ФИЗИКОХИМИЯ ПОВЕРХНОСТИ И ЗАЩИТА МАТЕРИАЛОВ, 2010, том 46, № 6, с. 657-663
ФИЗИКО-ХИМИЧЕСКИЕ ПРОБЛЕМЫ ЗАЩИТЫ МАТЕРИАЛОВ
УДК 620.197.3
THE INHIBITION EFFECT OF AMIDES ON ALUMINIUM CORROSION
IN CHLORIDE SOLUTIONS
© 2010 S. Zor*, H. Ozkazang
Department of Chemistry, Kocaeli University, 41380, Umuttepe Kocaeli, Turkey E-mail: merve@kou.edu.tr or szor2001@yahoo.com Поступила в редакцию 28.10.2009 г.
The inhibition effect of different concentrations of benzamide (BA), sulfanilamide (SA), and thioacetamide (TA) on the corrosion of aluminium in 0.1 M NaCl solution was investigated by the methods of potentiody-namic polarization, electrochemical impedance spectroscopy, and SEM (scanning electron microscope) in this study. Impedance measurements showed that the charge transfer resistance increased whereas double layer capacitance decreased with the increase in the inhibitor concentrations. Adsorption of these inhibitors followed the Langmuir adsorption isotherm. Thermodynamic parameters of adsorption (Kads, AGads) of studied amides were calculated by using Langmiur adsorption isotherm. The surface films of the aluminium, both in solutions with and without the inhibitors, were then investigated by SEM. The results obtained showed that thioacetamide was much more effective in aluminium inhibition.
1. INTRODUCTION
Aluminium is an important material for use in many industrial applications, such as production of automobiles, radiators, aviation, household appliances, pipes, air conditioners, and electronic devices due to its relatively low cost, high electrical and thermal conductivities, low density, and high corrosion resistance [1—3]. The corrosion resistance of aluminium arises from its ability to form a natural oxide film on its surface in wide variety of media [4—10]. Consequently, the corrosion mechanism of aluminium in chloride solutions has been investigated in a number of studies [11—14] as a popular research subject.
Among many methods of corrosion control and prevention, the use of organic inhibitors is the most frequently used one. Organic inhibitors are mostly used to protect aluminium and alloys against corrosion in aggressive electrolytes because they adsorb on the surface by acting as a protective film on anodic and cathodic areas simultaneously [12, 15]. Most of organic compounds contain polar groups such as nitrogen, sulfur, and oxygen [1—10]. The inhibition of organic compounds is based on the adsorption ability of their molecules. The inhibitor molecules are bonded on the metal surface as chemisorp-tions, by physical adsorption, or by complexation with the polar groups acting as the reactive centers in the molecules [9].
Adsorption characteristics of these inhibitors depend on a few factors; such as the surface charge of the metal, the chemical structure of organic inhibitors, the distribution ofcharge in the molecule, the type ofaggressive electrolyte, and the type of interaction between the organic molecules and the metallic surface [9, 10, 12, 16, 17].
Amides are organic molecules that contain atoms with high electron levels, such as N, O, and S, in their
molecular structures. Some amides and derivatives e.g. urea (U), thiourea (TU), thioacetamide (TA), and thi-osemicarbazide (TSC) have been found to be good inhibitors for mild steel in acid solutions [18, 19]. Relationship between molecular structures of these amides and their inhibition efficiencies have been studied in several research reports [18—21]. The inhibition efficiencies of amides on steel have been generally studied in the literature whereas their effects on aluminium have not been studied much. Therefore, the effects of benzamide (BA), sulfanilamide (SA), and thioacetamide (TA) in 0.1 M NaCl on aluminium corrosion were investigated by electrochemical methods (potentiodynamic polarization and impedance spectroscopy). Moreover, their suitable adsorption isotherms were determined by the examination oftheir adsorption characteristics. The change in the surface structure ofaluminium, both in the solutions with and without inhibitors, was determined with SEM.
2. EXPERIMENTAL
All chemicals were of analytical reagent grade (Merck). An aqueous solution of 0.1 M NaCl was used as a blank solution. Amides (Benzamide (BA), sulfanilamide (SA), and thioacetamide (TA)), which were obtained commercially, was added to the chloride solution ranging from 25ppm to 100 ppm. The molecular structures of the studied inhibitors are shown in Fig. 1. All experiments were carried out at room temperature with the electrolyte solution in equilibrium with the atmosphere (aerated solutions).
Electrochemical measurements were performed in a classical three electrodes assembly method; with aluminium as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode
H3C
Q benzamide
Chemical Formula: C7H7NQ Molecular Weight: 121.14
4-aminobenzenesulfonamide Chemical Formula: C6H8N2O2S Molecular Weight: 172.20
thioacetamide Chemical Formula: C2H5NS ^NH2 Molecular Weight: 121.14
Fig. 1. Structure of inhibitor molecules.
NH2
(SCE) provided with a luggin capillary as the reference electrode. The commercially obtained Al was used in all experiments. A cylindrical aluminium rod, whose exposed surface is 0.785 cm2, was inserted in a Teflon tube so that only the flat surface was in contact with solution.
Prior the electrochemical measurements, the WE was abraded with emery papers (grade 320—400—800—1200), washed with distilled water and methanol, dried at room temperature, and finally immersed to cell. After the immersion of the specimen, a stabilization period of60 min. was needed to attain a stable value that proves it to be sufficient for open circuit potential. The potentiodynamic polarization curves were obtained from —250 mV versus OCP to +250 mV versus OCP with a scan rate of 5 mV/s. Electrochemical parameters (Icorr, Ecorr, Rp, Corr Rate (mpy)) were obtained from polarization curves.
Impedance measurements were performed at the open circuit potential. Computer controlled EIS measurements carried out on steady state open circuit potential (OCP) disturbed with amplitude of 5 mV a.c Sine Wave, at frequencies between 105 Hz- 0. 1 Hz, were realized for impedance measurements. Electrochemical measurements, on the other hand, were carried out with a Gamry Instrument Potentiostat/ Galvonastate/Refer-ence 600. Echem Analyst Software was finally used for plotting, graphing, and fitting data.
2.1 SEM analysis
SEM photographs obtained from aluminium surface after specimen immersion in two sets of 0.1 M NaCl solutions, with and without 100 ppm of BA, SA, and TA, for 5 days.
3. RESULTS AND DISCUSSIONS 3.1. Potentiodynamic polarization measurements
Polarization curves of aluminium in 0.1 M NaCl solutions, both containing and not containing BA, SA, and TA at various concentrations, was given in Figures 2, 3, and 4 respectively. Anodic and cathodic current densities of all inhibitors decreased as can be seen from the figures. The decrease in cathodic current densities was observed to be more. Accordingly, for the inhibition effects of the molecules, though they seem as mixed inhibitors, cathodic effect is more dominant as depicted in tafel curves (Figs. 2—4). The decrease in the cathodic current density in TA is especially high (Fig. 4). This decrease was observed more also when the concentration of inhibitors
w и
_ a 0.1 MNaCl
b 25 ppm BA
c 50 ppm BA
- d 75 ppm BA
e 100 ppm BA
- e
- cb a
1 1 1 1 1 lllll 1 III Mill 1 1 1 1 lllll 1 1 1 1 lllll 1 III lllll 1 1 1
-0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0
1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 log I, (A/cm2)
Fig. 2. Polarization curves of aluminium in 0.1 M NaCl in the absence and presence of benzamide.
-0.4
-0.5
-0.6
W U
oo ^
-0.7
0.8
-0.9
1.0
a 0.1 MNaCl b 25 ppm SA c 50 ppm SA d 75 ppm SA e 100 ppm SA
.....I_I........I_I........I_I........I_I........I_I_» »
1E-7 1E-6 1E-5 1E-4 log I, (A/cm2)
1E-3
Fig. 3. Polarization curves of aluminium in 0.1 M NaCl in the absence and presence of sulfanilamide.
was increased. The blockage of metal surface by inhibitors decreased the cathodic reduction rate (Figs. 2—4).
Electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (/corr), polarization resistance (Rp), and corresponding inhibition efficiency (IE) values for different inhibitor concentrations are given Table 1. The inhibition efficiencies at different inhibitor concentrations were calculated from the equation (1);
IE% =
( icorr)o ( icorr)inh
x 100,
(1)
( ^corr)o
where (/corr)o and (/corr)inh are the corrosion current densities without and with addition of inhibitor.
Corrosion current densities decreased as the inhibitor concentrations increased as can be seen from Table 1. The decrease in corrosion current density of TA was more than the decreases in BA and SA (Table 1). Polarization resistance and corrosion current density are inversely proportional. The lowest value of corrosion current density (100 ppm TA Icorr = 0.25 ^A cm-2) corresponded to the highest value of polarization resistance (100 ppm TA Rp = = 33650 Q). The corrosion potential for benzamide and sulfanilamide did not change in significant amounts compared to the 0.1 M NaCl. The corrosion potential for thioacetamide, since the decrease in cathodic current density was more, shifted to more negative values (from -0.653 V to -0.721 V). Inhibition efficiency increased as the inhibitor concentration increased. This increase was the highest in 100 ppm thioacetamide with a percentage of 98.1% (Table 1).
Inhibition efficiency was calculated by polarization resistance with the equation below;
IE% =
Winh - (Ä,)
p/o
X100,
(2)
-G.4 -G.5
-G.6
E S
E, -G.8 -G.9 -l.G
a G.1 MNaCl b 25 ppm TA c 5G ppm TA d 75 ppm TA e 1GG ppm TA
—i........i_i........i
(^p)inh
where (Rp)inh and (Rp)o are the polarization resistances of solutions with and without inhibitors, respecti
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