Effects of Co-mingling Dissimilar Fastener Coatings on the Corrosion Behavior ofSteel Bolt Assemblies

摘要

Accelerated corrosion tests were carried out to assess the performance ofselected coated fastener assemblies representative of those used in the M1A1 tank and Bradley fighting vehicle (coated grade 8 steel bolts fastened to armor steel and AI 5083 blocks). The coatings tested included the currently used electroplated cadmium and zinc as well as proposed electroplates of Zn-Ni, Zn-Co, Sn-Zn and a zinc phosphate modified polyacrylic acid conversion coating. The effect of co-mingling cadmium with the other coatings in the various assemblies was determined. Salt spray testing was performed in accordance with ASTM B117. In addition salt water immersion tests were performed and continuously monitored by electrochemical potential measurements. Breakaway torque values obtained after completion of either immersion or salt spray tests were correlated with coefficient of friction measurements. Cadmium electroplate exhibited the best corrosion protection for grade 8 steel bolts in salt spray or immersion tests. The zinc-nickel coating was the best overall alternative to cadmium plate. The modified zinc phosphate conversion coating was clearly unacceptable. Co-mingling of cadmium with the other coatings did not create a galvanic corrosion problem. Loosening of the Zn and Zn alloy coated fasteners will not likely occur when Cd torque values are used.