Microstructure of Ni-based hard facing alloy and its polarization behavior in aqueous solution with different pH and chloride concentration
Rongguang Wang, Yasunori Satake, Yunhui Li, Tian Xiao, Masaharu Honda, Hideki Fukuhara, Seiya Hiramatsu
Abstract
To clarify the corrosion behavior of Ni-based hard facing alloy Colmonoy 56 and the preferential dissolution detail at the micrometer scale, the microstructure and the elemental distribution of a rod were analyzed, followed with the polarization measurement in aqueous solution with different pH and NaCl concentrations. This rod has Fe-, Si-, and Cr-solutioned [Ni] matrix with heterogeneously dispersed secondary phases of Cr23C6, CrB, Cr2B, and Cr5B3. The polarization curve largely changed with different pH: active in acid solution and passive in neutral and alkaline solution. The strongest passivation appeared at pH=11. Cl− promoted the corrosion attack to the [Ni] matrix. In the case of pH=11, Cl− caused the fast climb of anodic current from passivation at a less noble potential. Preferential dissolution of the [Ni] matrix occurred in most solutions, except in the case of pH=14 without NaCl, in which the secondary phases preferably dissolved.