The deposition problem on grading electrodes in the inner cooling system of HVDC converter valves will cause water pipe blockage, leakage and heat loss failure, even DC blocking accident, endangering the safe and reliable operation of HVDC system. In this paper, simulated corrosion and deposition experiments are conducted to determine the decisive factor which influence the charged property of sediment-forming ions. The existence form of sediment-forming ions in cooling water under different conditions is calculated based on thermodynamic theory. The results show that the corrosion of the metal heat sink is the root of sediment-forming ions, but the existence form of the sediment-forming ions in the cooling water is determined by the pH of the cooling water during the migration, instead of being determined by the original corrosion reaction. In weakly acidic environment, most of the sediment-forming ions exist in the form of positive aluminum complex ions, while in weakly alkaline environment, the sediment-forming ions almost all exist in the form of negative aluminum complex ions.