The role of chromate conversion coating in the filiform corrosion of coated aluminum alloys

The mechanisms of inhibition of chromate conversion coating on the filiform corrosion of coated aluminum alloy AA6016 was investigated using X-ray absorption near-edge structure (XANES) spectroscopy and Raman microspectroscopy. The results showed that Cr(VI) was preferably leached out at the opening of the filament, whereas Cr(VI) was reduced to Cr(III) at all other locations. The leached out Cr(VI) accumulated mainly in the pitting area, located in the scratch just outside the filament. The chromate released from the CCC was partly reduced to Cr(III) in the pit. The formation of an Al/Cr mixed oxide was observed in the pitting area. The repassivation of pits in the scratch by the chromate leaching from the CCC close to the scratch explains the low propagation of filiform corrosion observed for the chromated surfaces. (C) 2003 The Electrochemical Society.