There is a black film on some parts after using Metal Rescue. What is it?

Metal Rescue 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 Metal Rescue for longer periods of time. For these metals, reducing the time that the part is immersed in the liquid 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. Also, it is best to remove it immediately after removing parts from Metal Rescue and before the parts dry. The black oxide becomes more difficult to remove for parts that have soaked for a long period of time. It is best to soak rusted parts only as long as needed to remove rust.