低Cr管线钢在油气田CO2环境下耐蚀机理研究

More than 60% corrosion failure of oil/gas pipeline happened on carbon steel serving CO2. Adding appropriate amount of Cr to carbon steel can greatly suppress localized corrosion. So, anti-CO2 low Cr steel and its corrosion mechanism become research hot spot. The applicant found that active-passive transition region occurred when low Cr steel was anodic polarized. But stable broad passive region like typical passivation was not found. This phenomenon was called semi-passivation by the applicant. The applicant also proposed the mechanism of corrosion kinetics causing semi-passivation. In order to improve research efficiency, the influence of water chemistry, temperature and CO2 partial pressure on corrosion behavior of low Cr steel, which is conducted principally by researches home and abroad, is not studied in this project. Our study focus on the change of potential and solution pH near metal surface during the corrosion process. Monitoring the change of electrochemical property of metal/solution interface by multiple in-situ electrochemical methods. Mechanism of corrosion kinetics at the early stage of corrosion is thoroughly studied. Phase diagram of high temperature and high pressure saturate CO2-H2O system and concentration of Cr/Fe in the solution are also studied. The composition and layered structure of corrosion scale are investigated by autoclave tests. Based on the above results, the reason and requirement of semi-passivation are investigated. Then, mechanism of Cr suppressing localized corrosion is studied and the way of Cr enrichment under flowing condition of pipeline is discussed. The corrosion mechanism of low Cr steel is hopeful to be revealed, which provides theory support for the wide use of low Cr pipeline steel.

60%以上的油气管线腐蚀失效发生于含CO2的碳钢管线，在碳钢中添加适量Cr可显著抑制局部腐蚀，因而抗CO2低Cr钢及其腐蚀机理成为研究热点。申请者发现，低Cr钢阳极极化时出现了活化-钝化转变区，但没有典型钝化那样的稳定宽钝化区，将其称为准钝化，并提出准钝化的腐蚀动力学机制。为提高研究效率，本项目绕开了国内外学者曾大量开展的溶液成分、温度和CO2分压等因素对低Cr钢耐蚀性能的影响研究，以腐蚀过程中电位变化和钢表面附近溶液pH值变化为切入点，采用多种原位电化学手段，监测钢/溶液界面电化学特性的改变，挖掘腐蚀初期动力学机制；同时结合高温高压饱和CO2-H2O体系相图和溶液中Cr、Fe离子浓度测试结果，并与高压釜实验得出的腐蚀产物成分、分层结构等数据相对照，探讨准钝化的成因和条件。进而研究Cr抑制局部腐蚀机制和流动状态下Cr富集方式，有望揭示低Cr钢耐蚀机理，为其进一步推广提供理论支持。

60%以上的油气管线腐蚀失效发生于含CO2的碳钢管线，在碳钢中添加适量Cr可显著抑制腐蚀，但是低Cr钢耐蚀机理尚未建立起来，仅仅是在碳钢的腐蚀机制上增加了Cr影响腐蚀产物膜保护性。本项目以腐蚀过程中电位变化和钢表面附近溶液pH值变化为切入点，采用多种原位电化学手段，监测钢/溶液界面电化学特性的改变，挖掘腐蚀初期动力学机制；同时结合腐蚀阴极过程的深入分析，探究阳极-阴极过程的协同作用。进而研究Cr抑制局部腐蚀机制和流动状态下Cr富集方式，最终建立低Cr钢耐蚀机理。.结果表明，Cr影响耐蚀性能的主要过程是Cr首先通过溶解，进入溶液，改变了钢表面附近溶液的pH值，并且促进了Cr(OH)3的沉积，还改变了阴极反应，使得低Cr钢自发进入“半钝化”状态。Cr通过对阴极过程和阳极过程的共同影响来诱发基体的自发半钝化，从而大大提高其耐蚀性能。对于阳极过程，在基体表面生成一层均匀致密完整覆盖的三维固态薄膜，使基体与溶液隔绝，电化学腐蚀阳极过程受阻或减缓，阳极电流密度在极化过程中明显下降，极化曲线阳极区表现为明显的电流“回头峰”，即半钝化。对于阴极过程，随着基体的腐蚀溶解，基体最外层的Cr在溶液/基体界面处也不断发生溶解并水解，水解产生大量H+，导致试样表面溶液微小区域内快速酸化，当pH降至4.7或更低，3Cr钢阴极过程即发生转变，从较高pH范围内HCO3-的直接还原作为主导逐渐转变为较低pH范围内H+的直接还原占据主导，对应混合电位将明显正移，当正移幅度足够大，自腐蚀电位进入半钝化电位区间，基体即发生自发半钝化。而低Cr钢在CO2环境下能够自发进入半钝化状态，是低Cr钢耐蚀机理的精髓。.本项目以半钝化为基础，建立起低Cr钢腐蚀理论体系，从碳钢-低Cr钢-不锈钢这样Cr含量不断增加的材料体系里面，碳钢采用活化腐蚀溶解来指导、低Cr钢采用阳极半钝化+pH值发生变化+腐蚀产物膜生长动力学来指导、不锈钢采用钝化来指导，就形成了一套完整的理论体系，具有重要的科学意义。