Replacement of Chromium Electroplating on Gas Turbine Engine Components using Thermal Spray Coatings

Electrolytic hard chrome EHC plating is a technique that has been in commercial production for more than 50 years. It is a critical process used for applying hard coatings to a variety of aircraft components in manufacturing operations and for general re-build of worn or corroded components that have been removed from aircraft during overhaul. Chromium plating baths contain chromic acid, in which the chromium is in the hexavalent state, with hexavalent chromium hex-Cr being a known carcinogen. During operation, chrome plating tanks emit a hex-Cr mist into the air, that must be ducted away and removed by scrubbers. Wastes generated from plating operations must be disposed of as hazardous waste, and plating operations must abide by U.S. Environmental Protection Agency EPA emissions standards and Occupational Safety and Health Administration OSHA permissible exposure limits PEL. Recent studies have clearly shown that there are a significant number of excess deaths at the current PEL of 100 gm3. OSHA is currently under court order to establish a new hex-Cr PEL by January 2006, and the metal finishing industry expects it to be in the range of 1 gm3. A NavyIndustry task group concluded that the cost of compliance for all Navy operations that utilize hex-Cr i.e., not just plating would be in excess of 10 million if the PEL was reduced to less than 5 gm3. Previous research and development efforts had established that high-velocity oxygen-fuel HVOF thermal spray coatings are the leading candidates for replacing hard chrome. HVOF thermal spraying can be used to deposit both metal alloy and ceramicmetal cermet such as tungsten carbidecobalt WCCo coatings that are dense and highly adherent to the base material. They also can be applied to thicknesses in the same range as that used for EHC. Currently, there are HVOF thermal spray systems commercially available.