Study on formation mechanism and corrosion resistance of an environmentally protective insulating coating on non-oriented electrical steel

In the present work, we developed an environmentally protective insulating coating for non-oriented electrical steel, which was composed of phosphate film-forming agent, rare-earth passivating agent, silane coupling agent and polyurethane additive. The morphology, composition, electrochemical properties and corrosion resistance of the coating were characterized by scanning electron microscopy (SEM), confocal Raman microscopy (CRM), scanning vibrating electrode technique (SVET), and electrochemical measurements. The formation mechanism of the coating revealed that the synergistic effect of rare-earth passivating agent and silane coupling agent made the distribution of rare-earth oxides uniform and hindered the agglomeration of phosphate. Further addition of polyurethane formed cross-linked network structures with the original coating to enhance corrosion resistance. The electrochemical results indicated a relatively low corrosion current density (0.12 μA cm−2) and a relatively high polarization resistance (143 kΩ cm2). The SVET results illustrated a good passivation effect of the coating, contributing to a continuous protection to the substrate. In addition, the coating could withstand the annealing test for 2 h and resist the neutral salt spray test for 8 h. Consequently, the comprehensive properties of the coating were found to be higher than industrial standards, satisfying application requirements for non-oriented electrical steel.