Abstract
A test sample incorporating a coated and scribed Al alloy panel and uncoated through-hole fasteners was designed to provide accelerated response during atmospheric corrosion testing in the field or in laboratory chambers. Protective coatings that would take many months to show signs of degradation on normal scribed samples with no galvanic connection exhibited corrosion in much shorter times when uncoated Ti alloy or stainless steel fasteners were used, particularly in conjunction with scribes through the coating. The results after three weeks of exposure to ASTM B117 correlated well with long-term beach exposure, allowing rapid ranking of different coating systems. Of the fastener materials studied, visual observation indicated that the worst attack occurred near Type 316 (UNS S31600) stainless steel (SS316) fasteners even though Ti-6Al-4V fasteners are more noble. The volume of AA2024-T3 (UNS A92024) lost during the exposure, as determined by an optical profilometer, was also largest for samples with SS316 fasteners. Scanning Kelvin probe measurements across the scribes on one of the coating systems showed that, after exposure in an ASTM B117 salt fog chamber, the potential was lowest for scribes next to SS316 fasteners. The difference between SS316 and Ti-6Al-4V was explained by the available cathodic current, which was measured in chloride solution.
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