The treatment of rusted steel with mimosa tannin

Tannin treatments for rusted steel have been in use for some years now but their mode of action is poorly understood. Investigations including the use of scanning electron microscopy, electron probe analysis and i.r. spectrophotometry, have determined some fundamental aspects of the behaviour of tannin when protecting steel. It was found that the tannin reacted with the steel to form a crazed outer layer of ferric tannate. About half the rust was impregnated with the ferric tannate or unreacted tannin. Some of the rust was apparently converted to protective magnetite over a period of time. Excessive moisture caused breakdown of the film by removing the unreacted tannin and causing regrowth of the existing rust.