Sweet Iron is designed to oxidize.
Sweet Iron is made from black iron and copper, creating a metal that promotes moisture in the horse's mouth. The black iron and copper compound slowly oxidises and rusts. Slow-acting and harmless to horses, the oxidation has a sweet taste that horses like and which encourages salivation. This metal will almost always discolour. The cheeks and all the joints of the bits are made from stainless steel and will not rust or bind.