Effect of an Fe(II)-modified trivalent chromium conversion process on Cr(VI) formation during coating of AA 2024 alloy

A trivalent chromium conversion coating formed in an Fe(II)-containing solution on AA2024 alloy has been investigated by high resolution analytical electron microscopy, ion beam analysis, Raman spectroscopy and electrochemical tests. It is shown that preferential oxidation of Fe(II) species by H2O2 at cathodic sites on the alloy surface suppresses the oxidation of Cr(III) species, hence reducing the Cr(VI) content of the coating. The addition of Fe(II) species has only minor influences on the general composition of the coating, the kinetics of coating growth and the corrosion protection of the alloy.