J. Coat. Technol. Res., 9 (4) 483–493, 2012
DOI 10.1007/s11998-011-9379-1
The corrosion inhibition of maraging steel under weld aged
condition by 1(2E)-1-(4-aminophenyl)-3-(2-thienyl)prop-2-en-
1-one in 1.5 M hydrochloric acid medium
B. S. Sanatkumar, Jagannath Nayak, A. Nityananda Shetty
 ACA and OCCA 2011
Abstract The influence of 1(2E)-1-(4-aminophenyl)- Introduction
3-(2-thienyl)prop-2-en-1-one (ATPI) on the corrosion
behavior of weld aged maraging steel in 1.5 M Corrosion inhibition of metals by organic compounds
hydrochloric acid was studied by potentiodynamic results from the adsorption of molecules and ions on
polarization method and AC impedance (EIS) tech- the surface of the metals.1–4 Several groups of organic
nique at different temperatures. The results showed compounds have been reported to exert inhibitive
that the inhibition efficiency of ATPI increased with effects on the corrosion of steels. The extent of
the increase in the concentration of inhibitor and adsorption of an inhibitor depends on many factors
decreased with the increase in temperature. ATPI acts such as the nature and the surface charge of the metal;
as a mixed type inhibitor without affecting the mech- the mode of adsorption of the inhibitor; the inhibitor’s
anism of the hydrogen evolution reaction or iron chemical structure; and the type of the aggressive
dissolution. The adsorption of ATPI on a weld aged solution. The presence of hetero atoms (oxygen,
maraging steel surface obeys the Langmuir adsorption nitrogen, sulfur, and phosphorus), triple bonds, and
isotherm equation. Both activation and thermody- aromatic rings in the inhibitor’s chemical structure
namic parameters were calculated and discussed. ATPI enhances the adsorption process. It has been reported
inhibits the corrosion through both physisorption and that the order of the inhibitor efficiency (g) of
chemisorption on the alloy surface. The surface mor- heterocyclic organic compounds follows the sequence:
phology of the weld aged maraging steel specimens in oxygen < nitrogen < sulfur < phosphorus.5–13
the presence and the absence of the inhibitors was Maraging steel is an important category of material
studied by the respective SEM images. due to its high technological value and wide range of
industrial applications, especially in aerospace, auto-
Keywords Maraging steel, ATPI, Corrosion rate, mobile, and marine industries.14–17 The alloy is a low
EIS, SEM carbon steel that classically contains about 18 wt% Ni,
substantial amounts of Co and Mo, together with small
additions of Ti. However, depending on the demands
dedicated by the application, the composition of the
material can be modified.18 The high strength of
maraging steel is achieved by aging at 480C, where
precipitation of intermetallics takes place. Because of
the low carbon content, maraging steels have good
19
B. S. Sanatkumar, A. Nityananda Shetty (&) machinability. They have higher modulus of elasticity,
Department of Chemistry, National Institute of Technology lower thermal expansion coefficient, high strength,
Karnataka, Surathkal, Srinivasnagar 575 025, Karnataka, moderate toughness, and good weldability as compared
India with conventional alloy steels.20 Another important
e-mail: nityashreya@gmail.com property is its high thermal conductivity, which reduces
surface temperature during thermal loading and lowers
J. Nayak thermal stresses. Maraging steels are also used in
Department of Metallurgical and Materials Engineering,
preparation of surgical components, nuclear, and gas
National Institute of Technology Karnataka, Surathkal, 14–17
Srinivasnagar 575 025, Karnataka, India turbines applications. Thus, they frequently come
483
J. Coat. Technol. Res., 9 (4) 483–493, 2012
into contact with acids during cleaning, pickling, Table 1: Composition of the aged maraging steel
descaling, acidizing, etc. specimen
A search of the literature reveals only a few reports on
the corrosion studies of 18 Ni 250 grade maraging steel, Element Composition Element Composition
which is entirely in martensitic phase. Bellanger and (wt%) (wt%)
Rameau21 have studied the effect of slightly acidic pH
C 0.015 Ti 0.52
with or without chloride ions in radioactive water on the
Ni 18.19 Al 0.11
corrosion of maraging steel and have reported that
Mo 4.82 Mn 0.1
corrosion behavior of maraging steel at the corrosion
Co 7.84 P 0.01
potential depends on pH, and intermediates remaining
Si 0.1 S 0.01
on the maraging steel surface in the active region
O 30 ppm N 30 ppm
favoring the passivity. The effect of carbonate ions in a
H 2.0 ppm Fe Balance
slightly alkaline medium on the corrosion of maraging
steel was studied by Bellanger.22 Maraging steels were
found to be less susceptible to hydrogen embrittlement plates which are welded as described above and aged at
than common high-strength steels owing to significantly 480 ± 5C for 3 h followed by air cooling.
low diffusion of hydrogen in them.23 Poornima et al.17
have studied the corrosion behavior of 18 Ni 250 grade
maraging steel in a phosphoric acid medium and Preparation of test coupons
reported that the corrosion rate of the annealed sample
is less than that of the aged sample. Similar observations Cylindrical test coupons were cut from the plate and
also have been reported for the corrosion of 18 Ni 250 sealed with epoxy resin in such a way that the area
2
grade maraging steel in sulfuric acid medium.24 The exposed to the medium was 0.64 cm . These coupons
effect of 3,4-dimethoxybenzaldehyde thiosemicarba- were polished mechanically using emery papers of
zone in 0.5 M H2SO4 medium on corrosion of aged grade nos. 180, 400, 600, 800, 1000, 1500, and 2000 and
maraging steel was studied by Poornima et al.25 and finally on a polishing wheel using legated alumina to
reported good inhibitor efficiency. obtain a mirror finish, washed thoroughly with double
A few research reports revealed that the inhibition distilled water, and degreased with acetone before
efficiency of chalcone derivatives is much higher than being immersed in the acid solution.
that of corresponding aldehydes and amines, and this
may be due to the presence of a –C=C– group in the
molecules and the presence of hetero atoms.26,27 The Medium
planarity (p) and lone pairs of electrons present on
hetero atoms are the important structural features that Standard solution of 1.5 M hydrochloric acid was
determine the adsorption of these molecules on the prepared by diluting AR grade hydrochloric acid with
metal surface. double distilled water. Inhibitive action of ATPI on the
The objective of the present research article is to corrosion of weld aged maraging steel in 1.5 M HCl
study the adsorption and inhibition action of synthe- solution was studied by introducing different concen-
sized ATPI on the corrosion of maraging steel under trations of the inhibitor into the solution. The exper-
weld aged conditions in 1.5 M HCl medium at different iments were carried out at temperatures 30, 35, 40, 45,
temperatures using the potentiodynamic polarization and 50C (±0.5C), in a calibrated thermostat.
method and the AC impedance (EIS) technique. The
mode of adsorption and the corrosion inhibition
mechanism are also discussed. Inhibitor
The inhibitor 1(2E)-1-(4-aminophenyl)-3-(2-thienyl)
Experimental prop-2-en-1-one (ATPE) was synthesized as per the
reported procedure28 in one-step reaction of 4-amino-
Material acetophenone (0.40 g, 3 mmol) with thiophene-2-
carboxaldehyde (0.28 mL, 3 mmol) in ethanol
The material employed was 18% Ni M250 grade (30 mL) in the presence of 10% NaOH (aq) (5 mL).
maraging steel under weld aged condition. The com- The reaction mixture was stirred for 2 h at room
position of the 18% Ni M250 grade maraging steel is temperature. The resulting yellow solid was collected
given in Table 1. The maraging steel plates were by filtration, washed with distilled water, and dried.
welded by gas tungsten arc welding-direct-current The product was purified by recrystallization from
straight polarity (GTAW-DCSP) using filler material acetone and was identified by melting point (105–
of compositions 0.015% C, 17% Ni, 2.55% Mo, 12% 106C), elemental analysis, and infrared spectra. The
Co, 0.015% Ti, 0.4% Al, 0.1% Mn, 0.1% Si, with the molecular weight of the compound is 229.29. The
remainder being Fe. The specimen was taken from the synthesis scheme is given below.
484
J. Coat. Technol. Res., 9 (4) 483–493, 2012
O CH3
O
O
EtOH S
S H +
10% NaOH
NH2
NH2
thiophene-2-carbaldehyde 4-aminoacetophenone (2E)-1-(4-aminophenyl)-3-(2-thienyl)prop-2-en-1-one
Electrochemical measurements The immersion time of the electrode for the SEM
analysis was 3 h.
Electrochemical measurements were carried out by
using an electrochemical work station, Gill AC having
ACM instrument Version 5 software. The arrangement Results and discussion
used was a conventional three-electrode compartment
glass cell with a platinum counter electrode and a Tafel polarization measurement
saturated calomel electrode (SCE) as reference. The
working electrode was made of weld aged maraging The polarization studies of the weld aged maraging
steel. All the values of potential are referred to the steel were carried out in 1.5 M hydrochloric acid
SCE. The polarization studies were done immediately solution containing different concentrations of ATPI at
after the EIS studies on the same electrode without any different temperatures using the Tafel polarization
further surface treatment. technique. Figure 1 shows the Tafel polarization curves
for the corrosion of weld aged maraging steel in 1.5 M
HCl solution at 45C in the presence of different
Tafel polarization studies concentrations of ATPI. Similar results were obtained
at other temperatures also. The inhibition efficiency,
Finely polished weld aged maraging steel specimen was g (%), was calculated from the following equation (1).
exposed to the corrosion medium of 1.5 M hydrochlo-
ric acid in the presence and the absence of the inhibitor icorr  icorrðinhÞ
at different temperatures (30–50C) and allowed to g (%) ¼  100 ð1Þicorr
establish a steady-state open circuit potential (OCP).
The potentiodynamic current–potential curves were where icorr and icorr(inh) are the corrosion current
recorded by polarizing the specimen to 250 mV densities obtained in uninhibited and inhibited
cathodically and +250 mV anodically with respect to solutions, respectively. The corrosion rate is
the OCP at a scan rate of 1 mV s1. calculated using equation (2).
Electrochemical impedance spectroscopy
studies (EIS) 0
Blank
The impedance measurements were carried out in the 0.2 mM–100 0.4 mM
frequency range from 10 kHz to 0.01 Hz, at the rest 0.6 mM
potential, by applying 10 mV sine wave AC voltage. –200 0.8 mM
The double layer capacitance (Cdl) and the charge 1.0 mM
transfer resistance (Rct) were calculated from the –300
Nyquist plot.
In all the above measurements, at least three similar
–400
results were considered, and their average values are
reported.
–500
–600
Scanning electron microscopy (SEM)
1E–3 0.01 0.1 1 10
The surface morphology of the weld aged maraging log i (mA cm–2)
steel specimen immersed in 1.5 M HCl solution in the
presence and the absence of inhibitor was compared by Fig. 1: Tafel polarization curves for the corrosion of weld
recording the SEM images of the samples using JEOL aged maraging steel in 1.5 M hydrochloric acid containing
JSM-6380LA analytical scanning electron microscopy. different concentrations of inhibitor at 45C
485
E (mV/SCE)
J. Coat. Technol. Res., 9 (4) 483–493, 2012
   3270  M  icorr profiles are influenced simultaneously, almost to thet 1corr mm y ¼ ð2Þq  Z same extent, which indicate the influence of ATPI on
both the anodic and the cathodic reactions; hydrogen
evolution and metal dissolution.30 If the displacement
where 3270 is a constant that defines the unit of in corrosion potential is more than ±85 mV with
corrosion rate, icorr is the corrosion current density in
2 respect to the corrosion potential of the blank, theA cm , q is the density of the corroding material
3 inhibitor can be considered a distinctive cathodic or(g cm ), M is the atomic mass of the metal, and Z is
29 anodic type. However, the maximum displacement inthe number of electrons transferred per atom. this study is ±20 mV; and therefore ATPI can be
Corrosion parameters, such as corrosion potential considered a mixed-type inhibitor.31,32
(Ecorr), cathodic and anodic Tafel slopes (bc and ba), The data in Table 2 show that there is no significant
corrosion current density (icorr), and the inhibition change in the values of cathodic Tafel slope bc and
efficiency values g (%), calculated from Tafel plots are anodic Tafel slope ba with the increase in the concen-
listed in Table 2. As seen from the data, in the absence tration of the inhibitor. This suggests that the reduction
of inhibitor, weld aged maraging steel corrodes mechanism at the cathode and the oxidation mecha-
severely in 1.5 M HCl. The presence of inhibitor nism at the anode are not affected by the presence of
brings down the corrosion rate considerably. Polariza- inhibitor,33,34 and hence the corrosion reaction is
tion curves are shifted to a lower current density region slowed down by the surface-blocking effect of the
indicating a decrease in corrosion rate (tcorr). Inhibi- inhibitor. This indicates that the inhibitive action of
tion efficiency increases with the increase in ATPI ATPI may be considered due to the adsorption and
concentration. No definite trend is observed in the shift formation of barrier film on the electrode surface. The
of Ecorr values; both anodic and cathodic polarization
Table 2: Results of Tafel polarization studies on weld aged maraging steel in 1.5 M hydrochloric acid containing
different concentrations of ATPI
Temperature Conc. of Ecorr ba bc icorr tcorr g
(C) inhibitor (mM) (mV/SCE) (mV dec1) (mV dec1) (mA cm2) (mm y1) (%)
30 Blank 342 100 223 0.70 8.0
0.2 339 102 221 0.39 4.2 47.8
0.4 345 105 215 0.28 3.1 61.1
0.6 343 103 213 0.21 2.2 73.0
0.8 348 104 209 0.85 1.1 86.0
1.0 351 107 210 0.08 0.8 90.2
35 Blank 339 118 229 0.82 10.9
0.2 342 115 226 0.59 6.2 43.1
0.4 338 112 222 0.44 4.5 58.7
0.6 344 108 219 0.34 3.6 67.5
0.8 347 111 217 0.20 2.1 80.6
1.0 343 106 215 0.13 1.5 87.0
40 Blank 343 125 235 0.94 12.9
0.2 337 127 233 0.72 7.5 42.0
0.4 340 121 228 0.53 5.7 55.6
0.6 342 119 230 0.41 4.4 65.7
0.8 341 115 225 0.26 2.9 77.7
1.0 336 109 221 0.18 1.9 84.8
45 Blank 338 133 237 1.53 17.3
0.2 335 135 233 0.90 10.7 38.2
0.4 337 129 235 0.79 8.2 52.6
0.6 339 126 228 0.64 6.9 60.1
0.8 341 123 225 0.43 4.5 74.0
1.0 343 121 220 0.31 3.2 81.4
50 Blank 341 145 239 1.61 22.2
0.2 343 139 241 1.33 14.0 36.8
0.4 339 136 236 1.10 11.2 49.5
0.6 338 138 233 0.92 9.4 57.6
0.8 340 134 231 0.65 6.7 69.7
1.0 342 128 227 0.54 5.55 74.9
486
J. Coat. Technol. Res., 9 (4) 483–493, 2012
barrier film formed on the metal surface reduces the replacement of water molecules by the adsorption of
probability of both the anodic and cathodic reactions. the inhibitor molecules on the metal surface to form an
Thus, the inhibitor, ATPI, can be regarded as a mixed adherent film on the metal surface and thereby
type inhibitor. reducing the metal dissolution in the solution. As Rct
is inversely proportional to the corrosion current
density, inhibitor efficiency, g (%), was calculated
Electrochemical impedance spectroscopy (EIS) from the following relationship:
studies
R
¼ ctðinhÞ
 Rct
g (%)  100 ð3Þ
Figure 2 shows the Nyquist plots for the corrosion of RctðinhÞ
weld aged maraging steel in 1.5 M HCl solution at
45C in the presence of different concentrations of
where R
ATPI. Similar plots were obtained at other tempera- ct(inh)
and Rct are the charge transfer resis-
tances obtained in inhibited and uninhibited solutions,
tures also. The experimental results of EIS measure-
respectively.
ments obtained for the corrosion of weld aged
The corrosion current density i
maraging steel are summarized in Table 3. corr
can be calculated
using the charge transfer resistance value, R , using
As seen from Fig. 2 the Nyquist plots are semicir- ct
with the Stern–Geary equation (4).39
cular in the presence as well as in the absence of
inhibitor. This indicates that the corrosion of weld aged babc
maraging steel is controlled by a charge transfer icorr ¼ ð4Þ2:303ðb þ b ÞR
process and the addition of ATPI does not change a c ct
the reaction mechanism of the corrosion of sample in
HCl solution.35 ATPI inhibits the corrosion primarily
through its adsorption and subsequent formation of a Table 3: EIS data of weld aged maraging steel in 1.5 M
barrier film on the metal surface.36 This is in accor- hydrochloric acid containing different concentrations of
dance with the observations of Tafel polarization ATPI
measurements. It is seen from Fig. 2 that the Nyquist
Temperature Conc. of R C g
plots are not perfect semicircles. The deviation has ct dl
(C) inhibitor (mM) (X cm2) (lF cm2) (%)
been attributed to frequency dispersion, a phenome-
non often corresponding with surface heterogeneity 30 Blank 50 3028
which may be the result of surface roughness, disloca- 0.2 88 1719 43.2
tions, distribution of the active sites, or adsorption of 0.4 121 1260 58.7
molecules.37 0.6 156 573 68.0
The data in Table 3 reveal that the charge transfer 0.8 331 225 84.9
resistance (Rct) increases with the increase in the 1.0 418 197 88.0
inhibitor concentration, suggesting a hindrance to the 35 Blank 45 3278
charge transfer reaction (i.e., effective metal dissolu- 0.2 78 2317 42.0
tion).38 The increase in Rct values is due to the gradual 0.4 97 1825 53.6
0.6 119 1377 62.2
0.8 181 647 75.1
100 1.0 258 403 82.6
40 Blank 43 4049
Blank 0.2 73 2364 41.0
80
0.2 mM 0.4 91 1952 52.7
0.4 mM 0.6 112 1745 61.6
0.6 mM
60 0.8 149 925 72.10.8 mM
1.0 M 1.0 219 869 81.3
45 Blank 24 4301
40 0.2 40 3498 40.0
0.4 43 3235 44.1
0.6 55 3049 54.7
20
0.8 78 1597 67.5
1.0 107 1194 76.1
0 50 Blank 25 4516
0 20 40 60 80 100 120 0.2 38 3946 34.2
Z ' (Ω cm2) 0.4 47 3756 46.8
0.6 54 3451 53.7
Fig. 2: Nyquist plots for the corrosion of weld aged 0.8 69 2715 63.7
maraging steel specimen in 1.5 M hydrochloric acid con- 1.0 91 1938 72.5
taining different concentrations of inhibitor at 45C
487
–Z '' (Ω cm2)
J. Coat. Technol. Res., 9 (4) 483–493, 2012
45
1.3
1.75 966m
40 718m R2.35 s Q
534m
35 3.16
397m
30 4.24 R
5.71 295m
ct
25 Z , Msd.
7.67 220m Z , Calc.
20 10.3 163m
13.9
15 18.7 122m
10 33.7 25.1 95.4m
5 82 45.4
67.2m
100k
0
2.86k 50m
–5
–10
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Z ' (Ω cm2)
Fig. 3: Equivalent circuit used to fit the experimental EIS data for the corrosion of weld aged maraging steel specimen in
hydrochloric acid at 45C
The data in Table 3 also show that the value of Cdl Effect of temperature
decreases with the increase in the concentration of
ATPI. This decrease in Cdl could be due to the The study on the effect of temperature on the
decrease in the local dielectric constant and/or an corrosion rate and inhibition efficiency facilitates the
increase in the thickness of the electrical double layer calculation of kinetic and thermodynamic parameters
as a result of the adsorption of ATPI molecules at the for the inhibition and the adsorption processes. These
electrochemical interface and thereby thickening of the parameters are useful in interpreting the type of
electrical double layer. adsorption by the inhibitor. The results in Tables 2
The results obtained can be interpreted in terms of and 3 show that corrosion rate increases and the
the equivalent circuit of the electrical double layer inhibition efficiency of ATPI decreases with the
shown in Fig. 3 which has also been used previously increase in temperature. The decrease in inhibition
to model the iron/acid interface.40 The circuit fitment efficiency with the increase in temperature indicates
was done by ZSimpWin software version 3.21. The desorption of the inhibitor molecules from the metal
equivalent circuit (Fig. 3) is a parallel combination of surface on increasing the temperature.25 This fact is
the charge-transfer resistance (Rct) and the constant also suggestive of physisorption of the inhibitor mol-
phase element (CPE, Q), both in series with the ecules on the metal surface.
solution resistance (Rs). The CPE element is used The value of activation energy (Ea) was calculated
to explain the depression of the capacitance semi- using the Arrhenius law equation,42
circle. The CPE impedance (ZCPE) is given by the E
expression, lnðt acorrÞ ¼ B  ð7Þ
RT
1 1
ZCPE ¼  ð Þn ð5ÞQ jx where B is a constant which depends on the metal type,
R is the universal gas constant, and T is the absolute
where Q is the CPE coefficient, n is the CPE temperature. The plot of ln(tcorr) vs reciprocal of
exponent (phase shift), x is the angular frequency absolute temperature (1/T) gives a straight line with
(x = 2pf, where f is the AC frequency), and j is the slope = Ea/R, from which the activation energy
imaginary unit. When the value of n is 1, the CPE values for the corrosion process were calculated. The
behaves like an ideal double-layer capacitance (C ). Arrhenius plots for the corrosion of weld aged marag-dl
The correction of capacity to its real values is ing steel in the presence of different concentrations of
calculated from ATPI in 1.5 M hydrochloric acid are shown in Fig. 4.
The enthalpy and entropy of activation values for
C ¼ Qðx Þn1 ð6Þ the corrosion process (DH
# and DS#) were calculated
dl max from transition state equation8
   
where x is the frequency at which the imaginary ¼ RT DS
# DH#
max
 41 tcorr exp exp ð8Þpart of impedance ( Zi) has a maximum. Nh R R
488
–Z '' (Ω cm2)
J. Coat. Technol. Res., 9 (4) 483–493, 2012
3.2 Table 4: Activation parameters for the corrosion of
3.0
2.8 weld aged maraging steel in 1.5 M hydrochloric acid
2.6 containing different concentrations of inhibitor
2.4
Conc. of E DH# DS#2.2 a
2.0 inhibitor (mM) (kJ mol1) (kJ mol1) (J mol1 K1)
1.8
1.6
1.4 Blank 40.55 43.12 11.37
1.2 0.2 48.15 50.74 29.79
1.0 Blank 0.4 51.42 53.97 37.62
0.8 0.2 mM
0.6 0.4 mM 0.6 58.59 61.20 58.58
0.4 0.6 mM 0.8 70.61 73.22 93.08
0.2 0.8 mM 1.0 76.66 79.26 109.62
0.0 1.0 mM
–0.2
–0.4
0.00310 0.00315 0.00320 0.00325 0.00330
–1 on the electrode surface leads to the formation of a1/T (K )
physical barrier between the metal surface and the
Fig. 4: Arrhenius plots for the corrosion of weld aged corrosion medium, blocking the charge transfer, and
maraging steel in 1.5 M hydrochloric acid containing thereby reducing the metal reactivity in the electro-
different concentrations of inhibitor chemical reactions of corrosion. The decrease in the
inhibition efficiency of ATPI with the increase in
temperature can be considered to be because of the
decrease in the extent of adsorption of the inhibitor on
9.0
8.8 the metal surface with the increase in temperature, and
8.6 corresponding increase in corrosion rate as a greater
8.4 area of the metal surface is exposed to the corrosion
8.2
8.0 medium. The observations also support the view that
7.8 the inhibitor is adsorbed on the metal surface through
7.6 physisorption.25
7.4
7.2 The entropy of activation values in the absence and
7.0 the presence of ATPI are large and negative; this
6.8 Blank indicates that the activated complex formation in the
6.6 0.2 mM
6.4 0.4 mM rate-determining step represents an association rather
6.2 0.6 mM than dissociation, decreasing the randomness on going
6.0 0.8 mM
1.0 mM from the reactants to the activated complex.
43 It is also
5.8
5.6 seen from the table that entropy of activation
5.4 decreases with the increase in the concentration of
0.00310 0.00315 0.00320 0.00325 0.00330 ATPI. This could be the result of the adsorption of the
1/T (K–1) inhibitor molecules, which could be regarded as a
quasi-substitution process between the inhibitor com-
Fig. 5: Plots of ln(tcorr/T) vs 1/T for the corrosion of weld pound in the aqueous phase and water molecules at
aged maraging steel in 1.5 M hydrochloric acid containing electrode surface. In the present case, the more orderly
different concentrations of inhibitor arrangement of the inhibitor molecules on the metal
surface overweighs the solvent entropy resulting from
desorption of water molecules from the metal surface.
where h is Plank’s constant, and N is Avagadro’s
number. A plot of ln(tcorr/T) vs 1/T gives a straight line
with slope = DHa/T and intercept = ln(R/Nh) + DSa/R. Adsorption behavior
The plots of ln(tcorr/T) vs 1/T for the corrosion of weld
aged samples of maraging steel in the presence of The adsorption of ATPI molecules on the metal
different concentrations of ATPI is shown in Fig. 5. surface can occur either through donor–acceptor
The calculated values of activation parameters are interaction between the unshared electron pairs and/
recorded in Table 4. The results show that the value of or p electrons of inhibitor molecule and the vacant
Ea increases with the increase in the concentration of d-orbitals of the metal surface atoms, or through
ATPI indicating that the energy barrier for the electrostatic interaction of the inhibitor molecules with
corrosion reaction increases. It is also indicated that already adsorbed chloride ions.34 The adsorption of an
the whole process is controlled by surface reaction,3 organic adsorbate on a metal solution interface can be
since the activation energies of the corrosion process represented as a substitution process between the
are above 20 kJ mol1. The adsorption of the inhibitor organic molecules in the aqueous solution (Org(sol))
489
In(υ /T ) (mm y–1 –1corr  k ) In(υcorr) (mm y
–1)
J. Coat. Technol. Res., 9 (4) 483–493, 2012
and the water molecules on the metallic surface 0.0014
(H 252O(ads)) as represented below: 0.0013 30 °C
35°C
InhðsolÞ þ vH O 0.00122 ðadsÞ $ OrgðadsÞ þ vH2OðsolÞ ð9Þ 40°C
0.0011 45°C
50°C
where Org(sol) and Org(ads) are the organic molecules
0.0010
in the aqueous solution and adsorbed on the metallic 0.0009
surface, respectively; H2O(ads) and H2O(sol) are the 0.0008
water molecules on the metallic surface and in the
0.0007
solution, respectively; and v represents the number of
water molecules replaced by one molecule of organic 0.0006
adsorbate. Thus, in aqueous acidic solution, ATPI 0.0005
exists partly in the form of protonated species and 0.0004
partly as neutral molecules.44
0.0002 0.0004 0.0006 0.0008 0.0010
The information on the interaction between the
C  (M)
inhibitor molecules and the metal surface can be inh
provided by adsorption isotherm. The degree of
Fig. 6: Langmuir adsorption isotherms for the adsorption
surface coverage (h) for different concentrations of of ATPI on weld aged maraging steel in 1.5 M hydrochloric
inhibitor was evaluated from potentiodynamic polari- acid at different temperatures
zation measurements. The data were applied to various
isotherms including Langmuir, Temkin, Frumkin and
Flory–Huggins isotherms. It was found that the data fit Table 5: Thermodynamic parameters for the adsorption
best with the Langmuir adsorption isotherm according of ATPI on weld aged maraging steel surface in 1.5 M
to which, the surface coverage (h) is related to the hydrochloric acid at different temperatures
inhibitor concentration C 25inh by the following relation :
Temperature DG ads DHads DSads
Cinh 1 (C)
1
¼ ð Þ (kJ mol ) (kJ mol
1) (J mol1 K1)
Cinh þ 10h K
30 31.81
where K is the adsorption/desorption equilibrium 35 31.31
constant, Cinh is the corrosion inhibitor concentration 40 31.17 49 57
in the solution, and h is the surface coverage, which is 45 30.80
calculated using equation (11) 50 30.63
¼ g ð%Þh ð11Þ  
¼ 1 DG

100 K exp ads ð12Þ
55:5 RT
where g (%) is the percentage inhibition efficiency as where the value 55.5 is the concentration of water in
calculated using equation (1). The plot of Cinh/h vs Cinh solution in mol dm3, R is the universal gas constant,
gives a straight line with an intercept of 1/K. The and T is absolute temperature. Standard enthalpy of
Langmuir adsorption isotherms for the adsorption of adsorption (DHads) and standard entropies of adsorptionATPI on the maraging steel surface are shown in (DSads) were obtained from the plot of (DG
 ) vs T
Fig. 6. The plots are linear, with correlation coeffi- adsaccording to the thermodynamic equation (13).
cients ranging from 0.9832 to 0.9929, with an average
value of 0.9894. The slopes of the isotherms show   
deviation from the value of unity as would be expected DGads ¼ DHads  TDSads ð13Þ
for the ideal Langmuir adsorption isotherm equation.
This deviation from unity may be due to the interaction The thermodynamic data obtained are tabulated in
among the adsorbed species on the metal surface. The Table 5.
Langmuir isotherm equation is based on the assump- The negative values of DGads indicate the sponta-
tion that adsorbed molecules do not interact with one neity of the adsorption process and the stability of the
another, but this is not true in the case of organic adsorbed layer on the metal surface. Generally the
molecules having polar atoms or groups which are values of DGads less than 20 kJ mol
1 are consistent
adsorbed on the cathodic and anodic sites of the metal with physisorption, while those greater than 40 kJ
surface. Such adsorbed species may interact by mutual mol1 correspond to chemisorptions.45 The calculated
repulsion or attraction. values of DGads obtained in this study range between
The values of standard free energy DGads of 31.81 and 30.64 kJ mol1, indicating both physical
adsorption are related to K by the relation shown in and chemical adsorption behaviors of ATPI on the
equation (12).38 metal surface.46
490
Cinh/θ (M)
J. Coat. Technol. Res., 9 (4) 483–493, 2012
The negative sign of DHads in HCl solution indicates In a cathodic reaction, hydrogen evolution takes
that the adsorption of inhibitor molecules is an place as follows:
exothermic process. Generally, an exothermic adsorp-
þ þ
tion process signifies either physisorption or chemi- Fe þ H ! ðFeH Þads ð22Þ
sorptions while the endothermic process is attributable þ 
unequivocally to chemisorption. Typically, the stan- ðFeH Þadsþe ! ðFeHÞads ð23Þ
dard enthalpy of the physisorption process is less
1 ðFeHÞ þHþnegative than 41.86 kJ mol , while that of the chem- ads þ e ! Fe þ H2 ð24Þ
isorptions process approaches 100 kJ mol1. In the
present study, the value of DH 1ads is 49 kJ mol In a highly acidic solution, as in the present case, the
which shows that the adsorption of ATPI on weld aged ATPI molecule can undergo protonation at its amino
maraging steel involves both physisorption and chemi- group and can exist as a protonated positive species.
sorption phenomena. From the thermodynamic data The protonated species gets adsorbed on the cathodic
and also from the variation of corrosion inhibition sites of the metal surface through electrostatic inter-
efficiency with temperature it can be concluded that action, thereby decreasing the rate of the cathodic
the ATPI gets adsorbed on the metal surface through reaction. The presence of anions in the solution and
both physisorption and chemisorption, but physisorp- their adsorption on the metal surface play an important
tion is more predominant. The predominant physical role in the mechanism of inhibition exhibited by the
adsorption is also inconsistent with the decrease in organic compounds.49 In a highly acidic medium like
inhibition efficiency and with the increase in temper-
 the one in the present investigation, the metal surfaceature. The DSads value is large and negative, indicating is positively charged. This would cause the negatively
that an ordering takes place when the inhibitor gets charged chloride ions to become adsorbed on the metal
adsorbed on the metal alloy surface.17 surface, making the metal surface negatively charged.
The positively charged protonated ATPI molecules
can interact electrostatically with the negatively
Inhibition mechanism charged chloride adsorbed metal surface, resulting in
physisorption. The negative charge centers of the
Inhibitive action of ATPI on the corrosion of weld ATPI molecules containing a lone pair of electrons
aged maraging steel in acidic solutions can be and/or p electrons can electrostatically interact with
explained on the basis of adsorption. The adsorption the anodic sites on the metal surface and get adsorbed.
of ATPI molecules on the metal surface can be The neutral inhibitor molecules may occupy the vacant
attributed to the presence of electronegative elements adsorption sites on the metal surface through the
like nitrogen and sulfur and also to the presence of p chemisorption mode involving the displacement of
electron cloud in the benzene ring of the molecule. TPI water molecules from the metal surface and sharing of
inhibits the corrosion by controlling both the anodic electrons by the hetero atoms like nitrogen and/or
and cathodic reactions. sulphur with iron. Chemisorption is also possible by the
In HCl solution, the following mechanism is pro- donor–acceptor interactions between p electrons of the
posed for the corrosion of iron and steel.47 According aromatic ring and the vacant d orbitals of iron,
to this mechanism anodic dissolution of iron takes providing another mode of protection.50 The presence
place as follows: of ATPI in the protonated form and the presence of
  negative charge centers on the molecule are alsoFe þ Cl ! ðFeCl Þads ð14Þ responsible for the mutual interaction of inhibitor
ð Þ ! ð Þ þ  ð Þ molecules on the alloy surface. This is reflected in theFeCl ads FeCl ads e 15 deviation of slopes of Langmuir adsorption isotherms
 
ð Þ ! þ þ  ð Þ as discussed in the previous section.FeCl ads FeCl e 16ads
 
FeClþ ! Fe2þ þ Cl ð17Þ
ads Scanning electron microscopy
The nickel present in the maraging steel also The effect of corrosion on the surface morphology
undergoes anodic dissolution as follows48: of the maraging steel sample was assessed by recording
the SEM images of the alloy samples subjected to
Ni þ Cl ! ðNiClÞads ð18Þ corrosion in 1.5 M hydrochloric acid for 3 h in the
presence and the absence of ATPI. Figure 7 shows the
ðNiClÞads! ðNiClÞadsþe ð19Þ SEM images of the maraging steel sample. Figure 7a
  shows the facets due to the attack of hydrochloric acid
ðNiClÞads! NiCl
þ þe ð20Þ
ads on the metal surface with cracks and rough surfaces.
  Figure 7b shows the SEM image of the sample after
NiClþ ! Ni2þ þ Cl ð21Þ immersion in 1.5 M hydrochloric acid in the presence
ads
491
J. Coat. Technol. Res., 9 (4) 483–493, 2012
Fig. 7: SEM images of the weld aged maraging steel after immersion in 1.5 M hydrochloric acid (a) in the absence and (b) in
the presence of ATPI
of ATPI. It can be seen that the alloy surface is smooth C-Steel in 1 M H2SO4.’’ J. Appl. Electrochem., 39 391–402
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