張生生地銹o既電筒還原

corrosion is an irreversable reaction, once the metal has corroded an oxide would form depends on what metal you have got in your torch. If it is iron, the iron metal will turn to oxide which is really flake and therefore easy to fall off. even though there is a possible way to reduce the oxide, the context of the oxide would have not enough to form the original torch back. You may see a pit on your torch.