Corrosion and carburization behavior of high-chromium alloys in high-temperature supercritical CO₂ with 3.1% H₂O addition

The corrosion and carburization behavior of four high-chromium (∼20 wt%) Fe- and Ni-based alloys (Alloy X, Alloy 230, Alloy 800HT and Alloy 282) were evaluated in supercritical CO₂ containing 3.1 vol% H₂O at 750 °C and 25 MPa for up to 1000 h. All alloys formed thin, continuous chromia (Cr₂O₃) layers and the corrosion rates followed the sequence of Alloy 282 > Alloy 800HT > Alloy 230 ≈ Alloy X. Parabolic kinetics were observed and were governed by the type and concentration of minor alloying elements such as Mn, Ti, Si, and Al. MnCr₂O₄ and TiO₂ were formed at the outermost oxide layer, whereas SiO₂ was observed at the oxide/matrix interface as either discrete particles or a continuous amorphous layer, depending on the alloying element content. Subscale internal oxides, mostly Al-rich alumina with traces of TiO₂, extended to depths that matched the bulk Al and Ti levels. The presence of 3.1 vol % H₂O accelerated oxide growth, yet carbon enrichment at the oxide/matrix interface was not detected. TEM-EELS mapping and SIMS depth profiling revealed only trace carbon confined to voids within the scale. The absence of interfacial carburization was attributed to the combined barrier effect of a thicker chromia scale formed by H₂O addition at high temperature and the structural modification of chromia through the incorporation of minor alloying elements.