Traditionally, steel fasteners have been used with cadmium electroplated coatings followed by a hexavalent chrome rinse. This provides effective and long lasting corrosion protection. However, environmental hazards associated with both cadmium and hexavalent chromium are well established and significant legislation has been put in place recently to limit their continued usage. The overarching objective of this project was to complete the following tasks:
Two separate fastener coatings systems were selected for the proposed demonstration and validation program. Both coating systems included a sacrificial metal base coat, zinc or aluminum, and an organic electrocoat topcoat technology that eliminates environmental concerns associated with currently fielded cadmium and/or chromium containing coating systems.
Performance was assessed through lab testing of coated coupons and fasteners, as well as field testing of coated fasteners installed on Medium Tactical Vehicle Replacement assets in multiple locations. Performance measures included cyclic and neutral salt spray, hydrogen embrittlement, adhesion and torque tension among other tests. Both zinc rich and high purity plating systems performed nearly as well as the incumbent cadmium/hex chrome systems and significantly better than zinc phosphate-only fasteners. The zinc rich/electrodeposition topcoat option is cost effective for substantial corrosion improvement in applications where plain phosphated fasteners are currently being used. For applications where cadmium is currently being used, both of the candidates coating systems are potential replacement candidates depending on cost restraints and performance needs.
The electrodeposition topcoat is necessary over sacrificial coatings for lubricity and corrosion performance, however, this additional coating layer can exceed tolerances for fit, and impair hand threadability. The fastener supply chain is also exceedingly complex due to the number and type of fasteners utilized. Much of the difficulty lies in changing supply chain at the depot refit level, where most of the cadmium usage still occurs. Many original equipment manufacturers have already transitioned to alternative coatings. However, depots are bound by legacy drawings and National Stock Number designations. Therefore the task of implementing a new coating system remains daunting.