Influence of weld preparation procedure and heat tinting on sulfide stress corrosion cracking of duplex stainless steel

摘要

Corrosion resistant alloys such as duplex stainless steel (DSS) are increasingly the material of choice for oilfield applications as a result of the trend towards more extreme temperatures, higher pressures and elevated levels of hydrogen sulfide. However, the susceptibility of such materials to localized corrosion and sulfide stress cracking (SSC) is a major consideration, particularly in the vicinity of welds. A systematic investigation has been carried out into the effect of weld oxide on the SSC susceptibility of UNS S32205 duplex stainless steel by varying the oxygen content in the backing gas to simulate leaks that can occur during welding in the field in oil and gas applications. Four-point bend testing of welded specimens under conditions close to the pass/fail boundary in the domain diagram for the material revealed a greater tendency for pitting as a function of oxygen content in the backing gas. Pitting was concentrated in the heat-tinted region whereas cracking was mainly observed in the adjacent coarse ground parent material, with surface defects from the grinding process acting as primary crack initiation sites. The results of the SSC tests were correlated with analysis of oxide composition and near-surface mechanical properties using X-ray photoelectron spectroscopy, microhardness mapping and electron back scatter diffraction measurements. Electrochemical measurements on unstressed weld specimens under similar exposure conditions showed no influence of weld oxide on the pitting potential, the absence of any effect attributable to the coarse ground surface adjacent to the weld being the more susceptible location for pit initiation and stabilization. The implications for welding procedures in the field are discussed.