Electrochemical impedance study of water transportation in corona-aged silicone rubber: effect of applied voltage.

Silicone rubber is widely used as the housing material of composite insulator for outdoor high-voltage insulation. However, corona-induced aging in silicone rubber contributes to the ingress of water or moisture, which finally leads to fracture of a composite insulator. To investigate water transportation in silicone rubber helps to shed light upon the insulator fracture mechanism. In this work, water transportation behaviors in silicone rubber treated by different applied voltages of corona discharge were systematically studied using electrochemical impedance spectroscopy (EIS). Low voltages (3, 4 kV) of corona treatments result in growing value of fθmin<inline-graphic></inline-graphic> in Bode phase angle, indicative of severer degree of surface delamination. After 5 kV of corona treatment, water can finally diffuse through the silicone rubber, and then induces double-layer capacitance (Cdl<inline-graphic></inline-graphic>) and the charge transfer resistance (Rct<inline-graphic></inline-graphic>) in EIS spectra. For high-voltage (≥6kV<inline-graphic></inline-graphic>) corona-exposed samples, great number of surface cracks are observed, and then Warburg impedance appears in EIS spectra on initial period of immersion, suggesting a direct water transportation process. The polar groups generated in silicone rubber during corona exposure also facilitate this water diffusion process. Calculated water diffusion coefficients show a sharp increase between 5 and 6 kV, which serves as an indicator for percolation of cracks or defects in bulk of silicone rubber. [ABSTRACT FROM AUTHOR]