The replacement of chromate by molybdate in phosphoric acid-based etch solutions for aluminium alloys

Abstract

Corrosion inhibition of aluminium alloys in the acidic solutions is important in the aluminium finishing industry (stripping solutions for anodic films). Sodium molybdate is a potential replacement for chromate-containing species used in the stripping solutions. It was tested as the corrosion inhibitor of 2024 aluminium alloy in orthophosphoric acid solutions. A corrosion rate of the alloy as a function of an initial concentration of sodium molybdate and temperature was determined by using gravimetric and gasometric methods. The corrosion rate was significantly lower in the solutions containing over 5 mM of sodium molybdate when compared to the uninhibited solution. The inhibition efficiency decreased with the increase of the temperature for the constant concentration of sodium molybdate. Sodium molybdate reduced the corrosion of the alloy by adsorption as well as formation of insoluble corrosion products on the alloy.