Cadmium Replacement Alternatives for Corrosion and Hydrogen Embrittlement Protection of High Strength Steels

The Navy is working to eliminate the use of cadmium and hexavalent chromium in shipboard applications.  The use of cadmium plating by the Navy is undesirable due to occupational health and safety concerns and the potential for hydrogen embrittlement of steel as a result of either the plating process and/or the corrosion process.  Cadmium is a highly toxic metal and a known carcinogen.  Toxic fumes are emitted if used in the presence of hot work at temperatures above 320°C.  OSHA has established that workers should not be exposed to airborne concentrations of cadmium in excess of the permissible exposure limit of 5 µg/m³.  There is an absolute need for a new, long-lasting, advanced inorganic coating system that will provide sacrificial protection and help reduce concerns for hydrogen embrittlement.  Potential Navy applications for these alternative coating systems include fasteners and electrical connectors.  The thrust on the discovery and invention tasks requires innovative processes and materials to develop new coating systems to replace cadmium plating.  The substitute must offer good corrosion protection as a sacrificial coating that is anodic to steel and also serve as a strong barrier to hydrogen entry into the steel substrate.  The latter is particularly important in the prevention of hydrogen embrittlement.  Replacement of cadmium with materials like Zn-Ni-X where X = P, SiO₂, trivalent chromium, etc. or any other metal system that is non-toxic, offers good sacrificial properties and their corrosion products serve as effective lubricants.  Candidate alternative coating systems are being evaluated through laboratory testing in terms of corrosion protection properties and resistance to hydrogen embrittlement.  Results obtained from the laboratory evaluation are discussed in this paper.