The valve tray is the main device to support the voltage of HVDC. The faults of the cooling system of HVDC converter valve greatly impact the safe operation of the power grid. About 66% of the all faults are valve cold system stoppages which are caused by the electrode scaling, electrode gasket corrosion and the pipes clogged by scale which can lead to poor cooling. The scale was verified to be Aluminum compounds. This paper analyzes the cause of scale formation and offers some proposals to prevent scale formation.