Microstructure of Ni based hard facing alloy and its polarization behavior in aqueous solution with different pH and chloride concentration

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 solutions 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.