Hot Corrosion of Nickel-Chromium Alloys in a Molten Eutectic Salt Environment

摘要

Aircraft engines, turbines, and industrial machinery operating at high temperatures near marine environments are prone to accelerated degradation of their components due to corrosion under a thin film of fused salt ("hot corrosion"). Typical salts formed under these conditions are sodium sulfate from fuels and sodium chloride from the marine environment leading to low temperature hot corrosion (Type Ⅱ). In general, the addition of chromium to alloys is known to increase their corrosion resistance, especially in mitigating attacks from molten sulfates. However, systematic studies on the corrosion resistance of binary nickel-chromium alloys to molten salts as a function of their chromium content are sparse. In this study, the corrosion kinetics of Type Ⅱ hot corrosion at 700°C were determined for UNS N02200, Ni-Cr alloys with 2.5, 5, 7.5 and 10 wt% Cr as well as the UNS N06600 alloy in a Na_2SO_4 -30.8 wt% NaCl eutectic salt using DC electrochemical techniques in a stagnant air closed system and a flowing 1vol% SO_2 - air blend (g). Metallography was used to validate the reliability of high temperature electrochemistry as an accelerated test for hot corrosion. Although the electrochemical test could not discriminate the influence of chromium in the long term performance of Ni-Cr model alloys, metallography confirmed that increasing Cr-contents in the alloy resulted in the formation of a continuous protective chromia layer at Cr-contents greater than and equal to 7.5 wt%.