Key words: X90 pipeline steel near neutral soil environment electrochemical behavior stress corrosion crackingįund: Supported by National Basic Research Program of China (No.2014CB643300), National Natural Science Foundation of China (Nos.5147101001) and the Fundamental Research Funds for the Central Universities (No.FRF-TP-15-047A3)įig.1 Microstructures of welded X90 pipeline steel (HAZ-heat affected zone, MA-martensite austenite)(a) base metal (b) weld joint (c) fusion zone (d) coarse grain hardening zone(e) coarse grain softening zone (f) fine grain zoneįig.2 Slow strain rate testing (SSRT) curves of base metal (a) and weld joint (b) of X90 pipeline steel (OCP-open circuit potential)įig.3 Loses of reduction in elongation I δ (a) and area I ψ (b) of the base metal and weld joint of X90 steelįig.4 Base metal fractographs of X90 steel fractured by SSRT(a) in air (b) OCP (c) -800 mV (d) -900 mVįig.5 Weld joint fractographs of X90 steel fractured by SSRT(a) in air (b) OCP (c) -800 mV (d) -900 mVįig.6 Side surface SEM images of the SSRT samples near main fractures of base metal (a1~c1) and its weld joint (a2~c2) of the X90 steel at OCP (a1, a2), -800 mV (b1, b2) and -900 mV (c1, c2)įig.7 Cross-section SEM images of the stress corrosion cracking (SCC) of base metal (a1~c1) and its weld joint (a2~c2) of X90 steel at OCP (a1, a2), -800 mV (b1, b2) and -900 mV (c1, c2)įig.8 Fast and slow scanning rate polarization curves of base metal (a) and weld joint (b) of X90 steel in simulated solution of near neutral soil environment 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. 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 is apparent under the OCP due to a strong AD effect. 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 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. 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. 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. 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. GALVANIZED STEEL STRESS CORROSION CRACKING MECHANISM CRACKBetween them, the high pH SCC occurred mainly in CO 3 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. The external environment SCC of pipeline steel mainly includes two modes, high pH SCC and close to neutral pH SCC. Soil environmental stress corrosion cracking (SCC) of pipeline steel in the process of service operation is one of the biggest security hidden dangers. 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. 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.
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