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

Abstract

The corrosion of several Fe-Cr-Ni alloys was examined at 420 °C, 480 °C and 580 °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 °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.