When water films cover on a ceramic surface of HVDC insulators, the irreversible electrolytic corrosion occurs driven by DC leakage current, and the subsequent formation of zinc dendrite may appear near by the cathode. Zinc dendrite, which is a well-known problem in Zn-based batteries, is formed occasionally under a range of current densities. However, effect on electrolytic corrosion process due to zin dendrite growth is still an insoluble mystery.
The corrosion test is designed acoording to the China electric industry standard DL/T 1581. Two metal slices with distance of 2 cm were pasted on the surface of a ceramic insualtor as the anode and cathode. DC voltage was applied to the slices. Water droplets flowed from the anode to cathode, and a eletrolyte film covered the insulated surface and connected the two electrodes. Electrolytic corrosion occured and lead to current increase.
Dendritic channels were formed and grew with corrosion. The channels were zinc dendrites and consisted of metallic zinc. One of the dendrites grew quickly, corresponding to a sudden rise in current, and suppressed the others. A thin electrolyte film and a slow flow rate can promote dendrite growth, of which the speed can be up to 1cm/s~2cm/s. Dendrites shorten the eletrolyte path, decrease the resistance, and concentrate the current on the longest dendrite channel. Joule heat evaporates liquid from the surface, and dendrites exposes to air. Zinc dendrites are oxidized in a short time to form zinc oxide, which restrains the development of corrosion.
Formation of Zinc dendrites accelerates the rate of electrolytic corrosion, which should be taken into account for corrosion suppression.
 

zinc dendrites,electrolytic corrosion,ceramic insulators,HVDC