"However, if continuous or repetitive reverse voltage exceeding
about 0.5 to 1.5 volts such as reverse DC voltage or even the negative half cycle of an AC voltage waveform is applied to a polar electrolytic capacitor, protons— which are,
after all, much smaller than hydroxyl ions— will not be effectively
blocked, and they will readily flow (conduct) through the dielectric to the negatively biased anode foil. This will cause the water and possibly other ions in the electrolyte to dissociate, attempting to grow aluminum oxide onto the surface of the cathode foil
https://www.cde.com/resources/catalogs/AEappGUIDE.pdfand to liberate hydrogen at the anode foil. Therefore, application of reverse or AC voltage will rapidly cause heating, hydrogen gassing, pressure build-up, loss of capacitance and increase
in ESR in a polar electrolytic capacitor"