Formation of conversion Zn-Ni-Mn phosphate coatings on steel and corrosion behaviour of phosphated specimens in a chloride-contaminated alkaline solution

The formation of the crystalline Zn-Ni-Mn phosphate coating on carbon steel as well as the protective abilities of this coating in a chloride contaminated alkaline solution have been studied. The chemical composition and the morphology of the phosphate coating were evaluated by the X-ray diffraction and scanning electron microscopy techniques. The gravimetric study has shown that the formed phosphate coating (0.4 at.% of Ni and 1.1 at.% of Mn) was medium weight (1.9–3.4 g m–2) with the thickness of ~2 μm. According to XRD analysis data the coating was composed of three phases: hopeite Zn3(PO4)2 · 4H2O, phosphophyllite Zn2Fe(PO4)2 · 4H2O and Fe. The results of electrochemical measurements revealed that after immersion into a saturated Ca(OH)2 + 1 M NaCl solution for 1 h the corrosion current icorr of phosphated samples decreased about twenty-fold and the polarization resistance Rp increased seventeen-fold as compared to those of the substrate. The prolongation of exposure time of phosphated samples to the solution up to 15 days leads to an approximately thirty-fold increase in Rp values as compared to those obtained for bare steel. Therefore, the low porosity (~1%) medium weight crystalline Zn-Ni-Mn phosphate coating on carbon steel demonstrated effective protective properties in a chloride-contaminated alkaline solution.