THE CORROSION OF MO-AL ALLOYS IN A H-2 H2S/H2O GAS-MIXTURE AT 800-1000-DEGREES-C/

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 800-1 00 degrees C in a H-2/H2S/H2O gas mixture. Except for the Mo-50Al allo y consisting of a two-phase structure of Mo3Al and Mo(3)AI(8), other a lloys studied were single phase. The corrosion kinetics followed the p arabolic rate law in all cases, regardless of temperature and alloy co mposition. The parabolic rate constants increased with increasing temp erature but decreased with increasing Al content. The Mo-75Al alloy ex hibited the best corrosion resistance among all alloys studied, whose corrosion rates are 2.1-3.0 orders of magnitude lower than those of pu re Mo (depending on temperature). A layer exclusively of MoS2 formed o n pure Mo, while heterophasic scales were observed on Mo-Al alloys. Th e scales formed on Mo-28Al consisted of alpha-Al2O3 and MoS2 at T less than or equal to 900 degrees C and of alpha-Al2O3, MoS2, and Al0.55Mo 2S4 at 1000 degrees C, while the scales formed on Mo-50Al consisted of mostly alpha-Al2O3, and a minor amount of MoS2. The scales formed on Mo-75Al consisted exclusively of alpha-Al2O3, at T 900 degrees C and o f mostly alpha-Al2O3 and a minor amount of MoS2 at 1000 degrees C. The formation of alpha-Al2O3 is responsible for the significant reduction of the corrosion rates as compared with those of pure Mo.