Role of carbon in the chromium deposit electroplated from a trivalent chromium-based bath

Precipitation of fine chromium carbides have been considered to be the hardening mechanism of the anneal-hardened Cr–C deposit for a long time. However, according to this study, precipitation of carbon-based membranes may serve the hardening mechanism for annealed Cr–C deposit. The carbon-based membranes could be extracted from as-plated and annealed Cr–C deposits by etching in a solution of 33 vol.% HNO3 + 67 vol.% HCl. The structure of carbon-based membrane transforms from an amorphous carbon to a crystalline diamond-like structure with increasing the annealing temperatures. This leads to obvious increase in the hardness of annealed Cr–C deposits. Based on this mechanism, a hardened surface on Cr–C deposited steel specimens could be achieved by using reduction-flame heating for a short period (~ 1 s).