Corrosion Behavior of a Novel Low-chromium Alumina-forming Austenitic Stainless Steel in Supercritical Water

Owing to the Al addition, alumina-forming austenitic stainless steel (AFAs) exhibits excellent oxidation resistance in high-temperature oxidizing environments such as supercritical water (SCW) or steam by forming a continuous alumina scale. Therefore, AFAs have merged as candidate materials for cladding materials for supercritical water-cooled reactor (SCWR). However, AFAs intended for SCWR are scarce, which has limited its application. The general corrosion behavior of a novel low-chromium AFAs was investigated in 600 ℃, 25 MPa SCW based on the demand for structural material for SCWR. This low-chromium AFAs with nominal chemical compositions of 14Cr-24Ni-2.5Al-2Mo-0.05Mn, was prepared by vacuum induction melting. The ingots were solution-treated at 1 200 ℃ for 1 h, and quenched with water, and then aged at 700 ℃ for 100 h, cooled using air. The specimens with a dimension of 20 mm×10 mm×2 mm were machined from the heat-treated ingots and carefully abraded and polished to achieve mirror-surface conditions. All specimens were cleaned with ethanol prior to be exposure to SCW for a maximum of 1 000 h. Oxidation tests were performed in an autoclave connected to a water chemistry control loop. Changes in the weight of each specimen were measured and averaged to calculate the weight gain per unit area. SEM, EDS, and EBSD technologies were used to examine the morphologies, microstructures, and chemical compositions of oxide films. The results show that the change of weight gain with time follows a parabolic law. When exposed to SCW, the low-chromium AFAs has created a dual-layer structure with an outer layer of magnetite (Fe₃O₄) and an inner layer of mixed Cr-Al oxide film. The lower Cr and Al concentration is insufficient to maintain the formation of independent alumina film. In the absence of an effective protective oxide film, the Fe in the alloy can diffuse outward unrestricted to the surface of the specimen and form a columnar crystal of magnetite. The outer oxide film is replenished with sufficient Fe to allow the magnetite particles to continue to grow, eventually forming a porous outer magnetite layer. The porous outer Fe oxide film provides little obstruction to oxidizing specimen inward diffusion. Therefore, the low-chromium AFAs display poor long-term corrosion behavior with this dual-layer oxide film because it is less protective. This paper explains how low-chromium AFAs corrode when exposed to SCW and emphasizes the importance of materials with enough Cr and Al to maintain high corrosion resistance in this environment. The development of AFAs and the choice of structural materials for SCWR are also aided by the data provided by this study.