What makes electrolytic capacitors swell.

[G7PSK]

I have just been cleaning out my stores, I found six power factor correction capacitors that are part of a lot I had specially made by Philips capacitors back in the early 1980's.
They have not been used yet two of them have swollen, I always thought that it was gas pressure produced by the boiling of the electrolyte but as these have not been used it must be some form of corrosion of the aluminium foils inside, am I correct in this assumption or is there something else going on.

[madires]

Hi!

Electrolytes are quite complex chemicals and can decompose over time causing gas. Do you remember the bad cap scandal? That was similar, just in fast motion :-)

Cheers
 madires

[SeanB]

Pictures? These caps do deteriorate with time, especially if the seal let oxygen thorough to the inside, where it caused decomposition of the conductive electrolyte and gas formation.

[G7PSK]

The bulging ones just went straight into the skip as I did not have time for any thing else as I needed to get cleared up and out of the premises. I do have the remaining four though. I can do a picture of them.

[NiHaoMike]

Pictures? These caps do deteriorate with time, especially if the seal let oxygen thorough to the inside, where it caused decomposition of the conductive electrolyte and gas formation.

Wouldn't a leaky seal also let any pressure buildup to leak back out?

I suspect the capacitors were exposed to high temperature at some point and/or may be marginal units that had abnormally high pressure from the final testing process. The use of electrolytic caps for power factor correction is a new one for me, since AC electrolytics generally are not designed for continuous use. Maybe that's why they're not used for that purpose anymore?

[SeanB]

PFC capacitors are always made from aluminium, either as a thin foil with an insulating spacer ( and part of the dielectric) or a sputtered film on a substrate. They are filled with either an oil, or a proprietary formula as a dielectric and arc suppressor. This often can break down with time, as it is often not designed to be stable in the presence of oxygen from the air. Thus a leaky seal lets in oxygen, and may not let the reaction products out again, causing the cap to swell.