Oxidation behavior and Stress Corrosion Cracking Susceptibility of Fe27Ni16Cr3.5Al based AFA Alloy in Supercritical Water

Abstract

The oxidation behavior and stress corrosion cracking susceptibility of Fe27Ni16Cr3.5Al based AFA alloy and Fe27Ni16Cr Al-free alloy were investigated in SCW. The morphology of corrosion specimens, the compositions of the surface and cross section and the fracture morphology of SRTT specimens were characterized using SEM, EDS, and XRD. For comparison, Al-free alloy (Fe27Ni16Cr) was further studied under the same condition. The results showed that a double oxide scales formed on AFA alloy surface, consisting of an outer layer of Fe2O3 and Fe3O4, and an inner layer of (Al0.9Cr0.1)2O3 and FeCr2O4. However, the oxide layer spalled off after 1000 h exposure due to the forming of the compression stress between the two layers. For Al-free alloy, a double layer oxide structure also formed with an outer layer containing Fe2O3 and Fe3O4, an inner layer containing Ni(OH)2 and FeCr2O4, and the oxide layer exhibited scale spallation seriously during all the exposure time. The slow strain rate tension (SSRT) tests were performed in supercritical water (25 MPa/10 ppb) at 550 °C and 650 °C, and the results showed that the AFA alloy was not sensitive to the stress corrosion and exhibited higher strength, better ductility than Al-free alloy. Additionlly, the stress strength and elongation decreased with increasing temperature. In summary, the AFA alloy exhibited better oxidation at 650 °C and lower stress corrosion cracking (SCC) susceptibility at 550 °C and 650 °C in SCW. The ultimate tensile strength (UTS) of the new AFA alloy reached 813 MPa and 437 MPa at 550 °C and 650 °C, respectively.