Study on corrosion mechanism of laser-clad Ni-WxC coating by in-situ electrochemical method under high temperature and pressure environment

Abstract

Corrosion protection of oil and gas equipment by laser cladding technology has emerged as a crucial method to reduce costs and increase efficiency. The in-situ electrochemical method was employed to investigate the microstructure evolution and corrosion mechanisms of laser-clad Ni1725-WxC coatings under high temperature and pressure environment. The coating is composed of cellular γ-Ni (W, Cr, Fe), block W2C, WC/W2C eutectic structure with a shell of WC, and small amount of CrxCy (Cr7C3/Cr23C6) in Ni1725 matrix. The corrosion resistance of the coating is worsened with increasing temperature under in-situ electrochemical method owing to reduced reaction activation energy and increased transfer rate. Furthermore, the surface potentials of the coating follow the order: E1 (WC shell) > E2 (Ni1725 matrix (oxide)) > E3 (WC/W2C eutectic phase) > E4 (block W2C). Therefore, galvanic corrosion occurs between WC shell and Ni1725 matrix. W2C with lower potential corrodes preferentially as well at WC/W2C eutectic structure in WxC particle, which expands with increasing temperature to form a loose internal structure.