A scanning vibrating capacitor (SVC, Kelvin probe) is used to study hydrogenation of iron and steel under the conditions of atmospheric corrosion. It is shown that hydrogen that forms in the course of corrosion or under cathodic polarization diffuses through the membrane and interacts with its opposite side, causing a decrease in the surface Volta potential. It is proposed that atomic hydrogen reduces Fe3+ ions in the passive film. It is shown that the SVC technique is informative for registration of local regions of hydrogenated metal at very low hydrogen flow into steel.