High-temperature corrosion of austenitic alloys in HCl and H2S containing atmospheres under reducing conditions

The corrosion of several Fe-Cr-Ni alloys was examined at 420 degrees C, 480 degrees C and 580 degrees C in reducing atmospheres containing 3.8 vol% HCl and either 0.2 vol% H2S or 2 vol% H2S, denoted as mid-H2S mixture or high-H2S mixture, respectively. The corrosion behavior was compared with the findings achieved during previous studies in an atmosphere containing 3.8 vol% HCl and 0.02 vol% H2S, denoted as low-H2S mixture. Increasing the H2S level accelerated the corrosion of the alloys, particularly at 580 degrees C. In the low-H2S mixture, the alloying element nickel was identified to be helpful for corrosion protection and the evaporation of metal chlorides played a significant role in the corrosion process. With increasing H2S content conversion processes from metal chlorides to metal sulfides as well as sulfidation processes dominated the corrosion behavior of the tested alloys and chromium was found to be a beneficial alloying element against corrosive attack. In previous studies corrosion models for the low-H2S mixture were already identified. In this paper the effect of rising H2S amount on the corrosion behavior is investigated and a model of the course of corrosion is proposed by considering the role of alloying elements, vapor pressures of metal chlorides and the influence of H2S and HCl.