Characterization of as-deposited and annealed Cr-C alloy coatings produced from a trivalent chromium bath

Amorphous Cr-C alloy coatings were electrodeposited from a trivalent chromium bath containing formic acid. The effects of plating parameters such as pH and current density on composition, structure, morphology and corrosion behaviour of as-deposited coatings were investigated. Results showed that the suitable current density for Cr-C alloy electrodeposition is increased as the pH is decreased. Furthermore, by decreasing the pH, higher amount of carbon was introduced into the coatings. The Cr-C alloy coatings deposited at lower pH showed higher passive current density on Tafel plot. By annealing the amorphous Cr-C alloy coatings at 300 degrees C. a nanocrystalline structure having higher hardness was obtained. However, the corrosion resistance was seen to be impaired. Annealing at 600 degrees C led to the precipitation of Cr23C6 which resulted in a higher hardness. The formation of Cr23C6 enhanced the corrosion resistance of coatings due to an increase in corrosion potential. However, formation of micro-cells between the Cr23C6 precipitates and bulk chromium prevented the passive film formation. (C) 2010 Elsevier B.V. All rights reserved.