On the Mechanism of Aluminum Corrosion in Metallized Film AC Capacitors

Metallized film capacitors consist of alternating layers of polypropylene or other polymeric film dielectric and vapor deposited aluminum conductor. During operation above 250 V ac (RMS), circular islands of oxide develop in the aluminum conducting layers and reduce the capacitance. Increasing the aluminum layer thickness to more than the normal value of about 30 nm reduces the corrosion problem. The removal of moisture stops corrosion. Corrosion increases with temperature, ac voltage, and frequency to 3.5 kHz. A dc field, equivalent to the peak ac field, cannot initiate or sustain the corrosion process. These observations are consistent with a mechanism of anodic oxidation at the edge of defects in the aluminum layers. These discontinuities in the aluminum must be sufficiently large to enhance the local electric field and sustain ionic transport in the oxide. Fibrils on the polymer-film surface, and both electrical and electrochemical self-healing processes, are the most probable sources of corrosioninitiating conducting-layer defects.