Abstract
Stainless steels are widely used as metal components owing to self-protection in aggressive environments, provided by an extremely thin surface oxide film enriched in chromium oxide. Yet, despite decades of research, the mechanisms distributing the chromium enrichment at small length scale are poorly understood, although it may cause loss of stability and local failure of the corrosion resistance. Here, we apply high resolution surface analysis to investigate at small time and length scales the nucleation and growth mechanisms of the surface oxide on a model stainless steel. Starting from an oxide-free surface, we report the direct observation of the oxide nucleation and local oxidation of chromium, which governs the nanoscale heterogeneity of the growing surface oxide by chromium pumping from the atomic terraces to the steps for preferential Cr(III) oxide nucleation and subsequently by segregation from the atomic planes below to grow the Cr(III) layer incompletely saturating the stainless steel surface. This work provides new insight on corrosion chemistry, by evidencing local chemical and structural defects self-generated at the nanoscale by the building process of the protective oxide barrier, and affecting the passive film stability.
Highlights
Stainless steels (SSs) are widely used in various industrial sectors because of their high resistance to corrosion. This corrosion resistance comes from the passive film which is a surface oxide film only a few nanometers thick and strongly enriched in Cr(III)oxide species.[1,2,3,4,5,6,7]
Even though numerous studies have characterized the passivity of various SS grades at macroscopic level, only recent surface analytical works performed on austenitic SS have suggested that the origin of passivity breakdown, which can lead to the local failure of the corrosion resistance and to localized corrosion by pitting, would depend on the surface heterogeneity at microscopic level.[6,7,8]
In the regions adjacent to both the upper and lower edges of the multisteps, the terraces appear darker. These regions of the terraces are hereafter referred to as the “terrace borders”. Their darker appearance is assigned to the local presence of nitrogen, observed by X-ray photoelectron spectroscopy (XPS) to co-segregate with chromium during surface preparation.[9]
Summary
Stainless steels (SSs) are widely used in various industrial sectors because of their high resistance to corrosion. One can notice that the patterns formed by these new vacancy rows are for most of them connected to the initial periodic vacancy lines and/or to the modified terrace borders adjacent to the upper edges of the multisteps, indicating the formation of diffusion pathways leading to the multisteps and to the Cr(III)-rich oxide nuclei.
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