Effective Reduction of Chromium-oxy-hydroxide Evaporation from Ni-Base Alloy 690

Abstract

The corrosion of the Ni-base alloy 690 (60Ni, 30Cr and 10Fe) in humidified air was studied at 500–800 °C, and the rate of CrO2(OH)2 volatilization was measured quantitatively as a function of exposure time using a denuder technique. Different gas velocities were employed in exposures with a maximum duration of 200 h. Corrosion morphology was investigated by SEM/EDX using BIB-milled cross sections. The rate of chromium volatilization increased with increasing temperature and gas velocity. The rate of volatilization decreased with exposure time. Two oxide scale morphologies were observed, depending on temperature and gas velocity. In the 500–700 °C range, the scale consisted of chromia-rich corundum-type oxide, while exposures with high gas velocities at 800 °C produced an entirely different type of scale that included a Ni-rich and Cr-poor cap layer. The latter scale morphology is suggested to result from extensive chromium depletion of the alloy substrate which triggers a new mode of oxidation involving formation of NiCr spinel oxide. Continued volatilization of CrO2(OH)2 causes the NiCr spinel to decompose into a Ni-rich oxide that forms a cap layer on the scale surface. This cap layer is very efficient in decreasing the rate of chromium volatilization, allowing the chromium levels in the substrate to recuperate. We show that volatilization of chromium (VI) from the alloy can be mitigated by an oxidation pre-treatment that allows the Ni-rich cap layer to form.