Grain Refiner Effect of Black Wattle Tannin in Iron and Zinc Phosphate Coatings

Abstract

This article reports a study of the addition of black wattle tannin to a phosphating bath as an environmentally friendly grain refiner additive. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and electrochemical impedance spectroscopy analysis were carried out to verify the tannin effect. Adhesion was measured according to the ASTM D3359 standard. The presence of tannin in the zinc phosphating bath changed the direction of phosphate crystal growth and favored phosphophylite phase formation (XRD analyses). SEM images showed a reduction in the size of the zinc phosphate crystals when tannin was present in the bath. Because of this decrease in size, the adherence of the final coating improved. The optimal concentration of black wattle tannin in the zinc phosphating bath used in this work was 2 g L–1, as higher values reduced the corrosion resistance of the coating. Grain size reduction was also observed in iron phosphating.