Corrosion of Three Commercial Steels Under ZnCl2 –KCl Deposits in a Reducing Atmosphere Containing HCl and H2S at 400–500°C

Abstract

The corrosion of three commercial steels in a reducing atmosphere containing HCl plus H2S in the presence of ZnCl2-KCl deposits has been investigated at 400-500 degrees C and compared with the corrosion of the same materials in a similar gas mixture free from H2S. The presence of H2S in the gas accelerated the corrosion of the three commercial steels beneath ZnCl2-KCl deposits. All materials suffered from severe corrosion with partial detachment and spalling of the external scales. Degradation of the steels resulted from the penetration of chlorine-containing species through the scale formed initially down to the metal matrix, because chorine-rich species were detected close to the alloy/scale interface. Although the corrosion resistance generally increased with increasing Cr content, even the high-Cr stainless steel SS304 is still unable to provide good corrosion resistance against the ZnCl2-KCl deposits in the presence of H2S due to the bad adherence of the scales to the alloy. The mechanisms of attack are discussed on the basis of thermodynamic considerations and of the active-oxidation model.