2011 International Conference on Chemistry and Chemical Process
IPCBEE vol.10 (2011) © (2011) IACSIT Press, Singapore
Corrosion Inhibition of Al6061- SiCp Composite in 0.5 M Hydrochloric Acid
U Achutha Kini1, Prakasha Shetty 2 +, S Divakara Shetty 1 and M Arun Isloor 3
1
Department of Mechanical Engineering, 2 Department of Printing & Media Engineering, Manipal Institute
of Technology, Manipal University, Manipal- 576104, INDIA. 3 Department of Chemistry, National Institute
of Technology Karnataka, Surathkal – 575025, INDIA.
Abstract. The corrosion inhibition of Al6061-SiCp composite in the presence of 0.5 M HCl solution
containing propanoyl(1Z)-N-(2-hydoxyphenyl)-2-oxopropanehydrazonoate (PHOH) has been investigated
using potentiostatic polarization techniques and weight loss method. PHOH behaves like a cathodic inhibitor
for Al6061-SiCp composite in hydrochloric acid medium. The effect of concentration of PHOH and
temperature of the acid medium on the inhibition efficiency was investigated. The adsorption of the inhibitor
on the composite surface is found to obey Temkins’ adsorption isotherm.
Keywords: Potentiostatic polarization, Aluminium alloy composite, Temkins’ adsorption isotherm,
Corrosion inhibition.
1. Introduction
Aluminium matrix composites reinforced with silicon carbide particles show many prominent properties
such as light weight, low coefficient of thermal expansion, high thermal conductivity, high stiffness and
strength, better wear resistance and so on. They are widely used in electronic, aerospace and automobile
industry, and also in military applications. One of the main drawbacks of aluminium matrix composites is
that they are less resistant to corrosion compared to the Al base alloys for which a protective oxide surface
film imparts greater corrosion resistance. The presence of reinforcing phase in the composite material can
lead to heterogeneities and cause discontinuities in the surface film, increasing the number of active sites
available for corrosion to take place [1]. The attack is more aggressive in acidic medium. Hydrochloric acid
solutions are used for pickling, chemical and electrochemical etching and in various chemical process
industries wherein aluminium alloy composites are used. In such cases it becomes very important to use
corrosion inhibitors so as to protect the material against excessive corrosion. In the present work, inhibitive
action of PHOH on the corrosion behavior of Al6061-SiCp composite in 0.5 M HCl solution at four different
temperatures (30-60 °C) has been investigated using potentiostatic polarization techniques and weight loss
method.
2. Experimental
2.1 Material and Medium
Al alloy SiC composite specimens having 6061 aluminum alloy as the matrix and containing 15 vol. %
of silicon carbide particles of mean diameter 25µm in the form of cylindrical bars of length 120 mm and
diameter 40 mm were used. The base metal 6061 aluminium alloy has the chemical composition (%wt): 0.25
Cu; 1.0 Mg; 0.60 Si; 0.20 Cr and balance Al. The specimen mounted on a Teflon holder with an exposed
bottom surface area of 0.95 cm2 was polished with emery papers of different grades degreased with acetone
+
Corresponding author. Tel.: + 91 820-2925661: fax: + 91 820-2571071
E-mail address: [email protected]
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and then rinsed with distilled water and dried in air. The test solutions of 0.5M HCl were prepared using AR
grade HCl and double distilled water.
2.2 Inhibitor
Propanoyl(1Z)-N-(2-hydoxyphenyl)-2-oxopropane hydrazonoate (PHOH) was synthesized by dissolving
2amino-phenol (1.2g, 11.0 mmol) in dilute hydrochloric acid (9.0 ml 12 M HCl dissolved in 13.0 ml water)
and cooled to 0 0C in an ice bath. To this, a cold solution of sodium nitrite (1.69 g, 22 mmol in 10.0 ml water)
was added, with the temperature of the reaction mixture kept below 5 0C. The resulting diazonium salt
solution was filtered into a cooled solution of ethylacetoacetate (2 ml) and sodium acetate (3.7 g) in ethanol
(100 ml). The resulting solid was filtered, washed with ice cold water, dried in air and recrystallized from
methanol. (Yield 1.88 g, 68.5 %; m. p., 171-173 0C; mol. wt., 250)[2].
H 3C
O
CH3
N
O
O
NH
OH
Figure 1: Structural formula of PHOH
2.3 Polarization studies
The polarization studies were performed using a Wenking Potentiostat (LB95L) and a three electrode
cell system. The steady state Tafel extrapolation studies were made from -250 mV versus OCP to +250 mV
versus OCP in steps of 20 mV from the cathodic side and the corrosion currents were noted. The Tafel plots
of potential versus logI were drawn and the corrosion current density (Icorr) and the corrosion potential (Ecorr)
were determined. The corrosion rate, the degree of surface coverage (θ) and the percentage inhibition
efficiency (%IE) were calculated. The experiment was performed with 0.5 M HCl solutions at four different
temperatures (30-60 °C).
Linear polarization studies were carried out in the potential (E) range of -20 mV versus OCP to + 20 mV
versus OCP in steps of 5 mV from the cathodic side and the steady state corrosion currents (I) are noted.
From the slopes of the plots of E versus I, the corrosion current density (Icorr) and the corrosion rate were
calculated.
2.4 Weight loss method
Specimen with surface area of 1 sq cm was immersed in 100 ml of 0.5M HCl solution for 6 hours at
room temperature (30 ⁰C). Before exposure the surface is polished with different grades of emery paper and
rinsed with distilled water and acetone, then dried and finally accurately weighed. After exposure, again the
specimen is gently polished, rinsed with distilled water and acetone, then dried and accurately weighed. The
difference in weight gives the weight loss.
3. Results and Discussion
The electrochemical parameters (Table 1) for the inhibitive action of PHOH on the corrosion of Al6061SiCp composite in 0.5 M HCl solution are studied by Tafel extrapolation technique at four temperature levels.
The corrosion rate, the degree of surface coverage (θ) and the percentage inhibitor efficiency (%IE) were
calculated by using the following relations [3].
Corrosion rate (mpy) =
0.1288 × Eq. wt × I corr
D
(1)
where, Icorr is the Corrosion current density in µA/cm2, Eq.wt is the specimen eq.wt. in g, D is the specimen
density in g/cc, 0.1288 is the metric and time conversion factor.
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Table 1. Electrochemical parameters for the corrosion inhibition of Al6061-SiCp composite in 0.5 M HCl
Temp.
(°C)
30
40
Concentration of
PHOH
(10-4 molL-1)
0
1
2
4
5
6
0
1
2
4
5
6
Ecorr
-690
-715
-734
-748
-752
-740
-690
-755
-762
-760
-764
-740
CR
(mpy)
2054
185
140
98
66
93
3698
382
296
217
156
218
θ=
IE
(%)
90.99
93.19
95.23
96.79
95.49
89.67
92.00
94.11
95.91
94.11
Temp.
(°C)
50
60
Concentration of
PHOH
(10-4 molL-1)
0
1
2
4
5
6
0
1
2
4
5
6
Ecorr
-690
-743
-740
-740
-741
-732
-710
-748
-782
-785
-780
-760
CR
(mpy)
4930
575
452
374
269
370
5546
719
601
519
370
452
IE
(%)
88.33
90.83
92.42
94.55
92.50
87.04
89.16
90.65
93.33
91.35
(I corr - I corr (inh))
I corr
(2)
where, Icorr and Icorr(inh) are the corrosion current densities in µA/cm2 in the absence and presence of the
inhibitor respectively.
% I E = θ × 100
(3)
In linear polarization studies, the Icorr value was calculated using the relation,
I corr =
0.26 × 10 6
Slope × A
(4)
Where, A is the exposed area of the specimen (0.95 cm2).
The inhibition efficiency by weight loss method was calculated by using the following relation [4].
⎛ W − Winh ⎞
% IE = ⎜
⎟ × 100
⎝ W
⎠
(5)
Where, W and Winh are the values of weight loss of the specimen after immersion in test solution in the
absence and presence of inhibitor respectively.
The % IE values obtained by Tafel extrapolation technique were found to be in reasonably good
agreement with that obtained by linear polarization technique and weight loss method (Table 2).
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Figure 2: Tafel extrapolation plot for Al6061-SiCp composite in 0.5M HCl at 30 ⁰C
Table 2 Comparison of % IE obtained by Tafel extrapolation and weight loss methods at 30 0C
Concentration
of PHOH
(10-4 molL-1)
1
2
4
5
6
Tafel
extrapolation
technique
90.99
93.19
95.23
96.79
95.49
IE (%)
Linear
polarization
technique
88.67
91.25
93.17
95.65
94.86
Weight loss
method
84.98
88.03
90.49
91.48
90.98
In the absence of PHOH, the corrosion current and the corrosion rate increases very rapidly with increase
in temperature. This may be due to the increased diffusion rate of hydrogen ions to the composite surface. It
is clear from the results that there is a large negative shift in the corrosion potential (Ecorr) and a drastic
reduction in the corrosion current density (Icorr) and corrosion rate (CR) values. The shift in corrosion
potential in the negative direction (Fig. 2) indicates that PHOH act as a cathodic inhibitor. The % IE
increases with increase in concentration of PHOH at all temperatures. This may be due to the blocking effect
of the surface by both adsorption and film formation mechanism which decreases the effective area of attack
by the acid medium on the composite surface. The high efficiency exhibited by PHOH may be due to its
adsorption on the composite surface through polar groups as well as π electron cloud of aromatic ring. The
results indicate that PHOH is an effective corrosion inhibitor which gives IE values as high as 95 %.
The surface analysis of Al6061- SiCp composite was carried out using scanning electron microscope
(JEOL Model 8340LA). The scanning electron micrograph of a fresh specimen of Al6061- SiCp composite
is shown in Figure 3a. The micrographs for the specimens immersed in 0.5 M HCl solution at 30° C in the
absence and presence of inhibitor are shown in Figures 3b and 3c respectively. The corrosion of the
composite in HCl medium is presumably due to the anodic dissolution either at the grain boundaries or at the
metal-media interface. It is seen from the Figure 3c that the surface of the composite exposed to inhibited
solution is smoother than that exposed to uninhibited acid solution (Figure 3b). These observations suggest
that the inhibitor forms a protective layer on the composite surface, which prevent the attack of acid on the
surface.
(a)
(b)
(c)
Figure 3: The SEMs of Al6061-SiCp composite (a) Fresh specimen (b) Specimen exposed to 0.5 M HCl
(c) Specimen exposed to 0.5 M HCl containing 5×10-4 mol L-1 of PHOH.
To understand the mechanism of corrosion inhibition, the adsorption behaviour of the inhibitor
compound on the composite surface must be known. The surface coverage values (θ) were tested graphically
by fitting a suitable adsorption isotherm. The plot of θ versus log c (Figure 4) for different concentrations of
PHOH shows a straight line indicating that the adsorption of the compound on the composite surface follows
Temkins’ adsorption isotherm. The applicability of Temkins’ adsorption isotherm verifies the assumption of
130
mono-layer adsorption on a uniform homogeneous composite surface with an interaction in the adsorption
layer [5].
Figure 4: Temkins’ adsorption isotherm for the adsorption of PHOH
The thermodynamic parameters for the corrosion of Al6061-SiCpcomposite in the presence and absence
of PHOH are determined (Table 3) from the experimental data.
The values of activation energy (Ea) are calculated using Arrhenious equation [6]
ln(r 2 / r1) = −
Ea ΔT
( R × T 1 × T 2)
(5)
Where, r1 and r2 are the corrosion rates at temperatures T1 andT2 respectively, ∆T is the difference in
temperatures and R is the universal gas constant in joules.
The equilibrium constant (K) is determined by the relation:
K=
θ
(6)
c(1 − θ )
Where θ is the degree of surface coverage on the metal surface and c is the concentration of the inhibitor
in molL-1.
The free energy of adsorption (∆Gads) is calculated from the following equation [7].
ΔGads = − RT ln(55.5 K )
(7)
-1
Where 55.5 is the concentration of water in solution in molL and T is the temperature in Kelvin.
Table 3 Thermodynamic parameters for the adsorption of PHOH
Concentration
of PHOH
(10-4 molL-1)
Blank
5
Ea
( kJmol-¹ )
26.40
47.38
K
-ΔGads( kJmol-1)
30 °C
37.85
42053
40 °C
38.34
32014
50 °C
38.86
18141
60 °C
39.47
15271
The negative values of ∆Gads indicate spontaneity of the adsorption process and stability of the adsorbed
layer on the composite surface. The ∆Gads values obtained for the optimal concentration (4.5×10-4 molL-1) of
PHOH are less than 40kJmol-1. This indicates that the inhibition is governed by physical adsorption
mechanism [8]. The decrease in K values with increasing temperature also suggests that the inhibitor is
physically adsorbed on the composite surface [9]. On the other hand, the value of Ea obtained in the inhibited
solution is lower than that in the uninhibited solution, which can be attributed to the physical adsorption of
PHOH on the composite surface [10]
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4. Conclusions
The corrosion rate of Al6061-SiCpcomposite decreased drastically in the presence of PHOH in the acid
medium at 30-60 °C. PHOH has proved to be an efficient cathodic inhibitor for corrosion of Al6061-SiCp
composite in 0.5 M hydrochloric acid solution. The adsorption of the compound on the composite surface
was found to obey Temkins’ adsorption isotherm and governed by physical adsorption mechanism. The
inhibition efficiency values obtained by polarization techniques and weight loss method are in reasonably
good agreement.
5. Acknowledgements
The authors are thankful to Prof. Dr. Kumkum Garg, Director and Dr. N. Y. Sharma, Head, Department
of Mechanical Engineering, Manipal Institute of Technology for providing the facilities.
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