Effect of steam on high temperature oxidation behaviour of alumina-forming alloys

Alternative light water reactor fuel cladding materials are being investigated to replace Zircaloy for enhanced accident tolerance, which involves oxidation resistance to steam environments at ≥1200°C for short times. As chromia-forming alloys and Ni-containing alloys are both undesirable for this application, the focus has been on FeCrAl, although NiAl was used to evaluate the effect of steam oxidation at 1600°C for this study. For commercial and model FeCrAlY alloys, a critical Cr-Al composition was identified for 1 bar isothermal steam (100% H₂O) oxidation resistance at 1200°C, which differed for exposures in Ar-50%H₂O at the same temperature. Alloys with lower Cr and Al contents were not able to form a protective alumina scale under these conditions. To simulate the accident scenario, exposures were also conducted in steam with the temperature rising 5°C min21 to 1500°C for the most oxidation resistant alloys. Using thermogravimetry, the maximum use temperature for candidate alloys was determined for different Cr and Al contents. Minor additions such as Y and Ti appeared to be beneficial for oxidation resistance. Similar to prior studies, alumina scales formed in air and in steam appeared to have only subtle differences in microstructure.