Mangrove (Rhizophora apiculata) tannins: an eco-friendly rust converter

Rust converters are chemical formulations that can be applied to corroded surfaces to convert residual rust on iron/steel surfaces into harmless and adherent chemical compounds. The known hazardous effects of most synthetic rust converters have led to a search for safer, more environmentally friendly natural products. Tannins, a class of natural, non-toxic and biodegradable polyphenolic compounds, extracted from plant sources have been proposed as an alternative. The transformation of rust in the presence of 5 g L(-1) tannins extracted from mangrove (Rhizophora apiculata) barks was studied. Three different methods of pre-rusted sample preparations were tested. The rust composition of the steel was dependent of sample preparation. The pre-rusted samples prepared formed predominantly lepidocrocite and magnetite. Transformation of rust components to ferric-tannates, particularly lepidocrocite, in the presence of mangrove tannins that could be observed via Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy analysis indicated their potential as rust converters.