The Kinetics and Mechanism of the Selective Oxidation of 20Fe–40Ni–10Mn–30Cr Alloy

Abstract

Process environments of high carbon activity (a _c  > 1) and low oxygen partial pressures (p _O2) are often encountered in the petrochemical industry. Under these conditions a type of corrosion known as metal dusting occurs. Previous work has identified a 20Fe–40Ni–10Mn–30Cr high temperature alloy as a potential metal dusting resistant material. Knowledge of the kinetics and mechanism of the oxidation of the alloy under low p _O2 conditions would provide a better understanding of its corrosion resistant behavior and guide future alloy development. The present work examines the selective oxidation of Mn and Cr in the 20Fe–40Ni–10Mn–30Cr alloy under low p _O2 conditions. The conditions for oxidation were selected by fixing the p _O2 value such that only Mn and Cr would oxidize. These p _O2 values are representative of the conditions that exist in metal dusting type environments. A comparison of the oxide film chemistry and morphology with those formed under metal dusting conditions will inform whether the high carbon activity is playing a role in protective oxide film formation. Low p _O2 values are obtained by using a 90:10 CO:CO₂ mixture at 1,000 K. Under these conditions an oxide film rapidly forms on the surface of the alloy. Analysis of the oxidized alloy by electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy reveals that the oxide consists of a continuous spinel with the approximate stoichiometry of MnCr₂O₄, as well as islands of MnO and fibrillar structures of MnCr₂O₄ stoichiometry. The parabolic rate constant for the growth of the spinel is determined from thermogravimetric analysis [k _p  = (3.72 ± 0.01) × 10⁻¹⁴ g² cm⁻⁴ s⁻¹], TEM analysis [k _p  = (6 ± 3) × 10⁻¹⁴ g² cm⁻⁴ s⁻¹] and cross-sectional SEM [k _p  = (3 ± 1) × 10⁻¹⁴ g² cm⁻⁴ s⁻¹].