Experimental study of high temperature corrosion behavior of water-wall tube material in the near-wall reducing atmosphere of NH3-H2S-O2-CO-N2 under pulverized coal-ammonia cofiring condition

This study aimed to clarify the corrosion behavior of water-wall tube material (12Cr1MoV) at 400/500℃ in a near-wall reducing atmosphere of H₂S-NH₃-O₂-CO-N₂ bal. after exposure time of 168 h, which is closely associated with the conditions of pulverized coal boilers cofiring with ammonia. Corrosion of specimens at 500℃ was more severe than that at 400℃ in the reducing atmosphere with a same NH₃ concentration of 5 %. The sulfidation reaction of iron oxide with low valence states due to ammonia reduction was highly promoted by elevating temperature. With the NH₃ concentration increasing from 0.5 % to 5 % in the reducing atmosphere, the final mass gain of specimens at 400/500℃ after exposed time of 168 h was obviously increased. Oxidation reaction of iron and its sulfides was dominated during corrosion process at 400℃ in the exposure atmosphere with low NH₃ concentration. Higher NH₃ concentration of 10 % at low temperature could promote the penetration of N and S elements into base metal more deeply than O element. The penetrated NH₃ could directly react with iron matrix to generate Fe₄N, which is thermodynamically favored. The periodical alternation of NH₃ presence and absence per 24 h in exposure atmosphere accelerated the corrosion of specimens at 400℃. While the O₂ concentration increased from 0.1 % to 0.5 % in the reducing atmosphere, the corrosion rate of specimens at 400℃ could be slightly inhibited. The reduction pathway of corroded product Fe₂O₃ by ammonia follows a same step: Fe₂O₃→ Fe₃O₄→ Fe →Fe₄N at 400/500℃.