Abstract The effect of molybdate ion on the corrosion of mild steel in an aggressive, low hardness water was investigated electrochemically. Studies were conducted with molybdate concentration ranging from 50 to 5000 mg/L in waters of pH 3, 5, 7, and 9. The effect of temperature and aeration was also investigated. In deaerated solution, increasing molybdate concentration reduced anodic current densities at all pH values studied. Self passivation of steel requires an oxidizing agent, the strength of which is inversely proportional to molybdate concentration. Auger and XPS analyses of films formed on steel in basic and acidic molybdate solution were also conducted.