Degradation of a silicone-based coating in a substation application

An RTV silicone rubber coating applied to insulators in a 345 kV switchyard located near Long Island Sound has depolymerized and become putty-like after six years of service. Coating samples were taken from high creep insulators which were energized and weathered, weathered but not energized, and neither weathered or energized (warehouse storage). The unweathered and unenergized coating did not appear to degrade. The weathered but unenergized coating had degraded measurably but was still in an operational state. The energized and weathered coating has become putty-line and lost all physical integrity. This suggests aging by a combination of chemical, photochemical (weathering), and electrochemical mechanisms.