Influence of pre- and post-treatments on formation of a trivalent chromium conversion coating on AA2024 alloy

A comparison has been made of the influence of two alloy pre-treatments and two coating post-treatments on the formation, composition and corrosion protection of a trivalent chromium conversion coating on AA2024-T351 alloy. The investigation employed analytical electron microscopies, ion beam analysis, X-ray photoelectron spectroscopy (XPS) and electrochemical tests. The pre-treatments used alkaline etching followed by de-oxidizing in either nitric acid or a commercial de-oxidizer. The conversion coatings were formed in SurTec 650 chromitAL and revealed two-layers, comprising an inner aluminium-rich layer and an outer chromium- and zirconium-rich layer, with a Cr:Zr atomic ratio in the range ~ 0.73-0.93. XPS indicated a chromium-enriched near-surface region that contained ~ 2 at.% of Cr(VI) species. Potentiodynamic polarization and electrochemical impedance spectroscopy revealed an improved corrosion protection for a pre-treatment that left copper-rich sponges, probably de-alloyed S phase, and fewer residues of other intermetallic particles on the alloy surface. Post-coating immersion treatments in deionized water at 20 °C or 40 °C resulted in a significant difference in the zirconium species in the region adjacent to the coating surface that is accessible to XPS, with oxide and hydroxide dominating at the respective temperatures.