Question

red crust on the old metal​

Answer 1

Answer: Red Crust on Old metals is called rust. I think that's what you asked, or did you ask something else ?

Answer 2

The red crust on old metal and especially on Iron it is called rust. The rust is formed on iron due to oxidation of iron. When a metal comes in contact with the moisture and oxygen present in the air, it reacts with them and forms a corrosive layer just like dead skin cells on our body.

The layer formed on iron is made up of iron oxide and it is a product of redox reaction between iron and oxygen and moisture.