Zhanying Wang

Simplex-centroid mixture design (SCMD) is applied to change the combination of Na₂SiO₃, KF, NaOH and NaAlO₂ to examine the influences of electrolyte components and their interactions on the thickness and corrosion resistance of micro-arc oxidation (MAO) coating of AZ91D magnesium alloy. The results indicate that the obtained regression equations are very significant (p-value < 0.01) and have high prediction accuracy (R² = 0.9893, 0.9989). Pareto analysis shows that the interactions effect between Na₂SiO₃, KF and NaAlO₂ on the coating thickness and corrosion resistance are 70.03% and 92.35%, respectively, which quantitatively confirms that there are interactions among electrolytes. The analysis of response surface methodology (RSM) demonstrates that the optimum formula is high concentration of Na₂SiO₃, high concentration of KF and low concentration of NaAlO₂. When Na₂SiO₃ is compounded with NaAlO₂, the two will react to form aluminosilicate colloids, resulting in increased viscosity of the electrolyte, and the coating corrosion resistance is poor. When the main salt of electrolyte is single Na₂SiO₃ or NaAlO₂, the corrosion resistance is better. KF can significantly improve the coating thickness and corrosion resistance. Pearson correlation coefficient (PCC) reveals that there is a remarkable relationship between thickness and the corrosion resistance in acidic media (r = 0.88927), which was determined by the corrosion mechanism of the latter.

To explore the influence of the microstructure of plasma electrolytic oxidation (PEO) coating on the loading of corrosion inhibitors and the silanization treatment on its surface, PEO coatings were first prepared on the surface of AZ31B magnesium alloy under different voltages. Secondly, sodium tungstate (Na₂WO₄) was loaded into the micropores and onto the surface of the PEO coatings via vacuum impregnation, and which were subsequently subjected to silanization treatment. The phase composition of the coatings was studied by XRD, while the elemental composition and valence state were investigated by XPS. The surface and cross-sectional morphology of the coatings, as well as the composition and distribution of elements, were studied by SEM and EDS. Image J software was employed to analyze the thickness of the coatings. The results show that the microstructure of PEO coatings prepared under different voltages varies, which affects the loading of Na₂WO₄ on the surface of PEO coating and the sealing effect of silanization treatment, thereby influencing the corrosion resistance of the coatings. As the voltage increases, the coating thickness and roughness gradually increase, while the surface porosity first increases and then decreases, and the loaded content of Na₂WO₄ also follows a trend of first increasing and then decreasing. Meanwhile, at 300 V and 350 V, silanization treatment effectively seals the PEO coatings loaded with Na₂WO₄. However, when the voltage increases to 400 V, due to the uneven surface of the PEO coating, nonuniform distribution of micropores, and high roughness, the silanization treatment fails to completely cover the coating. This results in defects such as pits on the surface of the composite coating prepared at 400 V. Therefore, the composite coating prepared at 350 V exhibits the best corrosion resistance. After immersion in a 3.5 wt.% NaCl solution for 240 h, the composite coating formed at 350 V remains intact, and its low-frequency impedance modulus |Z|_0.01Hz is as high as 1.06 × 10⁶ cm². This value is approximately two orders of magnitude higher than that of the composite coating fabricated at 400 V and about three orders of magnitude higher than that of the pure PEO coating prepared at 350 V.

Sodium tungstate (Na₂WO₄) was filled into the micropores and onto the surface of a magnesium alloy microarc oxidation (MAO) coating by means of vacuum impregnation. Subsequently, the coating was sealed through silane treatment to synergistically boost its corrosion resistance. The phase composition of the coating was inspected using XRD. FTIR was utilized to analyze the functional groups in the coating. XPS was employed to study the chemical composition and valence state of the coating. The surface and cross-sectional morphology of the coating, along with its elemental composition and distribution, were investigated by SEM and EDS. Meanwhile, the thickness of the coating was analyzed using Image J software. Electrochemical impedance spectroscopy (EIS) was employed to determine the corrosion resistance of the coating. The results show that compared with an MAO coating, M-0.125W composite coating (only filled with sodium tungstate on the surface of the MAO coating), and M-SG composite coating (only receiving silanization treatment applied to the surface of the MAO coating), the corrosion resistance of the M-nW-SG composite coating (loaded with sodium tungstate on the surface of the MAO coating and then treated with silane) is significantly improved. This is mainly attributed to the fact that sodium tungstate can be combined with Mg²⁺ to form insoluble magnesium tungstate protective film, which blocks corrosion media. At the same time, silanization treatment further seals the MAO coating and increases the compactness of the coating. In addition, with the increase in the impregnation concentration of sodium tungstate, the content of sodium tungstate in the M-nW-SG composite coating improves, and the sealing effect of silanization treatment is better. When the impregnation concentration of sodium tungstate is 0.1 mol/L or above, the MAO coating with sodium tungstate can be completely sealed. When the impregnation concentration of sodium tungstate is 0.125 mol/L, M-0.125W-SG composite coating has the best corrosion resistance, and its impedance modulus value can be maintained at 8.06 × 10⁶ Ω·cm² after soaking in 3.5 wt.% NaCl solution for 144 h, which is about three orders of magnitude higher than those of MAO coating and M-0.125W and M-SG composite coatings.

Effect of V₂O₅ additive in silicate-containing electrolyte on AZ91D magnesium alloys treated by micro-arc oxidation (MAO) technology under different loading voltages was investigated. The results showed that vanadium was well up-taken into the coating chemically. Moreover, a new phase of MgV₂O₄ with spinel structure was obtained in MAO coatings due to V₂O₅ added into the electrolyte. The MgV₂O₄ phase was responsible for the coatings exhibiting brown color and also was beneficial to improving the anti-corrosion property. In spotting tests, the corrosion resistances of coatings prepared under the high voltage are about 6–9 times higher than those of the low voltage because of the thicker coatings of the former. In potentiodynamic polarization tests, the coatings’ corrosion resistances were improved with the addition of V₂O₅, which was more significant under the low voltage than that under the high voltage. When the concentration of V₂O₅ was 0.2 g/L, the corrosion current density of the coating was the lowest, which means that the coating’s corrosion resistance under the low voltage is the best. Hence, it is necessary to carry out targeted design of the coating’s microstructure according to the different applications.