The present work systematically investigated the initiation mechanism of localized corrosion induced by Al2O3-MnS composite inclusion in E690 steel under a simulated marine environment. The results showed that a micro-gap exists between the Al2O3-MnS inclusion and the matrix, and electron backscattered diffraction (EBSD) analysis revealed significant lattice dislocation zones around the Al2O3-MnS composite inclusion. The presence of the micro-gap and the lattice dislocation both promoted the localized corrosion initiation. The Volta potential of Al2O3 detected by scanning Kelvin probe force microscopy (SKPFM) was approximately 149.33 mV higher than that of the steel matrix, and the Volta potential of MnS was 10 mV lower than that of the steel matrix. The current-sensing atomic force microscopy (CSAFM) results showed that the Al2O3 was not conductive, while the MnS had good conductive properties. Therefore, it was not possible for a galvanic couple to be formed between Al2O3 and the adjacent steel matrix. A galvanic couple effect between the MnS and the adjacent steel matrix was directly demonstrated for the first time. The MnS acted as the anode phase for preferential dissolution in the corrosion process. The in situ immersion experiments and the Pourbaix diagram results confirmed that the dissolution of MnS was an electrochemical reaction process and the dissolution of Al2O3 was a chemical reaction.

No abstract

No abstract

The impact of heat treatment on the initiation and progression of localized corrosion in E690 steel in a simulated marine environment was investigated systematically. The primary cause of localized corrosion was the presence of inclusions, which led to the dissolution of the distorted matrix surrounding them. In the initial stages of corrosion, localized corrosion resulting from inclusions was the predominant form. The chemical and electrochemical mechanisms underlying matrix deformation and localized corrosion caused by inclusions were meticulously elucidated. As the immersion time was extended, the galvanic contributions at the ferrite-austenite interfaces, as well as the coarsened carbides, reduced the polarization resistance in the annealed specimen, accelerating the corrosion rate compared to the lath martensite in the as-received specimen. Consequently, the heat-treated sample promoted a transition from localized to uniform corrosion. Finally, a model was established to describe the corrosion behavior of E690 steel in the marine environment.