The radiochemical technique for the study of surface reactions has been investigated. This technique readily demarks on the metal surface areas that are cathodic to adjacent areas. The steel-copper and aluminum-copper couples were observed and the strong attraction of radiocobalt to the cathode member of the couple was measured. The sorption of radiocobalt ions on surface films is a complicating effect in the employment of this technique if not considered and understood. The extent of this effect varies with the surface film under consideration, being considerable for oxide and sulfide films on copper and small for oxide films on alloy steels and anodized films on aluminum.The sorption of ions from solution has a great significance in so far as corrosion mechanism is concerned as it would appear that ion concentration cells are common occurrences when part of a metal surface is covered with an oxide film and part is bare.The fact that atoms on a metal surface may exchange with ions of the same metal in solution has been known for a number of years and has been of particular interest to those working on the theoretical aspects of corrosion, catalysis, and other applications of surface chemistry. Recently pickup of radioactive ions from solution by metals has been used to study electrode reactions (1), adsorption processes (2), and corrosion mechanisms (3, 4).In this laboratory there has been a critical appraisal of the radiochemical technique with respect to its usefulness in studying corrosion mechanisms. There are several possible mechanisms by which tracers, as radiocobalt ions, may be transferred from solution to a metal surface:'(a) exchange between metal ions on surface and in solution; (b) sorption; (c) electrolytic action.The more recent publications on this technique have contained experiments interpreted in terms of the electrolytic action mechanism. However, some of the authors' earliest experiments demonstrated that the cobalt ion tracer was rather loosely held on the sm'face, suggestNe of a sorption mechanism.If this technique is to be of value for the study of corrosion processes there should be no ambiguity in the interpretation of the results obtained. The objective of this work was to contribute to the understanding of the mechanism and interpretation of the technique. EXPERIMENTALAll the radiochemical studies were carried out using solutions containing tracer concentrations of the radioactive isotope, cobalt-60. This isotope has a half-life of 5.27 years and emits 13-radiation of 0.306 mev energy as well as 1.33 mev and 1.17 mev "/-rays. The original solution was obtained from Oak Ridge National Laboratory and was in the form of CoCl: (7.1 mg Co/ml) in 0.34N HC1 solution. Separate dilutions of this solution were used for each of tbe metals investigated. Concentrations of cobalt ion and H + after dilution are shown in Table I. This table also records the activity of each solution. In all cases the metal samples in strip form were immersed in the tracer solution for 10 rain at a constant temperatur...