Electrode corrosion degradation in metallized polypropylene capacitors

One of the major failure mechanisms in metallized polypropylene capacitors is the loss of capacitance through a conversion of the aluminum electrode to an oxide (corrosion). This work examines the voltage endurance and corrosion growth characteristic in metallized polypropylene capacitors under different conditions of temperature, power frequency and applied voltage. The corrosion growth phenomena are studied both in full size 17 uf capacitors and in "stamp capacitors" consisting of a two parallel plate arrangement that allows real time observation of the corrosion growth. The effects of electrode thickness and impregnating fluid are investigated and optical and electron microscope observations are presented that quantify different corrosion characteristics. The experimental results indicate that the electrode corrosion process responsible for the loss of capacitance is not consistent with a partial discharge mechanism, but with an electrochemical corrosion involving ionic transport under alternating field conditions. Electron diffraction microscopy data suggests that at least some of the corroded aluminum electrode is converted to an insulating crystalline Al
2
O
3
of V form. The initiation and growth rate of the corrosion sites are shown to have a strong dependance on the frequency of the applied voltage and ambient temperature, with near zero growth under DC conditions, frequencies higher than ten KHz, and temperatures lower than −40°C.