When people pee on the iron gate, they will have a chemical reaction when they meet something.

Human urine has a PH of 5~7 and contains urea urate and ammonia. Therefore, in the range of slight acidity and alkalinity, iron gates will be corroded by absorbing oxygen in humid air.

The negative electrode Fe-2e===Fe2+

Anode O2+2h2o+4e = = 4oh-

To scare people who pee, you can install a camera on the door, or put a sign on the door, "This is the camera area, please pay attention to your civilized and noble image." Or you stay at home for a while, someone wants to pee, and you suddenly open the door. Or shock him with a low-voltage power supply.