采用慢应变速率拉伸实验和恒电位浸泡实验,分析了不同电位下X80管线钢在高pH环境中的应力腐蚀开裂(SCC)行为和机制.结合不同扫描速率下的动电位极化曲线的测量,从热力学角度对SCC的敏感电位区间进行了划分.在综合考虑材料的最大断裂强度和面缩率损失的前提下,提出了评价SCC敏感性的综合指标I_∑,并将适用于酸性土壤模拟溶液中的SCC敏感性判据拓展到高pH环境中对其进行了验证.结果表明, X80钢在本实验溶液中的SCC敏感电位区间可以划分为: -600 — -569 mV, 受阳极溶解(AD)机制控制,沿晶应力腐蚀开裂(IGSCC)较为敏感, 铁素体晶界优先溶解; 在开路电位E_ocp­ — -600 mV, SCC敏感性最低,以点蚀的发生和发展为特征,单纯的AD机制不会导致显著的穿晶应力腐蚀开裂(TGSCC)敏感性;在析氢电位-1046 mV—E_ocp,在AD和氢脆(HE)机制的共同作用下, X80钢的SCC敏感性最高,裂纹以TGSCC的形式扩展, 点蚀对TGSCC裂纹的萌生和扩展起到关键作用;低于-1046 mV, X80钢在HE的作用下断裂, 表现出较高的脆性. 经计算验证,采用I_∑作为评价SCC敏感性的指标是可行的.

Most of the current researches are only limited to the evaluation of high pH stress corrosion cracking (SCC) susceptibilities of pipeline steels at some certain potentials, but the division of the sensitive potential ranges controlled by different SCC mechanisms is rarely reported. When the results of SCC susceptibilities adopted by using the ultimate fracture strength, the reduction-in-area and the elongation rate separately are not consistent, how to evaluate the SCC susceptibilities taking into account both the loss of fracture strength and toughness also needs solving. In this work, the slow strain rate tensile tests were conducted to evaluate the SCC susceptibilities of X80 pipeline steel in concentrated carbonate/bicarbonate solution at different applied potentials. The immersion tests at several constant potentials were used to analyze the potential sensitivity of electrochemical corrosion behavior and its influence on the SCC controlling mechanisms. With the aid of potentiodynamic polarization tests and the thermodynamical calculation, the typical potential ranges of different SCC controlling mechanisms were divided. Considering both of the ultimate fracture strength and reduction-in-area loss, a comprehensive index I_∑ for estimating SCC susceptibility was established and verified. The main results showed that at -580 mV, the ferritic intergranular was corroded preferentially, and X80 steel exhibited remarkable intergranular SCC (IGSCC) with anodic dissolution (AD) as the controlling mechanism. At -750~mV, the ferritic grain was prone to be dissolved causing the occurrence of pitting, but the steel showed very low SCC susceptibility, which indicated that high transgranular SCC (TGSCC) susceptibility could not be triggered only by AD. The open circuit potential E_ocp could be seen as a transition potential between pitting and TGSCC zone, at which the SCC susceptibility was slightly enhanced. At -880 mV, the steel showed tremendous TGSCC susceptibility due to the synergistic effects of AD and hydrogen embrittlement (HE), although it was protected by the applied cathodic potential. And the SCC behavior mainly controlled by HE at -1200~mV, showing quasi-cleavage fracture mode. Therefore, the typical potential ranges could be divided into four categories: -600 —­ -569~mV for IGSCC, E_ocp— -600 mV for pitting, hydrogen evolution potential -1046 mV—E _ocp for TGSCC, and below -1046 mV for HE,i.e., quasi-cleavage cracking. By applying the criteria proposed in acidic soil simulated solution to illustrate the SCC susceptibility in this work, the comprehensive index I_∑ was proved to be reasonable, which provided a supplementary method for evaluating the SCC susceptibility.