Effect of Surface Preparation on Corrosion Resistance of NiCr, NiAl and NiCrAl Alloys in Supercritical Water

Abstract

The corrosive effect of supercritical water on a number of potential coating materials was investigated in this study. Various surface preparation methods were used to prepare four Ni- based alloys including Ni20Cr, Ni5Al, Ni50Cr and Ni20Cr5Al. The surface preparation methods employed were: sanding, polishing, vacuum furnace heat-treatment and air furnace heat- treatment. Corrosion testing was conducted in supercritical water at 500oC at a pressure of 27 MPa. After 500 h and 940 h exposured to SCW the ground and the polished (to mirror finishing using alumina paste) samples showed little weight gain. Vacuum furnace heat-treatment produced a very light surface film and all samples showed moderate weight gain. However, air furnace heat-treated samples formed surface scales and all samples suffered appreciable weight loss after testing in SCW, except for the Ni20Cr5Al sample. Of the samples tested, Ni20Cr5Al prepared by sanding showed the best performance in terms of weight change when tested at 500oC at a pressure of 27 MPa. This is believed to be associated with higher residual stresses in samples after sanding.