Influence of hexamine on the formation of zinc phosphate coatings on steel

Abstract

Addition of hexamine (0.1 – 2.0 g/l) to zinc phosphating solutions (H₃PO₄ 30.0 g/l, ZnO 7.0 – 7.5 g/l) operated at 60 °C increased the thickness and weight of zinc phosphate conversion coatings on mild steel. Photomicrographs showed that the coatings were comprised of a rough fine- grained matrix which contained numerous grains of a needle-like to more rectangular phase having a smooth surface and higher reflectivity. Hexamine increased the number of these grains in the matrix. Electron microprobe analysis of the highly reflecting grains detected the presence of zinc, iron and phosphorous with the zinc to iron ratio increasing in the presence of hexamine while the phosphorous content remained constant. The increase in the zinc to iron ratio is due to an enrichment of α-hopeite phase relative to the phosphophyllite phase in these grains.

The performance of hexamine was compared with that of sodium nitrate by the above methods as well as by potential measurements during phosphating and by the potentiostatic anodic polarization of phosphated steels in 0.6M NH₄NO₃ solutions. These studies showed that, as an accelerator for the zinc phosphating of mild steel, hexamine is as efficient as sodium nitrate.