Adsorption and inhibitor action of 4-(N,N-dimethylamino) benzaldehyde thiosemicarbazone on 6061 Al/SiC composite and its base alloy in sulfuric acid medium

Article abstract of DOI:10.1080/15533174.2010.538023

The inhibitive action of 4-(N,N-dimethylamino)benzaldehyde thiosemicarbazone (DMABT) on the corrosion behavior of 6061 Al -15 vol. pct. SiC(p) composite and its base alloy was studied at different temperatures in sulfuric acid medium containing varying co

Full text of DOI:10.1080/15533174.2010.538023

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:127–140, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2010.538023
Adsorption and Inhibitor Action of 4-(N,N-Dimethylamino)
Benzaldehyde Thiosemicarbazone on 6061 Al/SiC
Composite and its Base Alloy in Sulfuric Acid Medium
1
2
1
Geetha Mable Pinto, Jagannath Nayak, and A. Nityananda Shetty
1
Department of Chemistry, National Institute of Technology Karnataka, Srinivasnagar, India
Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka,
2
Srinivasnagar, India
media, can be combated using inhibitors.[
11–12]
A wide variety
The inhibitive action of 4-(N,N-dimethylamino)benzaldehyde of compounds have been used as inhibitors for aluminum and
thiosemicarbazone (DMABT) on the corrosion behavior of 6061
its alloys in acid media. These are mainly organic compounds
Al -15 vol. pct. SiC(p) composite and its base alloy was studied at
different temperatures in sulfuric acid medium containing varying
concentrations of it, using Tafel extrapolation technique and AC
impedance spectroscopy (EIS). Results showed that DMABT was
[13−16]
and critical use of these com-
containing N, S, or O atoms
pounds in industries has also been reviewed.[
their usefulness in several stages of process industries, organic
17−19]
Owing to
an effective inhibitor, showing inhibition efficiency of 70% in 0.5 M compounds containing both N and S atoms function as better
sulfuric acid. The adsorption of DMABT on both the composite
adsorption inhibitors because of their lone pair of electrons and
and base alloy was found to be through physisorption obeying
Langmuir adsorption isotherm. The thermodynamic parameters
such as free energy of adsorption and activation parameters were
calculated.
[20–21]
polar nature of the molecules.
Thiosemicarbazone and
their derivatives have continued to be the subject of extensive
investigation in chemistry and biology owing to their broad spec-
trum of anti tumor activity,[ and in many other applications
including corrosion and inhibition of metals.[
22]
23]
Keywords acid corrosion, aluminum, EIS, metal matrix composites,
polarization
The present work aims at investigating the inhibitive
action of 4-(N,N-dimethylamino)benzaldehyde thiosemicar-
bazone(DMABT) on the corrosion behavior of 6061 Al-15 vol.
pct. SiC(p) composite and its base alloy in sulfuric acid solutions
INTRODUCTION
Aluminium matrix composites (AMCs) have received con- at different concentration levels of the acids as well as at dif-
siderable attention for military, automobile, and aerospace ap- ferent temperatures. Sulfuric acid solution is used for pickling,
plications because of their low density, high strength, and high chemical, and electrochemical etching and in many chemical
stiffness.[
(
1−6]
Further, the addition of ceramic reinforcements process industries where aluminum composites are used. The
SiC) has raised the performance limits of the Al (6061) alloys.[
7]
study of corrosion behavior and control of corrosion of the base
It is known that aluminium matrix composites exhibited bet- alloy and the composite in sulfuric acid solution is particularly
ter resistance to mechanical wear than their base alloys and useful in the field where metal matrix composite is exposed to
hence they have specific strength for numerous weight sensi- the corrosion environment containing sulfuric acid. DMABT
tive applications.[
8–9]
One of the main disadvantages in the use with the presence of electronegative elements like nitrogen and
of metal matrix composite is the influence of reinforcement on sulfur and also the π electrons on the benzene ring is expected
corrosion rate. The addition of a reinforcing phase could lead to be capable of strongly getting adsorbed on the metal surfaces.
to discontinuities in the film, thereby increasing the number of The bulkiness of the molecule may provide a good surface cov-
sites where corrosion can be initiated and thereby making the erage. Both the above factors would result in a better corrosion
[
8,10]
composites more susceptible for corrosion.
However the inhibition by the compound.
high corrosion rates of these composites, particularly in acid
EXPERIMENTAL
Received 14 May 2010; accepted 18 August 2010.
Address correspondence to A. Nityananda Shetty, Department of
Chemistry, National Institute of Technology Karnataka, Srinivasnagar –
Material
The experiments were performed with specimens of 6061 Al-
15 vol. pct. SiC(p) composite and its base alloy in extruded rod
5
75 025, India. E-mail: nityashreya@gmail.com
127

128
G. M. PINTO ET AL.
TABLE 1
at the OCP and impedance data were analyzed using Nyquist
The composition of the base metal Al 6061 alloy
plots. The charge transfer resistance, R , was extracted from the
t
diameter of the semicircle in Nyquist plot.
In all the above measurements, at least three similar results
were considered and their average values are reported.
Element
Cu
Si
Mg
1.0
Cr
Al
Composition (Wt.%)
0.25
0.6
0.25
Bal
Synthesis of 4-(N,N-Dimethylamino)Benzaldehyde
Thiosemicarbazone
form (extrusion ratio 30:1). The composition of the base metal
061 Al alloy is given in Table 1. Cylindrical test coupons were
6
4-(N,N-Dimethylamino)benzaldehyde thiosemicarbazone
cut from the rods and sealed with epoxy resin in such a way
that the areas of the composite and the base alloy exposed to
was synthesized and recrystallized as per the reported
procedure.[ A mixture containing equimolar ethanolic 4- (N,
N-dimethylamino)benzaldehyde and thiosemicarbazone were
taken into a round bottom flask. The reaction mixture was
refluxed on a hot water bath for about 60 minutes. The light
yellow colored product obtained was separated by filtration
and dried. The product was recrystallized from ethanol. The
recrystallized product was checked by IR spectra, elemental
analysis, and melting point.
24]
2
2
the medium were 0.95 cm and 0.785 cm , respectively. These
coupons were polished as per standard metallographic practice,
belt grinding followed by polishing on emery papers, and finally
on polishing wheel using levigated alumina to obtain mirror
finish. It was then degreased with acetone, washed with double
distilled water, and dried before immersing in the corrosion
medium.
Medium
NH2
Standard solution of sulfuric acid was prepared from analyt-
ical grade (Nice) acids. The three solutions used for the study
were with the following concentrations of sulfuric acid: 0.5 M,
N
CHO
NH
S
0
.25 M and 0.05 M. Experiments were carried out using cal-
◦
◦
◦
◦
NH2CSNH2NH2
1hr
ibrated thermostat at temperatures 30 C, 35 C, 40 C, 45 C,
◦
◦
5
0 C (± 0.5 C). The inhibitive effect was studied by introduc-
∆
(1)
ing 50 ppm to 400 ppm (0.2 mM to 1.6 mM) of DMABT into
sulfuric acid solutions.
N(CH3)
2
N(CH3)2
Electrochemical Measurements
Tafel polarization studies
Electrochemical measurements were carried out by using an
electrochemical work station, Auto Lab 30, and GPES soft-
ware. Tafel plot measurements were carried out using conven-
tional three electrode Pyrex glass cell with platinum counter
RESULTS AND DISCUSSION
Potentiodynamic Polarization (PDP) Measurements
The polarization studies of aluminum specimens were car-
electrode and saturated calomel electrode (SCE) as reference ried out in 0.05 M–0.5 M sulfuric acid solutions separately in
electrode. All the values of potential are therefore referred to absence and presence of different concentrations of DMABT.
the SCE. Finely polished composite and base alloy specimens Figures 1 and 2 represents potentiodynamic polarization curve
were exposed to corrosion medium of different concentrations of 6061 Al–SiC composite and its base alloy in 0.5 M sulfuric
◦
◦
◦
of sulfuric acid at different temperatures (30 C to 50 C) and acid at 30 C solutions in the absence and presence of DMABT.
allowed to establish a steady state open circuit potential. The Similar results were obtained in the same concentrations of sul-
potentiodynamic current-potential curves were recorded by po- furic acid at four other temperatures and also in the other two
larizing the specimen to -250 mV cathodically and +250 mV concentrations of the sulfuric acid at the five temperatures stud-
anodically with respect to open circuit potential (OCP) at scan ied.
−1
rate of 1 mV s
.
The electrochemical parameters (Ecorr, icorr, ba, and bc) asso-
ciated with the polarization measurements at different DMABT
Electrochemical impedance spectroscopy studies (EIS)
concentrations for the composite and the base alloy in three dif-
◦
The corrosion behaviors of the specimens of the composite ferent concentrations of the sulfuric acid solution at 30 C are
and the base alloy were also obtained from EIS technique using listed in Table 2. The surface coverage θ of the inhibitor on the
electrochemical work station, Auto Lab 30, and FRA software. metal surface is calculated by the expression
In EIS technique a small amplitude ac signal of 10 mV and
frequency spectrum from 100 kHz to 0.01 Hz was impressed
θ = icorr(uninh) − icorr(inh) / icorr(uninh)
[2]

129

130
G. M. PINTO ET AL.
The anodic and cathodic Tafel slopes remain almost un-
changed for the uninhibited and inhibited systems. This indi-
cates that the inhibitive action of DMABT occurs by simple
blocking of the available surface area for corrosion attack. In
other words, the inhibitor decreased the surface area available
for hydrogen evolution without affecting the reaction mecha-
nism and only causes inactivation of part of the surface.[
28–29]
This fact is an important observation, since the presence of SiC
particles in the composite initiate cathodic sites, which is re-
sponsible for the higher corrosion of the composite than that
of the base alloy.[
8,10]
Therefore, blocking of these sites via
DMABT adsorption would result in decreasing the corrosion
rate of composite to a greater extent as compared to the base al-
loy. The comparison of inhibition efficiency of DMABT for the
composite and base alloy at different concentrations and differ-
ent temperatures shows that the inhibition effect is more on the
composite than on the base alloy. The increase in the efficiency
of the inhibitor in the case of composite may be due to its het-
erogenic nature, where the incorporation of silicon carbide acts
as the potential active site for the adsorption of the inhibitor.
The increase in inhibition efficiency with increasing inhibitor
◦
FIG. 1. Tafel curves of 6061 Al- SiC composite in 0.5 M sulfuric acid at 30 C
in the presence of different concentrations of DMABT.
The inhibition efficiency (% IE) is calculated by the relation concentration indicates that adsorption of inhibitor molecules
increases on the metal surface. Thus, the surface coverage in-
IE (%) = θ × 100
[3] creases as the inhibitor concentration increases.
The data in the table clearly show that the addition of
DMABT decreases the corrosion rates of both the composite Electrochemical Impedance Spectroscopy (EIS)
and the base alloy. Inhibition efficiency increases with increas-
Corrosion behavior of 6061 Al- SiC composite and its base
ing DMABT concentration. For the inhibited systems, the values alloy in 0.05 M–0.5 M sulfuric acid solutions, in the absence
of Ecorr are shifted to more negative direction and the cathodic and presence of different concentrations of DMABT, was also
branches of the curves are displaced to the left. These are typi- studied by electrochemical impedance spectroscopy. Figures 3
cal features of cathodic inhibitors, in agreement with the results and 4 represent Nyquist plots of 6061 Al – SiC compos-
obtained for other aluminum alloys.[
25−27]
ite and its base alloy, respectively, at various concentrations
of DMABT in 0.5 M sulfuric acid at 30 C. Similar results
◦
were obtained in two other concentrations of sulfuric acid,
and also at other temperatures studied. The electrochemical pa-
rameters obtained from the EIS measurements are tabulated in
Table 3.
As can be seen from Figures 3 and 4, the impedance diagrams
show semicircles, indicating that the corrosion process is mainly
charge transfer controlled. In the Nyquist plots for the 6061Al-
SiC composite, the impedance spectra consists of a large capac-
itive loop at high frequencies (HF) and an inductive loop at low
frequencies (LF). Similar impedance plots have been reported
in literature for the corrosion of pure aluminium and aluminium
[
30−32]
alloys in various electrolytes such as sodium sulphate,
sulfuric acid,[
31−32]
acetic acid,
[31],
sodium chloride,
21,29,35−39]
The general shape of the curve is
[33−34]
and
hydrochloric acid.[
similar for all individual samples of the base alloy, with large
capacitive loop at high frequencies (HF) and an inductive loop
at intermediate frequencies (IF), followed by a second capac-
itive loop at low frequency (LF) values. Similar results have
been reported in literature for the corrosion of pure aluminum
◦
FIG. 2. Tafel curves of 6061 base alloy in 0.5 M sulfuric acid at 30 C in the
presence of different concentrations of DMABT.
in acidic and neutral solutions.[
30−31]

CORROSION INHIBITION OF 6061 Al/SiC COMPOSITE
131
is in accordance with the observations obtained from potentio-
dynamic measurements.
However, in the case of Al/SiC composites, the obtained
semicircles in absence or in presence of inhibitor are depressed.
Deviation of this kind are referred to as frequency dispersion,
have been attributed to inhomogeneties of solid surfaces, as the
aluminum composite is reinforced with SiC particles. Mansfeld
et al.[
40−41]
have suggested an exponent n in the impedance
function as a deviation parameter from the ideal behavior. By
this suggestion, the capacitor in the equivalent circuit can be
replaced by a so-called constant phase element (CPE) that is a
frequency-dependent element and related to surface roughness.
The impedance function of a CPE has the following equation:[
31]
1
ZCPE =
[4]
(
Yo/ω)n
where the amplitude Y0 and n are frequency independent, and
ꢀꢀ
ω is the angular frequency for which –Z reaches its maximum
values, n is dependent on the surface morphology, with values,
−
1 ≤ n ≤1. Y0 and n can be calculated by the equations proved
FIG. 3. Nyquist plots of 6061 Al-SiC composite in 0.5 M sulfuric acid at
[39,40]
◦
by Mansfeld et al.
In the case of base alloy, due to the
3
0 C in the presence of different concentrations of DMABT.
homogenous surface, frequency dispersion is very less. There-
fore the obtained semicircles in the impedance spectra are not
depressed.
The high frequency capacitive loop could be assigned to the
charge transfer of the corrosion process and to the formation of
oxide layer.[
40−41].
The oxide film is considered to be a parallel
circuit of a resistor due to the ionic conduction in the oxide film
and a capacitor due to its dielectric properties. According to
Brett,[
36,38]
the capacitive loop is corresponding to the interfa-
cial reactions, particularly the reaction of aluminum oxidation
at the metal/oxide/electrolyte interface. The process includes
+
the formation of Al ions at the metal/oxide interface, and their
migration through the oxide/solution interface where they are
3+
−
2−
oxidized to Al . At the oxide/solution interface, OH or O
ions are also formed. The fact that all the three processes are rep-
resented by only one loop could be attributed either to the over-
lapping of the loops of processes, or to the assumption that one
process only dominates, excluding the other processes.[34−35]
The other explanation offered to the high frequency capacitive
loop is the oxide film itself.
The origin of the inductive loop has often been attributed to
surface or bulk relaxation of species in the oxide layer.[36] The
LF inductive loop may be related to the relaxation process ob-
tained by adsorption and incorporation of sulphate ions, oxide
ions, and charged intermediates on and into the oxide film.[
The second capacitive loop observed at LF values could be
33]
[32]
assigned to the metal dissolution.
The measured values of
polarization resistance (Rp) increase with the increasing con-
centration of DMABT in the solution, indicating the decrease in
the corrosion rate for the base metal with increase in DMABT FIG. 4. Nyquist plots of 6061 base alloy in 0.5 M sulfuric acid at 30 C in the
◦
presence of different concentrations of DEABT.
concentration up to critical concentration of the inhibitor. This

132
G. M. PINTO ET AL.
TABLE 3
Electrochemical parameters obtained from EIS measurements of 6061 Al-SiC composite and its base alloy at various
◦
concentrations of DMABT at 30 C
Composite
Base alloy
Conc. of
Conc. of
Conc. of
2
−2
2
−2
H2SO4 (M) inhibitor (ppm) Rt (ꢀ cm ) CPE (µF cm ) IE (%) inhibitor (ppm) Rt (ꢀ cm ) CPE (µF cm ) IE (%)
0
0
0
.5
0
0
00
00
00
0
93.9
125.4
148.5
221.0
340.8
213.0
279.3
312.9
392.7
617.8
401.0
499.4
612.9
884.5
222
199
168
140
120
179
158
132
111
99
0
25
50
100
200
0
25
50
100
360.1
451.0
523.7
671.5
799.2
456.7
551.1
682.9
839.4
27.1
26.1
26.1
24.7
22.3
26.4
24.1
23.7
21.4
5
25
45
58
72
20
31
46
55
1
2
4
.25
.05
2
5
5
0
23
32
46
67
17
33
46
1
2
00
00
0
101
79
64
0
5
10
25
720.0
862.3
979.9
16.7
15.0
13.5
12.8
10
25
50
20
35
55
17
27
46
58
1322.7
An equivalent circuit of five elements depicted in Figure 5a later is in series with the inductor L. he polarization resistance
was used to simulate the measured impedance data, as shown in Rp can be calculated from:[
5]
Figure 5b. In this equivalent circuit Rs is the solution resistance
and Rt is the charge transfer resistance. RLand L represent the
inductive elements. This also consists of constant phase element,
CPE (Q) in parallel to the parallel resistors Rt and RL, and the
R × R
L
t
Rp =
[5]
RL + Rt
FIG. 5. The equivalent circuit model used to fit the experimental data of the composite.

CORROSION INHIBITION OF 6061 Al/SiC COMPOSITE
133
FIG. 6. The equivalent circuit model used to fit the experimental data of the base alloy.
An equivalent circuit of nine elements depicted in Figure 6a the composite and its base alloy. It was also observed that the
was used to simulate the measured impedance data of the base value of constant phase element, Q, decreases, while the values
alloy, as shown in Figure 6b. In this equivalent circuit Rs is of Rt increase with increasing concentration of DMABT, indi-
the solution resistance and Rt is the charge transfer resistance. cating that the inhibition efficiency increases with the increase
RLand L represent the inductive elements. This also consists in concentration of DMABT. The double layer between the
of constant phase element; CPE (Q) in parallel to the series charged metal surface and the solution is considered as an elec-
capacitors C1, C2and series resistors R1, R2, RL and Rt. RL is trical capacitor. The adsorption of DMABT on the aluminum
parallel with the inductor L. The polarization resistance Rp and surface decreases its electrical capacity because they displace
double layer capacitance Cdl can be calculated from equations the water molecules and other ions originally adsorbed on the
(6) and (7):
surface. The decrease in this capacity with increase in DMABT
Rp = RL + Rt + R1 + R2
Cdl = C1 + C2
[6]
[7]
Since Rp is inversely proportional to the corrosion current
and it can be used to calculate the inhibition efficiency from the
relation,
ꢀ
ꢁ
ꢀ
R − Rp
p
IE % =
× 100
[8]
ꢀ
R
p
ꢀ
where R and Rp are the polarization resistances in the presence
p
and absence of inhibitors.
The Bode plots for 6061 Al/ SiC composite and its base alloy
with and without inhibitor, obtained at OCP, are presented in Fig-
ures 7 and 8. It is apparent that, in the case composite, the phase
angle maxima are quite broad after addition of DMABT where
as there is no significant change was observed for the base alloy.
It is seen from Table 3 that Rs (solution resistance) remains
◦
FIG. 7. Bode plots of 6061 composite in 0.5 M sulfuric acid at 30 C in the
almost constant, with and without addition of DMABT for both presence of different concentrations of DMABT.

134
G. M. PINTO ET AL.
for both the composite and the base alloy in 0.5 M sulfuric acid
at different temperatures. The results show that the inhibition
efficiencies determined by the two techniques are in acceptable
level of good agreement. Similar level of agreement was ob-
served at other two concentrations of the acid mixtures also.
Though the inhibition efficiencies obtained at lower tempera-
◦
◦
◦
tures (30 C, 35 C, 40 C) are quite high, it is bit lower at higher
◦
◦
temperatures (45 C and 50 C). Tables 2, 3 and 4 also reveal
that the optimum concentration of DMABT required for max-
imum efficiency is higher for the composite than for the base
alloy. Above the optimum concentration of DMABT given in
the tables, there was no appreciable increase in the inhibition
efficiencies. The inhibition efficiencies are as high as 71% for
the composite and 52% for the base alloy in 0.5 M sulfuric acid
solution, but the values are 52% and 43% in 0.05 M sulfuric
acid solution.
◦
FIG. 8. Bode plots of 6061 base alloy in 0.5 M sulfuric acid at 30 C in the
presence of different concentrations of DMABT.
Effect of Temperature and Activation Parameters
of Inhibition Process
The results obtained indicate that the rates of aluminum cor-
rosion in absence and presence of DMABT increased with tem-
perature while the inhibition efficiency decreased. This may be
attributed to the higher dissolution rates of aluminum at elevated
temperature and a possible desorption of adsorbed inhibitor due
to increased solution agitation resulting from higher rates of hy-
drogen gas evolution, which may also reduce the ability of the
inhibitor to be adsorbed on the metal surface. Such a behavior,
observed in both samples, suggests physical adsorption of the
concentrations may be attributed to the formation of a protective
layer on the electrode surface. The thickness of this protective
layer increases with increase in inhibitor concentration up to
their critical concentration and then decreases. The obtained
CPE (Q) values decrease noticeably with increase in the con-
centration of DMABT. In the case of composites, adsorption of
negatively charged heteroatoms and π electrons of the benzene
ring of DMABT at the Al/SiC interface occurs to a better extent
than the base alloy (Q is inversely proportional to the thick-
ness of the surface film or protective layer). In the case of base
alloys due to the homogenous surface, frequency dispersion is
very less. Therefore the obtained semicircles in the impedance
spectra are not depressed. Significant change in the CPE val-
ues of the base alloy is not observed after the addition of an
inhibitor.
[42−44]
DMABT on the corroding aluminum surface.
Plot of ln(corrosion rate) vs. 1/T for 6061 Al composite in
0.5 M sulfuric acid in absence and presence of various concen-
trations of DMABT is shown in Figure 9. As shown from this
figure, straight lines were obtained according to Arrhenius –
type equation:
A comparison of the maximum attainable inhibiting efficien-
cies obtained using a.c and d.c methods are listed in Table 4
Ea
In (corrosion rate) = A −
[9]
RT
TABLE 4
Maximum inhibitor efficiency obtained in 0.5 M sulfuric acid solution at different temperatures
Composite Base alloy
Inhibition efficiency (%) Inhibition efficiency (%)
Temperature of
Optimum
inhibitor
Optimum
inhibitor
the medium
Tafel
EIS
Tafel
EIS
◦
(
C)
concentration (ppm)
method
method
concentration (ppm)
method
method
3
3
4
4
5
0
400
400
400
400
400
71
67
56
48
44
72
70
58
50
47
200
200
200
200
200
52
51
47
42
38
54
52
49
45
41
5
0
5
0

CORROSION INHIBITION OF 6061 Al/SiC COMPOSITE
135
’
#
where h is Planck’s constant, N is Avogadro s number, ꢁH
#
is the activation enthalpy, and ꢁS is the activation entropy.
The calculated values of apparent activation energy, Ea, activa-
#
#
tion enthalpies, ꢁH , and activation entropies, ꢁS are given
in Tables 5 and 6. These values indicate that the presence of
inhibitor increases the activation energy, Ea, and the activation
#
enthalpies, ꢁH . The increase in the activation energies with
increasing concentration of the inhibitor is attributed to an ap-
preciable increase in the adsorption process of the inhibitor on
the metal surface with increase in the concentration of inhibitor.
The addition of inhibitor modified the values of Ea; this
modification may be attributed to the change in the mechanism
of the corrosion process in the presence of adsorbed inhibitor
molecules.[ The lower value of the activation energy of the
process in the presence of inhibitors when compared to that in
its absence is attributed to its chemisorption, while the opposite
45]
is the case with physical adsorption.[
46−47]
In the case of both
samples of aluminum, the inhibition efficiency decreases with
increase in temperature, which indicates desorption of inhibitor
molecule.[
FIG. 9. Arrhenius plots for the composite in 0.5 M sulfuric acid at different
concentrations of DMABT.
48]
#
The values of ꢁS are higher for inhibited solutions than
where A is constant depends on metal type and electrolyte, Ea is that for the uninhibited solutions. This suggested that an in-
the apparent activation energy, R is the universal gas constant, crease in randomness occurred on going from reactants to the
and T is the absolute temperature. Similar plots are obtained for activated complex. This might be the results of the adsorption
the base alloy also.
of organic inhibitor molecules from the acidic solution, which
Plot of ln(corrosion rate /T) vs 1/T for both the samples of could be regarded as a quasi-substitution process between the
aluminum 6061 in 0.5 M sulfuric acid in absence and presence organic compound in the aqueous phase and water molecules at
of various concentrations of DMABT is shown in Figure 10. As electrode surface.[ In this situation, the adsorption of organic
shown from this figure, straight lines were obtained according inhibitor is accompanied by desorption of water molecules from
49]
to transition state equation:
the surface. Thus, the increase in entropy of activation can be
attributed to the increasing in solvent entropy.[
50]
RT ꢁS#
ꢁH#
Corrosion rate = Nhe R e RT
[10]
Adsorption Isotherm
In order to understand the mechanism of corrosion inhibition,
the adsorption behavior of the organic adsorbate on the alu-
minum surface must be known. The degree of surface coverage
(
θ) for different concentration of inhibitor was evaluated from
potentiodynamic polarization measurements. A correlation be-
tween θ and inhibitor concentration in the corrosive medium
can be represented by the Langmuir adsorption isotherm.
θ
=
Kads C
[11]
(
1 − θ)
where Kads is the equilibrium constant of the inhibitor adsorption
process and C is the inhibitor concentration.
A straight line was obtained on plotting C/θ against C, sug-
gesting that the adsorption of the compound on aluminum sur-
face follows Langmuir adsorption isotherm model. The slopes
2
are close to 1, and the strong correlation (R > 0.99) proved
that the adsorption of DMABT on the metal surface obeyed
to the Langmuir’s adsorption isotherm. The high values of K
for the studied inhibitor indicate strong adsorption of inhibitor
FIG. 10. ln(corrosion rate/T) vs. 1/T for composite in 0.5 M sulfuric acid at
different concentrations of DMABT.

136
G. M. PINTO ET AL.
TABLE 5
Activation parameters of the corrosion of 6061 Al-SiC composite in absence and presence of different concentrations of DMABT
Medium
0.5 M H2SO4
0.25 M H2SO4
0.05 M H2SO4
Inhibitor concentration (ppm)
Inhibitor concentration (ppm)
Inhibitor concentration (ppm)
Activation parameters
Ea ( kJ mol−1)
0
50 100 200 400
0
25
50
100
200
0
10
25
50
68.8 74.7 80.3 86.5 98.4 66.0 74.8 76.7 77.2 79.5 53.0 58.4 60.9 67.5
65.5 71.3 76.8 83.1 94.9 63.4 72.2 74.1 74.9 76.9 50.3 55.8 58.3 64.9
82.7 99.7 114.7 133.5 168.7 69.0 95.6 100.6 102.1 105.6 22.4 38.2 41.6 64.8
#
−1
ꢁ
H ( kJ mol
)
#
−1
−1
ꢁS (J K mol
)
on the alloy surface. Adsorption isotherms for different concen-
The enthalpy of adsorption was calculated using rearranged
trations of DMABT, on the surface of 6061 Al- SiC composite form of Gibbs – Helmholtz equation
in 0.5 M sulfuric acid at different temperatures are shown in
0
0
0
ads
Figure 11. Similar plots were also obtained for the base alloy.
ꢁG
= ꢁH − TꢁS
[13]
ads
ads
0
The free energy of adsorption, ꢁGads was calculated using
the relation,
The variation of ꢁG0 with T gives a straight line with an in-
ads
0
ꢂ
ꢃ
tercept that equals ꢁH (Figure 12). Figure shows the good
ads
5
5.5 θ
◦
0
ads
ꢁ
G = −RT ln
[12] dependence of ꢁG on T, indicating good correlation among
C(1 − θ)
thermodynamic parameters. The thermodynamic data obtained
for DMABT adsorbed on aluminum composite and base alloy
where C is the concentration of the inhibitor expressed in mole
0
ads
are listed in Table 7. The negative sign of ꢁH in sulfuric
dm . The calculated values of ꢁG0 for DMABT on the com-
−3
ads
acid solution indicates that the adsorption of inhibitor molecule
is an exothermic process. Generally, an exothermic adsorption
posite and the base alloy were in the range of −27.11 to −28.59
−1
−1
kJ mol and −27.00 to −28.83 kJ mol , respectively. The
process signifies either physisorption or chemisorptions, while
0
ads
negative values of ꢁG suggest the spontaneous adsorption
[52]
endothermic process is attributable to chemisorptions.
Typ-
of DMABT on the composite and base alloy surfaces. Since
ically, the enthalpy of physisorption process is lower than that
0
ads
−1
the values of ꢁG of −40 kJ mol is usually accepted as
−1
4
1.86 kJ mol while the enthalpy of chemisorptions process
threshold value between chemisorption and physisorption, the
−1 [53]
In the present study, the absolute
approaches 100 kJ mol
.
0
obtained values of the ꢁG may be indicative of both physical
and chemical process.
−1
−1
ads
value of enthalpy is −62.45 kJ mol and −50.02 kJ mol
[
51]
for the composite and base alloy, respectively, which is an
FIG. 11. Langmuir adsorption isotherm for the adsorption of DMABT on the
composite.
0
FIG. 12. Plot of ꢁG vs. T for the composite.

CORROSION INHIBITION OF 6061 Al/SiC COMPOSITE
137
TABLE 6
Activation parameters of the corrosion of 6061 Al base alloy in absence and presence of different concentrations of DMABT
Medium
0
.5 M H2SO4
0.25 M H2SO4
Inhibitor concentration
(ppm)
0.05 M H2SO4
Inhibitor concentration
(ppm)
Inhibitor concentration
(ppm)
Activation parameters
Ea ( kJ mol−1)
0
25
50
100
200
0
25
50
100
0
5
10
25
58.7
56.2
40.7
61.1
58.6
47.4
61.3
59.6
51.5
69.6
67.1
71.5
76.6
74.2
93.1
63.9
61.2
54.7
65.0
62.4
56.5
71.1
68.5
74.8
72.7
70.1
78.1
59.8
57.2
37.4
63.1
60.5
45.7
64.5
61.9
50.7
65.1
62.5
54.3
#
−1
)
ꢁ
H ( kJ mol
#
−1
−1
ꢁS (J K mol )
intermediate case. The ꢁS0 value is large and negative, in- mechanism, anodic dissolution of Al follows steps
ads
dicating that an ordering takes place when the inhibitor gets
adsorbed on the metal alloy surface.
−
ꢂ
−
(ads)
[54]
Al + Cl ꢃ Al Cl
[14]
[15]
−
−
+
2
−
Al Cl( + Cl → Al Cl + 3e
ads)
The cathodic hydrogen evolution is according to the follow-
ing steps
Mechanism of Inhibition
The corrosion inhibition property of DMABT through ad-
sorption on the surface of the composite or the base alloy can
be attributed to the presence of electronegative elements like
nitrogen and sulfur, and also to the presence of π electrons
on the benzene ring. The metal surface in contact with a so-
+
−
H + e → H(ads)
[16]
[17]
H(ads) + H(ads) → H2
lution is charged due to the electric field that emerges at the
In acidic solution, all the nitrogen atoms including secondary
interface. This can be determined, according to Antrapov,[ by amino group can be protonated easily because they are all planar
comparing the zero charge potential and the rest potential of the and having greater electron density. The protonated molecules
metal in the corresponding medium. The value of pHZch, which can get adsorbed on the cathodic sites of aluminum in competi-
is defined as the pH at a point of zero charge is equal to 9.1 tion with the hydrogen ions (equation 16). Co-ordinate covalent
for aluminum.[ So aluminium is positively charged in highly bond formation between electron pairs of unprotonated S atom
acidic medium, as the ones used in this investigation. There- and metal surface can take place. Further, DMABT molecules
fore, sulphate ions and DMABT can be adsorbed on aluminum are chemically adsorbed due to interaction of π–orbitals with
surface via their negative centres. Also, DMABT can be pro- metal surface following deprotonization step of the physically
tonated in the highly acidic solution used in the investigation. adsorbed protonated molecules.
55]
56]
The mechanism of adsorption of protonated DMABT can be
As observed from Tables 2 and 3, the corrosion rate as well
predicted on the basis of the mechanism proposed for the cor- as the inhibition efficiency increases with increase in sulfuric
[57]
rosion of aluminum in hydrochloric acid.
According to this acid concentration, and is maximum in 0.5 M solution. The
TABLE 7
Thermodynamic parameters for the adsorption of DEABT on Al composite and base alloy in a mixture of 0.5 M sulfuric acid at
different temperatures
Composite
Base alloy
0
ꢁH0
((kJ mol
0
ꢁG0
(kJ mol
ꢁH0
((kJ mol
0
ꢁ
G
ꢁS
(J mol
ꢁS
ads
ads
ads
K
ads
−1
ads
ads
Temperature (K) K M 1 (kJ mol
−
−1
)
−1
)
−1 −1
)
K M
−1
)
−1
)
−1 −1
(J mol K )
303
308
313
318
323
1519.7
1223.6
970.9
793.7
694.4
−28.59
1782.1
1563.0
1478.5
1399.0
1100.2
−28.83
−28.51
−28.45
−27.50
−27.00
−28.21
−27.95
−27.32
−27.11
−62.45
−118.8
−50.02
−95.3

138
G. M. PINTO ET AL.
observations can be explained by the above adsorption mecha-
nism. The increase in the extent of adsorption of the inhibitor
on the alloy surface and in turn the increase in the inhibitor
efficiency can be attributed to the following two facts:
•
The increase in the concentration of the acid increases
the extent of protonation of the inhibitor molecules,
thereby facilitating their adsorption on the cathodic
sites.
•
With the increase in the concentration of the acid, the
anion of the acid (SO2 ) adsorb physically on the pos-
−
4
itively charged metal surface, giving rise to a net nega-
[21]
tive charge on the metal surface. This further facili-
tates the adsorption of protonated inhibitor molecules.
0
−1
In the present case, the value of ꢁG is −32.57 kJ mol
ads
−1
to −33.72 kJ mol , hence, indicating that the adsorption of
DMABT on the surface of aluminum involves both physical and FIG. 14. SEM image of the surface of composite after immersion for 2h in
◦
0
.5 M H2SO4at 30 C in presence of 400 ppm of DMABT.
chemical process. But, as can be seen from Table 7, the values
of ꢁG decreased with increasing temperature, indicating that
0
ads
adsorption of DMABT is not favored at higher temperature. This and resulting in the detachment of the particle from the matrix.
also supports the fact that DMABT is adsorbed predominantly Figure 14 shows more or less smooth surfaces of the composite
by physisorption on the surface of aluminum composite.
without pits in the presence of inhibitors.
Scanning Electron Microscopy
CONCLUSIONS
In order to evaluate the effect of corrosion on the surface 1. DMABT acts as a good corrosion inhibitor for 6061 Al- 15
morphology of the composite and the base alloy, SEM analysis
vol. pct. SiC(p) composite and its base alloy in sulfuric acid
was carried out on the samples subjected to corrosion in acid
medium.
mixture containing 0.5 M sulfuric acid in the presence and ab- 2. Corrosion inhibition efficiency of DMABT increases with
sence of the inhibitors. Figure 13 shows cavities formed due to
increasing concentration of inhibitor up to critical concen-
detachment of SiC particle from the composite when exposed
tration.
to the acid mixture medium. This may be attributed to the cor- 3. DMABT behaves as cathodic type inhibitor.
rosion at the interface between the particle and matrix due to 4. Inhibition efficiency of DMABT on aluminium composite is
the galvanic action, with the particle acting as a cathodic site
more than that of its base alloy.
5
. Inhibition efficiency of DMABT on aluminum composite
and its base alloy increases with increase in concentration
of sulfuric acid and decreases with increase in temperature
◦
◦
from 30 C–50 C.
6
. Inhibitor obeys Langmuir’s model of adsorption, and the
adsorption is predominantly through physisorption.
. The adsorption process is spontaneous and exothermic pro-
cess accompanied by an increase of entropy.
7
REFERENCES
1
. Sanders Jr., T.H.; Starke Jr., E.A. Influence of heat treatment on the coeffi-
cient of thermal expansion of Al (6061) based hybrid composites. Proceed-
ings of the Fifth International Conference on Al–Lithium alloys, Materials &
Composites Engineering Publications Ltd., Birmingham, UK., 1–40, 1989.
. Monticelli, C.; Zucchi, F.; Brunoro, G.; Trabanelli, G. Corrosion and corro-
sion inhibition of alumina particulate/aluminum alloys metal matrix com-
posites. J. Appl. Electrochem. 1997, 27(2), 325.
2
3
. Pardo, A.; Merino, M.C.; Merino, S.; Viejo, F.; Carboneras, M.; Arrabal,
R. Influence of reinforcement proportion and matrix composition on pitting
corrosion behavior of cast aluminum matrix composites (A 3xxxx/SiCp).
Corros. Sci. 2005, 47(6), 1750.
FIG. 13. SEM image of the surface of composite after immersion for 2h in
◦
0
.5 M H2SO4at 30 C in the absence of DMABT.

CORROSION INHIBITION OF 6061 Al/SiC COMPOSITE
139
4
. Da Costa, C.E.; Velasco, F.; Toralba, J.M. Aluminum powder metallurgy:
Develop and improve their alloys and composites. Rev. Metal. Madrid.
28. el Kadher, A.; El Warraky, J.M.; Abd el Aziz, A.M. Corrosion inhibition of
mild steel by sodium tungstate in neutral solution. Br. Corr. J. 1998, 33(2),
139.
2
000, 36, 179.
5
6
. Rohatgi, P.K. Cast aluminium–matrix composites for automotive applica-
tions. JOM. 1991, 43(4), 10.
. Pardo, A.; Merino, M.C.; Merino, S.; Lopez, M.D.; Viejo, F.; Carboneras,
M. Influence of reinforcement proportion and matrix composition on pit-
ting corrosion behavior of cast aluminium matrix composites in a humid
environment. Mater.Corros. 2003, 54(5), 311.
29. Khaled, K.F.; Al-Qahtani, M.M. The inhibitive effect of some tetrazole
derivatives towards Al corrosion in acid solution: Chemical, electrochemi-
cal and theoretical studies. Mater.Chem. Phys. 2009, 113(1), 150–158.
30. Sayed, S.; Rehim, A.; Hamdi Hassan, H.; Mohammed, A.A. Corrosion and
corrosion inhibition of Al and some alloys in sulphate solutions containing
halide ions investigated by an impedance technique. App. Surf. Sci. 2002,
187(3–4), 279–290
7
. Peel, C.J.; Moreton, R.; Gregson, P.J.; Hunt, E.P. Influence of heat treat-
ment on the coefficient of thermal expansion of Al (6061) based hybrid 31. Lenderink Linden, H.J.W.; De Wit, M.V.D. Corrosion of aluminium in
composites. Proceedings of the XII International Conference on Society of acidic and neutral solutions. Electrochem. Acta 1993, 38(14), 1989–1992.
Advanced Material and Process Engineering, SAMPE, Covina, CA. 1991, 32. Wit, J.H.; Lenderink, H.J.W. Electrochemical impedance spectroscopy as a
pp. 189.
tool to obtain mechanistic information on the passive behavior of aluminum.
8. Hutchings, I.M.; Wilson, S.; Alpas, A.T.; Clyne, T.W. (ed.). Wear of alu-
Electrochim. Acta. 1996, 41(7-8), 1111–1119.
minium based composites. Comprehensive Composite Materials, Vol.3; 33. Bessone, J.; Mayer, J.B.; Jutner, C.; Lorenz, W.J. Ac impedance measure-
Elsevier Science Ltd: UK., 2000, 501–505.
. Aylor, D.M. Corrosion of Metal Matrix Composites, Metals Hand Book,
Vol.13, Ninth Edition; ASM, 1987, pp. 859.
0. Trowsdate, A.J.; Noble, B.; Haris, S.J.; Gibbins, I.S.R.; Thomson, G.E.;
Wood, G.C. The influence of silicon carbide reinforcement on the pitting
behavior of aluminum. Corros. Sci. 1996, 38(2), 177.
1. Bentiss, F.; Treisnel; Lagrence, M. Inhibitor effects of triazole deriva-
tives on corrosion of mild steel in acidic media. Br. Corros. J. 2000, 35,
ments on aluminium barrier type oxide films. Electrochim Acta. 1983, 28(2),
171.
9
34. Frers, S.E.; Stefenel, M.M.; Mayer, C.; Chierchie, T. AC-Impedance mea-
surements on aluminium in chloride containing solutions and below the
pitting potential. J. Appl. Electrochem. 1990, 20(6), 996–999.
35. Metikos-Hukovic, M.; Babic R.; Grubac, Z. Corrosion protection of alu-
minium in acidic chloride solutions with nontoxic inhibitors. J. Appl. Elec-
trochem. 1998, 28(4), 433–469.
1
1
1
1
3
15.
36. Brett, C.M.A. The application of electrochemical impedance techniques
to aluminium corrosion in acidic chloride solution. J. Appl. Electrochem.
1990, 20(6), 1000–1003.
37. Lee E.J.; Pyun. The effect of oxide chemistry on the passivity of aluminium
surfaces. Corros. Sci. 1995, 37(1), 157–168.
2. Rao, S.A.; Padmalatha; Nayak, J.; Shetty, A.N. 3-Methyl-4- amino-5-
mercapto-1,2,4-triazole as inhibitor of corrosion of 6061 Al- 15 vol.pct.
SiC(p) composite. J. Met. and Mat. Sci. 2005, 47(1), 51–57.
3. Desai, M.N.; Rana, S.S.; Gandhi M.H.; C.B. Snah. Corrosion inhibitors
for aluminium and aluminum –base alloys. Anticorrosion Methods and
Materials 1971, 18(5), 4.
38. Brett, C.M.A. On the electrochemical behavior of aluminum in acidic chlo-
ride solution. Corros. Sci. 1992, 33(2), 203–210.
1
1
1
1
4. Desai, M.N.; Rana, S.S.; Gandhi, M.H.; Snah, C.B. Corrosion inhibitors
for copper. Anticorrosion Methods and Materials 1971, 18(2), 19.
5. Muller, B. Citric acid as corrosion inhibitor for aluminium pigment. Corros. 40. Mansfeld, F.; Lin, S.; Kim, S.; Shih, H. Pitting and surface modification of
Sci. 2004, 46, 159–167. SiC/Al. Corros. Sci. 1987, 27(9), 997.
6. Putilova, I.N.; Balezin S.A.; Arannik, B. Metallic Corrosion Inhibitors; 41. Mansfeld, F.; Lin, S.; Kim, S.; Shih, H. Electrochemical impedance spec-
39. Aytac, A.; Ozmen, U.; Kabasakaloglu, M. Investigation of some Schiff base
as acidic corrosion of AA 3102. Mater.Chem. Phys. 2005, 89(1), 176–181.
Pergamon: Oxford, 1960.
7. Trabanelli, G.; Montecelli, C.; Grassi, V.; Frignani, A. Electrochemical
troscopy as a monitoring tool for passivation and localised corrosion. West
u Korros. 1988, 39(11), 487.
study on inhibitors of rebar corrosion in carbonated concrete. J. Cem. Concr. 42. Migahed, M.A. Electrochemical investigation of the corrosion behavior
Res. 2005, 35(9), 1804.
of mild steel in 2 M HCl solution in presence of 1-dodecyl-4-methoxy
pyridinium bromide. Mater.Chem. 2005, 93(1), 48–53.
43. Abdallah, M. Antibacterial drugs as corrosion inhibitors for corrosion
of aluminium in hydrochloric solution. Corros. Sci. 2004, 46(8), 1981–
1996.
1
1
2
8. Rozerfeld, I.L. Corrosion Inhibitors; McGraw Hill: New York, 1981.
9. Nathan, C.C. Corrosion Inhibitors; NACE: Houston, 1973.
0. Hunkeler, F.; Bohni, H. Inhibition of pit initiation and pit growth on alu-
minum. Workstoffe u. Korrosion 1983, 34, 68.
2
2
2
2
2
2
2
1. Noor, E.A. Evaluation of inhibitive action of some quaternary N-
heterocyclic compounds on the corrosion of Al–Cu alloy in hydrochloric
acid. Mater. Chem. Phys. 2009, 114, 533–541.
44. Oguzie, E.E. Corrosion inhibition of aluminium in acidic and alkaline media
by Sansevieria trifasciata extract. Corros. Sci. 2007, 49(3), 1527–1539.
45. Mernari, B.; Elkadi, L.; Kertit, S. 2,5 bis(2-thienyl)-1,3,4-oxadiazole as
corrosion inhibitor of mild steel in acidic media. Bull. Electrochem. 2001,
17, 115.
46. Szauer, T. Brandt, A. Adsorption of oleates of various amines on iron in
acidic solution. Electrochim. Acta. 1981, 26, 1253.
47. El Sherbini, E.F. Effect of some ethoxylated fatty acids on the corrosion
behaviour of mild steel in sulfuric acid solution. Mat. Chem. Phys. 1999,
60, 286
2. Arab, S.T. Inhibition action of thiosemicarbazone and some of its ρ- substi-
tuted compounds on the corrosion of iron base metallic glass alloy in 0.5M
◦
H2SO4 at 30 C. Materials Research Bulletin 2008, 43(3), 510.
3. Abd El-Nabey, B.A.; Khamis, E. Effect of temperature on the inhibition of
the acid corrosion of steel by benzaldehyde thiosemicarbazone: Impedance
measurements. Surf. and Coat. Tech. 1986, 28, 83.
4. Shah, P.T.; Daniels, T.C. Synthesis and characterization of lanthanide(III)
complexes with N, N-diethylaminobenzaldehyde thiosemicarbazone. Rev. 48. Kumar Singh, A.; Quraishi, M.A. Effect of Cefazolin on the corrosion of
Trav. Chim. 1950, 69, 1545.
mild steel in HCl solution. Corros Sci. 2010, 52, 156.
49. Sahin, M.; Bilgic, S.; Yilmaz, H. The inhibition effects of some cyclic
nitrogen compounds on the corrosion of the steel in NaCl mediums. Appl.
Surf. Sci. 2002, 195, 17.
50. Ateya, B.; El-Anadouli, B.E.; El-Nizamy, F.M. The effect of thiourea on
the corrosion kinetics of mild steel in H2SO4. Corros. Sci. 1984, 24(6),
497–507.
5. Bethencourt, M.; Botana, F.J.; Cauqui, M.A.; Marcos, M.; Rodriguez, M.A.
Protection against corrosion in marine environments of AA5083 Al–Mg
alloy by lanthanide chlorides. J. Alloys Compd. 1997, 250, 455.
6. Abdel Rahim, S.S.; Hassan, H.H.; Amin, M.A. The corrosion inhibition
study of sodium dodecyl benzene sulphonate to aluminium and its alloys in
1
.0 M HCl solution. Mater. Chem. Phys. 2002, 78(2), 337.
7. Aramaki, K. Effects of organic inhibitors on corrosion of zinc in an aerated
.5 M NaCl solution. Corros. Sci. 2001, 43(10), 1985–2000.
51. Geler, E.; Azambuja, D.S. Corrosion inhibition of copper in chloride solu-
tions by Pyrazole. Corros. Sci. 2000, 42(4), 631–643.
0

140
G. M. PINTO ET AL.
5
5
5
2. Durnie, W. ; De Marco, R. ; Kinsella, B. ; Jefferson, A. A study of adsorption 55. Antropov, L.I.; Makushin, E.M.; Panasenko, V.F. Metallic Corrosion In-
properties of commercial carbon dioxide corrosion inhibitor formulations.
J. Electrochem. Soc. 2001, 31(11), 1221–1226.
hibitors; Kiev: Technika, 1981, pp.182.
56. Tschapek, M.C.; Wasowski, R.M.; Sanchez, T. The p.z.c. and i.e.
of γ -Al2O3 and TiO2. J. Electroanal.Chem. 1976, 74(2), 167–
176.
57. Awady, A.A.; Abd. El- Nabey, B.A.; Aziz, S.G. Thermodynamic and ki-
netic factors in chloride ion pitting and nitrogen donor ligand inhibition of
aluminum metal corrosion in aggressive acid media. J. Chem. Soc. Faraday
Trans. 1993, 89, 795–802.
3. Martinez, S.; Stern, I. Thermodynamic characterization of metal dissolu-
tion and inhibitor adsorption processes in the low carbon steel/mimosa
tannin/sulfuric acid system. Appl. Surf. Sci. 2002, 199(1-4), 83–89.
4. Tao, Z.; Zhang, S.; Li, W.; Hou, B. Corrosion inhibition of mild steel in
acidic solution by some oxo-triazole derivatives. Corros. Sci. 2009, 51(11),
2
588–2595.

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