Corrosion in acidic solutions frequently leads to equipment failure in industry. The ability to model corrosion in acidic solutions is thus necessary to increase asset integrity. A prerequisite for any corrosion model is the accurate knowledge of solution thermodynamics. In this study, a previously developed electrochemical module has been integrated with the Mixed-Solvent Electrolyte (MSE) thermodynamic model. MSE has been previously proven to accurately represent the behavior of solutions, including solubility limits, pH, and transport properties. Here, the accuracy of MSE in predicting speciation in acidic systems is demonstrated. The electrochemical module, which incorporates partial anodic and cathodic reactions, can model active corrosion, active-passive transition, as well as dissolution in the passive state. The model is shown to quantitatively predict the corrosion rates of two duplex stainless steels, i.e., 2507 (UNS S32750) and 255 (UNS S32550) in acid mixtures containing both oxidizing and non-oxidizing acids. The effects of aggressive (fluoride and chloride) and inhibitive (nitrate) species are also accurately predicted. Simulated polarization curves are presented for the mixed acid systems to illustrate the effect of different species on the anodic and cathodic processes.