A comprehensive model has been developed for simulating the effect of FeCO_3 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 CO_2 and H_2S on the formation of scales and corrosion rates.
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