Abstract: The converter valve is one of the core equipment in high voltage direct current (HVDC) system, and the water cooling system is often used for heat dissipation. However, the corrosion of radiator in the cooling system and the sediment on the grading electrode will lead to fault of converter valve. It's therefore essential to study the characteristics of corrosion and deposition in the water cooling system. Based on the mechanism of radiator corrosion and sediment deposition, this paper uses the finite element method to establish a grading electrodes deposition model considering the electrode surface deposition reaction and material transfer process. The built model can investigate the growth process of the sediment, and analyze the electric field near the surface of electrodes and the distribution of the corrosion ion concentration. The deposition behavior of sediment and the distribution of sediment on grading electrodes is studied based on the proposed model, and the effect of voltage level on the thickness of sediment is discussed. The results indicated that the thickness of sediments is proportional to the voltage level, the sediment is mainly concentrated on the high-potential electrode, and the amount of sediment on the electrode tip is much larger than that on the parallel position of the electrode. In addition, the proposed model is verified by the measurement results of actual converter valve's cooling system and the sediment on the electrode.