Recent investigations tend to indicate that the nature of the source of current (DC, AC low or high frequency) affects significantly the electrode wear mechanism during resistance spot welding (RSW) of steels. It has been also demonstrated that the electrical current affects microstructural transformations occurring during heat treatment of metallic materials, when performed using Joule heating systems. We explore in this work the influence of welding current (50 Hz AC or 1000 Hz Medium Frequency Direct Current, MFDC) on the degradation of the copper-steel interface during repeated instrumented RSW experiments on a automotive high strength steel. In order to analyze the contribution of current-sensitive microstructural transformations in the degradation process, model experiments are also performed on an idealized copper-steel system using controlled heat treatments with different heat sources, from lamps to Joule heating. The influence of the nature of welding current is then demonstrated, with strong potential implications on a wide variety of engineering systems exploiting Joule heating effect.