Answer:
chemical
Explanation:
Chemical change is a type of change that involves the formation of new substances. The process of forming this new substance is irreversible i.e. cannot be changed back into it's normal form. Chemical changes is characterised by color changes, gas evolution etc.
In this case, a RUST on a pipe is being described. This rust is a type of chemical change because it involves a change in color and also a new substance is formed.