Because the basic structure of aluminum electrolytic capacitors consists of an anode, aluminum oxide attached to an insulating medium, a cathode aluminum layer for the receiving electrode, and a true cathode electrolyte.
In fact, there are two capacitors inside an aluminum electrolytic capacitor, one formed by an anode foil, an anodic oxide film, and an electrolyte, and the other formed by a cathode foil, a natural oxide film, and an electrolyte. These two capacitors are connected in series to form the final equivalent capacitance.
Usually, the tolerance values of these two capacitors differ greatly because there is a thick oxide film on the surface of the anode foil, which is artificially oxidized through electrochemical reactions to obtain a relatively dense aluminum oxide film. This aluminum oxide is insulating and has a very large dielectric constant, making it easier to form larger capacitance values. In the capacitance of the cathode, the oxide film on the surface of the cathode is naturally formed by aluminum foil in the air and electrolyte working environment, and is very thin. So her withstand voltage ability will be very poor, about 1-1.5V, while the oxide film on the anode foil, which is the result of chemical treatment, is thicker and more uniform and stable, so the withstand voltage value is relatively high.
So why does the difference in withstand voltage indirectly lead to the easy explosion of aluminum electrolytic capacitors when they are reversed?
This is because reverse connection means applying a voltage beyond the withstand voltage range on the cathode. At this time, the moisture in the cathode and electrolyte will be electrically decomposed to produce oxygen, which reacts with the aluminum on the cathode surface to form an acidic film. In addition, due to the oxidation-reduction reaction, on the one hand, a large amount of hydrogen gas will be generated inside the capacitor, increasing the internal pressure, and on the other hand, heat will be released, causing the temperature to rise. As a result, the aluminum electrolytic capacitor begins to rapidly expand, causing the oxide film to fall off and eventually explode.