The negative difference effect in the case of steel can be explained in terms of corrosion by removal of blocks or "chunks" of metal containing perhaps only a few atoms. The "chunk effect" also explains the observed fact that, under conditions of high corrosion rate, minimum protective current can be much less than the calculated corrosion current. It is suggested that the chunk effect and anodic polarization occur on the same piece of metal. Either the positive or the negative difference effect may be observed depending on the corrodent.
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Marsh et al. (1960) studied this question.