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

Abstract

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 °C, a nanocrystalline structure having higher hardness was obtained. However, the corrosion resistance was seen to be impaired. Annealing at 600 °C led to the precipitation of Cr 23C 6 which resulted in a higher hardness. The formation of Cr 23C 6 enhanced the corrosion resistance of coatings due to an increase in corrosion potential. However, formation of micro-cells between the Cr 23C 6 precipitates and bulk chromium prevented the passive film formation.