海水中阴极极化对X80钢应力腐蚀及氢脆敏感性的影响

目的研究不同p H值海水中阴极极化对X80管线钢应力腐蚀及氢脆的抑制作用。方法采用慢应变速率拉伸试验、电化学测试、微观组织观察等分析方法。结果 X80钢在天然海水中的析氢电位约为-940m V(vs.SCE,下同),海水p H为3.5时析氢电位发生正移。其应力腐蚀敏感性与极化电位有很大关系,随着极化电位负移,X80钢的氢脆敏感性增加。天然海水中当极化电位负于-950 m V时,断口出现准解理断裂特征形貌。在-1050 m V极化电位下,钢材进入氢脆断裂区发生脆性断裂。海水p H为3.5时,-900 m V钢材有发生氢脆的危险。结论与天然海水相比,X80钢在p H为3.5的酸性海水中具有较高的应力腐蚀敏感性,两种海水介质中X80钢的应力腐蚀敏感性均随极化电位负移而增加。

Effects of Cathodic Polarization on Stress Corrosion Cracking and Hydrogen Embrittlement of X80 Steel in Seawater

Objective To study the inhibition effects of cathodic polarization of X80 pipeline steel to stress corrosion cracking and hydrogen embrittlement in seawater of different pH value. Methods The slow strain rate test, electrochemical test and SEM observation of fracture surfaces were applied. Results The hydrogen evolution potentials of X80 steel in seawater was about -940 mV (vs.SCE, similarly hereinafter). Shifting potential appeared in the positive direction when pH value of seawater was 3.5. The cathodic potential had strong influences on SCC susceptibility of the X80. It became more positive with the increase of cathodic potential. The susceptibility to hydrogen embitterment increased with shifting potential in the negative direction. When the polarization potentials were negative to -950 mV, the fracture surfaces exhibited quasi-cleavage fracture; at -1050mV, the steel reached into the dangerous zone of hydrogen embitterment, and the fracture was brittle. When the pH was 3.5, X80 steel was in danger of hydrogen embitterment. Conclusion Compared with natural seawater, X80 steel has a high sensitivity of stress corrosion cracking when the pH of seawater is 3.5. The sensitivity increases with the potential negative shift in both two kinds of seawater.