The term CORROSION SIGN can be solved by the four-letter word RUST. RUST is a form of corrosion that occurs when iron or its alloys, such as steel, are exposed to oxygen and moisture for a long period of time. RUST is an orange-brown flaky coating that forms on the surface of metals and is made up of iron oxide. RUST can cause significant damage to metal structures and is often seen as a sign of deterioration.