Corrosion occurs when metals are exposed to oxygen and form undesirable byproducts, effectively reducing the metal's lifespan. With water being such a commonly found and naturally occurring substance, it also serves as one of the most common causes of metal corrosion. Surface coatings can easily prevent metal corrosion, provided they can keep it away from the metal substrate it protects. ASTM D870 measures a coating's ability to prevent water from coming into contact with its substrate, effectively preventing corrosion from beginning. The coated surfaces are totally or partially immersed in distilled or deionized water in a corrosion-resistant container at a specific temperature for a certain amount of time. Test panels are subsequently evaluated for blistering, softening, loss of adhesion, or any other coating defect.