Effect of H₂S on High Temperature Corrosion of Fe–Ni–Cr Alloys in Carburizing/Oxidizing Environments

Abstract

This investigation involves the corrosion behavior of two Fe–Ni–Cr alloys containing different Si content at 1050 °C in carburizing-oxidizing environments (typical of ethylene pyrolysis) with varied concentration of H₂S. High-Si containing alloy could form thinner but less uniform oxide scale than low-Si alloy after pre-oxidation due to the barrier effect of continuous SiO₂ at interface of scale/substrate. Pre-oxidized alloy showed a better resistance to carburization/sulfidation attacks than the bare alloy in absence of pre-oxidation. It was found that carburization and sulfidation of the Fe–Ni–Cr alloys could be prevented in the environment with a ratio of $P_{{{\text{H}}_{ 2} {\text{S}}}} /P_{{{\text{H}}_{ 2} }}$ at 1.7 × 10⁻⁵. When the sulfur partial pressure was lower than this value, oxides were found to be converted to porous and non-protective carbides. When the sulfur potentials were increased, manganese or chromium sulfide on outer layer and internal sulfide stringers mixed with silicon oxide in substrate could be formed. Under high sulfur partial pressures, spallation of outer sulfide or oxide scale was observed on high-Si alloy due to less stability of oxide layer formed at surface which was converted to sulfide faster than on low-Si alloy.