Control of aluminium corrosion with amino acids in acidic solutions

In this study, the purpose was the prevention of corrosion of aluminium in citric acid solutions. For prevention of aluminium corrosion in 0.5 wt % of citric acid solution; aminoacids used are L-Arginine, L- Asparagin Monohydrat, D- Mandelic Acid, DL- Methionine, D-Valine, L-Leucin, D-Phenylalanine, N-Asetil-L-Cysteine, L-Cysteinhydrocloride and L-Aspartic Acid. The inhibition efficiencies and corrosion rates were determined with the use of Tafel extrapolation and Linear polarization method. Inhibitor of efficiencies were found to be closely related to the functional groups on amino acids involved in the adsorption process. The inhibition efficiency seems to follow the order of L-Arginine ? L- Asparagin Monohydrat ?D- Mandelic Acid ? DL- Methionine ?D-Valine ? L-Leucin ? D-Phenylalanine ?N-Asetil-L-Cysteine. Some of these amino acids behaved as cathodic inhibitors. Experiments were carried out at 35oC, 45oC and 55oC at 1000 ppm with the amino acids of high efficiency. In order to examine the dependency of inhibitor efficiency on concentration, experiments to were carried out for the inhibitor concentration varying between 500 ? 4000 ppm at 25oC. High inhibitor efficiency was obtained for L-Arginine and L- Asparagin Monohydrat at 3000ppm at 25oC which as %92.6 and %84.1, respectively; for DL- Methionine and D-Valine were % 88.8 and % 87.1 (25oC at 4000 ppm). I was thought that amino acids acts as inhibitor as result of adsorption. From the adsorption isotherms, it was seen that adsorption of inhibitors obeys generally Langmuir isotherms. With the use of Activation Energy (Ea), were determined Lni - 1/T graphs for absence or presence of inhibitor at constant inhibitor concentration. In this way, the type of adsorption is determined by calculating the activation energies. The activation energy of the solution without inhibitor were lower than the activation energies of solutions with inhibitor. This result indicates that dissolution of metal becomes harder at the presence of inhibitor, corrosion reaction becomes harder and consequently corrosion rate decrease. Thus, protection effect was supported with activation energy, too. In 0.5 wt % citric acid solution, the experiments were made with binary inhibitor blends which have high efficieny. L-Arginine, L- Asparagine-Monohydrat, DL- Methionine, D-Phenylalanine and N-Acetyl-L-Cysteine at 25°C were used in binary inhibitor blends at constant total inhibitor concentrations. The highest inhibitor efficiency was obtained for 500 ppm L-Arg + 500 ppm DL-Meth inhibitor blend as % 84.3.