What causes corrosion on high voltage electrodes

Summary:In the context of this work, suitable holders for high-voltage investigations were developed, which allowed an exchange of discharge-damaged Cu, Al, Ag, W and WCu electrodes after partial and spark discharges under an SF6-overpressure atmosphere. A multi-stage multi-method concept was developed for the analysis of the damaged electrodes, which provided information about the material changes on the electrodes both on the surface (nm range) and inside (µm range). The investigations could be implemented successfully with the help of the SEM-EDX, XPS, AES and SNMS in this order of the analysis. In addition, it was possible to develop a determination procedure for S, F and O for the SNMS with the help of laboratory standards. When carrying out the high-voltage experiments, the electrode materials used showed a differentiated electrical behavior in the event of spark discharges. Only with Al electrodes was the number of discharges limited by the progressive formation of a passivating insulating layer. In the case of W, Cu and WCu, the electrical properties were admittedly also impaired by the formation of corrosion products, but there was no total blocking of the conductivity as in the case of Al. In the case of Ag, corrosion processes did not affect the electrical behavior of this metal during the test period. With the help of micro-area analysis it could be shown that in the area of ​​individual spark strikes there is a differentiated distribution of S, F, O and C depending on the electrode material. The analysis of chemical products on the electrode surface showed a significant formation of sulphides, fluorides and oxides for each material both after partial and after spark discharges. In the inner electrode body, high concentrations of S, F, O and C could also be found in buried layers. The greatest penetration depths of S, F, O and C were found in Ag electrodes for partial and spark discharges, and the smallest in W electrodes. It was found that, as a rule, F penetrated deepest into the electrodes. When quantifying the amount of foreign matter in the electrodes, Ag turned out to be the material that was most extensively damaged by partial discharges and low-energy spark discharges. In the case of high-energy spark discharges, Cu proved to be the most susceptible to corrosion. The lowest amounts of S, F, O and C were found in Al for partial discharges and in W for spark discharges. When comparing the amounts of foreign elements in the gas and solid phase, significantly more proportions of Cu, Al, W and WCu were found in the gas after partial discharges (gas: solid = 3: 1 or more). Only in the case of Ag could a gas-solid ratio of approx. 1: 1 be observed after a long discharge time. In the case of spark discharges, with Cu, Al and W electrodes, especially with a higher number of discharges, a relative preponderance of the amounts of foreign elements in the gas was also found. Only in the case of high-energy spark discharges with Cu electrodes was the relative proportion of foreign elements in the solid body significantly higher than in the gas. With Ag electrodes, the solids content predominated under all spark discharge conditions, with WCu as the electrode material it was highest at 70-90%. The calculation of the energies occurring and chemically converted during the discharges showed that in the case of partial discharges approx. 50-90% of the energies introduced were converted into heat. For the formation of corrosive gaseous products, significantly more energy was required for partial discharges than for the introduction of foreign matter into the electrodes (approx. 20: 1). In the case of spark discharges, the proportion of energies converted into heat was at least 99% in all cases. In contrast to the partial discharges, with Ag and Cu electrodes the amount of energy converted during electrode corrosion was greater than that required for the formation of gaseous decomposition products. With Al, W and WCu electrodes, the energy ratio was again on the side of product formation in the gas, but not as clearly as with partial discharges.