Studying corrosion performance, microstructure and adhesion properties of a room temperature zinc phosphate conversion coating containing Mn2 + on mild steel

In this work, a series of low temperature zinc phosphate conversion coatings formed on mild steel. The surface treatment was carried out at different pHs in the phosphate solutions containing Mn2 +. An epoxy/polyamide coating was then applied on the phosphated substrates. Potentiostatic polarization test was carried out in order to evaluate the corrosion protection properties of the phosphated samples in 3.5 wt.% NaCl solution. The microstructure and morphology of the coatings were studied by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM) and atomic force microscopic (AFM). The adhesion strengths of the epoxy coatings applied on the surface treated samples were measured before and after 30 days of immersion in 3.5 wt.% NaCl solution by a pull-off test. Results showed that the corrosion resistance of the phosphate coating was significantly enhanced using Mn2 + (especially at 7 g/L). A less porous and more compact phosphate coating formed on the metal surface in the presence of Mn2 + compared with the one without additive. Adhesion properties were enhanced by addition of Mn2 + to the phosphate coating composition.