ABSTRACT
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.
INTRODUCTION
With the increasing demand of energy, the exploration of hydrocarbon in oil and gas industry has moved to deeper and deeper wells, which are frequently associated with high temperature, high pressure, and high sour gas content (H2S). High temperature H2S corrosion brings serious challenges to infrastructure integrity, materials selection, and corrosion mitigation.1-8
The effects of high temperature and exposure time on the H2S corrosion kinetics and FexSy transformation sequence have been studied in previous work.9-10 It was found that the formation of corrosion products was responsible for the initial rapid decrease in corrosion rates, which eventually stabilized over time at high temperatures.9 The observed formation and transformation sequence at high temperature was mackinawite ? troilite ? pyrrhotite ? pyrite. With the increase of temperature (80°C~200°C) and time (1~21 days), iron sulfide transformed to more thermodynamically stable phases.10 However, these tests were performed at a constant H2S partial pressure (0.1 bar) and the effect of H2S content (pH2S) on the corrosion rate of carbon steel and iron sulfide transformation at high temperature has not been studied. Generally, H2S plays a dual role. Firstly, it accelerates the corrosion rate by providing an additional cathodic reaction:
(Equation-1)