土壤中硫酸盐还原菌对高强度管线钢应力腐蚀作用机理研究

Studies of stress corrosion cracking of buried pipeline steels mainly focus on effects of soil chemical and electrochemical factors, however, effects of microbes on stress corrosion cracking are seldom reported. Amounts of cases on stress corrosion cracking of buried pipelines show that stress corrosion of pipeline steels is markedly influenced by microbes. Through field burying test and laboratory simulation test, the effects of sulfate-reducing bacteria (SRB) on stress corrosion cracking of high-strength pipeline steel and welded joint, and synergistic effects of SRB influenced corrosion and stress corrosion cracking are investigated in soils by using mechanics, electrochemistry, microbiology, chemical analysis, and surface analysis. Distributions of biofilm on different structures (such as texture, grain, grain boundary, granular second phase, inclusion, etc) of high-strength steels and welding heat-affected zone at different polarization potential, and effects of biofilm on hydrogen permeation of high-strength pipeline steel, effects of biofilm on initiation and expansion of pitting and stress corrosion at different evolutionary stages are studied. Functions of hydrogen, hydrogen sulfide, organic and inorganic acid, sulfide of corrosion products are also researched during the course of local chemical process at the tip of cracking. Theory and method of safety evaluation of stress corrosion cracking expansion of high-strength steels are established under the action of SRB in soils. The finish of this project will provide theoretical basis for performance optimization, protection of pipeline, and safety-evaluation of high-strength pipeline steels.

迄今有关地下管线钢应力腐蚀的研究，主要考虑了土壤化学和电化学因素的作用，尚少涉及土壤微生物对应力腐蚀的影响,而大量现役地下管线腐蚀开裂分析表明微生物参与并显著影响了管线钢的应力腐蚀行为。本项目拟开展现场埋设试验及室内模拟试验，利用力学、电化学、微电极、微生物、化学分析、表面分析等测试技术，研究土壤中高强管线钢及其焊接接头的硫酸盐还原菌腐蚀和应力腐蚀的协同作用规律；研究微生物膜对高强管线钢氢渗透影响机理；微生物膜在管线钢基材及其焊接接头等显微组织不同部位的分布规律；微生物活动产生的硫化氢、氢、有机酸、无机酸、胞外聚合物及腐蚀产物铁硫化物等对高强管线钢及其焊接接头点蚀及应力腐蚀裂纹萌生和扩展不同演化阶段的影响机制，以及在裂纹尖端局部化学过程的作用机理，建立土壤中硫酸盐还原菌影响下高强管线钢应力腐蚀裂纹扩展安全评价理论及方法，为高强管线钢的性能优化、防护及安全评价提供科学依据。

微生物腐蚀(MIC)和应力腐蚀开裂(SCC)是埋地管线腐蚀失效的重要形式，已经引起世界各国研究者的关注。现场调查和实验研究表明微生物参与并显著影响了管线钢的SCC过程。但是，微生物在管线钢SCC过程中的作用尚不清楚，土壤环境中微生物和外加应力下管线钢腐蚀行为和机理亟待澄清。.本文通过慢速应变速率试验、电化学氢渗透试验及恒载荷试验，研究了硫酸盐还原菌(SRB)和阴极保护(CP)对管线钢SCC敏感性的影响，氢在管线钢SCC过程中的作用， SRB对管线钢腐蚀动力学的影响，分析了SRB和静载荷共同作用下管线钢表面微裂纹萌生和扩展规律，提出了管线钢菌致开裂的“连续点蚀”机理。主要研究进展如下：.1. 研究了SRB和CP电位共同作用下管线钢SCC敏感性的变化规律。随CP电位负移，X80管线钢SCC敏感性升高；SRB对管线钢SCC有促进作用，特别是SRB处于对数生长期最为明显。SRB和CP电位共同提高管线钢SCC敏感性，随着CP电位负移，这种联合作用减弱。.2. 研究了X80管线钢微生物腐蚀中氢的渗透规律和作用。SRB和CP电位加速管线钢的氢渗透过程；管线钢SCC敏感性和表面可渗透氢浓度在双对数坐标中遵循同样的直线关系，表面可渗透氢浓度是判定管线钢SCC敏感性的重要指标；可以通过测量不同环境条件下表面有效氢浓度预测管线钢SCC敏感性；SRB的存在使得CP电位范围缩小，施加阴极保护的金属构件必须考虑SRB对氢渗透性及力学性能的影响。.3. 研究了不同载荷条件下SRB对管线钢腐蚀动力学的影响。SRB和外加应力不仅单独促进管线钢的腐蚀，还能协同加速管线钢的腐蚀。SRB及其代谢产物硫化物加速管线钢点蚀；外加应力造成低浅点蚀坑和微裂纹尖端的应力集中；SRB和应力协同诱发低浅点蚀坑底部的“连续点蚀”和裂纹壁上的“连续开裂”，进而协同加速管线钢腐蚀和开裂过程。.4. 腐蚀驱动力计算表明外加应力和SRB同时存在时管线钢阳极溶解驱动最大，这是管线钢菌致开裂的本质原因；阴极保护条件下氢渗透加快，管线钢菌致开裂行为受氢扩散的调制，而开路电位下管线钢的菌致开裂行为以点蚀机理为主。