Investigation of the passivity of iron by nuclear microanalysis using stable oxygen isotopes

A solution of 0.15N sodium borate and 0.15N boric acid in O18 enriched water was used to study the passivity of iron by means of nuclear microanalysis. It was established that the surface of the sample maintained under cathodic polarization is free of oxygen 18; while in open circuit, the presence of of oxygen 18 was detected on the sample which was previously cathodically reduced. Moreover, in the passivity region, the quantity of oxygen 18 fixed on the metal surface varies from as a linear function of potential. The value of the apparent electric field of formation was calculated and compared with the literature data deduced from ellipsometric and coulometric measurements. The cathodic reduction of the films was studied by determining the 018 losses of the passive films as a function of the charge passed in the circuit and of the temperature. The mechanism of the cathodic reduction is discussed on the basis of the experimental data.