On the dissolution rates and mechanisms of Al-Mg and Al-Cu alloys during acid pickling using element-resolved electrochemistry

Acid pickling is a one-step process prior to the conversion coating of aluminum alloys, an alternative to the two-step degreasing-deoxidizing. Selective dissolution may lead to enrichment or depletion of alloying elements with important consequences for corrosion resistance. In this work, the mechanisms of selective dissolution were explored using element-resolved electrochemistry for Al-3at.%Cu and Al-3at.%Mg, representing two extremes of the galvanic series. Pickling reactions were investigated in sulfuric and nitric acids with and without Fe(III) additives. The microstructure of the Al-3at.%Cu alloy was varied by heat treatment. Al-3at.%Mg showed congruent dissolution throughout the steady state in all electrolytes while Al-3at.%Cu underwent selective dissolution with surface enrichment of Cu and a significant release of detached Cu-rich particles in sulfuric acid. With the presence of Fe(III) in the electrolyte, Cu dissolved congruently, and the dissolution kinetics of these two alloys were also markedly enhanced. Experimental Evans diagrams were determined to clarify the electro-chemical nature of the reactions.