Effect of Surface Pretreatment on Galvanic Attack of Coated Al Alloy Panels

A test assembly consisting of a coated and scribed aluminum alloy 7075-T6 panel galvanically connected to uncoated through-hole noble fasteners has recently been utilized to quantify corrosion attack during corrosion testing in laboratory chambers. In this work, the corrosion morphology and extent of attack of Al alloy panels with five different surface pretreatments prior to painting and scribing were compared after exposure to ASTM B117 with galvanic connection to uncoated stainless steel fasteners. The nature of the attack for samples with different surface pretreatment samples was found to exhibit two different trends: they either penetrated deeply into the substrate at the scribes or spread out under coatings. The galvanic currents between the coated Al alloy panel and the bare 316 stainless steel fasteners were monitored during 21 days exposure in an ASTM B117 chamber. The current trends measured for different surface pretreatment panels were consistent with optical profilometry (OP) results. However, the galvanic current and OP techniques both underestimated the extent of corrosion attack because of local H₂ evolution and undercut attack, respectively. The results from both techniques were modified to compensate for these deficiencies. Acceleration factors associated with the galvanic interaction for the different surface pretreatment panels were determined.