A comprehensive model has been developed for simulating the effect of FeCO3 and FeS scale formation on general corrosion of iron and carbon steel. The model combines thermodynamic speciation with electrochemical computations based on the mixed-potential theory. The effect of scaling is modeled by considering surface reactions between the metal and active solution species, which lead to the formation and dissolution of scales. The electrochemical computations take into account various partial anodic and cathodic processes on the metal surface, such as oxidation of iron and reduction of protons, water molecules, carbonic acid and hydrogen sulfide. The rates of the electrochemical processes are related to the presence of surface scales. The model has been verified by comparing calculated corrosion rates with experimental data over substantial ranges of environmental conditions. Good agreement with the data has been obtained. In particular, the model quantitatively reproduces the effect of variations in temperature, pH and partial pressures of COz and H2S on the formation of scales and corrosion rates.
Formation of iron carbonate and iron sulfide scales has a profound influence on the corrosion behavior of carbon steels. Therefore, numerous experimental studies have been performed to elucidate the conditions that are conducive to the formation of FeCO~ and FeS and to explain their effect on corrosion. H9 The formation of FeCO3 and FeS scales depends on multiple factors including temperature, partial pressures of CO2 and HzS, pH, composition of the aqueous stream and flow characteristics. Therefore, it is desirable to develop a model that could rationalize the effect of these factors and to predict the effect of scale formation on corrosion rates.
In a previous study, 2° the stability of various solid phases that result from CO2/H2S corrosion was investigated. For this purpose, a comprehensive thermodynamic model was used in conjunction with a facility to generate stability diagrams, 2~ which help to visualize the conditions at which various solids are stable or metastable. In particular, it was shown, in a qualitative way, that the predicted formation of scales may explain the reduction in corrosion rates under some conditions.
The objective of this study is to develop a model that predicts the effect of scale formation on corrosion rates in a quantitative way. For this purpose, it is necessary to go beyond thermodynamic analysis and introduce an electrochemical model that simulates the kinetics of various phenomena at the metal/solution interface. In particular, this model should introduce a mathematical formalism that makes it possible to simulate the formation of scales on the metal surface and not only in the bulk solution. Also, the model should incorporate the kinetics of various partial anodic and cathodic processes on the metal surface and should predict how such processes are influenced by the formation of scales.
THERMODYNAMIC CALCULATIONS
The starting point for the analysis of scaling and corrosion is the computation of speciation in the investigated system. For this purpose, a realistic model of electrolyte systems is used. This model combines information about standard-state properties of all species that may exist in the system with a formulation for the excess Gibbs energy, which accounts for solution nonideality. The model has been described in detail by Zemaitis et al. 22 and Rafal et al. 23 In a previous study, this model was shown to predict the conditions under which various FeS and FeCO3 scales are stable. Here, the essential elements of the model are summarized in Appendix A.
The speciation calculations define whic
