Study on the electrochemical behavior of the weldment for SPV50Q steel in H₂S-containing environment

The difference in microstructures of the base metal (BM), weld metal (WM) and heat-affected zone (HAZ) in the weldment is one of the major reasons for the failure of the welded equipments, which can be essentially attributed to the non-homogeneous corrosion occurred electrochemically on the weldment. Therefore, it is necessary to explore the corrosion properties of weldment. In this paper, the electrochemical behavior of SPV50Q steel weldment was investigated. The polarization curves of BM, WM and HAZ in 5wt.%NaCl-0.5wt.%HAc solution containing H₂S were measured by potentiodynamic polarization. Interface characterization was also conducted by electrochemical impedance spectroscopy (EIS). The results show the anodic curves are almost same regardless of various pH or H₂S content, but the cathodic curves show some difference. Relatively large variation in corrosion current density (i_corr) obtained by fitting technique exists among BM, WM and HAZ. i_corr of WM and HAZ is larger than that of BM, and i_corr of WM is maximum. According to EIS results, polarization resistance (R _p) increases in the orders of WM, HAZ and BM. It is concluded that WM and HAZ are less resistant to corrosion than BM, which can be correlated to the premature failure of the weldment serviced in H₂S-containing environment such as sulfide stress corrosion cracking (SSCC) and /or stress oriented hydrogen-induced cracking (SOHIC) etc.