Corrosion is the breakdown of materials due to chemical reactions. It is usually oxidation with air molecules and often in the presence of water. Corrosion also occurs when an acidic/basic corrosive substance touches another material. When a material corrodes, its physical properties change. Problems with corrosion are mostly with metal, though other materials can corrode. Corrosion is a form of erosion. Some materials, such as stainless steel, are highly resistant to corrosion. Metals that may corrode can be protected by plating, painting, and other means.