On the role of water in the ageing of polymers in air-insulated electrical systems

Experimental results are presented on the direct interaction between corona and insulating surfaces, including the generation of water by corona oxidation of the polymer. Measurements of indirect corona-polymer interaction by corona exposure of distilled water with submerged polymer samples are also reported. It is shown that corona discharges in ambient air will form a water layer on any exposed polymer surface, both by bringing water to the surface made hydrophilic by the corona, and by producing water from the polymer itself, by oxidation. Water exposed to air corona becomes acid and oxidizing, and causes marked aging effects on polymer surfaces. Thus, any model of the action of air corona discharges on polymer surfaces must include the presence and effects of an intermediary aqueous electrolyte surface layer