High temperature oxidation of HP40 alloy under H₂–H₂O and air atmospheres: a surface study

Abstract

In the present work, high temperature oxidation of HP40 alloy was carried out at 1050°C under H 2 -H 2 O and air atmospheres; the influence of atmosphere on surface morphology and composition was studied. Octahedral crystals with considerable spalled regions are present on the surface of alloy oxidized under air, the oxide scale composes of MnCr 2 O 4 , Cr 2 O 3 and (Fe, Ni)Cr 2 O 4 and spalled regions exhibit base alloy and SiO 2 -rich regions. The surface of alloy oxidized under H 2 -H 2 O is fully covered by small granular crystals and blade-type structures without spallation, and the oxide scale composes of MnCr 2 O 4 and Cr 2 O 3 . Moreover, X-ray photoelectron spectroscopy analysis shows considerable difference in chemical valence states of Mn, Cr and O elements on both alloy surfaces, and hydroxyl compounds exist on the alloy oxidized under H 2 -H 2 O atmosphere.

Gasification of Radical Coke with Steam and Steam–Hydrogen Mixtures over Manganese–Chromium Oxides

“…Previous research efforts have sought to understand coke formation mechanisms and develop approaches to limit coke deposition onto the tube surfaces. ,− Three main coking mechanisms have been shown to occur: catalytic coking, radical coking, and droplet condensation coke deposition. , Steam cracking furnaces are typically composed of high performance Fe–Ni–Cr alloys, and they are pretreated under steam and/or air at high temperature to form a protective Cr 2 O 3 -enriched surface. , This oxide surface serves to prevent the migration of Fe and Ni species to the surface (which are known to catalyze coke formation) and helps limit carburization of the metal tubes. Prior investigations of these alloys showed that Mn (which is typically present at concentrations of 1–2 wt % in the bulk alloy) also migrates to the surface, due to its high diffusivity in the alloy and high thermodynamic affinity for oxygen. , The Mn and Cr on the surface can combine to form a relatively stable Mn–Cr–O spinel layer (mainly MnCr 2 O 4 ) that is believed to contribute to a decrease in the coking rates of the alloys. − …”