The adsorption of acid amides as organic corrosion inhibitors on metal surface was studied by corrosion tests and catalytic hydrogenations. Mild steel specimens were preliminarily treated with inhibitors and then corrosion tests were carried out with them in 5% HCl aqueous solutions at 50°C. The following corrosion inhibitors have been used: caprylic acid amide and N-methyl substituted acid amides. Corrosion rates were obtained from weight losses of the specimens. Corrosion rates which were measured in the corrosion tests with pre-filmed specimens prepared in isooctane solution of caprylic acid amide, decreased with the concentration increase of the inhibitor up to 0.01M/l. The minimum corrosion rate was attained at 0.01M/l. Judging from the infrared spectra of the filming solutions, the concentration of the solution which gave the minimum corrosion rate was the case that free-molecular acid amide was the most predominant. Further addition of the inhibitor lowered free-molecular parts because of intermolecular association of the inhibitor. These results demonstrated that the adsorption behavior of inhibitors on metal surface was highly dependent on the intermolecular hydrogen bonds of acid amides. Since an unshared pair of electrons of the inhibitor which was necessary for its chemisorption on metal surface was occupied by intermolecular hydrogen bonding, the inhibitor molecules were unable to donate their unshared pairs to metal surface and the formation of adsorption film was disturbed. When the concentration of the solution of caprylic acid amide was enhanced, it was difficult to be adsorbed on metal surface, due to its strong molecular association. However, intermolecular hydrogen bonds of acid amide could be released by adopting some polar solvents, e.g. dioxane. When its association was released, the adsorption film was easily formed on metal surface. In consequence, this film gave high resistance to corrosion. On the other hand, N, N-dimethyl-caprylic acid amide had no molecular association, so that it was easily adsorbed on metal surface, but it could not give good inhibition. One of these causes was steric hindrance of the N-substituents of acid amide.
The same conclusions were obtained from the evaluation of the reactivity of the catalytic hydrogenation. The catalytic hydrogenations were carried out by using Cu (R) and Cu-Cr-Ba oxide as catalysts which were preliminarily immersed in the solutions containing various amounts of caprylic acid amides. The rates of reaction were determined with gas chromatography of reaction products. Results showed that immersed Cu (R) inhibited the reactions considerably, but similarly treated Cu-Cr-Ba oxide could not do so remarkably. Therefore, it was assumed that acid amide was easily adsorbed on metal surface, but difficult on metallic oxide-surface. For the adsorption behavior of acid amides, these catalytic hydrogenations had intimate relation with the corrosion tests. These catalytic hydrogenations were, therefore, available for studying the adsorption behavior of the materials and their inhibition abilities.

View full abstract