Electrodeposition of Nanocrystalline Cobalt-Phos Coatings as a Hard Chrome Alternative

Replacement of hard chromium (Cr) plating in aircraft manufacturing activities and maintenance depots is a high priority for the U.S. Department of Defense.  Hard Cr is a technique that has been in commercial production for 50 years and is a critical process that is used both 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.  In particular, Cr plating is used extensively on hydraulic and pneumatic actuator wear surfaces. Chromium plating baths contain chromium in the hexavalent state, a known carcinogen.  Wastes generated from plating operations must be disposed of as hazardous waste and plating operations must abide by EPA emissions standards and OSHA permissible exposure limits (PEL).  OSHA recently reduced the PEL for hexavalent Cr and all hexavalent Cr compounds from 52µg/m3 to 5µg/m3. The rule also includes provisions for employee protection such as: preferred methods for controlling exposure, respiratory protection, protective work clothing and equipment, hygiene areas and practices, medical surveillance, hazard communication, and record-keeping.  Due to the expected increase in operational costs associated with compliance to the revised rules and the expected increased turnaround times for processing of components there is tremendous pressure to find an environmentally benign alternative to hard chrome.  Electrodeposited nanocrystalline cobalt-phosphorus (nCoP) coatings have been developed as an environmentally benign alternative to hard Cr coatings for non-line-of-sight (NLOS) applications under the Strategic Environmental Research and Development Program (PP-1152), and are currently going through demonstration-validation testing in a project under the Environmental Security Technology Certification Program (WP-0411).  Nanocrystalline CoP coatings show great potential as an alternative coating to hard chrome plating for NLOS as well as line-of-sight (LOS) applications due to: higher cathode efficiency, higher deposition rates, good sliding wear, and corrosion resistance properties.