Molybdate's inhibition of localized corrosion and passivation of AISI 1020 carbon steel in pH 9 and pH 10 solutions containing bicarbonate/carbonate and chloride anions was investigated under open- circuit conditions. The critical molybdate concentration required to prevent pitting corrosion in these solutions was determined. Using Design of Experiments and statistical analysis, the main and interaction effects of these anions on the polarization resistance were identified. Results show that the immersion time ( a measure of the degree of passivation) and the presence of other ions can affect the effectiveness of a corrosion inhibitor. A combination of XPS and electrochemical methods was used to assess the effect of ions on the passive film formation. Increasing chloride concentration in the presence of bicarbonate/carbonate ions increased the total charge passed ( extent of oxidation) but decreased the passive film thickness. This indicates that a significant part of the Fe atoms oxidized do not contribute to passive film formation, suggesting a harmful synergy between bicarbonate/carbonate and chloride. However, when molybdate is present, the total charge passed decreased and the passive film thickened slightly, showing the beneficial effect of molybdate on the passive film growth process, which is most probably due to competitive adsorption against bicarbonate/ carbonate and chloride. (C) 2017 The Electrochemical Society. All rights reserved.