Formation and breakdown of chromate conversion coatings on aluminum-zinc-magnesium-copper 7X75 alloys

The objective of this study was to characterize the formation and breakdown of chromate conversion coatings (CCCs) on aluminum alloys Al-Zn-Mg-Cu 7075 and 7475 with a focus on the effect of alloy temper, alloy purity and selected coating processing variables. Overall, results consistently pointed to a slight temper effect. Conversion coated AA7475-T7 was significantly more corrosion resistant than conversion coated AA7475-T6. In AA7075, there was only a slight difference in corrosion resistance between the two tempers. This was attributed to the effect of constituent particles on coating formation and breakdown, which are present to a much greater extent in AA7075 than in AA7475. The difference in the corrosion resistance between the T6 and T7 tempers in the coated and uncoated conditions is about the same suggesting that the origin of any "temper effect" in conversion coated materials is ultimately due to the intrinsic change in corrosion susceptibility of the alloy itself. Thicker coating formed on AA7475-T7 has the effect of increasing corrosion resistance, which could be associated with the 860 cm-1 Raman intensity band. Studies were also conducted with alloys in retrogression and reaged tempers and the W temper. Results with these tempers were mixed and no general conclusions could be drawn. In terms of electrochemically derived measures of corrosion resistance (electrochemical impedance, and pitting potential measurements), the magnitude of the temper effect was about the same as the effect due to the purity difference between AA7075 and AA7475. The temper effect was less significant than effects due to increasing coating time from 1 to 3 minutes, withholding certain substrate precleaning steps, or withholding key ingredients from the coating bath.; Scanning probe microscopy, scanning Kelvin probe force microscopy (SKPFM) and scanning electron microscopy were used to characterize coating formation in the vicinity of constituent intermetallic particles (IMPs) present in the alloys. Coatings formed on IMPS exhibited different morphologies and were much thinner that coatings formed on the matrix. Coatings were thinnest on S phase particles, and in post-coating exposure to aggressive chloride environments coating breakdown was almost always associated with these particles. The difference in coating thickness between particles and the surrounding matrix was established within the first tens of seconds of coating. The thickness differential was then observed to remain constant for the remainder of the coating immersion time. (Abstract shortened by UMI.)...