Critical temperature determination of detectable Cr diffusion enhancement by nanostructure through structural evolution analysis of the oxide films at 25–450 °C on 304 stainless steel

Highlights: • The 300 °C was critical temperature of detectable Cr diffusion enhancement in NC SS. • The structure of oxide film on SS underwent transient process formed at 25–450 °C. • Above 300 °C, Cr diffusion increase led to better compactness of oxide film on NC SS. - Abstract: The structural evolution of the oxide films at 25–450 °C on nanocrystalline (NC) and coarse crystalline (CC) 304 stainless steels (SS) was investigated. The structure of the oxide film on both NC and CC SSs was observed to undergo transient processes from a bi-layer to a single-layer and then back to a bi-layer when the temperature changed from the low range (25–150 °C) to the medium range (150–300 °C) and subsequently to the high range (300–450 °C), respectively. These formation mechanisms of the oxide films on SS were attributed to the different diffusion properties of Cr and Fe in the three temperature ranges. The thickness of the oxide films was similar between the NC and CC SSs below 300 °C due to their similar Cr_ox/Fe_ox concentration ratios in their oxide films at this temperature. Above 300 °C, Cr diffusion enhancement in the NC matrix led to a higher Cr_ox/Fe_ox ratio and better compactness of the oxide film, which resulted in a slower atomic diffusion rate in the oxide film and a thinner oxide film. Therefore, the temperature of 300 °C was concluded to be the critical temperature of the detectable Cr diffusion enhancement in the NC SS compared to the CC SS.