Influence of Applied Potential on the Stress Corrosion Behavior of X90 Pipeline Steel and Its Weld Joint in Simulated Solution of Near Neutral Soil Environment

Pipe is the main mode of transportation of oil and gas contemporary, and its security and reliability has an important influence on the smooth development of regional economy and even the security situation. For decades, quite a number of researches have been mainly focusing on various factors on the stress corrosion cracking (SCC) of both high and middle strength pipeline steels in soil or underground water conditions, but the division of the sensitive potential ranges which determines the different SCC mechanisms was rarely reported. Soil environmental stress corrosion cracking (SCC) of pipeline steel in the process of service operation is one of the biggest security hidden dangers. The external environment SCC of pipeline steel mainly includes two modes, high pH SCC and close to neutral pH SCC. Between them, the high pH SCC occurred mainly in CO₃^2-/HCO^3- under the coating of liquid, the mechanism of cracking is widely regarded as membrane rupture, crack tip anodic dissolution mechanism; near neutral pH SCC occurred mainly in the coating containing low concentration of HCO^3- resident fluid or groundwater environment. Due to pipe in the process of serving for a long time, pipeline external coating damage and strip defects are common, under the joint action of the applied potential and soil medium, SCC will generally occur in nearly neutral pH environment, which lead to a serious risk in nearly neutral pH SCC. As a new generation of high strength pipeline steel, the X90 steel probes into its SCC sensitivity at different applied potentials in a certain pH environment is of great significance. In this work, the SCC behavior as well as its mechanism of X90 pipeline steel and its weld joint in an simulated solution of the near neutral soil environment (NS4 solution) were studied by slow strain rate tensile tests (SSRT), potentiodynamic polarization tests and SEM observation of fracture surfaces. The results showed that both the as received X90 pipeline steel and its weld joint have obvious SCC susceptibilities, which initiated and extended in transgranular cracking mode under different applied potentials. Within the potential ranges from OCP to-1000 mV, the SCC mechanism of both X90 steel and its weld joint microstructures are a combined mechanisms of anodic dissolution (AD) and hydrogen embrittlement (HE), i.e. the AD+HE mechanism. The SCC susceptibility is apparent under the OCP due to a strong AD effect. At-800 mV, the SCC susceptibility comes to a minimum due to AD and HE being weaker, and it presents the highest SCC susceptibility at-900 mV because the HE effect was greatly enhanced. The SCC susceptibility of the weld organization is higher than that of the base metal, which may be related to organization phase transformation in the welds and metallurgical reaction.