An Investigation of the Hardening Mechanism of Annealed Trivalent Chromium-Carbon Deposit

• Main Authors: Yu Wai Liu, 劉育瑋
• Other Authors: C. A. Huang
• Format: Others
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/47937558507031291721

博士 === 長庚大學 === 機械工程學系 === 100 === Several researchers have pointed out that the hardness of a Cr-C deposit obtained from a trivalent Cr-based plating bath can be increased to 1600 Hv after annealing at 600 °C for 1 h. They believed that the precipitation of fine Cr-carbides is the main hardening mechanism for anneal-hardened Cr-C deposit. However, in this study, we provide evidence supporting our proposed hardening mechanism, the precipitation of a diamond-like structured membrane, for the hardening Cr-C deposited steel. The diamond-like structured membrane could be extracted from a Cr-C deposited steel by etching in a solution composed 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. And, according to our proposed hardening mechanism, a hardened Cr-C deposited steel can be accomplished by using induction- and flame-heating for a short period (~1 s). The corrosion behavior of Cr-C deposited steel specimen with Ni and Cu undercoats was studied in 0.1 M H2SO4 + 1 gl-1 NaCl solution . Experimental results show that the corrosion resistance of Cr-C deposited steel specimens strongly depends on undercoat materials. The Cr-C deposited steel specimen with a Cu undercoat has high corrosion resistance at 0 V, but it corroded easily at 0.5 V; however, high corrosion resistance of Cr-C deposited steel specimen with Ni undercoat was observed at 0.5 V, but poor corrosion resistance at 0 V. With a Ni or Cu undercoat, a soft coating between the Cr-C deposit and the steel substrate, the crack density in the Cr deposit could be markedly reduced, leading to having high corrosion resistance. The through-deposit cracks in the Cr deposit provided active paths for dissolution of Ni or Cu undercoat and even steel substrate during potentiostatic etching at 0 and 0.5 V. After potentiostatic etching at 0.5 V for ca. 900 s, some Cu deposits, which were developed by the reduction of Cu ions dissolved from the Cu undercoat, could be found adjacent to the surface cracks on the flame-heated Cr deposited specimen with a Cu undercoat. The reduction behavior of Cu ions was studied and discussed by means of immersion test.