Study of Ion Adsorption at Passive Iron by Using the Impedance and Photoadmittance Methods

Intensity-modulated photocurrent (photoadmittance) and electrochemical impedance of anodic-oxidized iron electrode in neutral nitrate solutions and in the presence of Ba2+, Ca2+, Cl-, and C6H5COO-( benzoate) are studied. It is shown that the ion adsorption at passive iron affects but slightly the system's impedance; by contrast, it affects the photocurrent value significantly: when adsorbing, the anions increase the photoeffect, while the cations decrease it. These effects are associated with the potential drop redistribution in the Helmholtz layer and the film. The dissimilar changes of the generation current in the presence of similarly charged ions at their equal concentration evidence their different adsorption activity. The correlation between the generation current and surface-active ion concentration in solution is found. The photoelectrochemical spectroscopy allows evaluating qualitatively the surface-active ion adsorption at the passive iron and judging on the ion adsorption by the dependence of the generation current on the ion concentration.