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Thisarticle reports a study of the addition of black wattle tanninto a phosphating bath as an environmentally friendly grain refineradditive. Scanning electron microscopy (SEM), X-ray diffraction (XRD)analysis, and electrochemical impedance spectroscopy analysis werecarried out to verify the tannin effect. Adhesion was measured accordingto the ASTM D3359 standard. The presence of tannin in the zinc phosphatingbath changed the direction of phosphate crystal growth and favoredphosphophylite phase formation (XRD analyses). SEM images showed areduction in the size of the zinc phosphate crystals when tannin waspresent in the bath. Because of this decrease in size, the adherenceof the final coating improved. The optimal concentration of blackwattle tannin in the zinc phosphating bath used in this work was 2g L–1, as higher values reduced the corrosion resistanceof the coating. Grain size reduction was also observed in iron phosphating. [ABSTRACT FROM AUTHOR]