Linking corrosion and catastrophic failure in low-power metallized polypropylene capacitors

Corrosion of the metallic film in cylindrical metallized film capacitors generally progresses from the end caps toward the center of the film. Promoted by the penetration of moisture from outside, the corrosion is usually more advanced in the outer layers where the windings are less compressed. This progression can result in increasing lengths of metallization being disconnected from direct connection to the zinc-sprayed end caps (schooping), resulting in long serial connection back to some point deeper in the spiral winding of the capacitor where contact to schooping still exists. In addition, vestigial remnants of metal, more resistant to corrosion, may remain at various points, bridging the gap between the schooping and the metallization. Theoretical analysis indicates that heavy series current at the still-connected end of the metal film and excessive current density in the vestigial links can result in localized heating that exceed the tolerance of the dielectric, resulting in catastrophic failure. Such heating and probable fusing of remnant links are prime suspects in the catastrophic failure of metallized polypropylene capacitors