Spot electrical contact welding of parts with coatings is considered. The aim of this work is a mathematical description of the thermal processes that occur during electrical contact welding of parts with coatings and determining the specifics of their heating. To describe the thermal processes occurring in these conditions, a mathematical model is proposed that allows, under certain assumptions, an assessment of the presence of low-melting coatings by a sudden change in the thermal and electric resistance, contact area of the welded parts, parts and electrodes. In the course of numerical experiments, when welding three parts of the same thickness, the above-mentioned parameters vary, as well as the heat flux in the radial direction. The obtained results, in general, are consistent with the available concepts and allow evaluating the dynamics of temperature growth in electrodes and parts, as well as an increase in the size of the welding core.