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The aim of this work was to establish the influence of the thickness of the anodic coatings on their mechanical properties and to understand the relation between their hardness and the abrasion resistance. The coatings were produced in the hard anodizing process onto the 6061-T6 aluminum alloy. Their thickness was in the range between 19 and 43 lm. The abrasion resistance was determined by using Taber abrasion test. The weight losses of the coatings obtained were in the range between 15 and 11 mg and decreased with their increasing thickness. It has been shown that the hardness measured on the cross sections of the coatings did not correspond to their abrasion resistance. Thus, the new approach has been proposed. The hardness of the coatings was estimated on the basis of the results of the scratch test performed at the constant load. The results obtained correspond to the abrasion resistance of the coatings.

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Plant extracts are intensively studied as green corrosion inhibitors of aluminum. Because these extracts are complex systems, the influence of their individual constituents on the corrosion of aluminum should be determined. In this work, gallic acid was tested for the first time as a corrosion inhibitor of aluminum in orthophosphoric acid aqueous solution. So far, its potential inhibiting properties in acidic solutions were only suggested based on promising results obtained for various plant extracts. Evaluation of the potential inhibiting properties of gallic acid was performed using electrochemical methods. The corrosion potential, polarization curves, and impedance spectra of aluminum in 0.5 M orthophosphoric acid, at T = 303 K, were determined. The corrosion potential, corrosion current density, and corrosion rate of aluminum in orthophosphoric acid were equal to −1.151 V vs. Ag|AgCl (3M KCl) reference electrode, 36 μA∙cm−2 and 0.39 mm∙year−1, respectively. These values did not change with the addition of gallic acid. The results obtained show that gallic acid does not inhibit aluminum corrosion. UV-Vis absorption spectra of gallic acid solutions and quantum mechanical calculations show that this organic compound did not adsorb onto the aluminum surface under the studied conditions.