Stress/dynamic straining effects on the corrosion and cracking behavior of a new AFA steel exposed to sCO2

Alumina forming austenitic stainless steel (AFA steel) shows high creep and corrosion resistance in high temperature environments, which makes it a promising candidate structural material for supercritical carbon dioxide (sCO2) Brayton cycle system. The compatibility of AFA steel with sCO2 should be carefully studied as stress corrosion cracking (SCC) poses high threat to the safe operation of the materials. In this work, the SCC of a new AFA steel was evaluated by constant load (CL) testing and slow strain rate tensile (SSRT) testing in 600 degrees C/20 MPa sCO2 for 185 h. General corrosion (GC) test was also carried out as a comparison. A protective Cr/Al/Si rich oxide layer was formed on the surface of GC specimen, which is also formed on the uncracked area of CL and SSRT tested specimen. The thickness of Cr/Al/Si rich oxide layer firstly increased and then decreased with the increase of nominal strain, which can be attributed to the change of diffusion mechanism from internal-stress controlled mechanism to defects/cracks-controlled mechanism. Intragranular cracking is the only cracking mode of the AFA steel exposed to sCO2, regardless of the testing method, and this result is ascribed to preferentially oxidation of the NiAl precipitates in the AFA steel, which makes it the weak point to be cracked.