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

摘要

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 g/m3. 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 g/m3. A Navy/Industry 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 g/m3. 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 ceramic/metal (cermet) such as tungsten carbide/cobalt (WC/Co) 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.