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].