Role of chromate, molybdate and tungstate anions on the inhibition of aluminiumin chloride solutions

The effect of CrO₄ ^2–, MoO₄ ^2– and WO₄ ^2– anions on the inhibition of aluminium corrosion in 0.5 M NaCl solution was investigated. The study comprised potentiodynamic polarization, potentiostatic current–time measurements complemented by SEM–EDAX and XPS investigations. It was found that, the pitting potential of an Al electrode in 0.5 M NaCl solution shifts in the positive direction by addition of CrO₄ ^2–, MoO₄ ^2– and WO₄ ^2– anions, and the shift in potential increases with increase in concentration. A pronounced inhibiting influence was achieved on addition of CrO₄ ^2–, MoO₄ ^2– and WO₄ ^2– anions to the electrolyte during potentiostatic current–time measurements. Chromate anions exhibit a great passivating influence during I/t measurements. This can be explained by the fact that the chromate anion, as a powerful oxidizing agent, is capable of oxidizing the corrosion sites to give a stable Al₂O₃ film. The inhibition observed on addition of molybdate anions is attributed to the adsorption and reaction of MoO₄ ^2– anions on the electrode surface forming a molybdate layer which selectively impedes the ingress of Cl^– ions and hence inhibits the pitting attack. The adsorption of WO₄ ^2– anions at flawed areas and developing pits was found to be the main factor for the observed inhibition.