Quantification of Accelerated Corrosion Testing of Coated AA7075-T6

Abstract

A test sample incorporating a painted Al alloy panel, uncoated through-hole fasteners, and scribes has recently been shown to provide accelerated response during atmospheric corrosion testing in the field and in laboratory chambers. In this paper, the galvanic current of an AA7075-T6 panel coupled with mixed SS316 and Ti-6Al-4 V fasteners was monitored using a zero-resistance ammeter during 3 weeks exposure in an ASTM B117 chamber or immersed in 5 wt% NaCl solution. SS316 fasteners provided more cathodic current than Ti in both environments and the current in ASTM B117 is higher than in 5 wt% NaCl solution due to greater oxygen availability. The integral of the anodic current with time and optical profilometery (OP) analysis were used to assess the corrosion attack quantitatively for two different coating systems. An acceleration factor was defined to represent the extent of accelerated corrosion for galvanically-connected fasteners. The acceleration factors were in the range of 20–50 for panels with SS316 fasteners and two different coating systems, both with and without a topcoat. The effects of SS316 fasteners were similar for the different coating systems even though the attack morphology was very different.