An induction coil is an important part of a medium-frequency induction furnace. The stability and reliability of the coil are important tasks for the design, manufacture and maintenance of the coil. Taking the medium-frequency induction furnace breakdown coil as the research object, the physical and chemical properties of the attachments on the coil surface are systematically analysed via multiscale characterization techniques such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), infrared spectroscopy and confocal Raman spectroscopy. The results show that the black attachment on the surface of the coil is mainly free carbon, and Raman spectroscopy confirms the presence of few layers of graphene in the carbon structure. A large amount of CO gas spillover during the smelting process and water vapour electrolysis are the main causes of a series of reactions. The CO hydrogen reduction reaction and CO disproportionation reaction are the main carbon deposition reactions that occur during the operation of an induction furnace. The mixed gas (CO, CO2 , H2 , H2 O, O2 ) is hindered by the Isoplan insulation plate during the outwards diffusion of free carbon, which is the reason why a large amount of free carbon is attached to the coil surface.