The mechanism of Zircaloy shadow corrosion was studied theoretically and experimentally. The theoretical study showed shadow corrosion, with a few exceptions, to be remarkably consistent with a galvanic corrosion mechanism. The exceptions may be explained as radiation effects. An experiment was therefore devised to test the galvanic hypothesis, based on the fact that galvanic corrosion requires a closed current loop. Three pieces of Zircaloy cladding with stainless steel counter electrodes were exposed in-pile. Two of the claddings were galvanically coupled to the counters. The third cladding was well insulated by fitting it to a 100 mm sapphire tube. The claddings were irradiated together in oxidizing water chemistry for one reactor period. The galvanically coupled claddings developed shadow corrosion, but not the well insulated cladding.