Bath contemplations.

[boater sam]

Navel gazing, what makes an electrolytic capacitor polarised?
I know that they are "activated" after construction, or reformed if old and still serviceable but why then do they explode if subjected to reverse voltage below the rated voltage?
I cannot understand the mechanism that makes them act as a semiconductor diode.
Indeed, how do they make non polarised electrolytics?

[DrStrangelove]

As I understand it, reversing the applied voltage destroys the oxide dielectric.

[Mike Watterson]

The insulating dielectric is aluminium oxide. The liquid is conductive. At the start there is no aluminium oxide and a small current causes oxidisation on the positive or anode. That's very thin and a good insulator so the capacitance is high.
With age the oxide degrades and becomes porous, so reforming is a voltage at a limited current till there is less than a microamp.

If you put AC or a reverse voltage the oxide dissolves in the electrolyte which decomposes producing gas and the part explodes. If it was current limited DC the capacitor might reverse polarise.

These seem good:
Wikipedia Aluminium Electrolytic Capacitor
and
Wikipedia Electrolytic Capacitor

However it doesn't explain bi-polar electrolytics, which are rare outside of loudspeaker crossover networks. They seem to develop high ESR so I often replace them with larger metpoly types.
I've never understood how they work and didn't care to find out. I suspect they can't be reformed if the oxide is created at manufacturing time by an anode layer between two cathodes, so there is in a sense a third wire not brought out. Two series electrolytics back to back with diodes is likely longer lived.
I had a quick search and I don't find any sane info on unpolarised/bipolar electrolytic caps than what I imagined.

If the foil for the anode is etched rather than plain you get a larger surface area and more capacitance. That might not be a good idea for higher voltage parts.

They don't work well as a diode, and not for long at higher current without exploding.