Based on electrochemical mechanisms, an equation is derived for the relation of to the critical current density necessary to passivate metals: where and λ are constants. Accordingly, a linear relation beween and is observed for Cr, Ni and 14% Cr‐Fe alloys. When passivity occurs by reason of dissolved oxygen, it is shown that usually equals the limiting diffusion current density for O2. A critical exists above which passivity is stable, but not below. This critical pH decreases linearly with the logarithm of dissolved O2 concentration. Calculated and observed values for the critical of 14% Cr‐Fe alloy are in reasonable agreement.