Electrochemical activity at the interface of dissimilar explosive joint of stainless steel with Zircaloy has been studied in 11.5 M nitric acid using scanning electrochemical microscopy. The reduction of nitric acid and oxidation of nitrous acid in the tip-substrate gap is proposed as a mechanism for the electron transfer reaction. Due to this reaction increase in tip current was observed, indicating that the positive feedback mode of SECM is under operation. The difference in tip current was correlated with the microstructure and the electrochemical activity of the dissimilar weld interface.