Pitting behavior of welded joint and the role of carbon ring in improving corrosion resistance

Pitting behavior and its transition process to cracking were investigated after immersion at 180 °C in pure water and 3.5% NaCl solution for NiCrMoV welded joint manufactured by narrow gap submerged arc welding technology. Dislocation outcrops in the interior of the grains formed small anodes, which mainly attributed to the pit growth for weld metal (WM) and heat-affected zone (HAZ). The serious pitting in WM was induced by strong strain concentration generating from welding thermal stress. Competition behavior between pitting and cracking for WM in pure water and chloride solution was also proposed. Chloride accelerating anodic dissolution caused serious pitting that predominated over cracking in chloride solution, while cracking preferentially formed and replaced mild pitting in pure water. In addition, carbon rings around pits decreased pitting susceptibility for HAZ. The formation of carbon rings generated from carbon diffusion was driven by strain aging and selective corrosion. Moreover, micro-Raman manifested the carbon ring was covered by protective oxide layer that inhibited pit growth and improved pitting resistance for HAZ. This work contributes to the understanding of pit growth and crack initiation for welded joint used in nuclear turbine. • Carbon rings surrounding pit in heat-affected zone protected matrix from serious dissolution and inhibited pit growth. • Strain aging facilitated carbon diffusion, and selective corrosion further drove the formation of carbon ring in heat-affected zone. • Strong strain concentration caused the higher sensitivity to pitting for weld metal. • Competition mechanism induced serious pitting in chloride solution and obvious cracking in pure water for weld metal.