materials

Article
Innovative Method for Coating of Natural Corrosion Inhibitor
Based on Artemisia vulgaris
Daniel Alejandro Pineda Hernández 1, *, Elisabeth Restrepo Parra 1 , Pedro José Arango Arango 1 ,
Belarmino Segura Giraldo 1 and Carlos Daniel Acosta Medina 2

1 Laboratorio de Física del Plasma, Unicversidad Nacional de Colombia sede Manizales,

Manizales-Caldas 170003, Colombia; erestrepopa@unal.edu.co (E.R.P.); pjarangoa@unal.edu.co (P.J.A.A.);
bsegurag@unal.edu.co (B.S.G.)
2 Calculo Científico y Modelamiento Matemático, Universidad Nacional de Colombia sede Manizales,
Manizales-Caldas 170003, Colombia; cdacostam@unal.edu.co
* Correspondence: dapinedah@unal.edu.co; Tel.: +57-323-439-8093

Abstract: In this work, the production of a novel methodology for the application of natural corrosion
inhibitors on steel, using an autoclave is presented. Tests were carried out using Artemisia vulgaris.
The inhibitor was produced with a simple soxhlet extraction process using 15 g of Artemisia vulgaris
and 260 mL of Ether. Once the inhibitor was produced, the steel was immersed in it, to form a coating
that protects the material against corrosion. Thermogravimetry analyzes (TGA) were performed on
the inhibitor, to determine the degradation temperature; it was observed that, at 321 ◦ C, the loss
of organic mass begins. After applying the inhibitor to the steel, the Fourier Transform Infrared
Spectroscopy (FTIR) technique was used to determine the vibrational bands and the difference
between the spectra for the steels before and after the coating was applied. For the evaluation of
the method efficiency, Electrochemical Impedance Spectroscopy (EIS) and polarization resistance



tests were performed, where Nyquist diagrams and Tafel curves were obtained, for steels with and


without treatment. In this case, an increase of 93% in the corrosion resistance, and an 88% decrease in
Citation: Pineda Hernández, D.A.; the corrosion rate were observed, proving that this methodology can be used to protect steel against
Restrepo Parra, E.; Arango Arango, corrosion and extend the steel’s useful life.
P.J.; Segura Giraldo, B.; Acosta
Medina, C.D. Innovative Method for
Keywords: EIS; organic coating; tafel; mild steel; corrosion
Coating of Natural Corrosion
Inhibitor Based on Artemisia vulgaris.
Materials 2021, 14, 2234. https://
doi.org/10.3390/ma14092234
1. Introduction
Received: 19 November 2020 Each minute, 300 tons of steel are dissolved around the world due to corrosion, gener-
Accepted: 24 December 2020 ating millions in losses for governments [1,2]. Due to this phenomenon, corrosion emerges
Published: 26 April 2021 as one of the biggest problems in the modern world, which makes it necessary to develop
more efficient protection and prevention solutions against corrosion. To counter this prob-
Publisher’s Note: MDPI stays neu- lem, researchers have proposed solutions such as coatings [3], paints [4], thin films [5],
tral with regard to jurisdictional clai- corrosion inhibitors [6–8], among others; however, currently, one of the most important re-
ms in published maps and institutio- quirements for anti-corrosion solutions is that they should be as little polluting as possible.
nal affiliations. In this field, corrosion inhibitors produced from natural sources are very promising.
Corrosion inhibitors are substances that, when added in small amounts to a corrosive
medium, can decrease the rate of deterioration of the material through passivation [7].
There are several commercial corrosion inhibitors, which are widely used by the industry.
Copyright: © 2021 by the authors. Li-
censee MDPI, Basel, Switzerland.
These inhibitors are a useful tool in the battle against corrosion, as they reduce costs and
This article is an open access article
improve the useful life of the material [8]. However, the composition of most of these
distributed under the terms and con- corrosion inhibitors is unknown; besides, these corrosion inhibitors are toxic and environ-
ditions of the Creative Commons At- mentally harmful substances. In addition, they could be dangerous for the personnel who
tribution (CC BY) license (https:// handle them. For this reason, in the last decade, governments have created regulations
creativecommons.org/licenses/by/ such as the Toxic Substances Control Act of the United States Environmental Protection
4.0/). Agency (EPA) and the European Union’s Restriction of Hazardous Substances Directive [9].

Materials 2021, 14, 2234. https://doi.org/10.3390/ma14092234 https://www.mdpi.com/journal/materials

Materials 2021, 14, 2234 2 of 13

These regulations demand that the products used in the industrial field must have the
minimum possible toxicity. In order to search for options to comply with these laws,
researchers worldwide are proposing the use of plant, fruit and/or flower extracts as
corrosion inhibitors [10].
The study of corrosion inhibitors from natural sources has been advanced through
systematic studies of plants and fruits on different types of metals, especially steel due to
its wide range of applications. Thanks to these studies, a new horizon has been discovered,
since these substances are very efficient under different corrosive conditions, with superior
ecological properties since they are biodegradable. For instance, N. Soltani and collabo-
rators [7] studied the inhibitory character of Salvia Officinalis in austenitic stainless steel
304; H. Herrera-Hernández et al. [8] investigated aloe vera gel on structural reinforcing
steel, finding that this type of inhibitors are highly efficient for the protection of steels in
different corrosive medium with protection efficiencies greater than 80% in comparison
with steels without inhibitor addition, and many reports show that this type of inhibitor
increases the useful life of materials by 80–90% [11–13].
Specifically referring to Artemisia vulgaris as a corrosion inhibitor, in 2012, Subhadra
Garai and collaborators, from the National Metallurgy Laboratory of Jamshedpur, India,
studied the inhibitory character of this plant. In this work, it was determined that the
methanoic extract of Artemisia vulgaris shows efficiencies of 93% in 1 mol L−1 HCl [6].
Artemisia vulgaris is a plant belonging to the Asteraceae family. Despite being considered
undergrown, this family of plants has been studied extensively due to their antibacte-
rial, antiseptic, and antioxidant properties. It grows in temperate climates and is native
to Europe [14].
Generally, applying this type of corrosion inhibitors to the corrosive medium shows a
considerable disadvantage at the application level against other types of solutions, such as
paints or coatings. Unlike previous works related to corrosion inhibitors from natural
sources, this work proposes an innovative, economical, and viable solution, which consists
of the inhibitor adsorption by the metal using a hydrothermal process. The process
generates optimal conditions for the creation of a natural extract layer that acts as a
corrosion inhibitor. For an initial test, the work carried out by N. Soltani and collaborators
was considered, since they used Artemisia vulgaris as a corrosion inhibitor for structural steel
in a 1 mol L−1 HCl solution, and based on this work, the autoclave method was applied.

2. Materials and Methods

2.1. Extraction and Inhibitor Application
Figure 1 shows a diagram of the methodology used to extract the corrosion inhibitor
from Artemisia vulgaris, and its application to the steel samples employing the autoclave.
To carry out the extraction, leaves were taken from the plant and a drying process was
carried out in a Humboldt MFG oven. The samples were in a Model 40 Go lab oven at
60 ◦ C for 24 h, afterwards, they were macerated until a fine powder was produced [15].
A sample of 20 g of this powder was taken and using a Soxhlet, the extract was prepared
in 260 mL of ether for 4 h. Afterward, the ether solution was distilled to concentrate the
extract until 20 mL of solution was acquired [16]. After that, a thermogravimetric analysis
of the extract was carried out, to determine the degradation temperature. The structural
steel samples to be coated must be partially polished and cleaned with acetone to remove
any dirt from the surface.
For optimal working conditions, a systematic study was conducted varying the tem-
perature from 60 ◦ C to 200 ◦ C with a step of 20 ◦ C in the hydrothermal process. It was
observed that at low temperatures, the coating was not formed and at high temperatures
the extract was calcined. The temperature at which the coating showed better characteris-
tics was 120 ◦ C; then, this temperature was selected for developing the process of coating
the steel.
After the extract was obtained and the steel samples were thoroughly cleaned, they were
put into the autoclave, and it was hermetically sealed and heated. This heating was carried
 the extract was calcined. The temperature at which the coating showed better characteris-
tics was 120 °C; then, this temperature was selected for developing the process of coating
Materials 2021, 14, 2234 the steel. 3 of 13
After the extract was obtained and the steel samples were thoroughly cleaned, they
were put into the autoclave, and it was hermetically sealed and heated. This heating was
carried out with a filling factor of 30%. In this filling factor, 20% of the volume corre-
out with a filling factor of 30%. In this filling factor, 20% of the volume corresponded to
sponded to the extract and approximately 10% to the metal immersed in it. The heating
the extract and approximately 10% to the metal immersed in it. The heating was carried
was carried out for 40 min. At the end of this time, the sample was extracted, dried under
out for 40 min. At the end of this time, the sample was extracted, dried under normal
normal environmental conditions, and the tests were subsequently carried out. To de-
environmental conditions, and the tests were subsequently carried out. To develop this
velop this experiment, a stainless-steel autoclave with a total volume of 100 mL was used;
experiment, a stainless-steel autoclave with a total volume of 100 mL was used; inside the
inside the autoclave,
autoclave, there container
there is a Teflon is a Teflon
to container toreaction
prevent the preventofthe reaction
external of externalduring
compounds com-
pounds during
the creation the coating.
of the creation of the coating.
Figure 1 showsFigure 1 shows
a diagram thata describes
diagram that describes the
the experimental
experimental process to make
process to make the coatings. the coatings.

Figure 1.
Figure Experimental setup.
1. Experimental setup.

2.2.
2.2. Materials
Materials Characterization
Characterization
Fourier
Fourier transform infrared
transform infrared spectroscopy (FTIR) was
spectroscopy (FTIR) was used
used to to determine
determine the the functional
functional
groups in the Artemisia vulgaris extract and the coatings on the
groups in the Artemisia vulgaris extract and the coatings on the structural steel. structural steel. For For this
this
analysis,
analysis, a BRUKER alpha platinum equipment with an ATR platinum Diamond 11 acces-
a BRUKER alpha platinum equipment with an ATR platinum Diamond acces-
sory
sory was
was used.
used. TheThe characterization
characterization waswas carried
carried outout with
with aa resolution
resolutionof of44cmcm−−11,, 32
32 steps
steps
and a measurement range from 400 to 4000 cm −−1
1 ; at the same
and a measurement range from 400 to 4000 cm ; at the same time, a thermogravimetric time, a thermogravimetric
analysis
analysis using
using aa TGA
TGA Q500
Q500 V6.7
V6.7 Build
Build 203
203 was
was performed
performed to to determine
determine the the degradation
degradation
temperature of the extract. A heating rate of 293.15 Kmin− 1 −1 was applied until reaching 800
temperature of the extract. A heating rate of 293.15 Kmin was applied until reaching 800 ◦ C.
°C. A nitrogen
A nitrogen atmosphere
atmosphere was was used
used with
with a flow
a flow ofof6060 mLmL minmin −1−1.. To
To evaluate
evaluate the the efficiency
efficiency
of
of the
the coating,
coating, electrochemical
electrochemical impedance,
impedance, and and polarization
polarization resistance
resistance spectroscopy
spectroscopy was was
performed
performed to todetermine
determinecorrosion
corrosionresistance
resistance andand corrosion
corrosion raterateusing
usinga Gamry
a Gamry 1000E po-
1000E
tentiostat/galvanostat.
potentiostat/galvanostat. EISEIS
tests were
tests performed
were performed in ainrange
a range of of
10610to6 10 −3 employing
to −310Hz Hz employing 0.1
mol L −
−1 HCl 1 as a corrosive medium, while TAFEL tests were performed
0.1 mol L HCl as a corrosive medium, while TAFEL tests were performed according to according to ASTM
G59
ASTM recommendations
G59 recommendations[17]. [17].

3. Results
3. Results
3.1. Compositional
3.1. Compositional Characterization
Characterization
3.1.1. Fourier Transform Infrared
3.1.1. Fourier Transform Infrared
Figure 2 shows the spectrum for the concentrated extract. A large number of CO,
Figure 2 shows the spectrum for the concentrated extract. A large number of CO, CH
CH and CC bonds, characteristic of organic substances, was observed. The results are listed
and CC bonds, characteristic of organic substances, was observed. The results are listed
in Table 1 [18,19].
in Table 1 [18,19].
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Figure 2. FTIR transmittance spectrum for Artemisia vulgaris extract.

Figure FTIR
Figure2.2.FTIR
Figure 2. FTIR transmittance transmittance
spectrum spectrum
for Artemisia
transmittance vulgaris
spectrum for Artemisia vulgaris extract.
forArtemisia
extract.
Figure 2. FTIR transmittance spectrum for Artemisia vulgaris extract. vulgaris extract.
Table 1. FTIR spectrum identification for Artemisia vulgaris extract.
Table 1. FTIR spectrum identification
Table1.
Table FTIRspectrum
1.FTIR for Artemisia
spectrum vulgaris
identification
identification extract.
Artemisia vulgaris extract.
forArtemisia
for
Table 1. FTIR spectrum identification for Artemisia vulgaris extract. vulgaris extract.
Wavenumber
Wavenumber
Wavenumber (cm−1 )−1
Wavenumber Compound
Wavenumber
Compound Notes Notes
(cm Compound Notes
(cm−1−1))
3399.29 (cm
Compound
(cm−1OH
)
Compound Notes Notes
Associated with a polymeric primary alcohol (carrier substance)
) Associated with a polymeric primary alcohol
3399.29 OH Associated withAssociated
a polymeric primary
with alcohol
a polymeric
3000 3399.29 OH
3399.29 Associated
OH with a polymeric
(carrier substance)primary Vs alcohol primary alcohol
3399.29 OH (carrier substance)(carrier substance)
2917.06 3000 (carrier Vs
substance) Vas
3000 3000 Vs Vs
3000
2849.23 2917.06
Vs
Vas Vs
2917.06 -CH - Vas
2917.06 2917.06 2 Vas Vas
1460.79 2849.23
2849.23 -CH22--
-CH Vs
Vs Scissor
2849.23 2849.23
-CH2- -CH2- Vs Vs
719.70 1460.79
1460.79 1460.79
Scissor
Scissor Rocking
Scissor
1460.79
719.70 Scissor
Rocking
719.70
1376.08 719.70 O-CO-CH3
719.70 Rocking Rocking
1376.08 O-CO-CH 3
Rocking
1376.08
1549.51 1376.08 O-CO-CH
1376.08 3 O-CO-CH3
1549.51 O-CO-CH3 Confirmed presence of
1549.51 1549.51
Confirmed
Confirmed presence ofaromatic
presence
aromatic
Confirmed
rings
of aromatic
presencerings with
rings
withwith benzene nucleus.
1549.51
1608.67 1608.67 Confirmed presence
benzene of aromatic
nucleus. ringsofwith
aromatic rings with
1608.67 benzene nucleus.
1608.67
1165.22 1165.22 1608.67 benzene nucleus.benzene nucleus.
1165.22 They
1165.22
1037.77 1037.77
1165.22 Theygive
give information
information
They giveThey
about
about
information
give
the
thebranches
about branches that
thatthatbranches
the branches
information about the ring presents.
1037.77 1037.77 They give information
the ring about the branches
presents. thatthe that
1037.77
967.94 967.94 the ring presents. the ring presents.
967.94 967.94 the ring presents.
967.94
1735.49
1735.49 1735.49
Esters
Esters Esters
1735.49 1735.49 Esters Esters

1708.87
1708.87 Carboxylic
Carboxylicacids
acidsCarboxylic acids
1708.87 1708.87 1708.87 Carboxylic acids
Carboxylic acids

Figure
Figure 33 shows
shows thethe comparison
comparison between
between the
the spectra
spectra for
for thethe coating,
coating, and
andthethe
the extract.
extract. and the extract.
Figure 3 toshows Figure
the 3 shows the
comparison comparison
between the between
spectra for thecoating,
the spectra andfor thecoating,
extract.
ItIt isis possible identify
possible to Itidentify the
Figure
the functional
3 shows
functional thegroups
comparison
groups in
in each of
between
each of the
the cases.
the The
spectra
cases. The conservation
for the coating,
conservation of
and
of the extract. It is
It is possible is possible
to identify the to identifygroups
functional the functional
in each groups
of the in each
cases. The ofconservation
the cases. The of conservation of
most of the compounds
possible in the
to steel
identify coated
the according
functional to
groupsthe in extract
each ofcan
the be observed.
cases. The This
conservation of most
most of the compounds
most of thethe most of the
compounds
in the
in the
steel coated
compounds
steel coated
according
in according
the to theaccording
steel coated
to the
extract can
extract can
be
tobe observed.
theobserved.
extract can This
be observed.
This Thisof
is because
is because the steel steel
the adsorbed
compounds
adsorbed several
in the
several substances
steel coated
substances from the
according extract.
to
from the extract. the In both
extract
In both cancases,
be some
observed.
cases, some This is because
is because is because
the steel adsorbed the several
steel adsorbed
substances several
fromsubstances
the −1extract. from the extract.
In both cases, In both cases, some
some
bands
bands correspond to
the hydroxy-phenolic
steel adsorbed several groups
substances(3336.4fromcm the−1), aromatic
extract. In groups
both (1653.94
cases, some bands
correspond
bands correspond bandsto hydroxy-phenolic
to hydroxy-phenolic
groups
correspond to hydroxy-phenolic (3336.4
groups (3336.4 −
cm
groups
cm
), aromatic
(3336.4
−1), aromatic groups
cm −1
groups), (1653.94
aromatic
(1653.94 groupscorrespond
(1653.94
to hydroxy-phenolic groups (3336.4 cm 1 ), aromatic groups (1653.94 cm−1 ), groups that,
according to the literature, are the ones that improve corrosion inhibition [6,20–22]. On the
Materials 2021, 14, x FOR PEER REVIEW 5 of 14

Materials 2021, 14, 2234 5 of 13

cm−1), groups that, according to the literature, are the ones that improve corrosion inhibi-
tion [6,20–22]. On the other hand, changes in the intensities of some peaks of the extract
were
otherobserved once applied
hand, changes in the to the steel. of
intensities This effect
some is because
peaks of the the steelwere
extract adsorption of spe-
observed once
cies is not total and therefore, the concentration of each substance is lower. The
applied to the steel. This effect is because the steel adsorption of species is not total and preserva-
tion of the compounds
therefore, that inhibit
the concentration of eachcorrosion
substanceon is the surface
lower. of the steel coating
The preservation is because
of the compounds
a that
thermogravimetric
inhibit corrosionanalysis
on thewas previously
surface carried
of the steel out toisfind
coating the temperature
because at which
a thermogravimetric
the extractwas
analysis begins to degrade.
previously Once
carried outthe
to degradation temperature
find the temperature was known,
at which thebegins
the extract coatingto
process was carried out at a temperature lower than the one found
degrade. Once the degradation temperature was known, the coating process was carriedin the thermogravi-
metric
out atanalysis.
a temperature lower than the one found in the thermogravimetric analysis.

FTIR
Figure3.3.FTIR
Figure transmittance
transmittance spectrum
spectrum for (a) (a) Artemisia
for Artemisia vulgaris
vulgaris extract
extract (b) coating
(b) coating of Artemisia
of Artemisia
vulgaris.
vulgaris.

3.1.2. Thermogravimetric Analysis

3.1.2. Thermogravimetric Analysis
In Figure 4, the thermal decomposition curve obtained by the thermogravimetric
In Figure 4, the thermal decomposition curve obtained by the thermogravimetric
analysis of the Artemisia vulgaris extract is presented. The decomposition of the extract
analysis of the Artemisia vulgaris extract is presented. The decomposition of the extract as
as a function of temperature can be observed, showing critical points of mass loss. It can
a function of temperature can be observed, showing critical points of mass loss. It can be
be deduced that the first significant loss of mass, corresponding to 20.25 wt% of the
deduced that the first◦ significant loss of mass, corresponding to 20.25 wt% of the total,
total, begins at 52.2 C with the surface water and ends between 100–105 ◦ C with bound
begins at 52.2 °C with the surface water and ends between 100–105 ◦°C with bound water
water [23]. Subsequently, at higher temperatures, between 125–350 C, 29.22% of the mass
[23]. Subsequently, at higher temperatures, between 125–350 °C, 29.22% of the mass is lost,
is lost, corresponding to the C=O bonds and the decomposition of the OH groups [11,18].
corresponding to the C=O bonds and the decomposition of the OH groups [11,18].
Table 2 shows in detail the loss of mass with the increase of temperature, however,
since in this investigation it is required for the extract to remain as intact as possible,
the working temperature must be lower than the first critical point of mass loss, that is,
less than 325 ◦ C. For this reason, it was decided to coat the steel with the extract inside the
autoclave, at a temperature of 120 ◦ C, at which the compound has not been degraded.
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ThermalDecomposition
Figure4.4.Thermal
Figure DecompositionCurve
Curveof
ofthe
thecorrosion
corrosioninhibitor
inhibitor produced
produced from
from the Artemisia vul-
the Artemisia
garis.
vulgaris.

2. Mass
TableTable loss percentage.
2 shows in detail the loss of mass with the increase of temperature, however,
since in this investigation it is required for the extract to remain asTemperature
% Weight
intact as possible,
(◦ C)
the
working temperature must be lower than the first critical point of mass loss, that is, less
A 11.38 52.24
than 325 °C. For this reason, it was decided to coat the steel with the extract inside the
B 20.25 288.28
autoclave, at a C
temperature of 120 °C, at which29.22the compound has not been degraded.
321.64
D 13.07 618.45
Table 2. Mass loss percentage.

3.2. Electrochemical Measurements % Weight Temperature (°C)

A
Figure 5 shows the Nyquist diagram from 11.38the electrochemical impedance
52.24 spectroscopy
technique. The B black and red curves correspond20.25 to uncoated and coated steel samples,
288.28
respectively. From
C this figure, it is observed that the coated steel exhibits
29.22 a much greater
321.64
radius of curvature
D than the uncoated steel, which indicates a noticeable
13.07 increase in the
618.45
corrosion resistance. The efficiency of the coating was calculated using Equation (1),
where
3.2. % ef is the coating
Electrochemical efficiency, SS and SR the corrosion resistance of the steel and the
Measurements
coated steel, respectively, [24–27], finding an increase of 92.91% in the corrosion resistance,
Figure 5 shows the Nyquist diagram from the electrochemical impedance spectros-
as shown in Table 3.
copy technique. The black and red curves SScorrespond
− SR to uncoated and coated steel sam-
%e f =
ples, respectively. From this figure, it is observed ∗ 100%
that the coated steel exhibits a much (1)
SR
greater radius
The of curvature
behavior of treatedthan
andthe uncoated
untreated steel,
steel can which indicates
be modeled a noticeable
from electronicincrease
compo-
in the corrosion resistance. The efficiency of the coating was calculated
nents to understand better the electrical behavior of the corrosion process. The using Equation (1),
equivalent
where % ef is the coating efficiency, SS and SR the corrosion resistance
circuits for untreated and treated steel, respectively, are shown in Figure 6a,b. of the steel and the
coatedFigure
steel,6 respectively,
shows the different components that model the electrochemical behaviorre-
[24–27], finding an increase of 92.91% in the corrosion of
sistance, as shown
the structural steelinsurface
Table 3.with and without treatment. Ru is related to the resistance of
the aqueous solution. Y0 is a constant phase element associated with the electronic double
layer capacitance, created on the interface surface of the working and solution electrode;
however, since it is not an ideal capacitance due to roughness and possible pores on the
surface of the steel, with an ideality factor α, the gerischer element G, that is found only
in the equivalent circuit of the treated steel, is an indication of the porosity of the organic
coating with a porosity factor K, the gerischer element is necessary to explain the reaction
 Materials 2021, 14, 2234 7 of 13

Materials 2021, 14, x FOR PEER REVIEW 7 of 14

of the organic coating, because the reactions between the two surfaces, the surface of the
steel and the organic coating, cannot be distinguished from each other [28]. Finally, Rp is
the polarization resistance or corrosion resistance of the material [27,29,30]. This variable is
the one of interest for the study. Tables 3 and 4 show the values of these elements.

Figure 5. Nyquist plot of mild steel in 0.1 mol L−1 HCl with and without the coating of Artemisia
vulgaris extract.

𝑆𝑆 𝑆𝑅
%𝑒𝑓 ∗ 100% (1)
𝑆𝑅
The behavior of treated and untreated steel can be modeled from electronic compo-
nents
Figureto5.understand
Nyquist plot better
of mildthe electrical
steel L−1 HCl of
in 0.1 molbehavior theand
with corrosion process.
without the coatingThe equivalent
of Artemisia vul-
Figure 5. Nyquist
circuits
garis forplot
extract. of mild steel
untreated andin 0.1 molsteel,
treated L HCl
−1 with and without
respectively, the coating
are shown of Artemisia
in Figure 6a,b.
vulgaris extract.

𝑆𝑆 𝑆𝑅
%𝑒𝑓 ∗ 100% (1)
𝑆𝑅
The behavior of treated and untreated steel can be modeled from electronic compo-
nents to understand better the electrical behavior of the corrosion process. The equivalent
circuits for untreated and treated steel, respectively, are shown in Figure 6a,b.

(a)

(a)
(b)
Figure
Figure 6.
6. Equivalent
Equivalentcircuits
circuitsfor
for (a)
(a) uncoated
uncoated steel
steel (b)
(b) coated
coated steel.
steel.

By using the potentiodynamic curves, different data can be obtained regarding the
phenomena occurring in the corrosion and inhibition processes once the extract is applied to
the Steel. In addition to obtaining information on the adsorption of the inhibitor employing
an autoclave. Figure 7 shows the Tafel polarization curves corresponding to uncoated
and coated steel samples. This figure shows an inhibitory behavior of an anodic nature
since it moves towards (b) the anodic part of the curve. This anodic inhibition behavior is
Figure 6. Equivalent circuits for (a) uncoated steel (b) coated steel.
Materials 2021, 14, 2234 8 of 13

related to the formation of films on the steel surface due to external printed currents [31,32].
Furthermore, it is observed that the corrosion rate is reduced by 32.6%, with respect to the
corrosion rate of the untreated steel. This decrease is because the inhibitor generates an
oxidoreduction reaction process, delaying the release of ions from the Steel, which is proof
of the efficiency of the coating. Table 5 show the corrosion resistances (Rcorr) and corrosion
rate of uncoated and coated steel with the efficiency shown for each parameter.

Table 3. Value of equivalent circuit components for uncoated steel.

Element Value ±Error Units

Ru 6.607 40.05 × 10−5 Ω
Y0 454.4 × 10−6 19.95 × 10−6 S × sα
α 841.6 × 10−3 7.136 × 10−3 -
Rp 35.53 289.7 × 10−3 Ω

Table 4. Value of equivalent circuit components for coated steel.

Element Value ±Error Units

Ru 6.101 97.19 × 10−3 Ω
Y0 59.29 × 10−6 5.415 × 10−6 S × sα
α 890 × 10−3 12.38 × 10−3 -
1
G 1232 × 10−3 55.50 × 10−6 S × s( 2 )
K 181.5 77.64 s−1
Rp 501.2 13.24 Ω

Figure 7. Potentiodynamic curves of mild steel in 0.1 mol L−1 HCl with and without coating of
Artemisia vulgaris extract.
Materials 2021, 14, 2234 9 of 13

Table 5. Results from the electrochemical tests.

Sample\Charact. Uncoated Steel Coated Steel %Efficiency

Rcorr (Ω) 35.53 ± 289.7 × 10−3 501.2 ± 13.24 92.91%
Corrosion Rate (mpy) 52.3 6.34 88.31%

3.3. Visual Inspection

Figure 8 shows the micrographs of the structural steel samples with and without coat-
ing before the corrosion measurements. Figure 8a,b show the polished structural steel with
the characteristic shine of the steel. In Figure 8c,d, the steel coated with Artemisia vulgaris is
presented. When these two samples are compared, a difference in their brightness is ob-
Materials 2021, 14, x FOR PEER REVIEW
served, the sample in Figure 8c,d shows an opacity in addition to a jade green pigmentation 10 o
developed due to the coating.

(a) (b)

(c) (d)
Figure 8. Micrographs
Figure 8.before corrosion
Micrographs of structural
before corrosion ofsteel (a) polished
structural steel (a)with magnification
polished 1×, (b) polished
with magnification with magnifi-
1×, (b) polished
cation 5×, (c) coated with magnification 1×, and (d) coated with magnification 5×.
with magnification 5×, (c) coated with magnification 1×, and (d) coated with magnification 5×.

Figure 9 showsFigure
the9 micrographs
shows the micrographs
correspondingcorresponding to the
to the samples samples
after after the corros
the corrosion
measurements. Figure 9a shows the corrosion of the mild steel
measurements. Figure 9a shows the corrosion of the mild steel sample, where localized sample, where localiz
sources ofare
sources of corrosion corrosion
present.are present.
When theseWhen thesepoints
corrosion corrosion points
are seen are closely
more seen morein closely
Figure 9b, characteristic
Figure 9b, characteristic pitting
pitting corrosion is corrosion
evident inisthis
evident in steel.
type of this type
On of
thesteel.
otherOn the other ha
hand,
in itFigure
in Figure 9c,d 9c,d itthat
is observed is observed
the sample that the sample
coated coatedvulgaris
with Artemisia with Artemisia
changedvulgaris
its colorchanged
color from jade green to an opaque brown, as a consequence of the reaction between
phenols and flavonoids of the coating with the medium corrosive [33].
Materials 2021, 14, 2234 10 of 13

Materials 2021, 14, x FOR PEER

from REVIEW
jade green
to an opaque brown, as a consequence of the reaction between the phenols 11 o

and flavonoids of the coating with the medium corrosive [33].

(a) (b)

(c) (d)
Figure 9. Micrographs
Figure 9.after corrosionafter
Micrographs measurements of structural steel
corrosion measurements (a) polished
of structural steelwith magnification
(a) polished 1×, (b) polished
with magnifica-
with magnification 5×, (c) coated with magnification 1×, and (d) coated with magnification 5×.
tion 1×, (b) polished with magnification 5×, (c) coated with magnification 1×, and (d) coated with
magnification 5×.
Figure 10 shows the coating inhibition method. The protection provided by Artem
Figure vulgaris
10 showscoating is possible
the coating thanks
inhibition to the
method. ability
The of phenols
protection to trap
provided oxygen vul-
by Artemisia and hydrog
garis coatinginistheir
possible
freethanks to [33,34],
radicals the ability of phenols
phenols beingtopart
trapofoxygen and hydrogen
the oxidation inwhile
process their the stee
free radicalsprotected.
[33,34], phenols being part of the oxidation process while the steel is protected.
 Materials 2021, 14, 2234 11 of 13
Materials 2021, 14, x FOR PEER REVIEW 12 of 14

Figure 10. Schematic description of the phenomenology occurring for the prevention of deterioration
Figure 10. Schematic description of the phenomenology occurring for the prevention of deteriora-
by coating with natural corrosion inhibitors; (a) corrosion phenomenology with coating; (b) corrosion
tion by coating with natural corrosion inhibitors; (a) corrosion phenomenology with coating; (b)
phenomenology without without
corrosion phenomenology coating. coating.

4. Conclusions
4. Conclusions
A natural corrosion inhibitor coating was obtained from the Artemisia vulgaris plant
usingA natural corrosion
the autoclave inhibitor
method. TGAcoating
analyseswas obtained
showed from
that the the Artemisia
material begins vulgaris plant
its degradation
using the◦ autoclave method. TGA analyses showed that the material
at 325 C. Then, lower temperatures must be used. Using FTIR, the vibrational bands begins its degrada-
tion at 325
in the °C. Then,
coating and lower temperatures
the Artemisia must
vulgaris be used.
extract wereUsing FTIR, theobserving
determined, vibrationalhydroxy-
bands
inphenolic
the coating andcm
(3336.4 −
the Artemisia
1 vulgaris
) and aromatic extractcm
(1653.94 −
were)determined,
1 groups that, observing
according to hydroxy-phe-
the literature,
nolic
help(3336.4 cm−1) and
the inhibitory aromatic
character of (1653.94 cm−1) groups
these substances. that, according
Corrosion resistancetowas the determined
literature,
help
usingtheNyquist
inhibitory character
diagrams, whereof these
a 59.95%substances.
of increase Corrosion resistance
in corrosion was was
resistance determined
obtained.
using Nyquist diagrams, where a 59.95% of increase in corrosion resistance
It is important to highlight that there is good adsorption of the inhibitor by the steel; was obtained.
It this
is important to highlight
can be evidenced in that
the there
FTIR is good adsorption
analyzes and in the of corrosion
the inhibitor by the
tests, steel; thisin
specifically
canEIS,be where
evidenced the in the FTIR
reaction of analyzes
the coating andandin the
thecorrosion
substrate tests,
withspecifically
the medium in EIS, wherebe
cannot
the reaction of theIncoating
differentiated. addition,anditthewas substrate
determinedwith that
the medium cannotcurrent
the corrosion be differentiated.
decreased and In
addition,
the voltage it was determined
increased, whichthatis anthe corrosion
indication of current
an increasedecreased and the
in the useful lifevoltage in-
of the steel,
creased,
showing which is an indication
an improvement in theofcorrosion
an increase rateinofthe useful life of the steel, showing an
88.31%.
This method
improvement in the was innovative
corrosion rate of and effective in protecting structural steels since it
88.31%.
provides a homogeneous
This method coating
was innovative andof effective
the surface exposed tostructural
in protecting the treatment
steelsandsinceshould
it pro-be
studied
vides to obtain a better
a homogeneous coatingunderstanding
of the surfaceofexposed
the adsorption mechanisms.
to the treatment and should be stud-
ied to obtain a better understanding of the adsorption mechanisms.
Author Contributions: Conceptualization, D.A.P.H. and P.J.A.A., methodology, D.A.P.H.; validation,
D.A.P.H.;
Author formal analysis,
Contributions: D.A.P.H. and E.R.P.;
Conceptualization, investigation,
D.A.P.H. D.A.P.H.
and P.J.A.A., and E.R.P.;
methodology, resources,
D.A.P.H.; B.S.G.
valida-
andD.A.P.H.;
tion, C.D.A.M.; data curation,
formal D.A.P.H.;and
analysis, D.A.P.H. writing—original draft preparation,
E.R.P.; investigation, D.A.P.H. andD.A.P.H. and E.R.P.;
E.R.P.; resources,
visualization,
B.S.G. D.A.P.H.
and C.D.A.M.; dataand E.R.P.; D.A.P.H.;
curation, supervision, E.R.P. All authors
writing—original drafthave read andD.A.P.H.
preparation, agreed to the
and
published version of the manuscript.
E.R.P.; visualization, D.A.P.H. and E.R.P.; supervision, E.R.P. All authors have read and agreed to
the published version of the manuscript.
Funding: This research received no external funding.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data available on request due to restrictions eg privacy or ethical The
dataAvailability
Data presented inStatement:
this study are available
Data availableonon
request from
request duethe
tocorresponding
restrictions eg author.
privacyThe data are
or ethical not
The
publicly
data available
presented duestudy
in this to these
areresults are on
available associated
request with
fromantheactive project at the
corresponding National
author. TheUniversity
data are
of publicly
not Colombia.available due to these results are associated with an active project at the National Uni-
versity of Colombia.
Acknowledgments: The authors gratefully acknowledge financial support from the Universidad
Nacional de Colombia. The authors express their gratitude to the research program entitled “Re-
Materials 2021, 14, 2234 12 of 13

construcción del tejido social en zonas posconflicto en Colombia” SIGP code: 57579 with the project
entitled “Competencias empresariales y de innovación para el desarrollo económico y la inclusión
productiva de las regiones afectadas por el conflicto colombiano” SIGP code 58907. Contract number:
FP44842-213-2018.
Conflicts of Interest: The authors declare no conflict of interest.

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