Investigations of the Passivation of Iron in Aqueous Solutions by the ″Oxygen Phthalate″ System.

The passivating properties of dissolved oxygen on iron and structural steel in potassium hydrogen phthalate solutions were investigated by plotting potentiodynamic polarization curves and by potential/time measurements. It is shown that the critical passivation current in a solution of constant phthalate concentration decreases with increasing dissolved oxygen concentration and the solution pH. The transition from active dissolution to passivation, associated with a potential shift toward more noble values, depends on critical minimum values of pH and dissolved oxygen concentration in the solution. Under these conditions the oxygen diffusion current is greater than the critical passivation current. This effect is interpreted in terms of interactions of phthalate ions at the ion/solution interface.