Military aircraft are often exposed to environmental conditions that accelerate corrosion of metallic components. For more than 70 years, cadmium coatings have been a key technology used to reduce corrosion of steel aircraft components. Depositing cadmium by electroplating is a relatively inexpensive and simple process; however, because cadmium is a toxic metal and human carcinogen, strict environmental and occupational safety and health regulations drive the search for alternative coatings. Several alternatives to cadmium have been identified although none meet all of the processing and functional performance characteristics of cadmium plating. Aluminum-manganese plating is a non-line-of-sight technology that was expected to closely retain the functional performance of cadmium compared to other alternatives. The objective of this project was to demonstrate the feasibility of molten salt plating as an alternative to cadmium electroplating at a Department of Defense (DoD) depot.
Aluminum-manganese molten salt plating is conducted in a salt bath with a nominal weight percent composition of 79% aluminum chloride, 10% sodium chloride, 10% potassium chloride, and 1% manganese chloride. Application of direct current to 1100 aluminum alloy anodes causes an aluminum-manganese deposit to form on the steel component surface. The deposit provides anodic protection of the steel, a good surface for paint adhesion due to hill-and-valley type surface, and lubricity similar to a cadmium-plated surface.
In the early stages of this ESTCP project, a 1-gallon molten salt plating process was demonstrated at Naval Air Systems Command, Patuxent River. The laboratory-scale process showed potential for a scaled-up process. Characteristics such as longevity and durability of the plating solution, corrosion protection provided by the deposit, and the uniform, nodular structure of the deposit all indicated the high potential for success of aluminum-manganese plating.
A 200-gallon molten salt plating system was designed and installed for demonstration at Naval Aviation Depot North Island. An initial charge of 25 gallons of plating bath chemicals was prepared and loaded into the depot system. However, the system was shut down before plating could be attempted due to visible plating bath fume generation and subsequent issues that could not be resolved without substantial time and funding. It was determined that the plating bath would require an inherent fume suppressant for the process to be feasible in a production size system. A scrubber would be sufficient to capture and treat the fumes but would not address the crust formation on ventilation and plating tank inner surfaces that results from the fuming. Future pursuits of molten salt plating should include a research and development effort to identify a suitable bath chemistry that eliminates fuming.
Several benefits from implementation of aluminum-manganese molten salt plating can be realized by DoD if a plating bath chemistry is developed that eliminates visible fuming. The bath constituents and coating deposit are environmentally preferable compared to cadmium. In addition, this technology shows an improved coating performance compared to other cadmium alternatives specifically in the areas of sacrificial corrosion, in-service hydrogen embrittlement, and lubricity. (Project Completed – 2006)