The protective abilities of four crystalline Zn (PZn), Zn-Ca (PZnCa), Zn-Ni (PZnNi) and Zn-Ni-Mn (PZnNiMn) phosphate coatings on carbon steel 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 (XRD) and scanning electron microscopy (SEM) techniques, while the corrosion and porosity parameters of samples were determined by electrochemical measurements. According to XRD analysis data all investigated coatings were composed of three phases: hopeite Zn₃(PO4)₂ · 4H₂O, phosphophyllite Zn₂Fe(PO₄)₂ · 4H₂O and the substrate metal Fe. The results of electrochemical measurements revealed that after immersion into a saturated Ca(OH)₂ + 1 M NaCl solution for 1 h carbon steel samples covered with the phosphate coatings possessed from fourto seventeen-fold higher R_p values in comparison to those of bare steel samples. The porosity values obtained by electrochemical impedance spectroscopy (EIS) were in total agreement with the values evaluated by linear polarization measurements. To summarize the results of measurements, the low porosity medium weight PZnNiMn coating on carbon steel demonstrated effective protective properties in the chloride-contaminated alkaline solution.

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