Corrosion Behavior of Cr2O3— Former Alloys in H2-H2S-H2O Atmospheres

Abstract

The corrosion behavior of Ni, Co, and Fe alloyed with Cr (25 Wt%) was investigated in H2/H2S/H2O atmospheres at 1200 and 1300 K. Reaction of Ni-25 Cr and Co-25 Cr at 1200 K gave rise to scales which were not affected by pO2 in the range 10−20 >pO2 (atm) > 10−16. In addition, Co-25 Cr formed a scale similar to that in oxygen free atmospheres consisting of an outer Co-S layer, in intermediate Co-Cr-S solid solution, and internal Cr-S precipitates. A multilayered scale Ni-S (liquid) Cr2O3/Cr-S formed on Ni-25 Cr. The intermediate porous Cr2O3 layer grew by reaction of Cr dissolved in Ni-S with oxygen. At 1300 K and with the exception of Fe-25 Cr at pO2 ≥ 10−14 atm which maintained a protective Cr2O3 scale containing small amount of dissolved sulfur during the period of the investigation (16 hours), the course of the reaction followed the same pattern for all alloys. Initial formation of Cr2O3 film was followed by localized growth of sulfides at both sides. Cr-S or Co-Cr-S solid solution grew beneath the Cr2O3 film, while sulfide of the noble metal (FeS, Ni-S, Co-S) formed at the outer surface. This localized attack is attributed to transport of sulfur via lattice and structural imperfections in the Cr2O3 film. Samples preoxidized in H2/H2O and subjected to H2/H2O/H2S exhibited similar behavior.