Preparation and characterization of Mg-P coatings on Mg-Zn-Y-Nd alloy for corrosion protection

To improve the corrosion resistance of Mg-Zn-Y-Nd alloy, Mg-P coatings were prepared on the surface by a simple chemical conversion method. The Mg-P coatings were optimized by regulating the reaction conditions such as the pH value of the conversion solution (4.2, 5, 6, and 7), treatment temperature (20 °C, 40 °C, and 80 °C), and treatment time (1 h, 2 h, and 3 h) in this study. Scanning electron microscopy (SEM) showed that the optimized Mg-P coating has a uniform surface structure. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) results demonstrated that the Mg-P coating consists of MgNH4PO4·6H2O and MgHPO4·3H2O. The cross-sectional morphology revealed that the Mg-P coating has a rough structure with a thickness of 8 μm. The effect of Mg-P coating on the corrosion resistance of Mg-Zn-Y-Nd alloy was investigated by in vitro immersion test, and it was found that the corrosion rate was effectively decelerated within 10 days. In summary, we studied the effects of reaction conditions for preparing Mg-P coatings on the surface of Mg-Zn-Y-Nd alloy by a chemical conversion method and effectively improving the corrosion resistance of Mg-Zn-Y-Nd alloy.


Introduction
Biomedical materials play an indispensable role in safeguarding human life and health, among which, surgical implant materials have a huge demand for the application.Traditional medical metal materials such as stainless steel and titanium alloys are almost nondegradable and need to be removed after implantation, which causes additional damage to the human body [1,2].Research and development of biodegradable implant materials are becoming urgent.Magnesium (Mg) has the potential to be used as biodegradable material because of its comprehensive advantages in degradation properties, mechanical properties, and biological properties [3].However, the degradation rate of Mg and Mg alloys in the physiological environment is too fast, limiting their clinical application [4,5].
To improve the corrosion resistance of Mg, various surface treatment techniques have been applied to Mg alloys, such as chemical conversion [6,7], micro-arc oxidation [8], atomic layer deposition [9], electrodeposition [10], sol-gel methods [11].Among them, the chemical conversion method is a simple way to generate in-situ coating and improve the corrosion resistance of Mg alloys.Phosphate coatings have good biocompatibility and can be easily prepared on Mg alloys by chemical conversion method [12].Mg-P coatings have good chemical continuity and bonding properties with the substrate due to the participation of Mg in the substrate in the formation of the coatings.Zhao et al. prepared Mg-P coating composed of MgHPO43H2O and MgNH4PO46H2O on AZ31 alloy by a simple two-step method of first immersed in H3PO4 solution and then followed by (NH4)2HPO4 solution.Electrochemical experiments showed that the coating could reduce the corrosion current density of AZ31 alloy in simulated body fluid (SBF) by 20 times [13].Jayaraj et al. found that Mg-P coatings could also be prepared on AZ31 alloy by immersion in NH4H2PO4 solution, and the morphology of the coatings could be controlled by adjusting the pH of the solution [14].As the solution pH increased from 4.5 to 7.5, phosphate coating became more uniform and had fewer defects, thus improving the anticorrosive properties of the coating.Zai et al. found that the reaction temperature had a great impact on the thickness of Mg-P coatings, and the coating treated at 80 °C had a thicker thickness and better corrosion resistance [15].
Therefore, in this paper, Mg-P coatings were prepared on the surface of Mg-Zn-Y-Nd alloy by a sample chemical conversion method, and the effects of reaction conditions such as pH value of the conversion solution, treatment temperature, and treatment time were systematically investigated in this study.Finally, the Mg-P coated samples were compared with the uncoated samples to study the effect of Mg-P coating on the corrosion resistance of Mg-Zn-Y-Nd alloy.

Materials and preparation of samples
The as-extruded Mg-Zn-Y-Nd alloy (Zn 1 wt%, Y 0.23 wt%, Nd 0.5 wt%) samples with the dimension of Ф8 × 2 mm were used as substrate materials.All Mg alloy samples were polished to 2000# with SiC sandpaper, cleaned by ultrasonic in ethanol for 10 min, and dried by warm air.Samples were treated in 1 mol/L NH4H2PO4 solution to obtain Mg-P coatings.To study the effect of the conversion solution pH, treatment temperature, and treatment time on the quality of Mg-P coatings, different reaction conditions were set up.The numbering and reaction conditions of the different Mg-P coated samples are shown in Table 1.
Table 1.Numbering and reaction conditions of Mg-P coated samples.

Sample Solution pH
Treatment temperature (℃) Treatment time (h)

Characterization of samples
Field emission scanning electron microscope (SEM, Regulus8230, USA) was used to examine the micro-morphologies of samples.X-ray diffraction (XRD, Bruker D8, Germany) was used to examine the phase composition of the samples.The 2θ data ranging between 20° and 80° were collected using Cu Kα radiation.The chemical composition of the coatings was studied by Fourier transform infrared spectroscopy (FTIR, Nicolet is50, USA) in the range of 400-4000 cm -1 at a resolution of 4 cm -1 .
Immersion tests were carried out in Hank's solution to investigate the long-term degradation behaviors of the bare and Mg-P coated Mg alloy.The ratio of the solution volume to the sample surface area was approximately 50 mL/cm 2 .During the immersion test, H2 evolution from each sample was collected by a burette every 24 h.

Effects of conversion solution pH on Mg-P coating
The surface morphology of the samples prepared by four different pH value (pH=4.2, 5, 6, and 7) of conversion solutions was shown in Fig 1 .There were a lot of cracks on the surface of A20-1 sample, and the Mg substrate was severely corroded (Fig 1a).When the pH value of the conversion solution was 5, there were still cracks on the surface of the sample, but the width of the cracks decreased, and lamellar crystals were formed on the surface (Fig 1b).When the pH value of the conversion solution was increased to 6, the surface of the sample was completely covered by the formed crystals, and the crystals were coarser with cracks at the junction of different crystals (Fig 1c).As the pH value of the conversion solution was further increased to 7, there were almost no cracks on the surface of the sample, but the crystals formed were poorly uniform (Fig 1d).In summary, the coating can be successfully formed on the surface of the Mg alloy under the condition of pH=6, and was selected for the follow-up study.

Effects of treatment temperature on Mg-P coating
The Mg-P coatings were prepared at pH=6, treatment time of

Effects of treatment time on Mg-P coating
In order to investigate the effect of treatment time on the morphology of Mg-P coatings, Mg-P coatings were prepared at pH=6, treatment temperature of 40 °C, and treatment time of 1 h, 2 h, and 3 h, respectively, and the surface morphology of the samples was shown in Fig 3 .With the increase of treatment time, the shape of the crystalline did not change significantly, and the crystallite size gradually increased.The bigger crystalline is beneficial to improve the thickness and density of the coating.However, the crystal has a limited size, prolonging the treatment time after reaching the limited size will cause space competition of crystalline and crack formation on the surface.Therefore, the treatment time of 2 h was selected as the best treatment time for subsequent testing and research.Among the factors affecting the Mg-P coatings, the pH value of the conversion solution is the key factor determining the formation of the coating and has the most significant effect on the microstructure of the coatings.The low pH value (pH=4.2, 5) leads to severe corrosion of the Mg substrate and high pH (pH=7) will lead to the crystalline inhomogeneity.The treatment temperature is also one of the main factors influencing the morphology and properties of Mg-P coatings, mainly in terms of morphology and homogeneity of the coatings.The increase in temperature accelerates the rate of dissolution of the Mg substrate and the deposition of the Mg-P coatings.The coatings prepared at 40 °C had a homogeneous and dense morphology.The effect of treatment time on the morphology of Mg-P coatings is relatively weak, and prolonged treatment time only causes the crystalline growth of Mg-P coatings.Therefore, considering the uniformity, integrity, and density of the Mg-P coatings, the final preparation parameter of the Mg-P coating on Mg-Zn-Y-Nd alloy was pH value of 6, treatment temperature of 40 ℃, and treatment time of 2 h.

Coating characterization and immersion test
The phase composition of the Mg-P coating was examined by XRD, as shown in Fig 4a .In addition to the α-Mg phase present in the Mg substrate, peaks belonging to the MgNH4PO46H2O phase and the MgHPO43H2O phase were detected in the Mg-P coated samples.Therefore, the Mg-P coating consisted of MgNH4PO46H2O and MgHPO43H2O.Fig 4b showed the FTIR spectra of the Mg-P coated Mg alloy.The absorption peak at 879 cm -1 was ascribed to the HPO4 2-derived from MgHPO43H2O.The absorption peaks at 1436, 1020, 973, and 569 cm -1 were ascribed to the NH4 + and PO4 3-derived from MgNH4PO46H2O.The FTIR results were in agreement with the XRD results.
As shown in Fig 4c, the thickness of the Mg-P coating was about 8 μm, with a large roughness.The rough surface structure limited the further preparation of composite coatings on the surface of Mg-P coating.
The corrosion resistance of the uncoated samples and Mg-P coated samples in Hank's solution was compared by immersion H2 evolution experiment, and the results were shown in Fig 4d .During the 10 days' immersion, the volume of H2 evolution increased with the increase of immersion time for both groups, and the volume of H2 evolution for the Mg-P coated samples was always less than that for the uncoated samples.The total amount of H2 evolution volume was 11.5 ml for the uncoated samples and 7.3 ml for the Mg-P coated samples for 10 d, so the corrosion rate of Mg alloy was reduced by 36.5% after being coated with Mg-P coating.Therefore, the Mg-P coating prepared under the conditions of conversion solution pH of 6, treatment temperature of 40 °C, and treatment time of 2 h can effectively improve the corrosion resistance of Mg-Zn-Y-Nd alloy.

Conclusion
Mg-P coatings were prepared on the surface of Mg-Zn-Y-Nd alloy by chemical conversion method, and the effects of reaction conditions on the coating morphology were investigated.The following main conclusions were drawn: (1) The optimal preparation parameter for the preparation of Mg-P coating on the surface of Mg-Zn-Y-Nd alloy by chemical conversion was: pH value of conversion solution was 6, treatment temperature was 40 °C, and treatment time was 2 h.
(2) The Mg-P coating consists of MgNH4PO46H2O and MgHPO43H2O, and the coating thickness was about 8 μm.
(3) The immersion experiment indicated that the Mg-P coating could effectively reduce the corrosion rate of Mg-Zn-Y-Nd alloy in Hank's solution by 36.5%.
(4) However, the rough surface structure limited the further preparation of composite coatings on the surface of Mg-P coating.
1 h, treatment temperature of 20 ℃, 40 ℃ and 80 ℃ respectively, and the surface morphology of the samples was shown in Fig 2. The Mg-P coated samples obtained at 20 ℃ had coarse crystallization, and cracks existed at the intersection of different crystals (Fig 2a).The Mg-P coated samples obtained at 40 ℃ had fine crystallization, uniform distribution, and no obvious cracks (Fig 2b).The Mg-P coated samples obtained at 80 ℃ had very poor uniformity, with large-sized crystals growing in the vertical direction in the center and crystals with broken structures growing in the horizontal direction around (Fig 2c).In summary, the coating prepared at 40 °C had the best uniformity and compactness, and was selected for the next study.