Chenggong Chang

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.

To clarify the deterioration behavior of magnesium oxychloride cement (MOC) under conditions of high humidity and high temperature, we first placed MOC slurry samples in a simulated environment with a relative humidity of 97 ± 1% and a temperature of 38 ± 2 °C; then, we observed the changes in the macroscopic and microscopic morphology, water erosion depth, bulk density, phase composition, and mechanical properties of the samples. The results show that, over time, under the promotion of high temperature, water molecules infiltrate the MOC samples. This results in the appearance of cracks on the macroscopic surface of the MOC samples due to the volume expansion caused by the hydrolysis of P5 (5Mg(OH)₂·MgCl₂·8H₂O) and the hydration of unreacted active MgO in the samples. The microscopic morphology of the samples changes from needle/gel-like, to flake-like, and finally leaf-like. Simultaneously, the major phase composition turns into Mg(OH)₂. Since the structure of the samples becomes looser and the content of the main strength phase decreases, the overall compressive strength and flexural strength are both reduced. The compressive strength of the MOC slurry samples (0 day) is 93.2 Mpa, and the flexural strength is 16.4 MPa. However, after 18 days of treatment, water molecules reach the center of the MOC samples, and the MOC samples completely lose their integrity. As a result, their compressive and flexural strengths cannot be obtained.

To reveal the effects of acidic environment on structure and property of magnesium oxychloride cement (MOC), the evolution of macro- and micro-morphologies, phase composition and mechanical property of MOC samples with soaking time in 5 wt.% H₂SO₄ solution were studied by using SEM, XRD, FT-IR and microelectromechanical electrohydraulic servo-pressure testing machine, respectively. The results show that the deterioration of MOC sample under acidic environment is mainly attributed to the consumption of the sample caused by the reaction between H⁺ ions and the substances in sample. The longer the soaking time, the more the sample consumed by H⁺ ions, and the more serious the deterioration of the sample. After dipping in H₂SO₄ media for 12h, the mass loss rate of MOC sample is up to 77.5%, and its volume decreases by nearly 3/4. Meanwhile, the surface morphology of MOC sample is changed from gel and needle rod to granular, and the content of P5, Mg(OH)₂ and MgO reduce, resulting in the compressive strength of MOC samples declining by 92.2%. However, due to the volume effect, H⁺ ions do not invade the inner core of the sample, so that the micro-morphology and phase composition of the inner core are almost unchanged. In short, MOC sample gradually deteriorates under acidic environment through being consumed by H⁺ ions from the outside to the inside.