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.

Stress corrosion crack (SCC) behavior of Type 310S stainless steel (SS) in supercritical water was evaluated at 400°C to 550°C. Crack growth rates (CGR) were obtained as a function of temperature and corrosion potential. Results show that Type 310S SS exhibits 100% intergranular SCC, and creep plays an important role in the overall growth rate. The SCC CGR increases with temperature between 400°C and 550°C, and is higher at a medium-low corrosion potential (argon deaerated water) than in water with dissolved O2 or dissolved H2. A mechanism for SCC of Type 310S SS in supercritical water is proposed to explain the combined effect of SCC and creep.

The corrosion behavior of alumina-forming austenitic (AFA) stainless steels with different Nb additions in a supercritical carbon dioxide environment at 500 °C, 600 °C, and 20 MPa was investigated. The steels with low Nb content were found to have a novel structure with a double oxide as an outer Cr₂O₃ oxide film and an inner Al₂O₃ oxide layer with discontinuous Fe-rich spinels on the outer surface and a transition layer consisting of Cr spinels and γ’-Ni₃Al phases randomly distributed under the oxide layer. Oxidation resistance was improved by accelerating diffusion through refined grain boundaries after the addition of 0.6 wt.% Nb. However, the corrosion resistance decreased significantly at higher Nb content due to the formation of continuous thick outer Fe-rich nodules on the surface and an internal oxide zone, and Fe₂(Mo, Nb) laves phases were also detected, which prevented the outward diffusion of Al ions and promoted the formation of cracks within the oxide layer, resulting in unfavorable effects on oxidation. After exposure at 500 °C, fewer spinels and thinner oxide scales were found. The specific mechanism was discussed.