Coupling effect of microstructure and hydrogen absorbed during service on pitting corrosion of 321 austenitic stainless steel weld joints

Abstract Pitting corrosion of austenitic stainless steel weld joints served in a hydrocracking unit for 6 years was examined. The microstructure of weld metal (WM) and fusion line (FL) was austenite and ferrite. After the immersion test, many pits were observed on the FL, whereas few pits were observed on the heat-affected zone (HAZ). Pits were microscopically initiated at ferrite/austenite boundaries. Hydrogen concentrations at FL and WM were higher than those of HAZ, and hydrogen segregation occurred at ferrite/austenite boundaries, which was consistent with the pit position. Hydrogen enrichment conditions varied under different microstructures, which induced the difference in pitting susceptibility.