Traditionally, high-strength steel fasteners have been used with cadmium electroplated coatings followed by a hexavalent chrome rinse. The environmental and health hazards associated with cadmium and hexavalent chromium are well established, and significant legislation has been put in place to limit their continued usage. It is critical that suitable replacement technologies be identified that can provide these high performance characteristics in a cost-effective manner.
The objective of this project was to develop new environmentally benign coating technologies that could eliminate the need for cadmium or hexavalent chromium coatings on high-strength fasteners. These technologies would provide a coatings system for high-strength, corrosion resistant fasteners for use on both legacy weapon systems and new weapon platforms.
In this project, a multilayer coating system was developed that consisted of a conventional zinc phosphate pretreatment followed by a zinc flake-based primer, then a cationic electrodeposition topcoat. Additionally, a silver aluminum-based topcoat was developed for application where electrodeposition equipment was not available. The bulk of the work focused on developing novel inorganic binders and zinc flake technology for the sacrificial zinc primer. This primer is superior over current zinc rich primers for corrosion performance and adhesion when used with the cationic electrodeposition topcoat. The topcoats were formulated for adequate torque/tension performance and color variations.
The multilayer coating system exhibits excellent corrosion resistance and represents a viable, environmentally friendly alternative to cadmium/chromium coatings for high-strength fasteners. The primary system includes zinc phosphate conversion coating, dip-spin application of an inorganic zinc rich primer, followed by an epoxy topcoat applied by cationic electrodeposition. An alternative aluminum rich topcoat has also been developed for dip-spin application where electrocoat is not available. Both coating systems meet the minimum 1000 hours of salt spray performance, meet torque/tension requirements, and are free of cadmium, chromium, lead, and nickel. The electrodeposition topcoat offers an environmentally benign coating alternative and is an excellent method for bulk application such as fasteners.
The coatings and processes were tested in production-scale environments and are ready for commercial distribution. Testing and approvals to automotive, industrial, and construction specifications are under way with positive results. The zinc rich plus electrocoat finish has been used commercially in a dip-drain application successfully.
Eliminating hazardous materials currently used in anticorrosion coatings for high-strength fasteners will improve the life-cycle costs as well as environmental and health issues associated with the manufacture and maintenance of DoD weapons systems. The environmentally benign coating technologies developed would impart high-end sacrificial corrosion resistance without the use of hazardous chrome or cadmium and without embrittlement of fasteners.