Fastener Assembly Galvanic Corrosion Testing

This test programme assessed the ability of Gentoo Coating to reduce galvanic corrosion in real fastener assemblies — evaluating both bare and cadmium-plated steel fasteners on painted aluminium alloy panels under accelerated salt fog conditions.

Test Configuration

316 stainless steel screws and washer assemblies were prepared — a subset bare and a subset coated with Gentoo. The fasteners were attached to primed and topcoated 7075-T6 aluminium alloy (UNS A97075) coupons and exposed to ASTM B117 accelerated corrosion conditions.

Fastener assemblies consisted of ¼-20 x 1 inch full-thread screws with corresponding mated flat washers. Aluminium coupons (approximately 1 × 1.375 × 0.032 inch) were deoxidised, pre-treated with a non-chromate adhesion promoter, primed with MIL-PRF-23377 epoxy primer, and topcoated with MIL-PRF-85285D grey polyurethane. Coupons were hand-scribed prior to fastener attachment to expose fresh bare metal — simulating the surface damage that occurs in service.

Additional fastener sets included cadmium-plated B7 steel fasteners (ASTM F1941 FE/CD 5A) alongside bare B7 steel fasteners on identical aluminium panels. All assemblies were tested in triplicate. Following exposure, panels were solvent stripped per ASTM G1 guidelines and assessed for weight loss and pit depth measurements.

Results — Bare B7 Steel Fastener Assemblies

After 648 hours of ASTM B117 salt fog exposure, significant corrosion product was evident on the bare B7 steel fastener assemblies on the control (untreated) panels. Gentoo-treated assemblies showed measurably less corrosion development at the same exposure duration — visually confirmed and quantified through weight loss and pit depth measurements of the stripped aluminium panels.

Results — Cadmium-Plated B7 Steel Fastener Assemblies

Cadmium-plated B7 steel fasteners are more corrosion-resistant than bare steel — representing a step up in baseline protection. Despite this, after 1,340 hours of ASTM B117 exposure, significant corrosion product was evident on the control (untreated) cadmium-plated assemblies. Gentoo-treated assemblies demonstrated significantly less corrosion at the same exposure duration.

Following stripping and cleaning per ASTM G1, weight loss and pit depth measurements confirmed that Gentoo treatment of both the fasteners and the painted aluminium panels produced improved corrosion resistance across both bare and cadmium-plated assembly types.