A Modified Thermodynamic Model for the Prediction of Mild Steel Corrosion Product Formation at High Temperature in an Aqueous H 2 S Environment

The prediction of phase identity and characteristics of corrosion products formed on the corroding metal surface is of prime importance to understanding the corrosion mechanisms and the protectiveness conferred by the formed layers. Pourbaix diagrams, developed for CO2 environments, are typically successful in predicting (depending on pH, steel potential, temperature, pCO2, etc) the most stable forms of corrosion products. In H2S environments, however, it is more difficult to build a representative thermodynamic model (Pourbaix diagram) due to the formation of various iron sulfide polymorphs and phases which is a strongly kinetically controlled phenomenon. In addition, high temperature studies have also shown that a thermodynamically less stable but kinetically favored inner Fe3O4 layer developed under the iron sulfide layer and greatly affected the corrosion rate. In this paper, experiments performed at high temperature at different partial pressures of H2S (pH2S=0.10~2.0 bar) were conducted to investigate polymorphous iron sulfide formation and determine if the inner Fe3O4 corrosion product layer would fully convert to iron sulfide if the right conditions were met. The results show that the Fe3O4 layer is not a transient corrosion product layer, as previously thought, since it was always present in all the experimental conditions tested. A modified thermodynamic model was proposed by reconsidering the Fe3O4 stability zone in the Pourbaix diagram. The current model shows better agreement with the experimental results because of these changes.