薄层液膜下不锈钢在缝隙和应力协同作用下腐蚀行为研究

Corrosion of stainless steel bolts in Marine atmosphere is not only a key problem affecting the safety and stability of offshore platforms, but also the fundamental issue of corrosion science under the synergistic effect of crevice and stress. In this project, the in-situ electrochemical test device under the coexistence of crevice and stress in the thin electrolyte layer is established. The synergistic corrosion mechanism of stainless steel under the coexistence of crevice and stress is investigated by the electrochemical tests, water chemical tests and surface analysis technology. Firstly, the evolution of the water chemical characteristics and passivation film in the crevice are clarified under the thin electrolyte layer, and then the crevice corrosion mechanism is revealed under the thin electrolyte layer. Then, the component and microstructural changes of the passivation film and the changes of the interface reaction kinetic parameters with the applied various stress are investigated under the thin electrolyte layer, and the growth and breakdown mechanism of the passivation film is analyzed by the point defect model (PDM). Finally, the relationship of electrochemical corrosion mechanism and the evolution of the passivation film and water chemical characteristics is clarified under the coexistence of crevice and stress. The synergistic effect between crevice and stress on corrosion of stainless steel is revealed under thin layer electrolyte. Therefore, it is anticipated that this research will provide a theoretical basis for the corrosion and protection of stainless steel bolts in the marine atmosphere.

海洋大气环境中不锈钢螺栓腐蚀问题，不仅是影响海洋平台安全性和稳定性的关键问题，也是缝隙和应力协同作用下的腐蚀科学问题。本项目以304不锈钢为研究对象，通过搭建薄液膜环境中缝隙和应力协同作用下原位电化学测试装置，结合缝隙内水化学监测、电化学测试和表面分析技术开展相关研究。首先，弄清薄液膜环境中缝隙内水化学演化特性和缝隙内电极表面钝化膜性能演变规律，揭示薄液膜环境中缝隙腐蚀机制；然后，探明薄液膜环境中应力作用下钝化膜微观结构变化及界面反应动力学参数变化，结合点缺陷模型（PDM）剖释钝化膜生长、破坏机制；最后，探明缝隙和应力协同作用下不锈钢腐蚀电化学机制与钝化膜结构组成、水化学特性演化机制的内在关系，揭示缝隙和应力交互作用机制。从而为海洋大气环境中不锈钢螺栓的腐蚀与防护提供理论依据。

X70碳钢在pH=2和pH=12的溶液中都能发生缝隙腐蚀，而在pH=6.8的溶液中则不能发生缝隙腐蚀。在pH=2的溶液中，由于缝隙内pH和Cl-浓度的增加，促进了缝隙内碳钢的阳极溶解，从而发生缝隙腐蚀。在pH=12的溶液中，随着pH的降低和Cl-的增加，缝隙内碳钢的钝化膜破裂，从而引发缝隙腐蚀行为。O2和Cl-并不是引发碳钢缝隙腐蚀的必要因素。不过，O2和/或Cl-的存在可以明显促进碳钢在酸性溶液中的缝隙腐蚀。原因在于O2的存在促进了缝隙外碳钢的阴极反应，Cl-的存在可以促进缝隙内碳钢的阳极反应，O2和/或Cl-的存在都可以加强缝隙内外碳钢之间的电偶腐蚀效应。O2和Cl-对碳钢在酸性溶液中的缝隙腐蚀具有协同作用。. 13Cr 缝隙腐蚀过程包括三个阶段：诱导期、快速发展期和稳定发展期。且随着浸泡时间的延长，缝隙内13Cr不锈钢维钝电流密度越大，点蚀破裂电位越低，这可能与缝隙内不断积聚的H+和Cl-有关。在外加应力作用下13Cr 不锈钢容易发生缝隙腐蚀，且随着应力增大，诱导期时间越短，这可能是由于外加应力能够增强金属的腐蚀活性，从而降低钝化膜的耐点蚀能力，同时加剧了的H+和Cl-积聚过程。.外加应力能够增加304不锈钢钝化膜中缺陷密度，降低其点蚀电位和再钝化电位，增加其点蚀敏感性。在不同的外加应力和不同的极化电位条件下304不锈钢能发生延迟引发和立即引发的缝隙腐蚀类型。这两种类型的缝隙腐蚀都是由亚稳态点蚀的演化及发展而引起。亚稳态点蚀的寿命随应力的增加而延长，此外，在400 MPa的外加应力下可观察到二次亚稳态蚀。因此，外加应力能够延长亚稳态点蚀寿命，促进亚稳态点蚀生长，使亚稳态点蚀转变为稳定点蚀的概率增加，进而促进缝隙腐蚀的发展。