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.