Corrosion inhibition studies of the combined admixture
of 1,3-diphenyl-2-thiourea and 4-hydroxy-3-methoxybenzaldehyde on mild steel in
dilute acid media
Estudios de inhibición de la corrosión de la mezcla
combinada de 1,3-difenil-2-tiourea y 4-hidroxi-3-metoxibenzaldehido en acero dulce
en medio ácido diluido
Estudos de inibição da
corrosão da mistura combinada de 1,3-difenil-2-tioureia e
4-hidroxi-3-metoxibenzaldeído em aço macio em meio ácido diluído
Roland
T. Loto1, 2*
https://doi.org/10.15446/rev.colomb.quim.v46n1.59578
1Department of Mechanical Engineering,
Covenant University, Ota, Ogun State, Nigeria
2Department of Chemical,
Metallurgical & Materials Engineering, Tshwane University of
Technology,
Pretoria, South Africa
*tolu.loto@gmail.com
Abstract
Mild steel corrosion is responsible
for economic and industrial losses due to accelerated deterioration.
Synthesized derivatives of thiourea and phenolic aldehydes containing
heteroatoms are capable of inhibiting corrosion by adsorption onto the steel
through formation of strong bonds. Thus, the electrochemical corrosion
inhibition properties of the combined admixture of 1,3-diphenyl-2-thiourea and
4-hydroxy-3-methoxybenzaldehyde on mild steel in 1 M H2SO4
and HCl acid media were studied through weight loss analysis, potentiodynamic
polarization method, optical microscopy and IR spectroscopy. Results showed that
the organic mixture effectively inhibited the corrosion of mild steel in both
solutions with an optimal inhibition efficiency of 97.4% and 97.47% in H2SO4
from weight loss and potentiodynamic polarization test, while the corresponding
values in HCl were 94.71% and 89.73% respectively. Thermodynamic calculations
showed that the compound chemisorbed onto the steel surface blocking the
diffusion of corrosive anions. Observations from micro-analytical images
confirmed the effective inhibition property of the compound and its presence on
the surface topography of the steel. Infrared spectra revealed the presence of
the functional groups of the organic compound responsible for corrosion
inhibition. The adsorption of the compound was deduced to obey the Langmuir,
Frumkin and Freundlich adsorption isotherm.
Keywords: adsorption, corrosion, mild steel, inhibitor, hydrochloric acid, sulphuric acid
Resumen
La corrosión de
acero dulce es responsable de pérdidas económicas e industriales debido a su deterioro
acelerado. Los derivados sintetizados de tiourea y aldehídos fenólicos con
heteroátomos inhiben la corrosión por adsorción sobre el acero mediante la
formación de enlaces fuertes. Por tanto, se estudiaron las propiedades de
inhibición de la corrosión electroquímica de la mezcla combinada de
1,3-difenil-2-tiourea y 4-hidroxi-3-metoxibenzaldehído sobre acero dulce en
medios de H2SO4 y HCl 1 M mediante análisis de pérdida de
peso, método de polarización potenciodinámica, microscopía óptica y espectroscopia
IR. Los resultados mostraron que la mezcla inhibe eficazmente la corrosión del
acero dulce en ambas soluciones con una eficacia de inhibición óptima de 97,4%
y 97,47% en H2SO4, mientras que los valores
correspondientes al HCl son 94,71% y 89,73%. Los cálculos termodinámicos demostraron
que el compuesto quimiosorbido sobre la superficie de acero bloquea la difusión
de aniones corrosivos. Las imágenes micro-analíticas confirmaron la efectiva propiedad
de inhibición del compuesto y su presencia en la topografía superficial del
acero. Los espectros infrarrojos revelaron la presencia de los grupos
funcionales del compuesto orgánico responsable de la inhibición de la
corrosión. La adsorción del compuesto se dedujo siguiendo las isotermas de
adsorción de Langmuir, Frumkin y Freundlich.
Palabras clave: adsorción, corrosión, acero dulce, inhibidor, ácido
clorhídrico, ácido sulfúrico
Resumo
A corrosão de
aço macio é responsável por perdas econômicas e industriais devido à
deterioração acelerada. Os derivados sintetizados de tioureia e aldeídos
fenólicos com heteroátomos inibem a corrosão por adsorção sobre o aço através
da formação de ligações fortes. Por tanto, estudaram-se as propriedades de
inibição da corrosão eletroquímica da mistura combinada de
1,3-difenil-2-tioureia e 4-hidroxi-3-metoxibenzaldeído em aço macio em meios de
H2SO4 e HCl 1 M através de análise de perda de peso,
método de polarização potenciodinâmica, microscopia óptica e espectroscopia de
IV. Os resultados mostram que a mistura inibiu eficazmente a corrosão de aço
macio em ambas as soluções com uma eficiência de inibição óptima de 97,4% e
97,47% em H2SO4, enquanto os valores correspondentes ao
HCl são respectivamente 94,71% e 89,73% . Os cálculos termodinâmicos mostram
que o composto quimisorvido sobre a superfície de aço bloquea a difusão de
aniões corrosivos. As imagens micro-analíticas confirmam a propriedade de
inibição do composto e sua presença na topografia superficial do aço. Os espectros
de infravermelho revelaram a presença dos grupos funcionais do composto
orgânico responsáveis pela inibição da corrosão. A adsorção do composto foi
deduzida seguindo às isotermas de adsorção de Langmuir, Frumkin e Freundlich.
Palavras-chave: adsorção, corrosão, aço macio,
inibidor, ácido clorídrico, ácido sulfúrico
Introduction
Mild steel is one
of the most applicable construction materials, extensively used in chemical,
petroleum, automotive, energy generating and allied industries for applications
that are exposed to acids, alkalis and salty environments such as acid
cleaning, pickling, descaling, industrial acid cleaning, cleaning of oil
refinery equipment, heat exchangers and oil well acidizing (1). It is the cheapest, most common and
most versatile form of steel serving for every application that requires huge
amount of steel as it provides material properties that are acceptable for many
applications. However it is weakly resistant to pitting and general corrosion,
thus, it is continually replaced after being severely degraded in the corrosive
environment during application.
Corrosion represents a significant
cost burden and major industrial setback to the economy of every country; it is
the largest single cause of plant and equipment breakdown in process
industries. For a variety of industrial applications, it is possible the selection
of construction materials which are completely resistant to corrosion from
corrosive fluids, but the cost of such an approach is most often restrictive (2). Current reviews show that the most
realistic cost of corrosion could be as high as 3% of the gross domestic
product (
The mechanism of inhibitor adsorption
and the relationship between inhibitor molecular structures and their
adsorption properties is of great importance in corrosion inhibition studies (8). Chemical compounds with functional groups
containing heteroatoms within their molecular structure are capable of donating
lone pair of electrons, important attribute of organic compounds in metal
corrosion inhibition.
The
use of organic compounds for corrosion inhibition of ferrous alloys in
different acidic medium has been studied by different authors. The corrosion
inhibiting property of these compounds is attributed to their molecular
structure (9-11). Bouklah et al.
(12) and Bentiss et al. (13) showed that
the adsorption of organic inhibitors mainly depends on physicochemical and
electronic characteristics of the inhibitor molecule, associated with their
functional groups, steric effects, electron density of donor atoms, and the π-orbital
character of donating electrons. A
good inhibitor decreases the anodic and/or cathodic reaction of the
corrosion process, the transport rate of the corrosive anions to the surface of
the metal, and the potential difference at various sites on the metal surface.
Inhibitors are basically easy to apply and offer the advantage of in-situ application.
To further contribute to the study of the use of low cost chemical
compounds for corrosion inhibition of ferrous alloys and deeper understanding
of their inhibition mechanism, this research aimed to investigate the
inhibiting influence of the synergistic effect of
4-hydroxy-3-methoxybenzaldehyde and 1,3-diphenyl-2-thiourea on mild steel
corrosion in 1 M H2SO4 and HCl acid solution through
weight loss analysis, potentiodynamic polarization test and optical microscopy.
Materials and Methods
Material
Mild steel was purchased from the Steel Works,
Owode, Nigeria and analyzed at the Materials Characterization Laboratory,
Department of Mechanical Engineering, Covenant. This mild steel gave an average
nominal composition of nominal per cent (w/w
%) composition, shown in Table 1. The steel had a cylindrical dimension of 16
mm diameter.
Table 1. Nominal
composition percentage of mild steel
|
Element Symbol |
C |
Si |
Mn |
P |
S |
Cu |
Ni |
Al |
Fe |
|
% Composition (w/w) |
0.401 |
0.169 |
0.440 |
0.005 |
0.012 |
0.080 |
0.008 |
0.025 |
98.86 |
Inhibitor
Combined mixture of
1,3-diphenyl-2-thiourea and
4-hydroxy-3-methoxybenzaldehyde (VTU), a solid white powdery substance obtained in synthesized form from
SMM Instrument, South Africa was the
inhibiting compound mixture used. Their structural formulas are shown in Figure
1, and the properties in Table 2.
(a)
(b)
Figure 1. Chemical
structure of (a) 1,3-diphenyl-2-thiourea (b) 4-hydroxy-3-ethoxybenzaldehyde.
Table 2. Chemical
properties of the inhibiting compounds.
|
Compound |
Molecular Formula |
Molar Mass (g/mol) |
|
1,3-diphenyl-2-thiourea |
C13 H12 N2
S |
228.31 |
|
4-hydroxy-3-methoxybenzaldehyde |
C8H8O3 |
152.15 |
VTU
was prepared in different molar concentrations of 3.29 x 106 M, 6.57
x 106 M, 9.86 x 106 M, 1.31 x 105 M, 1.64 x 105
M and 1.97 x 105 M respectively per 200 mL each of the test media.
Acid test media
1 M HCl and H2SO4 acid
media were prepared by dilution of an analytical grade H2SO4
(98% w/w) and HCl (37% w/w) with distilled water and used as
the corrosive test environment.
Preparation of mild steel samples
The mild
steels were machined into 14 test samples test specimens with an average length
of 5 mm and a diameter of 15 mm. The two exposed surface ends of the
cylindrical rod were metallographically prepared with silicon carbide abrasive
papers of 80, 120, 220, 800 and 1000 grits, before being polished with 6 μm to
1 μm diamond liquid, rinsed with distilled water and acetone, dried and later
stored in a desiccator for weight-loss analysis, open circuit potential
measurement and potentiodynamic polarization resistance technique.
Weight-loss analysis
Weighed
steel samples were individually immersed entirely into 200 mL of the dilute
acid media for 432 h at ambient temperature of 25 oC. Each sample
was removed from the solution at 24 h interval, rinsed with distilled water and
acetone, dried and re-weighed according to ASTM NACE/ASTMG31-12a (14). Graphical illustrations of
corrosion rate, ɤ (mm/yr) and
percentage inhibition efficiency (h) versus exposure time T were plotted from the
data obtained during the exposure hours. The corrosion rate (ɤ) calculation is defined as [1] (15).
ɤ =
Where
ῶ is the weight loss in mg, D
is the density in g/cm3, A
is the total area in
cm2 and 87.6 is a constant.
Inhibition
efficiency (h) was calculated from [2].
h
=
Where
ῶ1 and ῶ2 are the weight loss with
and without specific concentrations of VTU; h was calculated at all VTU concentrations throughout
the exposure period.
Surface
coverage is determined from [3] (16, 17).
Where θ is the amount of VTU mixture, adsorbed
per gram of the mild steel; ῶ1 and ῶ2 are
the weight loss of the mild steel coupon with and without predetermined
concentrations of VTU in the acid solutions.
Potentiodynamic polarization
technique
Potentiodynamic
polarization test was performed with cylindrical mild steel electrodes mounted
in acrylic resin with an unconcealed surface area of 154 mm2.
The steel electrode was prepared according to ASTM G59-97(2014)(18). The studies were performed at 25 oC
at ambient temperature with Digi-Ivy DY2300 potentiostat and electrode cell
containing 200 mL of the acid media, with and without VTU mixture. Platinum was
used as the counter electrode and silver chloride electrode (Ag/AgCl) was
employed as the reference electrode. Potentiodynamic measurement was performed
from -1.5V to +1.5 V at a scan rate of 0.0016 V/s according
to ASTM G102-89(2015) (19). The
corrosion current density (jcorr) and corrosion potential (Ecorr)
were calculated from the Tafel plots of potential versus log current.
The corrosion rate (ɤ) and the percentage inhibition efficiency (h2) were from equation [4].
ɤ =
Where jcorr is the current density in µA/cm2; D is the density in g/cm3; Eq is the specimen equivalent
weight in grams. 0.00327 is a constant for corrosion rate calculation in mm/yr (20, 21).
The percentage inhibition
efficiency (h2) was calculated from corrosion
rate values using the equation [5].
h2 = 1 –
Where ɤ1 and ɤ2 are the corrosion rates with and without VTU
inhibitor
Optical microscopy characterization and infrared spectroscopy
Optical micrographs of the surface morphology
and topography of the uninhibited and inhibited mild steel sample was studied
after weight-loss analysis with the aid of Omax trinocular optical metallurgical microscope at the Physical Metallurgical
Laboratory, Covenant University, Ogun state, Nigeria. The VTU/acid solution,
before and after the weight loss test were exposed to a range of infrared ray
beams from Bruker Vertex 70/70v spectrometer. The transmittance and reflectance
of the infrared rays at different frequencies was translated into an IR
absorption plot consisting of spectra peaks. The spectral pattern was analyzed
and matched according to IR absorption table to identify the functional group
contained in the compound.
Adsorption
Isotherm
Adsorption mechanisms are surface
phenomenon by which multi-component solutions diffuse towards the surface of
metallic alloys and adhere through physical or chemical adsorption at a
constant temperature and pH (22, 23).
To further understand
the mechanism of interaction between the organic compound and metallic alloy,
the adsorption behavior of the organic compound on the metal surface was
delineated (24). Langmuir,
Freundlich and Frumkin isotherms had the best fits for the data obtained for
VTU in H2SO4 and while in HCl only Langmuir produce the
best fit.
The isotherms are of the general form [6]
kc = g(θ,x)exp(-fθ)
[6]
where g(θ,x) is the configurational factor
subject to the physical model and assumptions involved in the emanation of the
isotherms. The
general form of the Langmuir equation is shown below
rearranging equation [7], [8]
results
where θ is the value of surface coverage on the steel alloy, C is VTU concentration in the acid
solution, and Kads
is the equilibrium constant of the adsorption process.
Frumkin
isotherm assumes unit coverage at high inhibitor concentrations and that the
electrode surface is inhomogeneous, i.e., the lateral interaction effect is not
negligible. In this way, only the active surface of the electrode, on which
adsorption occurs, is taken into account. Frumkin adsorption isotherm can be
expressed according to equation [9].
Log
{C ˟ (
Where K
is the adsorption-desorption constant and α is
the lateral interaction term describing the interaction in adsorbed layer.
Freundlich
isotherm states the quantitative relationship of the inhibiting compound and
the molecular concentration of inhibitor molecules absorbed onto the steel
which varies at specific concentrations according to equations [10] and [11] (25).
θ
= KadsCn
[10]
logθ = n
log C + log Kads
[11]
Where n is a constant subject to the
properties of the adsorbed molecule; 0 < n
< 1, Kads is the
adsorption-desorption equilibrium constant connoting the interaction strength
within the adsorbed layer. Absolute and higher results of Kads suggest strong interaction between the organic
molecule and the metal surface.
Results and discussion
Weight-loss measurements
Results
for weight loss (ῶ), corrosion rate (ɤ) and percentage inhibition efficiency
(ɲ) for VTU mixture and mild steel from the weight loss experiments in H2SO4
and HCl are presented in Tables 3 and 4. Figures 2 (a, b) and 3 (a, b)
show the graphical illustration of corrosion rate and percentage inhibition
efficiency versus exposure time in the acid media. The results for weight loss,
corrosion rate and inhibition efficiency in both acid solutions are generally similar
indicating similar electrochemical reaction. VTU mixture displayed similar
corrosion inhibition characteristics on the redox electrochemical process
basically through adsorption. Its presence in the acid media stifled the oxygen
reduction, hydrogen evolution and oxidation reaction mechanism responsible for
corrosion. Adsorption of VTU molecules onto the mild steel surface blocked the
active sites where the dissolution and release of metal cations into the
solution occurs as a result of the action of sulphates and chloride anions. The
surface charge on metal oxides in contact with aqueous solutions arises from
structural charge associated with the terminal oxygen and metal atoms at the
mineral surface that have unsatisfied valence, as well as ions from the solution
that associate with these terminal atoms in order to saturate this unsatisfied
valence (26).
The
reduction process was inhibited through increase in surface impedance of the
steel whereby the dissociated hydrogen ions are unable to recombine to give of
hydrogen gas. The presence of hydrogen on the metallic
surfaces significantly accelerates their deterioration because hydrogen
diffuses into the metal degrading their mechanical and chemical properties. The
high corrosion rate and rate of hydrogen evolution for 0% VTU can be
rationalized on the basis that H2SO4 and HCl react with
mild steel and forms metal sulphates and chlorides, which are soluble in
aqueous media. VTU inhibits the electrochemical reaction involving the release
of atomic hydrogen (27). Figures 2(a) and (b) show a steady
increase in corrosion rate for the mild steel sample in 0% VTU acid solutions
till the end of the exposure period. However, with the addition of specific VTU
concentrations the corrosion rates decline drastically with minimal values till
the end of the experiment. The same phenomenon is observed in Figures 3 (a) and
(b) for the inhibition efficiency values. The values ranged from 96.0-97.4% in
H2SO4 and 96.2-97.2% in HCl.
VTU
belongs to the group of organic compounds consisting of electron rich heteroatoms
which are centers of Lewis acid-base interaction with the steel (28). They act by forming a protective
film over the entire exposed area of the steel. The film chemisorbs onto the
steel inhibiting the reaction of corrosive anions with the steel (29). This prevents the passage of
metallic cations consisting of Fe2+ into the solution. The values of
surface coverage (Tables 3 and 4) show that virtually the entire sample area
were covered. This is due to the fact that the surface
coverage of the VTU cations on the steel through adsorption increases with the
increase in concentration (30). The VTU cations adhere themselves
onto the steel surface through adsorption in the acid solution inhibiting the
electrochemical reactions responsible for the deterioration of the steel. The
chemisorb adsorption is due to the donor–acceptor interaction between electrons
of donor atoms and reactive sites of the inhibitors and the acceptor. Adsorption onto the steel surface can
also be in the form of positively
charged species which interact electrostatically with the metal cations and
preadsorbed chlorides and sulphates (31).
Visual
observation of the steel samples in the test solutions can deduce that cathodic
inhibition plays a significant role in the inhibition characteristics of VTU.
Comparison of the uninhibited carbon steels (0% VTU) in H2SO4
and HCl solution with the inhibited solutions (0.13-0.75% VTU) in Tables 3 and 4
evidently shows that VTU at all concentrations effectively reduced the
corrosion rates of the steel, thus protecting it.
Table 3.
Results for mild steel in 1 M H2SO4 at predetermined
concentrations of VTU
|
Samples |
Weight Loss (g) |
Corrosion Rate (mm/yr) |
VTU Inhibitor Concentration (%) |
VTU Inhibitor Concentration (M x 10-3) |
VTU Inhibition Efficiency (%) |
Surface Coverage (θ) |
|
A |
7.5415 |
0.0292 |
0 |
0 |
0 |
0 |
|
B |
0.3037 |
0.0012 |
0.13 |
3.29
x 10-6 |
96.0 |
0.960 |
|
C |
0.2726 |
0.0011 |
0.25 |
6.57
x 10-6 |
96.4 |
0.964 |
|
D |
0.2233 |
0.0009 |
0.38 |
9.86
x 10-6 |
97.0 |
0.970 |
|
E |
0.1937 |
0.0007 |
0.50 |
1.31
x 10-5 |
97.4 |
0.974 |
|
F |
0.1933 |
0.0007 |
0.63 |
1.64
x 10-5 |
97.4 |
0.974 |
|
G |
0.2076 |
0.0008 |
0.75 |
1.97
x 10-5 |
97.2 |
0.972 |
Table 4.
Results for mild steel in 1 M HCl at predetermined concentrations of VTU
|
Samples |
Weight Loss (g) |
Corrosion Rate (mm/yr) |
VTU Inhibitor Concentration (%) |
VTU Inhibitor Concentration (M x 10-3) |
VTU Inhibition Efficiency (%) |
Surface Coverage (θ) |
|
A |
8.9772 |
0.0347 |
0 |
|
0 |
0 |
|
B |
0.3045 |
0.0012 |
0.13 |
3.29 x 10-6 |
96.6 |
0.966 |
|
C |
0.3201 |
0.0012 |
0.25 |
6.57 x 10-6 |
96.4 |
0.964 |
|
D |
0.3446 |
0.0013 |
0.38 |
9.86 x 10-6 |
96.2 |
0.962 |
|
E |
0.4751 |
0.0018 |
0.50 |
1.31 x 10-5 |
94.7 |
0.947 |
|
F |
0.2277 |
0.0009 |
0.63 |
1.64 x 10-5 |
97.5 |
0.975 |
|
G |
0.211 |
0.0008 |
0.75 |
1.97 x 10-5 |
97.7 |
0.976 |
(a)
(b)
Figure
2. Graph illustration of (a) corrosion rate
versus exposure time (b) inhibition efficiency versus exposure time in 1 M H2SO4
(a)
(b)
Figure
3. Graphical illustration of (a) corrosion
rate versus exposure time (b) inhibition efficiency versus exposure time in 1 M
HCl
Adsorption
Isotherm
The plots of
(a)
(b)
Figure 4. Plot of
Figure 5. Frumkin isotherm model of VTU concentrations
in 1 M H2SO4
Figure 6. Freundlich isotherm model of VTU
concentrations in 1 M H2SO4
Thermodynamics
of the corrosion process
The quantity of metal loss due to
corrosion deterioration is proportional to the degree of surface coverage of VTU
mixture over the mild steel surface. It is suggested that the steel surface is
covered with water dipoles, thus for adsorption of the cations of the organic
compound to occur the water dipoles must be replaced by the cation in the
electrochemical reaction as follows (34,
35).
nH2O electrode
+ Organic solution ≈ Organic electrode + nH2O
solution [12]
As earlier mention in the discussion
on adsorption isotherms, the thermodynamics of the replacement process is
subject to the numbers of water molecules (n) displaced by VTU mixture. The
values of the Gibbs free energy (ΔGoads)
for the adsorption process as shown in Tables 5 and 6 can be evaluated from equation
[13].
ΔGads = - 2.303RT log [55.5Kads]
[13]
Where 55.5 is the molar
concentration of water in the solution, R is the universal gas constant, T is the absolute temperature and Kads is the equilibrium constant of adsorption. Kads is related to surface
coverage (
The
heterogeneous characteristic (presence of flaws, impurities, cracks, vacancies,
etc.) of the steel surface is responsible for the proportional relationship
between the ΔGoads
of VTU and surface coverage (34, 36-37).
This relationship is caused by the changes in adsorption energies as shown in the
tables. The negative values of ΔGoads
shows the adsorption is spontaneous. Values of ΔGoads around -20 kJ/mol is consistent with
physisorption reactions, while ΔGoads
around -40 kJ/mol is consistent with chemisorption reactions which basically
involves charge sharing or transfer between the inhibitor cations and the
valence electrons of the metal forming a co-ordinate covalent bond. The ΔGoads values in H2SO4
ranged from -49.10 kJ/mol at 0.13% VTU to 45.63 kJ/mol at 0.75% VTU while in
HCl the values ranged from -49.54 to
-46.04 kJ/mol at 0.13-0.75% VTU. The high negative value of ΔGoads shows that
in H2SO4 and HCl acid medium chemisorption of VTU on the
mild steel surface occurs (38, 39).
Table 5. Results for Gibbs free energy, surface coverage and
equilibrium constant of adsorption for 0-7.5% VTU in 1 M H2SO4
|
Samples |
Surface
Coverage (θ) |
Equilibrium
Constant of Adsorption (K) |
Gibbs Free
Energy (∆G) |
|
A |
0 |
0 |
0 |
|
B |
0.960 |
7253719.8 |
-49.10 |
|
C |
0.964 |
4057998.1 |
-47.66 |
|
D |
0.970 |
3325012.5 |
-47.17 |
|
E |
0.974 |
2886467.7 |
-46.81 |
|
F |
0.974 |
2314078.6 |
-46.27 |
|
G |
0.972 |
1792071.7 |
-45.63 |
Table 6.
Results for Gibbs free energy, surface coverage and equilibrium constant of
adsorption for 0-7.5% VTU in 1 M HCl
|
Samples |
Surface
Coverage (θ) |
Equilibrium
Constant of Adsorption (K) |
Gibbs Free
Energy (∆G) |
|
A |
0 |
0 |
0 |
|
B |
0.966 |
8668940.4 |
-49.54 |
|
C |
0.964 |
4115814.1 |
-47.69 |
|
D |
0.962 |
2541581.7 |
-46.50 |
|
E |
0.947 |
1361696.0 |
-44.95 |
|
F |
0.975 |
2339102.2 |
-46.29 |
|
G |
0.976 |
2107544.0 |
-46.04 |
Potentiodynamic Polarization
studies
The corrosion polarization
behaviour of VTU inhibiting compound on mild steel in 1 M H2SO4
and HCl are shown in Figures 7 and 8. Tables 7 and 8 show the potentiodynamic
data obtained. Table 6 shows the significant change in corrosion rate in the
presence of VTU (0.13- 075% VTU) in comparison to the concentration without
VTU. The corrosion rate decreased significantly at 0.15% VTU and continues to
decrease progressively with increase in VTU concentration. The inhibition
efficiency at the lowest VTU concentration (0.13% VTU) is 92.49%, the values
continues to increase till 97.47% at 0.63% VTU, after which it decreased to
94.05% at 0.75% VTU. The results further confirm that VTU effectively inhibits
the corrosion of mild steel in H2SO4 at all the
concentrations studied. The corrosion current also decreased significantly. The
inhibition efficiency of VTU is slightly dependent on the values of its concentration
acid solution.
The same phenomenon was observed in
Table 7 for the electrochemical influence of VTU in HCl, however the maximum
inhibiting effect of VTU is 89.73% at 0.75% VTU. This shows that VTU molecules
which protonates in the acid solutions is more effective inhibiting the
diffusion of SO42- anions compared to Cl-
anions, probably due to the small size of Cl- ions which enables
selective penetration through the protective film. The anodic and cathodic
polarization plots in Figure 7 shows active-passive behavior in the presence of
VTU inhibitor in H2SO4 media.
The plots displayed similar
electrochemical behavior with the corrosion potential shifting majorly to
anodic potentials suggesting that the mechanism of inhibition is through film
formation by adsorption. This prevents the anodic dissolution and deterioration
of the steel sample through surface coverage of the reaction sites. The
coverage decreases the number of surface metal atoms at which corrosion
reactions can occur. Anodic dissolution process of is considered to occur at
specific dislocations in the metal surface, where metal atoms are less firmly
held to their neighbors than in the plane surface. The anodic and cathodic
Tafel slopes were moderately affected with changes in VTU concentration
suggesting that the oxidation and reduction reactions were simultaneously
inhibited however as earlier mentioned from corrosion potential values anodic
inhibition tends to predominate.
The polarization plots in Figure 8 shows
greater tendency for cathodic inhibition as observation of the corrosion
potential values indicates a significant shift to negative potentials. This
shows that the mechanism of inhibition in HCl is through stifling of the
hydrogen evolution and oxygen reduction reactions whereby VTU cations
selectively precipitates on the cathodic reaction sites increasing the surface
impedance of the steel. The anodic and cathodic Tafel slopes remained generally
the same at all VTU concentrations. The maximum change in corrosion potential
in H2SO4 is 52 mV in the anodic direction while in HCl it
is 34 mV in the cathodic direction, thus VTU is a mixed type inhibitor in both
acids (40, 41).
Corrosion of metallic alloys is
complex mechanism due to the presence of numerous anodic and cathodic reaction
sites on the metal surface. VTU inhibitor interacts with the reaction sites
retarding electrochemical corrosion reactions and preventing the diffusion of
reactive corrosive species from solution through the metal solution interface.
As earlier mentioned the heteroatoms of VTU mixture are the adsorption center
for its interaction with the steel surface via electrostatic interaction
between a negatively charged surface, through a specifically adsorbed anion (Cl-)
on the steel, and the cation molecule of VTU inhibitor (42, 43). VTU mixture has nitrogen, oxygen and sulphur atoms in its
molecular structure and adsorption occurs through the formation of an
iron–nitrogen coordinate bond or pi electron interaction between them (44).
Table
7. Potentiodynamic polarization results for
mild steel in 1 M H2SO4
|
Sample |
Inhibitor
Concentration (%) |
Corrosion Rate
(mm/yr) |
Inhibition
Efficiency (%) |
Corrosion
Current (A) |
Current
Density (A/cm2) |
Corrosion
Potential (V) |
Polarization
Resistance, Rp (Ω) |
Cathodic Tafel
Slope, Bc (V/dec) |
Anodic Tafel
Slope, Ba (V/dec) |
|
0 |
0 |
4.35 |
0 |
5.78 x 10-4 |
3.75 x 10-4 |
-0.327 |
44.44 |
-7.990 |
13.870 |
|
1 |
0.13 |
0.33 |
92.49 |
4.34 x 10-5 |
2.82 x 10-5 |
-0.334 |
59.25 |
-4.273 |
9.532 |
|
2 |
0.25 |
0.31 |
92.88 |
4.11 x 10-5 |
2.67 x 10-5 |
-0.292 |
49.01 |
-3.046 |
8.188 |
|
3 |
0.38 |
0.24 |
94.53 |
3.16 x 10-5 |
2.05 x 10-5 |
-0.277 |
81.29 |
-2.588 |
16.220 |
|
4 |
0.50 |
0.16 |
96.25 |
2.17 x 10-5 |
1.41 x 10-5 |
-0.275 |
118.60 |
-3.923 |
18.740 |
|
5 |
0.63 |
0.11 |
97.47 |
1.46 x 10-5 |
9.49 x 10-6 |
-0.311 |
175.80 |
-5.737 |
14.540 |
|
6 |
0.75 |
0.26 |
94.05 |
3.44 x 10-5 |
2.23 x 10-5 |
-0.290 |
74.72 |
-4.017 |
17.090 |
Table
8. Potentiodynamic polarization results for
mild steel in 1 M HCl
|
Sample |
Inhibitor Concentration (%) |
Corrosion Rate (mm/yr) |
Inhibition Efficiency (%) |
Corrosion Current (A) |
Current Density (A/cm2) |
Corrosion Potential (V) |
Polarization Resistance, Rp (Ω) |
Cathodic Tafel Slope, Bc (V/dec) |
Anodic Tafel Slope, Ba (V/dec) |
|
0 |
0 |
5.60 |
0 |
7.44 x 10-4 |
4.83 x 10-4 |
-0.324 |
34.55 |
-8.639 |
11.270 |
|
1 |
0.13 |
0.82 |
85.40 |
1.09 x 10-4 |
7.05 x 10-5 |
-0.326 |
236.60 |
-6.933 |
9.942 |
|
2 |
0.25 |
0.80 |
85.76 |
1.06 x 10-4 |
6.88 x 10-5 |
-0.358 |
242.50 |
-7.834 |
10.380 |
|
3 |
0.38 |
0.83 |
85.11 |
1.11 x 10-4 |
7.19 x 10-5 |
-0.338 |
223.00 |
-7.057 |
10.730 |
|
4 |
0.50 |
0.80 |
85.75 |
1.06 x 10-4 |
6.88 x 10-5 |
-0.334 |
242.40 |
-6.005 |
10.150 |
|
5 |
0.63 |
0.67 |
87.96 |
8.96 x 10-5 |
5.82 x 10-5 |
-0.329 |
286.90 |
-6.286 |
10.130 |
|
6 |
0.75 |
0.58 |
89.73 |
7.64 x 10-5 |
4.96 x 10-5 |
-0.343 |
336.20 |
-8.287 |
10.580 |
Figure 7. Anodic and cathodic polarization curve for mild steel in 1
M H2SO4 acid
Figure
8. Anodic and cathodic polarization curve
for mild steel in 1 M HCl acid
Optical Microscopy
Analysis
The
micro-analytical images of the mild steel samples before and after corrosion
are presented from Figures 9(a) to 10(d). Figures 9(a)-(d) show the images of
the steel samples before the corrosion test at magnifications of 4X, 10X, 40X and
100X. The image presents the samples as received after metallographic
preparation of their surfaces. Figure 10(a-d) shows the micro-analytical image
of the control specimens after the corrosion test. Topographic degradation and
significant deterioration of the surface morphology of the sample is clearly
visible as a result of the electrochemical action of corrosive anions present
in the acid media. The anions react with the metal surface through the redox
corrosion mechanism resulting in the loss of valence electrons and passage of
Fe2+ cations into the acid solution. This was clearly observed
during the exposure hours whereby there was a gradual buildup of sediments of
iron compounds and significant discoloration of the acid solution. Figures 10(a-d)
also shows that mild steel is unsuitable for applications in such environments
as rapid deterioration occurs. The image in Figure 10 contrasts the image in
Figure 9.
The
presences of large voids due to severe corrosion are visible since mild steel
is known to undergo general corrosion. Figures 11 (a-d) shows the images of the
mild steel specimens from the acid solution with VTU inhibiting compound after
the corrosion test. Based on results from weight loss and potentiodynamic
polarization, the images show the surface of well-protected steel specimens.
VTU molecules acting through adsorption from electrostatic attraction covers
and possibly builds up on the steel surface and reacting with it through the
chemisorption mechanism to effectively protect the steel from corrosion. The
images in Figure 11(a and b) are generally the same but closer magnification
reveals the presence of the inhibiting compound which strongly adheres to the
steel surface protecting it from deterioration.
(a) (b)
(c)
(d)
Figure 9.
Microanalytical images of mild steel before corrosion (a) 4X, (b) 10X, (c) 40X,
(d) 100X.
(a) (b)
(c)
(d)
Fig. 10.
Microanalytical images of mild steel after corrosion without VTU mixture (a)
4X, (b) 10X, (c) 40X, (d) 100X.
(a)
(b)
(c)
(d)
Figure 11.
Micro-analytical images of mild steel after corrosion study with VTU mixture (a)
4X, (b) 10X, (c) 40X, (d) 100X.
IR
spectroscopy
IR spectroscopy was used to study the
properties and center of adsorption of VTU mixture within its molecular
structure. Figures 12(a) and (b) show the spectra peaks for VTU mixture in H2SO4
before and after the corrosion test (without and with the mild steel sample). Figure
12(c) shows the superimposition of Figures 12(a) and (b), while Figures 13(a) and
(b) show the spectra peaks for VTU mixture in HCl before and after the
corrosion test (without and with the mild steel sample). Superimposition of Figures
13(a) and (b) is shown in Figure 13(c).
Characteristic IR absorptions are
presented in Table 9. Observation and comparison of Figure 12(a) with Table 9
shows the spectra peaks at 3345.78 cm−1 (N-H stretch bond), 1636.38 cm−1
(N-H bend bond), 1190.38 cm−1 (C–H wag (–CH2X) bond) and
1050.08 cm−1 (C-N stretch bond). These consist of primary and
secondary amines and amides, primary amines, aliphatic amines and alkyl halides
functional groups. The comparison of Figure 12(b) with Table 9 shows similar
functional groups with Figure 12(a) at spectra peaks of 3359.26, 1631.36,
1182.56, and 1047.57 cm−1. Spectra peaks of 871.62 cm−1
(=C–H bend, N–H wag and C–H "oop" bonds) consists of alkenes, primary
and secondary amines, and aromatics functional groups while spectra peak of 577.15
cm-1 (C–Cl stretch and C–Br stretch bonds) consists of alkyl halides
functional groups.
Superimposition of Figures 12(a) and (b)
in Figure 12(c) shows the differences between the VTU compounds involved in and
not involved in the inhibition of mild steel samples in H2SO4.
The decrease in transmittance for Figure 12(b) in comparison to Figure 12(a)
shows that the functional groups earlier mentioned were actively involved in
the inhibition of the steel by adsorption through chemisorption mechanism.
The groups are responsible for the
formation of stable complex between the iron constituents and functional groups
present in the VTU mixture forming covalent or coordinate bonds between the
anionic components of VTU and vacant Fe d-orbital. The metal-inhibitor bond
usually leads to corrosion inhibition through adsorption (45). The corrosion retarding mechanism through stable complex
formation dominates at all VTU concentrations. The corrosion retarding
mechanism is due to strong adsorption resulting from the donation of lone pair
of electrons on oxygen and nitrogen to vacant d orbital of the metal which
leads to the formation of metal complexes.
The spectra peaks of 3341.60 cm-1
(N-H stretch, O–H stretch and H–bonded
bonds), 2094.47 cm-1 (–C(triple bond)C– stretch bond), 1632.32
cm-1 (N–H bend bond) and 1291.99 cm-1 (N–O symmetric stretch, C–N
stretch, C–O stretch and C–H wag
(–CH2X) bonds) in Figures 13(a) and (b) consists of alcohols,
phenols, primary, secondary amines and amides, alkynes and aromatics functional
groups responsible for corrosion inhibition by VTU in HCl acid. However, super
imposing Figures 13(a) and (b) in Figure 13(c) shows that the spectral diagrams
are basically the same. It is suggested that VTU essentially inhibited the mild
steel corrosion through film formation by blocking the active sites on the
surface but not necessarily affecting the mechanism of the corrosion process.
(a)
(b)
(c)
Figure
12. IR spectra of VTU inhibiting compound
(a); VTU mixture in H2SO4 before mild steel corrosion (b); VTU
mixture in H2SO4 after mild steel corrosion (c).
Superimposition of Figures 11(a) and (b)
(a)
(b)
(c)
Figure
13. IR spectra of VTU inhibiting compound
(a); VTU mixture in HCl before mild steel corrosion (b); VTU
mixture in HCl after mild steel corrosion (c). Superimposition of Figures 12(a)
and (b)
Table 9.
Table of characteristic IR absorptions (Extracted)
|
Wavenumber (cm–1) |
Bond |
Functional Group |
|
3400–3250
(m) |
N–H
stretch |
primary,
secondary amines, amides |
|
3500–3200
(s,b) |
O–H stretch, H–bonded |
alcohols,
phenols |
|
2260–2100
(w) |
–C
(triple bond) C–
stretch |
alkynes |
|
1650–1580
(m) |
N–H
bend |
primary
amines |
|
1300–1150
(m) |
C–H
wag (–CH2X) |
alkyl
halides |
|
1360–1290
(m) |
N–O
symmetric stretch |
nitro
compounds |
|
1335–1250
(s) |
C–N
stretch |
aromatic
amines |
|
1320–1000
(s) |
C–O
stretch |
alcohols,
carboxylic acids, esters,
ethers |
|
1300–1150
(m) |
C–H
wag (–CH2X) |
alkyl
halides |
|
1250–1020
(m) |
C–N
stretch |
aliphatic
amines |
|
1000–650
(s) |
=C–H
bend |
alkenes |
|
910–665
(s, b) |
N–H
wag |
primary,
secondary amines |
|
900–675
(s) |
C–H
"oop" |
aromatics |
|
850–550
(m) |
C–Cl
stretch |
alkyl
halides |
m=medium,
w=weak, s=strong, n=narrow, b=broad, sh=sharp
Conclusions
Corrosion
inhibition study of VTU (1,3-diphenyl-2-thiourea and
4-hydroxy-3-ethoxybenzaldehyde) on mild steel in acidic
environment was evaluated and the results showed that it is a potent inhibitor.
VTU performed effectively with inhibition efficiencies above 90% at all
concentrations evaluated in H2SO4 and HCl acid solutions.
The inhibition characteristics of VTU was determined to be mixed type due to
its influence on the redox electrochemical process, however it showed greater
tendency for anodic inhibition in H2SO4 and cathodic
inhibition in HCl acid. VTU being a mixture of organic compounds with
heteroatoms protonates in the acid solution, forms cationic molecules which
reacts with the charged steel surface, forming in turn a chemically adsorbed
protective layer as shown from thermodynamic calculations. Infrared spectra
confirmed the presence of functional groups of the organic compound responsible
for corrosion inhibition. The adsorption mechanism aligned with Langmuir, Frumkin
and Freundlich adsorption isotherms. The corrosion inhibition results were
confirmed from micro-analytical images through optical microscopy. The
difference in surface topography and morphology was clearly distinct.
Acknowledgement
The author is grateful to the
Department of Mechanical Engineering, Covenant University for the provision of
facilities for the research work.
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