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A galvanic test panel design incorporating a painted and scribed Al alloy panel and uncoated through-hole noble fasteners has recently been utilized to compare effects of different surface pretreatments on galvanic attack in laboratory chambers. In this work, corrosion of galvanic panels composed of a coated AA2024-T3 panel and uncoated 316 stainless steel fasteners was quantified after exposure to ASTM B117 and a beach field site. Galvanic currents were continuously monitored between the panel and stainless steel fasteners in the field and in the laboratory chamber, exhibiting current transients and stable high value currents, respectively, associated with the two different environmental conditions. For all coating systems, exposure in the laboratory chamber resulted in larger galvanic current and greater extent of corrosion than what was generated by field exposure. However, the nature of the corrosion in the field was different, even though the galvanic current measured during field exposure was similar for the two coating systems. The galvanic current and analysis by optical profilometry provided quantification of the attack and allowed for the determination of acceleration factors to describe the influence of either galvanic coupling or an environment to accelerate coating degradation relative to the condition of no galvanic coupling or of another environment. An acceleration factor for the oxygen reduction reaction was also determined by comparing cathodic currents of a bare SS316 fastener in the different environments. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]