The Influence of Electrode-Fluorine Reactions on Corona and Glow Discharges in SF6

In a sulphur hexafluoride (SF6) discharge, regions of metal fluoride develop with time on the electrode surfaces as a result of chemical reaction between the electrode material and fluorine produced in the discharge. The influence of these fluorides on discharge behavior has been examined for several electrode materials: brass, copper, stainless steel, aluminum, and nickel, in SF6 positive corona discharges at a pressure of 200 torr and in SF6 glow discharges at a few torr. For observation and photography of the low-intensity corona discharges, an image intensifier system was used. For most of the electrode materials, the surface buildup of metal fluoride causes the structure of the discharges to be modified with time, and bright spots or filaments develop at the electrodes in times of the order of minutes. For nickel electrodes, the electrode-fluorine reaction is very slow, and little change in discharge behavior is observed, even after several hours. Nickel clearly represents the best choice of electrode material for any discharge device using a fluoride medium, particularly when uniformity and stability are required over long periods of time.