Effect of ultrasonic wave on coating and corrosion of ZK60 magnesium alloy

. In this paper, hydroxyapatite coating was successfully deposited on ZK60 magnesium alloy surface by ultrasound-assisted electrodeposition method, and the macroscopic structure, chemical composition and corrosion properties of ZK60 magnesium alloy sample and HA coating were analyzed. The results show that the quality of ZK60 magnesium alloy surface coating is improved by ultrasonic power, and the HA coating is uniform, smooth and compact. XRD and FTIR analysis show that the main composition of the coating obtained by ultrasonic assisted electrodeposition is hydroxyapatite. The results show that ultrasonic treatment is beneficial to the corrosion resistance of SBF solution.


Introduction
It has been reported [1] that more than 2 million artificial joint replacement operations are performed every year, and the number continues to increase rapidly as time goes by, which makes the demand for metal materials to replace human bones growing. Among them, traditional orthopedic implant materials that appeared earlier, such as 316L stainless steel, cobalt-chromium alloy, Ti-6Al-4V, etc., will generate products that have toxic effects on cells in the human body, even cause immune rejection and inhibit the growth of osteoblasts [2]. In recent years, some of the rise of new biodegradable materials is more suitable for materials for medical applications, including ZK60 magnesium alloy as a kind of Mg -Zn-Zr alloy, with high strength, good biocompatibility and widely research [3], the in vitro degradation of performance and biological response experiments also show that ZK60 magnesium alloy can be used as a biological material [4,5]. However, despite the ZK60 magnesium alloy as human bone material has many advantages, however, the degradation of magnesium in the body too fast speed, resulting in graft site, alkaline and hydrogen release speed too fast, the formation of airbags, affect tissue healing, therefore, how to slow down and control the degradation of magnesium alloys become the key to its clinical application [5]. At present, the preparation of HA coating on magnesium alloy surface by scholars are widely applied to improve the corrosion resistance of magnesium alloy, its main methods include electrochemical plating, conversion coating, anodizing and deposition process and so on, these methods can effectively balance on the mechanical properties of the magnesium alloy and biological properties of HA, make its application in bone implant material [6]. In addition, the study of ultrasound-assisted phosphate [7] on nickel base [8] copper base [9] shows that at low temperature, ultrasound-assisted reaction can improve ion diffusion, current efficiency and reduce cathode polarization due to cavitation, thus speeding up the reaction speed. Moreover, ultrasonic also has a positive effect on the second phase doped metal coating [10]. In addition, a large number of studies [11,12] show that, the addition of ultrasonic treatment to the electrodeposition process helps to eliminate bubbles and produce a smooth coating. However, there are few reports on the use of ultrasonic assisted electrodeposition technology to obtain uniform HA coating and improve the corrosion resistance of ZK60 magnesium alloy. Therefore, ZK60 magnesium alloy in Mg-Zn-Zr alloy was selected in this study to explore the corrosive effect of ultrasonic assisted electrodeposition on HA coating on its surface. The HA coating with different ultrasonic power and its changing behavior in bionic anti-corrosion research were studied, and its influence on biological mineralization ability was further studied in human body simulation fluid (SBF).

Material
The reagents used in the experiment were pure chemical reagents for Chinese medicine analysis, and were used directly without special treatment. The base material is ZK60 magnesium alloy, after cutting it into samples with a size of 5cm×5cm×1cm, Grind and polish them step by step with sand paper of size 600, 800, 1000, 1200, 1500 and 2000 respectively. A piece of 0.5 mol/L citric acid corroded for 2 min was used as the cathode, and then a piece of uncorroded sample was selected. Ultrasonic cleaning with ethanol, acetone and deionized water for 10 min, and air drying as anode. The electrolyte used for electrodeposition is composed of CaCl 2 , KH 2 PO 4 analytical pure reagent and ultra-pure water. After the configuration, the concentration of Ca 2+ and PO 4is 0.04mol/L and 0.024mol/L, making the Ca/P ratio 1.67. Then add 0.5mL/L hydrogen peroxide prepared in 2000mL volumetric bottle, constant volume, 1500mL was taken as the solution required for an ultrasonic electrodeposition [12]. After electrodeposition of HA coating on ZK60 magnesium alloy, the bionic corrosion resistance of ZK60 magnesium alloy was tested by foaming in human bionic solution SBF solution. SBF solution is supersaturated for apatite, therefore, improper preparation method may lead to hydroxyapatite precipitation in the solution [12] . In this paper, according to the given quality in Table 1, reagents were added successively from left to right to prepare SBF solution. The configuration process was completed on the magnetic stirrer, and the next reagent must be added after the reagent was completely dissolved each time. Meanwhile, the internal temperature of the solution should be kept at 37℃ during the dissolution process. After the solution was dissolved, the pH of the solution was adjusted to 7.4±0.1 by Tris and HCl buffer solution.

Experimental Process
The prepared ZK60 magnesium alloy was cleaned and sealed with adhesive tape to make the area of the exposed part of the reaction 1cm*1cm. The area of the exposed part of the reaction was put into 1500 ml electrolyte with electrodes 20 mm apart. Then, the constant piezoelectric deposition experiment was carried out under the constant current and constant voltage power supply (Shenzhen Solid Measurement Electronic Technology Co., LTD., NPS605W), and the experimental voltage was 1.5V. The schematic diagram of the experimental device is shown in Fig.1 During the reaction process, the electrolyte was placed in a low temperature constant temperature tank, the reaction temperature was kept at 60℃, and the electrochemical deposition time was 1500 s. In order to explore the influence of ultrasonic wave on electrodeposition, 2s was treated by ultrasonic wave and 2s was closed (Wuxi Woxin Instrument Manufacturing Co., LTD., V0Shin-501D), and the influence rules of different powers (0%, 15%, 25%, 40%, 80%) on coating and corrosion were studied.
In the bionic anti-corrosion performance experiment of SBF solution, half of the electrodeposited ZK60 magnesium alloy sample was sealed with tape, and then the sample was put into SBF solution (40 mL) for reaction. Comparison has been made. During the reaction, the container was sealed to avoid pollution or reaction with air. Then the whole product was placed in a water bath (MPLR-702) to maintain 37±1℃, and a new solution was changed every day during the reaction process to ensure the availability of fresh reaction ions. After seven days, the sample was removed from the solution, washed with ultrapure water, and dried in a drying oven at 80℃ for 24 h, and then analyzed by XRD and FTIR.

Characterization
Shimazu 6100Lab X-ray diffractometer was used to detect the sample phase. The tube voltage was 40kV, the tube current was 100mA, the continuous scanning range was 10-90 °, the step size was 0.02°, and the scanning speed was 10°/min. Infrared spectral analysis (FTIR) was performed by Thermo Fisher is5, with an infrared spectral scanning range of 400-2000 cm −1 and spectral resolution of 4cm −1 . Fig.2(a) shows the macroscopic morphology of HA coating electrodeposited on ZK60 magnesium alloy at different ultrasonic powers, visible, in the absence of ultrasound (that is, the ultrasonic power was 0%), only a small amount of ZK60 magnesium alloy surface HA coating on the majority of coating fall off in the electrolyte, it shows that the adhesion strength of the electrodeposition coating on ZK60 magnesium alloy is not very good, And with the increase of ultrasonic power, the quality of coating on the surface of the ZK60 increases, visible ultrasound is helpful to improve the surface of hydroxyapatite in ZK60 deposits, in addition, the ZK60 surface coatings are also more level off, confirmed that it has been reported in the HA material surface smooth, easy to induce cell adhesion, and convex concave surface, cell adhesion is poor [13], therefore, Ultrasonic wave is beneficial to the deposition of HA coating and the quality of biomimetic material. The sample of electrodeposited ZK60 magnesium alloy soaked in SBF solution for seven days is shown in Fig.2(b). The upper part of the red line is divided into the reaction area, and the lower part of the red line is divided into the tape sealing part. From the perspective of the reaction region, there is only a very small amount of crystal on ZK60 magnesium alloy without ultrasonic electrodeposition, while with the increase of ultrasonic power, the coating surface thickness becomes larger and larger. From 5.92 mg/cm 2 without ultrasonic wave to 23.32 mg/cm2, 27.52 mg/cm 2 , 28.42 mg/cm 2 , 30.52mg/cm 2 (corresponding to 15%, 25%, 40%, 80% of ultrasonic power respectively), the surface morphology is more and more dense, visible, Ultrasonic treatment is beneficial to the biocompatibility of ZK60 magnesium alloy coating. The sealed part of the tape (the part below the red line) is still reacting where the tape is not firmly sealed due to the HA coating which has been electrodeposited and the strong agitation of ultrasonic waves. From corrosion, no ultrasonic electrodeposition ZK60 magnesium alloy surface is uneven, severe corrosion and corrosion were not only at the bottom of the gap, and the central part is corroded, and with the increase of ultrasonic power, hydroxyapatite coating is obvious thickening, also some improved at the bottom of the corrosion rate, and a central part in addition to ultrasonic power 40%, also to have certain to improve, This could have been caused by accidentally knocking off the coating on the central part during the experiment.

Fig.2. ZK60 magnesium alloy coated by electrodeposition and soaked in SBF solution for 7 days
The coating was scraped off the ZK60 magnesium alloy and spread on the surface of silicon powder for X-ray diffraction analysis, and the potassium bromide tablet for Fourier transform infrared spectroscopy analysis, the results are shown in Fig.3. Fig.3(a) shows the XRD patterns of electrodeposited coating on ZK60 magnesium alloy. The diffraction peak 2θ = 25.9° and 32.9° are HA characteristics [6]. It can be seen that without ultrasonic treatment, the HA peak of coating is not obvious, and the crystallinity of hydroxyapatite after ultrasonic treatment is significantly enhanced. Fig.4 is a Fourier infrared image. The wavelengths of 474, 564, 602, 972, 1031, 1086 and 1095 cm −1 are characteristic peaks of PO 4 3- [12,14], while 634cm −1 is characteristic peak of hydroxyl (OH) [15]. Its peak is generally thought to be produced by the presence of PO 4 3and OHin hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 . As can be seen from Fig.3(b), the introduction of ultrasonic wave significantly enhanced the infrared characteristic vibration peak of hydroxyapatite, which was consistent with the XRD examination results, and there was weak hydrogen phosphate (HPO 4 2-) groups at absorption peaks 525, 1075 and 1110cm -1 [16]. This is generally believed to be caused by the formation of the calcium-phosphate phase (OCP) [17], due to the decomposition of hydroxyl radicals in water during ultrasonic treatment, which in turn promotes the formation of the HA phase rather than other, less stable OCP phases. In addition, bands related to adsorbed water were also found at map 1625 and 1672cm -1 [18]. The infrared spectra of 870, 1391 and 1400 to 1500cm -1 show CO 3 2- [19], which is because the electrolyte is exposed to CO2 in the air during the deposition process, and the dissolved CO 3 2reacts with Ca 2+ in the electrolyte to generate a small amount of CaCO 3 . CaCO 3 does not affect the binding of hydroxyapatite coating to cells [20]. In the process of electrodeposition without ultrasonic treatment, equation (1)~(4) will occur in water during electrolysis, and a large amount of hydrogen will be precipitated at the cathode [20], so that the bonding force of the formed hydroxyapatite coating is not strong, and it is easy to fall off into the solution. In the case of electrical deposition at the same voltage, ultrasonic wave has a strong stirring effect, which makes hydrogen discharge faster and reduces the blocking effect of bubbles, thus making HA coating uniform and smooth with dense surface, which is similar to the dense needle-like structure formed in literature [21]. At the same time, the hydroxyl radicals on ZK60 magnesium alloy forming hydroxyapatite as shown in formula (5), when the consumption of alcohol is more near cathode ion to supplement, can affect the crystallization of hydroxyapatite, and the role of water in ultrasonic processing will produce hydroxyl, able to replenish the hydroxyl free radicals [18], effectively promote the electrodeposition, Improve surface roughness [22].

Conclusion
(1) Ultrasonic power is beneficial to increase the surface coating quality of ZK60 magnesium alloy, making the HA coating uniform, smooth and compact.
(a) (b) (2) XRD and FTIR analysis show that the coating obtained by ultrasonic-assisted electrodeposition is mainly composed of hydroxyapatite.
(3) The results of SBF solution blistering showed that ultrasonic treatment was beneficial to the corrosion resistance of samples.