Effect of minor elements and a Ce surface treatment on the oxidation behavior of an Fe–22Cr–0.5Mn (Crofer 22 APU) ferritic stainless steel

This paper details the oxidation behavior of coupons from three different Crofer 22 APU sheets. Oxidation tests were conducted at 800∘C in moist air for 2300 h. The major features of the scales were similar in all three coupons, with a continuous Cr–Mn spinel layer forming at the gas–oxide interface and a Cr2O3 layer forming at the oxide–metal interface. Also, at the Cr2O3–metal interface, large Cr–Mn oxide particles formed in the base metal. Metal extrusions in the oxide scale were observed in the vicinity of these Cr–Mn oxide particles. All three alloys had internal oxide particles forming in the base metal beneath the oxide scale, and the extent of this internal oxidation was related to the Al and Si-content in the coupons (as also reported by Piron-Abellan J, Huczkowski P, Ertl S, Shemet V, Singheiser L, Quadakkers W. In: Fuel cell seminar, 2004. San Antionio, TX, November 1–5, 2004). Of note, discontinuous Ti-oxide and Mn-oxide particles formed above the outer spinel layer on the coupons with relatively high Al and Si contents. The effectiveness of rare-earth surface modification on improving oxidation resistance of this alloy was demonstrated. Pack cementation-like processes were used to treat the surface of the coupons with Ce. A continuous Cr–Mn outer oxide layer and a continuous inner Cr2O3 layer formed on the surface of the treated coupons during oxidation. However, the thickness of the scales and amount of internal oxidation were significantly reduced with the treatment, leading to the decrease in oxidation rate. The amount of internal oxidation was also reduced with the treatment. The treatment appeared to suppress the formation of SiO2 at the scale–metal interface in high Si containing alloys. ASR measurements indicate that the treatment decreases the electrical resistance, which is a benefit for SOFC interconnect application.