X80管线钢焊接接头非均匀梯度特征对H2S腐蚀的影响机理研究

Hydrogen sulfide (H2S) corrosion is one of the main threats that the pipeline faces. The H2S corrosion for every zone in welded joint is diverse because of the microstructure gradients and residual stress gradients. The local corrosion perforation and breakage can be caused by the corrosion inhomogeneity in welded joint, which threatens the pipeline operation safety seriously. Many researchers studied the corrosion behavior in every zone of welded joint separately and ignored the interaction effect among different zones. In the present study, the X80 pipeline steel welded joint will be immersed in the H2S environment entirely to study the distinction for corrosion process in different zones. The electrochemical corrosion kinetics for welded joint will be analyzed and the influence mechanism of galvanic effect, introduced by microstructure gradients, on corrosion process will be studied. In addition, the hydrogen distribution regular in welded joint will be obtained by hydrogen permeation tests combined with calculated results. Effects of the different corrosion products formed on different zones on hydrogen permeation will be analyzed and effects of residual stress on hydrogen traps, like dislocations and grain boundaries, will be studied. What’s more, the effect mechanisms of inhomogeneity in welded joint on the sulfide stress corrosion cracking will be studied to analyze the effects of microstructure gradients, hydrogen distribution regular and corrosion product films on the origin and extension of cracking. This research will help clarify the H2S corrosion failure mechanism in high strength pipeline steel welded joint, and provide the theoretical basis to improve weld quality of pipelines.

H2S腐蚀是管线面临的主要威胁之一。显微组织梯度和残余应力梯度的存在使焊接接头各区域的H2S腐蚀存在差异，接头腐蚀的不均匀性可造成局部穿孔或断裂的发生，严重威胁管线运行安全。以往的研究往往将接头各区域分开单独研究，忽略了各区域之间的相互影响。本项目采用X80管线钢焊接接头整体浸泡的方法，研究不同区域腐蚀过程的差异性，解析焊接接头电化学腐蚀动力学，分析组织梯度引起的电偶效应对腐蚀过程的作用机理。通过氢渗透实验结合软件分析，获得焊接接头氢分布规律，分析各区域腐蚀产物膜特征的差异性对氢渗透的影响，并探究残余应力对位错、晶界等氢陷阱的束缚氢能力的影响。研究焊接接头的非均匀性对硫化物应力腐蚀开裂的作用机理，分析显微组织梯度、氢分布规律和腐蚀产物膜特征对裂纹起源和扩展的影响。该研究成果有助于揭示高强管线钢焊接接头在H2S环境中的腐蚀失效机理，为管线的焊接质量控制提供理论依据。

本项目以X80管线钢焊接接头为研究对象，研究了各区域显微组织及M/A组元特征的差异，并分析了峰值温度、冷却速度等对粗晶区、临界粗晶区显微组织的影响；研究了焊接接头存在的电偶效应，采用整体浸泡的方式研究了各区域在H2S环境中电化学腐蚀行为的差异；探究了焊接热影响区氢渗透行为的差异及其机理；分析了焊接接头硫化物应力腐蚀开裂行为及其机理。研究发现，焊接接头各区域显微组织差异明显；随着峰值温度的升高，焊接热影响区M/A组元的含量逐渐降低；一次峰值温度的差异会影响临界粗晶区M/A组元严重粗化的临界温度，超过上临界温度后，临界粗晶区M/A组元不再粗化，但其含量不变；冷却速度越快（冷却时间越短），M/A组元粗化程度越严重。在H2S环境整体浸泡过程中，各区域电化学腐蚀行为差异较大且与浸泡时间有关；在24-72h内时耐蚀性顺序为热影响区>焊缝>母材，腐蚀96h后转变为焊缝>母材>热影响区，这与各区域腐蚀产物特征的转变有关；在各区域均存在一定的腐蚀不均现象，但不存在点蚀孔，腐蚀凹槽主要位于M/A组元处。与母材相比，热影响区更容易引起氢的聚集；随着峰值温度升高，热影响区氢的聚集程度降低，这与M/A组元含量减少有关；增加冷却速度，粗晶区氢的聚集程度上升，主要是出现了部分马氏体，且铁素体含量降低所致；发生M/A组元严重粗化的临界粗晶区的氢聚集程度远大于未粗化的，且冷速越快，聚集程度越大。在H2S环境中，X80管线钢母材和焊接接头的塑性和强度损失均较大，且焊接接头的硫化物应力腐蚀开裂敏感性远高于母材；一般断裂在热影响区，这与其电化学腐蚀速率较快且不均匀、氢容易聚集有关。基于上述创新性成果，项目资助发表论文9篇，录用2篇，申请或授权专利6项，培养硕士生8名，其中3名已毕业。项目投入直接经费24万元，支出15.1357万元，各项支出均按照预算进行，剩余经费8.8643万元，计划用于本项目研究的后续支出。