The so called Corona Effect is a phenomena of partial discharges (electrical sparks breakdown) between the layers (micocavities of air) that erode the metal layer when the inception or corona starting voltage is higher than the electric air field but not high enough to cause the dielectric breakdown. Inception voltage will as much lower as moisture is penetrated inside the wound element and temperature in not too high. The partial discharges happen at any cycle and even not associated at a very high energy they, due to the high repetition rate can determine, dot by dot, the evaporation of metal and then the reduction of the capacitance value. The phenomena can be controlled with film thickness, moistore barrier and proper manufacturing process.