.38 mJ/m2 ). Moreover, the coating
exhibited a smaller water diffusion coefficient (8.12 × 10−10 cm2 /s) and water uptake vol-
ume fraction (24.5 %), demonstrating low water permeability and a good physical barrier
performance. As a result, the corrosion current density of PAPTMS/PP coating exhibited
approximately three orders of magnitude lower than that of the AZ31 substrate, suggesting
excellent corrosion resistance. Similarly, Ahadi Parsa et al. used vinyl tri-ethoxy silane
(VTES) as a pretreatment and then coated it with hydroxyapatite (HA) on the surface [101].
The composite coating also had good corrosion resistance in a similar manner. Li et al. pre-
pared a novel catechol/lysine (CA/Lys) polymeric sol-gel coating (CA/Lys@Sol-gel) [26].
Experimental results showed that the unmodified sol-gel coating failed after only 3 days
of testing, while the CA/Lys@Sol-gel provided up to 18 days of corrosion protection at
aqueous 0.1 M NaCl. The main reason was that the polymerized CA/Lys was adsorbed
by the sol-gel coating, and then the compactness of the coating was greatly improved by
filling the micro or nano defects.

3.2. Sol-Gel Coating as the Surface Layer

In conversion coating, the surface film/coating is produced by chemical or electro-
chemical treatment. This treatment converts the surface of the metal into a thin film of metal
oxides/other compounds that are chemically bonded to the surface [3,11,102]. However,
most chemical conversion coatings or anodizing treatments (like plasma electrolytic oxida-
tion (PEO) or micro-arc oxidation (MAO)) have cracks and pores, which cannot provide
ideal corrosion protection effects, while sol-gel can provide functions such as sealing pores
or providing biocompatibility [103,104]. This section mainly reviews the influence of the
combination of chemical conversion coating, electrochemical oxidation coating and sol-gel
on the anticorrosion effect of Mg alloys.

3.2.1. Chemical Conversion Coating/Sol-Gel

Chemical conversion treatments are usually applied on the surface to improve the
corrosion resistance of Mg alloys and improve the adhesion of coatings [105,106]. Hu et al.
first prepared a layer of molybdenum acid conversion coating on Mg alloy, and then silica
sol was repeatedly applied to the surface three times [107]. This sol-gel coating can cover the
cracks on the molybdate conversion coating, and the formed composite coating can greatly
improve the corrosion resistance of the AZ91D Mg alloy. Yue et al. used a dihydrogen
phosphate solution to treat the surface of AZ31 and coated it with silane KH560 [108]. A
sol-gel coating of about 2 µm was formed on Mg alloy. The improvement of the corrosion
resistance of phosphating Mg AZ31 by sol-gel treatment was mainly due to the sealing
effect of sol-gel on the micropores of the phosphating coating. A similar investigation
was carried out by Murillo–Gutiérrez et al. [109]. Pereira et al. studied the corrosion
behavior of cerium conversion coating (CeP), hybrid sol-gel coating (Hyb), and CeP-Hyb
composite coating deposited on the surface of WE43 Mg alloy in brine electrolyte [105].
The results showed that the surface structure of the produced cerium conversion layer
was not uniform. The CeP-Hyb composite coating improved the corrosion resistance as a
result of more uniform surface morphology. Local electrochemical impedance spectroscopy
experiments in the mapping mode (LEIM) experiments showed that CeP-Hyb samples
exhibited a self-healing ability. The Ce in the bottom conversion layer migrated to the
defect site, inhibiting the development of corrosion activity.
Molecules 2023, 28, 2563 11 of 30

Nezamdoust et al. applied a composite coating consisting of Ce-V conversion coating/

Ti-Zr conversion coating and sol-gel coating on AM60B Mg alloy [110,111]. Scanning
electron microscopy and energy dispersive X-ray spectroscopy (XPS) analysis showed that
the Ce-V conversion coating had many cracks. Similarly, a layer of cracked conversion
coating was formed on Mg alloy after Ti-Zr impregnation. Then, a dense thin sol-gel film
completely sealed all cracks in the conversion coating. Potentiodynamic polarization and
EIS experiments in Harrison solution showed that the Ce-V conversion coating/Ti-Zr con-
version coating provided limited protection against corrosion, while the composite coating
significantly improved the corrosion resistance of Mg alloys. The sol-gel film provides
protection against corrosion by sealing the cracks of the Ce-V conversion coating and acting
as a barrier. Durán et al. mainly evaluated the effect of fluorine-based pretreatment time on
the protective degradation mechanism of TEOS/GPTMS hybrid sol-gel coating [112]. The
experimental results show that the long-term pretreatment was beneficial to the formation
of the hydroxy magnesium fluoride layer with a higher F/O ratio, thereby improving the
corrosion resistance of the coating on WE54 Mg alloy.
Ashassi–Sorkhabi et al. applied cerium lanthanum permanganate (CLP) conversion
coatings on Mg alloys prior to the sol-gel process [113–115]. CLP coatings prevented severe
corrosion of Mg in acidic sols. This pretreatment stabilized Mg and led to better adhesion of
the sol-gel coating. The results showed that the coatings with corrosion inhibitors had much
fewer cracks and pores after immersion in aqueous 3.5wt.% NaCl. The best-performing
coating contained 1 wt.% L-aspartic acid. This observation originated from the structure of
L-aspartic acid, which had three active adsorption sites, enabling its strong adsorption on
the surface and providing a better anti-corrosion performance. When salt was used as a cor-
rosion inhibitor, potassium hypophosphite showed better corrosion resistance under a short
immersion time, while manganese acetate showed stronger corrosion resistance under long-
term immersion. At the same time, when nanoparticles were added to the sol-gel coating
as a surface modifier, the anti-corrosion effect of the coating was better. The role of nanopar-
ticles was to increase the roughness of the coating surface, thereby reducing the number of
surface pores and cracks. Guo et al. prepared Mg(OH)2 /PMTMS/CeO2 hybrid coatings by
a hydrothermal method [116]. The results showed that the magnesium hydroxide coating
was sealed by PMTMS and CeO2 . The thickness of the Mg(OH)2 /PMTMS/CeO2 coating
was about 12.86 ± 0.01 µm, which significantly improved the corrosion resistance of the
AZ31 alloy.

3.2.2. Anodizing/Sol-Gel
Anodizing is the process of using metals or alloys as anodes to form an oxide film
on the surface via electrolysis. Bestetti et al. prepared a porous oxide layer of MgO by
anodizing, followed by a single or multilayer SiOx coating by the sol-gel method [117]. The
anodizing of Mg improved the adhesion of the sol-gel layer, and the sol-gel layer sealed
the pores of MgO. Hence, the corrosion resistance of Mg was well-improved. Lamaka
et al. also performed a similar investigation [118]. While Afsharimani et al. prepared an
anodizing/sol-gel coating, they added graphene nanosheets to the sol-gel to improve the
corrosion resistance of Mg alloys in 0.05 M NaCl [119]. The corrosion performance of the
sol-gel coating containing graphene nanoplatelets (icorr = 0.01 µA/cm2 ) was better than
that of the anodized coating without graphene nanoplatelets (icorr = 1.00 µA/cm2 ), which
was due to the better coating quality and barrier properties.
PEO is a popular process for forming porous ceramic oxide layers on metal substrates.
Although PEO can solve the problem of insufficient adhesion of organic coatings and
form a harder, thicker, and stronger coating, the pores and microcracks in the PEO coating
structure reduce its corrosion resistance. The coating porosity is of vital concern because the
interconnected pores can form direct pathways connecting the Mg alloy substrate surface
with aggressive media, reducing the protective barrier effect of the PEO coating [120]. To
solve this problem, combining PEO with a sol-gel coating allows the pores in the coating to
be sealed by the sol-gel, thereby improving corrosion protection performance.
Molecules 2023, 28, 2563 12 of 30

Ivanou et al. prepared an inhibitor-loaded PEO layer with a TiO2 -doped sol-gel coating
to the ZE41 Mg alloy [121]. The scanning vibrating electrode technique (SVET) test results
showed that the composite coating had an effective anti-corrosion performance. During
immersion in 50 mM of NaCl solution, the corrosion rate was reduced by a magnitude
of 3 to 100 times. In this composite self-healing coating, a thin, porous PEO layer can be
successfully used as a reservoir for corrosion inhibitors, in addition to providing barrier
protection. In this case, the corrosion inhibitor was pinned to the metal substrate where
corrosion begins, and a thin sol-gel coating on top of the PEO layer slowed its leaching rate.
Similar studies were also done by Shang et al. [122], Cui et al. [123], Pezzato et al. [124],
Chen et al. [76], Merino et al. [125], and Chen et al. [126]. The difference is that these scholars
loaded corrosion inhibitors or nanoparticles in the porous PEO layer or in the sol-gel, which
can get a better protective effect. Those works came to the same conclusion: the sol-gel
layer can effectively reduce the porosity of PEO coatings and form a dense hydrophobic
outer layer. The hydrophobic properties of the composite coating may be related to the
siloxane network (Si-O-Si) formed on the surface. After the silane-based sol-gel treatment,
the corrosion resistance of the PEO-based coating was improved significantly.
Jiang et al. prepared N-doped graphene quantum dots (N-GQDs)/PMTMS composite
sol-gel coating on AZ91D Mg alloy [127]. Compared with the bare Mg alloy, the N-
GQDs/PMTMS coating showed a significant enhancement in its corrosion resistance.
According to the EIS test, the value of Rct was more than three orders of magnitude than
that of bare Mg alloy. The mechanism was mainly due to the strong chemical bonding
between the N-GQDs coating and the sealing layer of the PMTMS sol-gel.
It has been well addressed in the literature that when the chemical conversion coating
and anodizing are used together with the silane-based sol-gel coating, the sol-gel can fill
the defects and cracks in the coating beneath the sol-gel. The composite coating becomes
Molecules 2023, 28, x FOR PEER REVIEW
denser, blocking the possible path from the aggressive external medium 13 of 30 to the substrate.
−
The corrosion factors, such as Cl , do not easily reach the substrate, thereby improving
the protection of the Mg alloy. This paper summarizes the anti-corrosion principle of the
protection of the Mg alloy. This paper summarizes the anti-corrosion principle of the com-
composite sol-gel coating, as shown in Figure 6.
posite sol-gel coating, as shown in Figure 6.

Figure
Figure 6. The
6. The schematic
schematic diagram
diagram of the anticorrosion
of the anticorrosion principle
principle of the compositeof the composite
sol-gel coatings. sol-gel coatings.

3.3.Multilayer
3.3. Multilayer Hybrid
Hybrid Coating
Coating
AsAsreported
reportedin thein
previous sections, sections,
the previous the sol-gel layer can be used
the sol-gel as both
layer can abe
pretreat-
used as both a pretreat-
ment layer and a surface-sealing layer. However, when applied as a pretreatment layer,
ment layer and a surface-sealing layer. However, when applied as a pretreatment layer, the
the sol-gel coating sometimes needs to be hydrolyzed and condensed under acidic condi-
sol-gel
tions. Whencoating
directlysometimes
coated on theneeds
surface,tothe
beMghydrolyzed and and
alloy will corrode condensed
generate aunder
small acidic conditions.
When directly
amount of hydrogen,coated on the surface,
consequently resultingthe Mg alloy
in more defectswill
and corrode and generate
poor adhesion. When a small amount
used as a surface layer,
of hydrogen, sol-gel coatings
consequently are cracked
resulting due todefects
in more the rapidandevaporation of resid- When used as a
poor adhesion.
ual water and solvents during heat treatment. To overcome the above problems, some
surface layer, sol-gel coatings are cracked due to the rapid evaporation of residual water
researchers proposed the application of multilayer sol-gel coatings. The first layer is a
and solvents
chemical during heat
or electrochemical treatment.
conversion film. The Toapplication
overcome of athe above
second problems,
sol-gel layer pro- some researchers
proposed the application of multilayer sol-gel coatings. The first
vides a chemically neutral surface that facilitates the addition of another thick coating. layer is a chemical or elec-
Toorani et al. studied morphology, surface properties, and corrosion
trochemical conversion film. The application of a second sol-gel layer provides resistance of a chemically
PEO/sol-gel/epoxy three-layer coatings [128, 129]. The presence
neutral surface that facilitates the addition of another thick coating. of APTES in the sol-gel
resulted in the formation of an amine-rich surface. The amino functional groups act as a
molecular bridge that enhance the adhesion between the organic and PEO coatings. Ad-
ditionally, the amino groups form a new reaction site, which is a strong nucleophile that
readily reacts with the epoxy group through a ring-opening reaction. Each amino group
can undergo a ring-opening reaction with an epoxy group to form a covalent bond be-
tween the two layers. There is also the possibility of hydrogen bonding between the silane
Molecules 2023, 28, 2563 13 of 30

Toorani et al. studied morphology, surface properties, and corrosion resistance of

PEO/sol-gel/epoxy three-layer coatings [128,129]. The presence of APTES in the sol-gel
resulted in the formation of an amine-rich surface. The amino functional groups act as
a molecular bridge that enhance the adhesion between the organic and PEO coatings.
Additionally, the amino groups form a new reaction site, which is a strong nucleophile
that readily reacts with the epoxy group through a ring-opening reaction. Each amino
group can undergo a ring-opening reaction with an epoxy group to form a covalent bond
between the two layers. There is also the possibility of hydrogen bonding between the
silane layer and the epoxy layer. The study on the protective properties of epoxy coatings
showed that the addition of a sol-gel layer between PEO and epoxy coatings can improve
the corrosion resistance of the coating system. In addition, the authors provided evidence
that the presence of cerium nitrate in the PEO coating and the presence of 8-HQ in the
sol-gel layer gave the coating system better protective performance.
Wang et al. prepared a (APS/Bis [3-(triethoxysilyl)propyl]tetrasulfide (BTESPT))/
graphene/(APS/BTESPT) three-layer composite coating [130]. Compared with bare AZ31B
Mg alloy, the corrosion current density of the composite coating in 3.5 wt.% NaCl solution
was reduced by four orders of magnitude. This was due to the excellent corrosion resistance
and strong adhesion of the sol-gel coating on the surface of the Mg alloy, as well as the
barrier of graphene oxide to the permeation path of corrosive media, such as Cl− and H2 O.
The results indicated that the composite coating still had good corrosion resistance after
immersion in 3.5 wt.% NaCl solution for 7 days.
The full names and abbreviations of the sol-gel precursors used in the cited documents
in Section 3 are summarized in Table 2.

Table 2. The full names and abbreviations of the silane precursors used in the literature cited in
Section 3.

Chemical Name Abbreviation Ref.

1 γ-Glycidoxy propyl trimethoxy silane γ-GPS [97,126]
2 Bistriethoxysilylethane BTSE [98]
3 3-Amino-propyltrimethox-ysilane γ-APS [98,129,130]
4 Poly(3-aminopropyl)trimethoxysilane PAPTMS [100]
5 Vinyl tri-ethoxy silane VTES [101]
6 Tetraethoxysilane TEOS [26,99,107,110–115,117,119,122,124–126,128,129,131,132]
7 3-Glycidoxypropyltrimethoxysilane GPTMS [26,76,107–112,118,119,121,125,131]
8 Phenyl-trimethoxysilane PTMS [110,121]
9 3-Methacryloxypropyl trimethoxysilane - [117]
10 Methyltriethoxysilane MTES [99,113–115,124,132]
11 γ-Amino propyltriethoxysilane APTES [128]
12 Bis [3-(triethoxysilyl)propyl]tetrasulfide BTESPT [130]
Methylmethoxysilane MTMS [116,123,127]

4. Conclusions and Outlooks

Due to the various industrial applications of Mg alloys, much research has been
done to improve its anti-corrosion performance. Over the past few decades, the sol-gel
methods have proven to be a good way to increase corrosion protection on Mg alloys. In
addition to its own good barrier function, the versatile sol-gel coating can be modified by
many approaches, which were divided into four categories in this paper: (1) Bare sol-gel;
(2) Nanoparticles; (3) Corrosion inhibitors; and (4) Sol-gel-based composite coatings. All
the above methods have been proven to significantly improve the barrier and protection
effects of the sol-gel coatings.
In this paper, the existing modification methods of silane-based sol-gel coatings were
reviewed. The sol-gel precursors of the cited article were summarized in Tables 1 and 2. It
can be seen that TEOS and GPTMS are the most commonly used silane precursors. The
thickness and corrosion resistance of different sol-gel coatings in the cited article was
included in Table 3.
Molecules 2023, 28, 2563 14 of 30

Table 3. Thickness and corrosion resistance of the different sol-gel coatings.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
SiO2 (TV) sol: the molar ratio of
TEOS:VTEO: ethanol: water:
acetic acid is equal to icorr Mg = 1.29 × 10−5 A/cm2
2008 AZ91D Bare sol-gel coating 0.25:0.75:10:4:0.01. - 3.5 wt.% NaCl icorr SiO2 (T) = 1.15 × 10−6 A/cm2 [56]
SiO2 (T) sol: the molar ratio of icorr SiO2 (TV) = 2 × 10−6.8 A/cm2
TEOS: ethanol: water: acetic acid
is equal to 1:10:4:0.01.
Harrison’s solution
The molar ratio of the silanes was |Z|0.01Hz : uncoated silica coated
2006 AZ31B Bare sol-gel coating 600–700 nm (0.35 wt.% (NH4 )2 SO4 [57]
1:2 (PHS: TEOS). coated with PHS: TEOS film.
and 0.05 wt.% NaCl)
Mixing TEOS and GPTMS
icorr is about an order of magnitude
precursors in a molar ratio of 3:1
lower compared to magnesium alloys,
2017 AZ31 Bare sol-gel coating and using ethanol and acetic acid 0.7–2.5 µm 0.1 M NaCl [58]
simultaneously, a protection range up to
as solvent and catalyst,
150 mV.
respectively.
Mixing GPTMS, VETO, distilled
icorr Mg = 1.29 × 10−5 A/cm2
Bare sol-gel coating water, and ethanol in 1:3:12:30
2010 AZ91D - 3.5 wt.% NaCl icorr sol-gel = 8.64 × 10−7 A/cm2 [63]
(Ce3+ ) molar ratios. Ce(NO3 )3 ·6H2 O was
icorr sol-gel (MPD) = 5.75 × 10−8 A/cm2
added to yield 0.01 M of Ce3+ .
Mixing the starting precursors
consisting of TEOS and MAP,
The hybrid film exhibited a high resistive
deionized water, and ethanol with
modulus (105 –106 Ω cm2 ) during the
a molar ratio of 11:1:60:80. The
Bare sol-gel coating first few hours of immersion, and the
2015 Elektron 21 (El21) alloy production of cerium-doped sols 1 µm 0.05 M NaCl [64]
(Ce3+ ) addition of cerium at a concentration of
was performed by adding cerium
0.01 M to the sol significantly increased
nitrate (Ce(NO3 )3 · 6 H2 O) at four
the durability of the film (2 days).
different concentrations: (0.005,
0.01, 0.05, and 0.1) mol/L.
Mixing TEOS and GPTMS in a
molar ratio of 3:1 that was
dissolved in ethanol. Then use an Hybrid coatings achieved a reduction of
inhibitor solution of 2.5 mol% the corrosion current density by about
Ce(NO3 )3 (relative to the two and three orders of magnitude with
Bare sol-gel coating
2020 AZ31 precursor) and a catalyst of 0.9–3.3 µm 0.1 M NaCl regard to the undoped coated specimen [37]
(Ce3+ )
2.5 vol% AcOH. Obtain mixed sols and the AZ31 alloy respectively, also
by mixing the two solutions at a exhibiting a protection range of up to
volume ratio of 4.5:1. The hybrid 160 mV.
sol was obtained by mixing both
solutions in a volume ratio of 4.5:1.
Molecules 2023, 28, 2563 15 of 30

Table 3. Cont.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
Solution A: zirconium (IV)
propoxide (70% solution in
2-propanol) and ethylacetoacetate
After 14 days immersed,
with volume ratio 1:1. Solution B:
|Z|0.01Hz sol-gel ≈2 × 105 Ω cm2
Bare sol-gel coating GPTMS and 2-propanol with 1:1
2010 AZ31 3 µm 0.005 M NaCl |Z|0.01Hz sol-gel(8-HQ)≈ 1 MΩ cm2 [61]
(8-HQ) volume ratio. The final solution:
Rct sol-gel = 687 kΩ cm2
solutions (A + B) with a volume
Rct sol-gel (8-HQ) = 1649 kΩ cm2
ratio of 1:1. Inhibitor-doped
sol-gel films were prepared
adding 0.26 wt.% of 8-HQ.
Mixing GPTMS and TEOS (molar
ratios = 5:1), the solvent is an icorr Mg = 1.31 × 10−4 A/cm2
Sol-gel coating + appropriate amount of distilled Simulated Body Fluid icorr sol-gel = 4.29 × 10−6 A/cm2
2010 ZE21B - [67]
corrosion inhibitor water and ethanol. The above (SBF) icorr sol-gel (PCTyr)
solution was doped with corrosion = 3.64 × 10−6 A/cm2
inhibitor (PCTyr Schiff base).
The mixed precursors were
GPTMS and TMOS with molar
ratio of 3:1 in acetic acid solution
of 0.05 mol/L. The molar ratio of
icorr Mg = 7.10 × 10−3 A/cm2
Sol-gel coating + GPTMS: acetic acid is 60:1. The
2009 AZ91D - Harrison’s solution icorr sol-gel = 2.41 × 10−7 A/cm2 [70]
corrosion inhibitor above solution was doped with
icorr sol-gel (MPD) = 4.5 ×10−10 A/cm2
0.001 mol/L MPD. The inhibitor
was pre-resolved in 20 mL of
distilled water and then added
into sol solution.
Phytic acid and γ-APS (mole
Sol-gel coating + ratios were 1:1) were added into icorr Mg = 49.41 µA/cm2
2017 AZ31 - SBF [71]
corrosion inhibitor 40 mL of mixed solution with icorr sol-gel (Phytic acid) = 3.57 µA/cm2
water/ethanol volume ratio of 3:2.
The silane sols consisted of three
different precursors: MTES, Rf : the overall resistance of the coating
GPTMS, and TEOS in equal response.
volumes (6.6% V/V) in a The Rf of silane coating modified with
Sol-gel coating +
2021 AZ31 combination of 10% distilled water 3.81 µm SBF solution HA nanoparticles reached 41 kΩ cm2 , [74]
corrosion inhibitor
and 70% ethanol. 1000 mg/L which was more than 100 times higher
hydroxyapatite (HA) than that without modification after
nanoparticles were added to the being soaked for 4 days.
sol.
Molecules 2023, 28, 2563 16 of 30

Table 3. Cont.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
Rp sol-gel = 207.5 kΩcm2
500 ppm OH-MWCNTs were Rp sol-gel (OH-MWCNTs) =
Sol-gel coating + added to PTMS, and the mixture 368.6 kΩ cm2
2018 AM60B 1.4–1.5 µm Harrison’s solution [78]
nanoparticles was ultrasonically agitated for 1440min later.
about 20 min. Rp sol-gel = 22.6 kΩ cm2
Rp sol-gel (OH-MWCNTs) = 44.1 kΩ cm2
icorr Mg = 49.90 µA/cm2
The GPTMS/GO was prepared by icorr sol-gel = 0.25 µA/cm2
Sol-gel coating + mixing 0.25 mL GO, 10 mL icorr sol-gel (GO) = 0.016 µA/cm2
2021 AZ91 10 µm 3.5 wt.% NaCl [79]
nanoparticles ethanol, 10 mL GPTMS, and Rct Mg = 0.87 kΩ cm2
79.85 mL deionized water. Rct sol-gel = 3.9 kΩ cm2
Rct sol-gel (GO) =5.02 kΩ cm2
Mixing 0.02 mol TEOS, 0.02 mol
GPTMS 0.14 mol water (the pH after being soaked for 1440 min.
Sol-gel coating +
2020 AM60B was formulated to 1.5 with HCl). 1.5–2 µm 3.5 wt.% NaCl Rp Mg = 1.405 kΩ cm2 [80]
nanoparticles
Then, 100 mg/L oxidized fullerene Rp sol-gel (OF) = 500.018 kΩ cm2
was added into the sol.
Mixing 0.02 mol TEOS, 0.02 mol
GPTMS 0.14 mol water (the pH
was formulated to 1.5 with HCl). After being soaked for 48h.
Sol-gel coating +
2021 AM60B Then, 500 ppm F-SDS (the SDS 3 µm 3.5 wt.% NaCl Rp sol-gel = 6 kΩ cm2 [81]
nanoparticles
molecules were stabilized on the Rp sol-gel (F-SDS) = 23 kΩ cm2
fullerene C60 nanoparticles) was
added into the sol.
Mixing 0.02 mol TEOS and 0.02
mol GPTMS. Acidic water (pH = 1,
HCl) was added to the sol with 1:1 After 3 h immersion,
Sol-gel coating +
2020 AM60B alkoxy to H2 O molar ratio. Then, 0.7–0.8 µm Harrison’s solution icorr sol-gel = 2.202 µA/cm2 [82]
nanoparticles
0.01 wt.% of the hydroxylated icorr sol-gel (HND) = 0.476 µA/cm2
nanodiamonds was added into the
sol.
The hydrolysis and
polymerization of GPTMS and After 3 h immersion,
Sol-gel coating + TEOS were under acidic condition. icorr Mg = 7.143 × 10−5 A/cm2
2020 AZ31B - 3.5 wt.% NaCl [85]
nanoparticles After a certain amount of icorr sol-gel (SiO2 ) =
F-ATP@SiO2 particles were added 5.519 × 10−8 µA/cm2
to the sol.
Molecules 2023, 28, 2563 17 of 30

Table 3. Cont.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
nA/cm2
icorr sol-gel = 1168.1
icorr sol-gel (cysteine +TiO2 ) =
Adding TEOS and TEVS with a
Sol-gel coating + 25.0 nA/cm2
molar ratio of 1:3. 0.5 wt.%
2019 AZ91 inhibitor + 450 nm 0.05 M NaCl Rct Mg = 0.238 kΩ cm2 [86]
cysteine and 1.0 wt.% TiO2 were
nanoparticles Rct sol-gel = 5.554 kΩ cm2
added to the sol.
Rct sol-gel (cysteine +TiO2 ) =
224.090 kΩ cm2
For the silane coating modified with the
untreated CNTs, the anodic current
densities attained values around
The BAS was prepared by 60 µA/cm2 for all the test period (up to
dissolving 5% (vol/vol) of silane 24 h) and the cathodic currents attained
Sol-gel coating + in a mixture of methanol (10% values around −60 µA/cm2 . The silane
2008 AZ31 inhibitor + vol/vol) and 85% (vol/vol) of 5.5–6 µm 0.05 M NaCl coatings modified with the CNTs treated [87]
nanoparticles distilled water. Modified by with cerium revealed the lowest
Ce(NO3 )3 or La(NO3 )3 CNTs were corrosion activity. After 24 h of
then added to the sol. immersion, the activity decreased and
both anodic and cathodic current
densities ranged between 2 and
−2 µA/cm2 .
The organic-inorganic hybrid After being soaked for 24h,
Sol-gel coating + matrix sol was prepared by icorr Mg =6.0 × 10−5 A/cm2
2017 AZ91D inhibitor + hydrolysis of GPTMS with TEOS - 3.5 wt.% NaCl icorr sol-gel = 1.4 × 10−6 A/cm2 [88]
nanoparticles in molar ratio of 3:5:1 with 0.1 N icorr sol-gel (Ce3+ /Zr4 +halloysite
HCl as catalyst. nanotubes) = 0.9 × 10−6 A/cm2
GPTMS and TEOS were taken in
After being soaked for 120 h,
Sol-gel coating + molar ratio of 3:5 and hydrolysed
icorr sol-gel = 1.513 × 10−5 A/cm2
2018 AZ91D inhibitor + in presence of 0.1 N HCl as 2.5 ± 0.5 µm 3.5 wt.% NaCl [89]
icorr sol-gel (Ce3+ /Zr4 +halloysite
nanoparticles catalyst, to synthesize the hybrid
nanotubes) = 5.602 × 10−7 A/cm2
organic-inorganic matrix sol.
After the introduction of zinc nitrate for
The silane sol was synthesized by
48 h, the resistance value increased and
mixing GPTMS, TEOS, distilled 0.005 M NaCl + zinc
2010 AZ91D Bare sol-gel coating - arrived at about 180 kΩ, which exceeded [94]
water, and ethanol in 3:1:13:40 nitrate
that of the sample initially immersed in
molar ratios.
undoped solution for 1 h (about 140 kΩ).
Molecules 2023, 28, 2563 18 of 30

Table 3. Cont.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
|Z|0.01Hz sol-gel/ Mg-rich epoxy
primer: higher than 1011 Ω cm2 ;
For magnesium-rich primer of AZ91D
Adding 10 wt.% γ-GPS to a 1:8 alloy without pre-treatment, EIS results
Composite sol-gel
mixture of methanol and distilled show that the alloy substrate is corroded
2012 AZ91D coating (sol-gel/ - 3 wt.% NaCl [97]
water. Glycerol (0.15 vol.% of the after 840 h immersion; for magnesium
Mg-rich epoxy primer)
total silane solution) was added. rich primer of AZ91D alloy pretreated
with silane, the EIS results show that the
substrate will corrode after being soaked
for 1800 h.
icorr Mg = 8.32 ± 0.63 µA/cm2
icorr sol-gel = 2.69 ± 0.31 µA/cm2
Composite sol-gel BTSE or γ-APS solution was icorr sol-gel /sol-gel =
2013 AZ31 coating (sol-gel/ prepared by mixing 5% silane, 90% - SBF solution 0.90 ± 0.24 µA/cm2 [98]
sol-gel) ethanol, and 5% Milli-Q water. Rp Mg = 2650 ± 538 Ω cm2
Rp Mg-B = 7788 ± 2572 Ω cm2
Rp Mg-B-A =13635 ± 2745 Ω cm2
TEOS +
MTES/Isopropanol/water:
1/5/10. To obtain the initial sols,
TEOS and MTES were mixed in After being soaked for 24 h,
Single sol-gel coating + molar fraction of 40 %/60 %. icorr Mg = 6.6 × 10−6 A/cm2
2020 AZ31B 2.2 µm 3.5 wt.% NaCl [99]
nanoparticles 0.005 wt.% COOH-GNPs was icorr sol-gel =1.2 × 10−6 A/cm2
added to isopropyl alcohol, and icorr sol-gel /sol-gel = 5.2 × 10−7 A/cm2
the final concentration of nano
charges measured in the coating
was 0.046 wt.% of COOH-GNPs.
icorr Mg = 4.96 × 10−5 A/cm2
icorr sol-gel = 1.95 × 10−6 A/cm2
Composite sol-gel PAPTMS: (APTMS: ethanol: icorr sol-gel/ PP = 9.08 × 10−8 A/cm2
2020 AZ31 About 60 µm 3.5 wt.% NaCl Rp Mg = 190.9 Ω cm2 [100]
coating (sol-gel/ PP) deionized water = 3:22:75, V/V/V)
Rp sol-gel = 7578 Ω cm2
Rp sol-gel/ PP = 2.80 × 105 Ω cm2
2.5 mL of VTES was added to a icorr Mg= 36.1 ± 0.1 µA/cm2
Composite sol-gel mixture of 5 mL DIW, 5 mL of icorr sol-gel/ HA = 0.9 ± 0.1 µA/cm2
2022 AZ31 - 3.5 wt.% NaCl [101]
coating (sol-gel/ HA) acetone, and 95 mL of ethanol Rp Mg = 253 Ω cm2
under magnetic stirring. Rp sol-gel/ HA = 12155 Ω cm2
Molecules 2023, 28, 2563 19 of 30

Table 3. Cont.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
Silicon sol was prepared from
Sol-gel: after being soaked for 4 days,
GPTMS, TEOS, deionized water,
Rct = 1.030e3 .
Composite sol-gel and ethanol, mixed in a volume
CA/Lys @Sol-gel: after being soaked for
2023 AZ31 coating ratio of 3:1:1:5. Then, add 9 ± 0.5 µm 0.1 M NaCl [26]
4day, Rct = 1.344e6 . After 18 days of the
(sol-gel/(CA/Lys)) Ce(NO3 )3 to make the
test, the value of Rct was still as high as
concentration of Ce(NO3 )3 reach
105 Ohm·cm2 .
0.01 M.
icorr Mg= 1.29 × 10−5 A/cm2
icorr conversion coating =
Silicon sol was prepared from 1.76 × 10−5 A/cm2
Composite sol-gel
TEOS, GPTMS, and ethanol, icorr conversion coating/sol-gel =
2009 AZ91D coating - 3.5 wt.% NaCl [107]
which were mixed in a molar ratio 3.80 × 10−5 A/cm2
(molybdate/sol-gel)
of 0.25:0.75:10. Rp conversion coating = 552 Ω cm2
Rp conversion coating/sol-gel =
4.5× 104 Ω cm2
Mixing starting precursors After being soaked for 192 h,
Composite sol-gel consisting of GPTMS and |Z|0.01Hz Mg ≈3 × 103 Ω cm2
2013 Elektron21 coating (phosphate aluminum-tri-sec-butoxide, 7 µm 0.05 M NaCl |Z|0.01Hz sol-gel≈3 × 103 Ω cm2 [109]
/sol-gel) deionized water, and propanol in |Z|0.01Hz conversion coating /sol-gel ≈
a molar ratio of 2:1:1:10. 3 × 104 Ω cm2
Composite sol-gel Rtot conversion coating = 2227 Ω cm2
2013 AZ31 coating (phosphate Silane solution: 10 g/L KH560 4.9 µm 3.5 wt.% NaCl Rtot conversion coating/sol-gel = [108]
/sol-gel) 5.6 × 103 Ω cm2
After being soaked for 24 h,
icorr Mg = 10.9 µA/cm2
The inorganic TEOS (10% V/V) icorr conversion coating = 3.0 µA/cm2
Composite sol-gel and organic GPTMS (20% V/V) icorr conversion coating/sol-gel =
2022 WE43 coating precursors were added together to 2.06 ± 0.05 µm 0.1 M NaCl 0.6 µA/cm2 [131]
(Cerium/sol-gel) a mixture of ethanol (10% V/V) Rct Mg = 3177 Ω cm2
and distilled water (60% V/V). Rct conversion coating = 4363 Ω cm2
Rct conversion coating /sol-gel =
22485 Ω cm2
Molecules 2023, 28, 2563 20 of 30

Table 3. Cont.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
icorr Mg = 310.9µA/cm2
icorr conversion coating = 145.8 µA/cm2
Mixing 0.04 mol TEOS, 0.02 mol
icorr conversion coating/sol-gel =
Composite sol-gel GPTMS, and 1.23 mol acidic water
4.6 µA/cm2
2017 AM60B coating (Cerium so that the molar ratio of the water 2 µm Harrison’s solution [111]
Rct Mg =21.4 Ω cm2
vanadate /sol-gel) molecules to alkoxide groups was
Rct conversion coating = 114.5 Ω cm2
about 5:1.
Rct conversion coating /sol-gel =
3750.0 Ω cm2
icorr Mg= 9.670 µA/cm2
icorr conversion coating = 5.692 µA/cm2
icorr conversion coating/sol-gel =
Composite sol-gel Here, 0.02 mol TEOS and 0.02 mol 0.027 µA/cm2
2019 AM60B 1.5–2 µm 0.05 M NaCl Rp Mg = 4.7× 103 Ω cm2 [110]
coating (Ti-Zr/sol-gel) GPTMS precursors were mixed.
Rp conversion coating = 7.9× 103 Ω cm2
Rp conversion coating/sol-gel =
858.5 × 103 Ω cm2
Hybrid sols were synthesized by
mixing TEOS and GPTMS in a icorr Mg = 1.78 × 10−5 A/cm2
Composite sol-gel molar ratio of 3:1, employing icorr conversion coating =
2021 WE54 coating ethanol as solvent and an acidic - 0.1 M NaCl 1.84 × 10−6 A/cm2 [112]
(fluoride/sol-gel) mixture of acetic acid and nitric icorr conversion coating/sol-gel =
acid as catalysts in a volume 1.86 × 10−7 A/cm2
proportion of 2.5:1.
Adding TEOS and MTES with a
molar ratio of 2:3 to an acidic Rtotal Mg = 0.119 kΩ cm2
Composite sol-gel solution of nitric and acetic acids Rtotal conversion coating =0.681 kΩ cm2
2019 AZ91 850 nm 3.5% NaCl [113]
coating (CLP/sol-gel) in 1:5 vol ratio. Then, 1 wt.% Rtotal conversion coating /sol-gel=
L-Aspartic was added to the sol as 85.417 kΩ cm2
corrosion inhibitors.
A mixture of TEOS and MTES
with a molar ratio of 2:3 was
Rtotal Mg = 0.119 kΩ cm2
hydrolyzed in a solution of acetic
Composite sol-gel Rtotal conversion coating =0.681 kΩ cm2
2019 AZ91 and nitric acids in a 5:1 vol ratio. 800 nm 3.5% NaCl [114]
coating (CLP/sol-gel) Rtotal conversion coating /sol-gel=
Then, 0.5 wt.% of cloisite Na+ and
434.731 kΩ cm2
0.5 wt.% of methionine were
added to the sol.
Molecules 2023, 28, 2563 21 of 30

Table 3. Cont.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
A mixture of TEOS and MTES
with a molar ratio of 2:3 was
hydrolyzed in a solution of acetic Rtotal Mg = 0.119 kΩ cm2
Composite sol-gel and nitric acids in a 5:1 vol ratio. Rtotal conversion coating = 0.681 kΩ cm2
2019 AZ91 - 3.5% NaCl [115]
coating (CLP/sol-gel) Then, 0.5 wt.% potassium Rtotal conversion coating /sol-gel =
hypophosphite and 0.5 wt.% of 127.382 kΩ cm2
cloisite 20A nanoparticle were
added to the sol.
icorr Mg = 1.51 ± 0.08 × 10−5 A/cm2
Composite sol-gel PMTMS/CeO2: a mixture of icorr Mg(OH)2 /sol-gel =
2017 AZ31 coating (Mg MTMS, ethanol and water (3:10:20, 12.86 ± 0.01 µm 3.5 wt.% NaCl 2.46 ± 0.07 × 10−8 A/cm2 [116]
(OH)2 /sol-gel) V/V/V), cerium nitrate (10−3 M). Rct Mg = 854.4 Ω cm2
Rct Mg(OH)2 /sol-gel = 4.03 × 105 Ω cm2
Mixing together with TEOS (4.7 g),
3-metacryloxypropyl
icorr Mg = 3 × 10−5 A/cm2
Composite sol-gel trimethoxysilane (10.4 g),
2010 AM60B 4 µm 3.5 wt.% NaCl icorr AO = 2 × 10−6 A/cm2 [117]
coating (AO/sol-gel) ethylalcohol (15.8 g), distilled
icorr AO/sol-gel = 7 × 10−9 A/cm2
water (4.9 g), and
tert-butylhydroperoxide (1.9 g).
TEOS, zirconyl chloride icorr Mg = 3.395 × 10−5 A/cm2
Composite sol-gel
2009 AZ91D octahydrate (ZrOCl2 ·8H2 O), and 5 µm 3.5 wt.% NaCl icorr MAO = 3.921 × 10−7 A/cm2 [122]
coating (MAO/sol-gel)
ethanol were mixed together. icorr MAO/sol-gel =1.577 × 10−9 A/cm2
The desired amounts of TEOS, icorr Mg = 2.2 × 10−5 A/cm2
C2H5OH, NH4OH, and H2O were icorr MAO = 2.5 × 10−7 A/cm2
mixed with a molar ratio of icorr MAO/sol-gel =2.6 × 10−8 A/cm2
Composite sol-gel
2012 NZ30K 1:30:1:1. The calculated amount of 3.5–7 µm 3.5 wt.% NaCl |Z|0.01Hz Mg ≈ 103 Ω cm2 [132]
coating (MAO/sol-gel)
MTES (molar ratio of MTES/TEOS |Z|0.01Hz MAO ≈ 105 Ω cm2
= 1/2) was added dropwise into |Z|0.01Hz MAO/sol-gel ≈
the mixed solution. 3 × 106 Ω cm2
Silane sol: mixing GPTMS and
PTMS (volume ratio was 1: 1);
metal organic: mixing TPOT After 7 days of immersion,
Composite sol-gel (70 wt.% in 2-propanol) and Sol-gel: about 50% of the coating was
2016 ZE41 7.8–8.4 µm 3% NaCl [121]
coating (PEO/sol-gel) acetylacetone in stoichiometric exfoliated from the surface.
proportion. Both metal organic |Z|0.01Hz MAO/sol-gel = 3 × 108 Ω cm2
and silane sols were mixed
together.
Molecules 2023, 28, 2563 22 of 30

Table 3. Cont.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
10−5
icorr Mg = 1.37 × A/cm2
Polymethyltrimethoxysilane
Composite sol-gel icorr MAO/sol-gel = 2.86 × 10−8 A/cm2
2017 AZ31 (PMTMS): (MTMS: ethanol: DI 13.65 µm 3.5 wt.% NaCl [123]
coating (MAO/sol-gel) Rct Mg = 275.30 Ω cm2
water = 3:10: 20)
Rct MAO/sol-gel = 2.24 × 106 Ω cm2
Ethanol: silica precursors: water:
PEO/sol-gel that is characterized by
Composite sol-gel hydrochloric acid (a molar ratio) = 0.1 M Na2 SO4 + 0.05 M
2019 AZ80 22 µm currents about two orders of magnitude [124]
coating (PEO/sol-gel) 2:1:4:0.01. The ratio between TEOS NaCl
lower than the untreated one.
and MTES were fixed at 30:70.
icorr Mg = 1.61 × 10−5 A/cm2
icorr PEO = 2.64 × 10−7 A/cm2
Mixing 0.5 mol TEOS, 0.5 mol icorr PEO/sol-gel = 2.80 × 10−8 A/cm2
GPTMS, and 0.54 mol of a Rp Mg = 207.3 Ω cm2
Composite sol-gel colloidal SiO2 nanoparticles Rp PEO =31432.5 Ω cm2
2021 AZ31B 3.5 µm 3.5 wt.% NaCl [125]
coating (PEO/sol-gel) suspension. Ethanol containing 0.1 Rp PEO/sol-gel =31,546.8 Ω cm2
mol of 1-Methylimidazole (MI) PEO/sol-gel includes an additional
were added. diffusive resistance (68716 Ωcm2 )
(non-faradaic resistance) between the
sol-gel coating and PEO oxide layer
A molar fraction of 40% TEOS and
60% MTES. Diluting in
isopropanol and 0.1 M of HCl
acidulated H2 O in a molar ratio of After being soaked for 24 h,
Composite sol-gel 1 mol of the mixture of precursors, Hanks’ solution icorr Mg = 1.5 × 10−6 A/cm2
2021 AZ31 36.7 µm [103]
coating (PEO/sol-gel) 5 mol of isopropanol, and 10 mol (pH = 7) icorr PEO = 1.6 × 10−7 A/cm2
of acidulated H2 O. In addition, sol icorr PEO/sol-gel = 2.50 × 10−8 A/cm2
was doped with 0.005 wt.% Grade
4 −COOH functionalized GNPs
(COOH−GNPs).
Mixing two different sols using
Composite sol-gel Rp PEO = 3.37 ×105 Ω cm2
2022 AM6 controllable hydrolysis of γ-GPS 19.3 µm 3.5 wt.% NaCl [126]
coating (PEO/sol-gel)
and TEOS. Rp PEO/sol-gel = 3.58 × 109 Ω cm2
Composite sol-gel
PMTMS: (MTMS: ethanol: DI Rct Mg = 78.3 Ω cm2
2020 AZ91D coating 19 µm 3.5 wt.% NaCl [127]
water = 3: 10: 20) Rct N-GQDs /sol-gel = 1.7 × 104 Ω cm2
(N-GQDs/sol-gel)
Molecules 2023, 28, 2563 23 of 30

Table 3. Cont.

Substrate Coatings Sol-Gel Solution Composition Thickness Electrolyte Anti-Corrosion Effect * Ref.
Composite sol-gel T50/A50: (TEOS: APTES: Water: After being soaked for 28 day, Rcoat
2020 AZ31 coating Ethanol =2.14:2.14:2:8(Volume - 3.5 wt.% NaCl PEO/epoxy ≈ 2 × 106 Ω cm2 , Rcoat [128]
(PEO/sol-gel/epoxy) ratio) PEO/sol-gel/epoxy ≈ 1 × 108 Ω cm2
After being soaked for 28 days in
T50/A50: (TEOS: APTES: Water:
3.5 wt.% NaCl, log |Z|0.01Hz
Ethanol =2.14:2.14:2:8(Volume
Triplex ≈ 7.5, log |Z|0.01Hz
Composite sol-gel ratio)
3.5 wt.% NaCl/0.5 wt.% Triplex-Ce-HQ ≈ 8.8;
2021 AZ31 coating 5 ppm of organic inhibitors 8-HQ, - [129]
NaCl Log |Z|0.01Hz Triplex = 4.88, log
(PEO/sol-gel/epoxy) I3C, 2- MBO, and DDTC were
|Z|0.01Hz Triplex-Ce-HQ = 6.06 with
added individually to silane
artificial defects, immersed in 0.5 wt.%
solutions.
NaCl solution for 48 h.
The molar ratio of mixed
Composite sol-gel APS/BTESPT was 12 , and the icorr Mg = 2.1852e−4 A/cm2
2019 AZ31B coating volume ratio of mixed 1100 nm 3.5 wt.% NaCl icorr sol-gel/GO/sol-gel = [130]
(sol-gel/GO/sol-gel) silane:deionized water:ethanol = 1.381e−8 A/cm2
1:1:8.
* According to EIS test, Rtot = Rf + Rct values are calculated as the sum of all the faradaic resistance by using the fitted data, |Z|0.01Hz is the low-frequency impedance, Rp is polarization
resistance, and Rct is related to the charge transfer resistance. icorr is the corrosion current density.
Molecules 2023, 28, 2563 24 of 30

As concluded in the present works, the sol-gel coating has a good protective effect on
Mg alloys. However, there are still some issues that need further study including:
1. Most of the synthetic methods reviewed in this paper were carried out under labora-
tory conditions.
2. The durability of the coated surface is considered to be the most important aspect that
should be further enhanced in future work. Although the corrosion inhibitor/nano-
filler silane hybrid coating has improved its protective effect, it is far from enough to
be used in the industry. The sol-gel composite coating with long-lasting corrosion
protection should be addressed in a future study.
3. There is a lack of work considering the mechanical properties of the coating, such as
ductility and hardness. These properties are worth paying attention to in the practical
application of coatings in industrial applications.
4. The versatility of the coating is also very important. Apart from the anti-corrosion
aspect, sol-gel coatings also need to provide oxidation resistance, abrasion resistance,
water resistance, biocompatibility, and many other useful properties. With the in-
depth study of sol-gel technology and related characterization techniques, sol-gel
coatings will have wider and more practical applications.

Author Contributions: Conceptualization, J.L., H.B. and Z.F.; methodology, J.L. and Z.F.; software,
J.L. and Z.F.; validation, J.L., H.B. and Z.F.; formal analysis, J.L., H.B. and Z.F.; investigation, J.L.,
H.B. and Z.F.; resources, J.L., H.B. and Z.F.; data curation, J.L., H.B. and Z.F.; writing—original draft
preparation, J.L. and Z.F.; writing—review and editing, J.L., H.B. and Z.F.; visualization, J.L., H.B. and
Z.F.; supervision, J.L., H.B. and Z.F.; project administration, J.L., H.B. and Z.F.; funding acquisition,
J.L., H.B. and Z.F. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Guangdong Basic and Applied Basic Research Foundation,
grant number 2022A1515110115.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Acknowledgments: The authors acknowledge the financial support from Guangdong Basic and
Applied Basic Research Foundation (2022A1515110115).
Conflicts of Interest: The authors declare no conflict of interest.

Glossary
|Z|0.01Hz Low frequency impedance
8-HQ 8-hydroxyquinoline
CNTs Carbon nanotubes
EIS Electrochemical impedance spectroscopy
FATP Fluorinated attapulgite particles
GNPs Graphene nanoplatelets
HA Hydroxyapatite
HA Hydroxyapatite
HNDs Hydroxylated nanodiamonds
icorr Corrosion current density
LEIM Local electrochemical impedance spectroscopy experiments in the mapping mode
MAO Microarc oxidation
Mg Magnesium
N-GQDs N-doped graphene quantum dots
OF Oxidized fullerene
OH-MWCNT Hydroxylated multi-walled carbon nanotube
Molecules 2023, 28, 2563 25 of 30

OIH coating Organic-inorganic hybrid coating

P-B ratio Pilling-Bedworth ratio
PEO Plasma electrolytic oxidation
PP Polypropylene
Rct The charge transfer resistance
Rp Polarization resistance
Rtot : Rf + Rct values are calculated as the sum of all the faradaic resistance by using the fitted data
SBF Simulated Body Fluid
SDS sodium dodecyl sulfate
SVET Scanning Vibrating Electrode Technique
XPS X-ray spectroscopy

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