Three different ironchromium alloys have been studied in pure 0.5 M H2SO4 and with an addition of chloride. The investigations have been performed in the active and activepassive range with a combination of Electrochemical Impedance Spectroscopy and a rotating ringdisc electrode to give quantitative information about the charge and information about the adsorption of reacting intermediates. In sulphuric acid the emission efficiency ralative to ferrous ions is totally located in the positive range, which means that iron species have a passivating role. The emission efficiency in sulphuric acid with chloride is located in the negative range. The iron containing surface species never have, therefore, a passivating role but behave as an intermediate in a dissolution path of the alloy. It has been found that it is possible to discriminate between the individual contributions of different species to the surface process by selecting the proper detection on the ring.