The effects of metal cations on corrosion of mild steel in model fresh water were investigated by electrochemical techniques and immersion tests. Analysis of X-ray photoelectron spectroscopy showed that metal cations have large hardness of cations, X, are incorporated in passive films. The electrochemical and immersion results showed that X is not suitable as a corrosion indicator of mild steel. A novel corrosion indicator, “corrosion inhibitory effect of cations Y”, that consists of X and molar volume ratio, ΔV, is proposed, and it is shown that the novel indicator can estimate the corrosion rate of mild steel in fresh water.