On the role of Al/Nb in the SCC of AFA stainless steels in supercritical CO2

Abstract

Abstract SCC of a series of AFA stainless steels with different Al and Nb contents were studied in supercritical CO2 by SSRT. The results show that Nb element plays a precipitation strengthening on the mechanical properties, while it shows few effects on the corrosion properties. The surface oxide film of the Al-free material only consisted of amorphous Cr2O3 and Cr-rich spinel. With the addition of Al, the Al2O3 layers are formed and significantly decreases the element diffusion, thus inhibiting the initiation of SCC. Fe3O4 fills the interior of cracks of both Al-free and Al-containing materials. The Al2O3 layer is formed at the crack tip of Al-containing materials. Because the matrix grains are large, the protective Al2O3 layer can only be formed at the crack tip, which cannot completely hinder the outward diffusion of ions on the crack walls and its protective effect on the crack propagation is limited.