Geothermal energy systems present corrosion challenges due to severe downhole operating conditions that involve high temperatures, high flow rates, solids, acid gases, high chloride concentrations, oxidizing species, heavy metals, and pH values below 4. Therefore, judicious selection of corrosion-resistant alloys (CRAs) is essential for safe and economical operation of geothermal energy systems. OLI has developed a localized corrosion model, which makes it possible to understand and quantify the effects of corrosive species in complex aggressive environments. The corrosion resistance of CRAs relies upon the formation of a very thin oxide film. If the oxide layer on the surface fails locally, corrosion will occur rapidly in the form of localized corrosion, which is one of the main types of failure in equipment made of CRAs in geothermal energy applications. The occurrence of localized corrosion can be modeled by calculating the corrosion potential (Ecorr) and the repassivation potential (Erp). If Erp < Ecorr, localized corrosion is expected to happen; conversely, if Erp > Ecorr, the passive layer is stable, and no localized corrosion is anticipated. A semi-empirical model has been developed for predicting both Erp and Ecorr values for stainless steels and nickel-base alloys. Recently, it has been extended to selected grades of Ti alloys. The new model accounts for competitive adsorption of aggressive and inhibitive species at the interface between the alloy and the occluded site environment and has been verified for various aggressive and inhibitive species.
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