Corrosion and Carburization Behaviour of Ni-xCr Binary Alloys in a High-Temperature Supercritical-Carbon Dioxide Environment

Pure Ni and Ni-xCr (x = 7, 14, 22 and 27 wt%) binary alloys were exposed to supercritical-carbon dioxide environment at 600 degrees C and 20 MPa for 200 h. For pure Ni, a thick NiO layer was formed on the surface. Meanwhile, for Ni-7Cr alloy, an inner oxide layer consisted of rather irregular chromia and NiO was formed below the outer NiO layer. When Cr content was greater than 14%, a continuous chromia layer was formed, resulting in much lower weight gain and oxide thickness. However, amorphous carbon layers had developed along the oxide-matrix interface when chromia was formed. The presence of the carbon layer was explained in view of the high C activity corresponding to the low equilibrium oxygen potential of chromia.