The bi-layer structure and the higher compactness of a passive film on nanocrystalline 304 stainless steel

•The lower Uox(Cr) of chromium results in the Cr2O3 rich inner layer to form initially.•The Fe2O3 with higher Uox(Fe) forms latterly and predominates the outer layer.•Higher Cr2O3 content in the inner layer is responsible for its higher compactness for NC-304SS.

The structure and compactness of the passive films on nanocrystalline (NC) and coarse crystalline (CC) 304 stainless steels (SSs) are investigated. A bi-layer passive film composed of an inner Cr2O3-rich layer and an outer Fe2O3-rich layer is obtained on both NC and CC 304SSs. The lower oxide formation potential Uox(Cr) of chromium causes the Cr2O3-rich inner layer to form initially, and when the surface potential of the passive film increases due to Cr2O3 formation, Fe2O3 with a higher formation potential Uox(Fe) forms and predominates in the outer layer as a result of the higher diffusion rate of Fe than that of Cr across the inner layer. Compared with the CC-304SS, the inner layer of the passive film on the NC-304SS possesses a higher Cr2O3 content, which is attributed to the greater nucleation sites and faster growth rate of Cr2O3 in NC-304SS and is believed to be responsible for the higher compactness of the passive film on NC-304SS.