The Corrosion of Mo‐Al Alloys in a H 2 / H 2 S / H 2 O Gas Mixture at 800–1000°C

Abstract

The corrosion behavior of pure Mo and three Mo‐Al alloys containing 28, 50, and 75 atom % Al was studied over the temperature range of 800–1000°C in a gas mixture. Except for the Mo‐50Al alloy consisting of a two‐phase structure of and , other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature but decreased with increasing Al content. The Mo‐75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 2.1–3.0 orders of magnitude lower than those of pure Mo (depending on temperature). A layer exclusively of formed on pure Mo, while heterophasic scales were observed on Mo‐Al alloys. The scales formed on Mo‐28Al consisted of and at T ⩽ 900°C and of , and at 1000°C, while the scales formed on Mo‐50Al consisted of mostly and a minor amount of . The scales formed on Mo‐75Al consisted exclusively of at T ⩽ 900°C and of mostly and a minor amount of at 1000°C. The formation of is responsible for the significant reduction of the corrosion rates as compared with those of pure Mo.