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<p>Zn–Ca composite phosphate coatings were successfully prepared on 30CrMo low-alloy steel via current-assisted phosphating in a phosphate solution containing non-polluting accelerators. The microstructure and chemical composition of the coatings prepared under different current densities (3, 6, 9, 12&#xa0;mA/cm<sup>2</sup>) were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FT-IR). The composite coatings primarily consisted of CaZn<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>·2H<sub>2</sub>O, Zn<sub>2</sub>Fe(PO<sub>4</sub>)<sub>2</sub>·4(H<sub>2</sub>O), Zn<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>·H<sub>2</sub>O, and metallic zinc. With increasing current density, both the thickness of the coatings and the metallic zinc content increased significantly. However, the excessively high current density (12&#xa0;mA/cm<sup>2</sup>) reduced the uniformity and increased the porosity of the coatings. Electrochemical tests and immersion experiments in NaCl solution indicated that the composite coatings enhanced the corrosion resistance of the steel through the synergistic effect of cathodic protection by metallic Zn and the physical barrier provided by the phosphate coatings in the early stage of corrosion. During the mid-to-late stages of corrosion, a dense layer formed by Zn<sub>5</sub>(OH)<sub>8</sub>Cl·H<sub>2</sub>O and phosphate compounds offered physical protection to the substrate. Notably, the coating formed under a current density of 6&#xa0;mA/cm<sup>2</sup> showed the highest impedance and the lowest hydrogen evolution, indicating superior corrosion resistance.</p> Graphical abstract <p></p>

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Zn–Ca phosphating composite coating prepared by current assisted method on 30CrMo steel and its corrosion resistance

  • 期刊论文

  • 发表日期：

    2026年2月25日

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  • Yong Tian,
  • Yulai Song,
  • Hao Chen,
  • Nan Wang,
  • Yanbo Tao,
  • Jiayuan Hou

摘要

Zn–Ca composite phosphate coatings were successfully prepared on 30CrMo low-alloy steel via current-assisted phosphating in a phosphate solution containing non-polluting accelerators. The microstructure and chemical composition of the coatings prepared under different current densities (3, 6, 9, 12 mA/cm2) were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FT-IR). The composite coatings primarily consisted of CaZn2(PO4)2·2H2O, Zn2Fe(PO4)2·4(H2O), Zn3(PO4)2·H2O, and metallic zinc. With increasing current density, both the thickness of the coatings and the metallic zinc content increased significantly. However, the excessively high current density (12 mA/cm2) reduced the uniformity and increased the porosity of the coatings. Electrochemical tests and immersion experiments in NaCl solution indicated that the composite coatings enhanced the corrosion resistance of the steel through the synergistic effect of cathodic protection by metallic Zn and the physical barrier provided by the phosphate coatings in the early stage of corrosion. During the mid-to-late stages of corrosion, a dense layer formed by Zn5(OH)8Cl·H2O and phosphate compounds offered physical protection to the substrate. Notably, the coating formed under a current density of 6 mA/cm2 showed the highest impedance and the lowest hydrogen evolution, indicating superior corrosion resistance.

Graphical abstract