Left unchecked, corrosion can cause structural failure, but because the appearance of corrosion varies with metals, it can go undetected. In some types of metal, pitting and etching indicate corrosion, while copper and copper alloys show corrosion as green or red deposits. Corrosion is part of the normal wear and tear of an aircraft, and minor corrosion may not significantly alter the strength of the metal. However, leaving the corrosion unchecked could result in cracks, which is why addressing aircraft corrosion is critical to pilot safety. Treating corrosion depends on the type of metal, the part of the aircraft that becomes corroded and what caused the corrosion. Corrosion can be effectively controlled if action is taken early.