Tinned copper is primarily used for protection against oxidation and corrosion. In climates where copper has long-term exposure to water, the oxygen will combine with the metal and form copper oxide, weakening the bonds of the metal. It is worse if the wire is in contact with salt water. As more of this oxide forms, it weakens the integrity of the copper making it brittle and crumbly. Corrosive copper wire is less efficient at moving electricity and can cause safety hazards.