The scanning reference electrode technique (SRET) has been used to examine corrosion in a galvanic 'Kelocouple' system, being that of an explosively bonded joint comprising a tri-metallic sandwich of steel/aluminium/aluminium alloy. Quantitative results of local current densities in 4% sea water at room temperature have been obtained. These have been compared with results from two conventional macroscopic techniques for measuring galvanic current, namely zero resistance ammetry and polarisation curves. The SRET results were found to be at least an order of magnitude higher than the average results obtained using the macroscopic methods. Comparison between SRET current densities and corrosion (as obtained by the amount of aluminium in solution) showed good agreement. Whether attack predominated on the aluminium (nearer the steel) or the aluminium alloy (further from the steel) depended on the concentration of the sea water