Influence of metallurgy on the protective mechanism of chromium-based conversion coatings on aluminum-copper alloys

Several important aspects relating to the mechanism of formation of protective Cr‐based oxide films on aluminum alloy 2024‐T3 generated from CrO3 + NaF‐containing solutions were observed with electrochemical, AES and XPS measurements. Although the film deposition rate and surface composition were very much influenced by the presence of both Fe‐ and Cu‐containing intermetallic phases, a uniform composition and thickness was eventually reached with increased coating time. This behavior is believed to be responsible for obtaining corrosion‐resistant films on such heterogeneous surfaces. Surface Cr was consistently found to be in both the 3+ and 6+ oxidation states in an approximately 40:60 ration, respectively, if x‐ray beam reduction of Cr(VI) was accounted for. The enriched Cu layer found on the surface of polished 2024‐T3 was found to remain intact during CrO film formation. The presence of Cr(VI) provides a ‘self‐healing’ aspect to the film's protective mechanism by remaining available for reduction to Cr(III) during oxidative attack. Addition of potassium ferro‐ or ferricyanide to the bath resulted in a film which obtained Fe, C and N enriched at the surface on both the matrix and intermetallic regions. These constituents were found, however, to be enriched and distributed throughout the entire depth of the film on the high Cu‐bearing intermetallic regions, suggesting that the formation of a Cu–ferrocyanide complex is responsible for the benefit derived from these compounds.