Abstract: Rust converters are chemical formulations that can be applied to corroded surfaces causing the passivation and elimination of possible further attack after the application of a coating. Tannins extracted from mangrove, chestnut, quebracho and mimosa as rust converters were evaluated. Two different types of pre-rusted sample preparations were adopted. The pre-rusted samples were immersed in 5 g/l of the various tannin solutions and the phase transformations were analyzed via Fourier Transform Infrared (FTIR), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The rust components were mainly lepidocrocite (γ-FeOOH) and magnetite (Fe3O4) and were found to be dependent on sample preparations. Transformation of rust components to ferric-tannates, particularly lepidocrocite, in the presence of various tannins could be observed. The relation between the partial transformation of pre-rusted samples and sample preparation is correlated...