Selective Brush Plating a Tin-Zinc Alloy for Sacrificial Corrosion Protection
By Zhimin Zhong, Sidney Clouser and Derek Vanek, Sifco Applied Surface Concepts
Editor's Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2013 in Rosemont, Ill., on June 11, 2013. A printable PDF version is available by clicking HERE.
A tin-zinc selective brush plated coating has been developed to provide sacrificial corrosion protection without the use of toxic chemicals. The coating is 80% tin, 20% zinc by weight. Neutral salt spray tests on the brush plated coatings have shown basis metal protection up to 500 hr. with a 0.5 mil coating. When combined with the appropriate conversion coating, trivalent or chromium-free, the formation of white zincate corrosion products is prevented for 96 hr. of salt spray. When the deposition is carried out at >50°C tests with Type 2a hydrogen embrittlement (HE) rings and Type 1a.1 notched bars meet the ASTM 519 requirement without a post-plating HE relief bake. Plating at lower electrolyte temperature requires an HE relief bake after plating. The research work identified a neutral nickel coating that can be applied prior to the tin-zinc coating plated at room temperature which eliminates the need for a post plating HE relief bake for high strength steel.
Keywords: selective plating, tin-zinc alloys, hydrogen embrittlement, ASTM 519, sacrificial corrosion protection
Sacrificial coatings are applied to surfaces both by bath and by selective brush plating to protect the underlying substrate from corrosion. The sacrificial coating corrodes in preference to the substrate, a property which is especially important when the substrate is scratched or damaged. Cadmium has long been used to provide this protection for structures, equipment and fasteners. It is easy to plate and it has desirable properties that include corrosion protection, lubricity, anti-galling, electrical conductivity and low hydrogen embrittlement.
Due to cadmium’s toxic nature, there is a tremendous amount of pressure to stop its use. Environmental concerns and regulatory mandates are driving manufacturers to find alternatives for cadmium. Therefore, identifying a suitable, non-embrittling cadmium replacement is of great interest. Tin-zinc and other zinc alloys are finding use in several industries for this purpose.
Tin-zinc alloys offer excellent sacrificial corrosion protection for steel by combining the barrier protection of tin and the galvanic protection of zinc without the bulky corrosion product associated with a simple zinc coating.1-16 In addition to corrosion protection, the tin-zinc alloy provides good lubricity, wear resistance and excellent solderability. Sifco Applied Surface Concepts has developed a new, non-embrittling, tin-zinc brush plating process. The sacrificial coating can be selectively applied to variety of surfaces including high strength steels in aircraft landing gear (Fig. 1).