Objective

Electroplated cadmium is utilized in numerous Department of Defense coating applications to provide corrosion protection, low coefficient of friction, and/or solderability. However, cadmium as a metal and cadmium deposition processes are extremely toxic to the environment and to humans, and release of cadmium is strictly controlled both by the Environmental Protection Agency and the Occupational Safety and Health Administration. Environmental concerns and an increase in the cost of cadmium finishing have triggered initiatives focused on the development of environmentally-benign materials and processes as a substitute in cadmium applications.

This project sought to develop novel, low-cost, and environmentally-benign electrodeposition processes for the production of alloy coatings based on manganese (Mn) and/or tin (Sn), which combine high-corrosion protection performance, good tribological behavior, and suitable mechanical properties, for use as an alternative to cadmium.

Technical Approach

The only known alternatives to cadmium for the production of coatings which offer sacrificial protection to steel are zinc and manganese. New chemistries, complexing agents, and additives were employed in the development of electrolytes and electrodeposition processes for the production of manganese-based binary and ternary alloys. Initial efforts were dedicated to the investigation of Mn-Sn electrodeposition, where the deposition potential of tin was shifted in the cathodic direction by experimenting with a series of complexing agents. Of particular interest among these is tin methanesulfonate (CH3SO3)2. This complex is environmentally friendly, yields deposits of very high quality, and is being substituted for established processes in the electronic industry for tin and lead electrodeposition. In order to develop selected electrolytes and electrodeposition processes to the point where they could be successfully transferred to manufacturing lines, it was important to optimize the throwing power of the electrolyte. Complex parts (e.g., threaded fasteners, boxes, springs, shims with through holes, and couplings) were plated with the alloy coating of choice, and the variation of coating thickness with position was determined by the use of metallographic cross sections. The results were compared with current standards of thickness uniformity for cadmium plates.

Results

Mn-Sn alloy coatings combine the sacrificial behavior of Mn with the protection afforded by Sn. However, coatings are often heterogeneous and their corrosion performance is likely limited by the formation of corrosion couples. In fact, coatings with a high percentage of the intermetallic Mn1.77Sn offer the best corrosion behavior. Copper (Cu)- Mn composition coatings increased the stability of the ductile crystalline structure and present the capability of forming a partially protective layer to slow alloy corrosion. In addition, the tribological properties of Cu- Mn coatings appear comparable, if not superior, to those of cadmium. However, the corrosion resistance of Cu-Mn is still not comparable to that of cadmium reference films, and further improvements in alloy formulation might be needed. A route towards further improvements of these alloys is the investigation of ternary Cu-Mn-Sn coatings of suitable composition. Although difficult to synthesize reliably, the potential characteristics of these alloys warrant further research.

Benefits

Successful development and implementation of this technology is expected to eliminate the toxic fumes, hazardous waste streams, and increasing cost associated with the production of cadmium plating. (SEED Project Completed – 2002)

  • Coating