Conversion coating growth on 2024-T3 Al alloy. The effect of pre-treatments

The cleaning pre-treatments, generally based on alkaline etching and acid pickling, provide a surface standard condition before the application of conversion coatings. In this work, the effect of cleaning pre-treatments on the surface morphology and activity of 2024-T3 Al alloy and on chromium-free Ce-based conversion coatings formation has been studied. Variations in the open circuit potential of the alloy of ~ 1.5 V were registered during the cleaning pre-treatment, accounting for the complex electrochemical phenomena involved. Even though, the presence and concentration of additives such as H2O2 was found to be essential to a fast kinetics coating formation, effective cleaning pre-treatments are demonstrated to be associated to the production of grained well structured coatings, allowing necessary redox reactions for conversion coating formation. The effect of copper-rich areas and its effect on the conversion coating growth rate and morphology were also studied using benzotriazole additions at different stages in the cleaning pre-treatments.