There is a black film on some parts after using Metal Rescue GEL, what is it?

Metal Rescue GEL may leave a black film on some parts. The black is from iron oxide II, often referred to as black oxide. Black oxide is a corrosion inhibiting film that is a more stable state of oxidized iron. It is not detrimental to the metal surface and is only cosmetic in nature. It can form on certain steel alloys and high carbon steel when left in contact with GEL for long periods of time. For these metals, reducing the time will minimize the black film. Much of the black oxide film can be removed simply by rinsing with mild detergent or wiping with a cloth. It is best to remove the black film immediately after using GEL and before the metal parts dry. It is best to treat rusted parts only as long as needed to remove the rust.