Citric Acid as an Alternative to Sulfuric Acid for Hard Anodizing in Aluminum Alloys Used in the Aeronautical Industry

Abstract

In the aircraft industry, Al-Cu alloys are widely used as a structural material in the manufacture of commercial aircraft due to their high mechanical properties and low density. Al-Cu alloy have low resistance to corrosion in aggressive substances; as a result, these alloys are surface treated to increase their resistance to corrosion. Anodizing was done hard in aluminum alloy AA 2024 in a solution composed of citric and sulfuric acid for 60 minutes with constant agitation using current densities of 3 and 4.5 A/dm2. Afterwards, a sealing procedure was performed for 60 minutes in water at 95 °C. Scanning electron microscopy (SEM) was used to characterize the microstructure and Vickers microhardness (HV) as a mechanical test of the material. The electrochemical evaluation was carried out using cyclic potentiodynamic polarization curves (CPP). The results indicate that the corrosion resistance in citric acid solutions with a current density of 4.5 A/dm2 was the best, with corrosion current densities of 2 x 10-8 and 2 x 10-9 A/cm2.