Preparation of degradable superhydrophobic Mg/P/Z/F/H composite materials and their anticorrosion

: In this study, the degradable superhydrophobic Mg/P/Z/F/H (magnesi-um/poly(-caprolactone)/zinc oxide/1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES)/ heating process) composite materials were prepared through one-step method for enhancing the corrosion resistance of AZ91D magnesium alloys. Electrochemical measurements showed that Mg/P/Z/F/H materials significantly improved the corrosion resistance of magnesium alloys in 3.5 wt.% NaCl. The self-cleaning, adhesion and stability tests suggested that Mg/P/Z/F/H composite materials had well self-cleaning properties, good adhesion strength and stability in wet atmosphere.

As one of the lightest metal, with a density two-thirds that of aluminium and one-quarter that of steel, magnesium alloys thus have the great potential to improve system performance and energy efficiency in aerospace, auto industry, shipbuilding, mobile electronics, and bioengineering applications because of the magnesium's excellent chemical, mechanical, biological and physical properties. [23][24][25][26][27] However, because of magnesium's poor corrosion resistance especially in the corrosive medium environment of Cl -, the magnesium alloys' applications are limited. [27][28][29][30][31] Furthermore, containing rare heavy metal elements such as Cu, Ni and Pb, the magnesium alloys are environment friendly and used as the biodegradable metals. [25] As a typical aliphatic polyesters, poly (ε-caprolactone) (PCL) is nontoxic and ecofriendly to living organisms which is widely used in food packaging and pharmaceutical industry. [25,[32][33][34][35][36] ZnO powders are used in electronic and optoelectronic devices, solar cells and pharmaceuti-cal engineering because of the high safety, low price, extraodinary opto-electronic property and lacking of polluting effects as a newer type of promising candidate. [35,37,38] In this study, degradable and superhydrophobic M/P/Z/F/H composite materials were prepared through one-step method to solve the poor corrosion resistance of magnesium. As a widely applied and simple surface treatment technology, dip-coating can produce a relatively adherent, stable and uniform films on the surface of materials. [39][40][41][42] Using dip-coating method the rough Mg/P/Z/F/H structure was constructed, which can be prepared simply and protect the magnesium alloy better. Furthermore, 30minutes' heating process at 50 o C can not only repair defects of the composite Mg/P/Z/F materials' surface but also rearrange the PCL and PFDTES molecules to make the composite surface from hydrophobic (96.5 o ) to superhydrophobic (159.0 o ). The surface morphology, microstructure, adhesion strength, corrosion resistance and self-cleaning of the superhydrophobic Mg/P/Z/F/H composite materials were tested.

Sample preparation
In this work, die-casted Mg alloy (AZ91D) with a chemical composition (wt.) of 8.77% Al, 0.74% Zn, 0.18% Mn, 90.31% were cut into 25mm × 25mm × 5mm. Before used, the samples were polished by silicon carbide papers from 150 to 1000 mesh, then ultrasonically cleaned in acetone and rinsed by de-ionized water. Finally, the samples were dried at 60 o C.

Preparation of Mg/P/Z/F/H composite materials
By dip-coating method the Mg/P/Z/F composite materials were prepared as was shown in Scheme 1 with suitable concentrations of PCL solutions. Under magnetic stirring for 5 h, PCL (5 wt.%) granules were dissolved in 60mL dichloromethane (DCM) solvent. Thenmixed the ZnO powders with PCL polymer solution (5 wt.%)and stirred continuously. Dropt 1.5mL PFDTES and stirred continuously for 10 h. The prepared samples were immersed into the mixed solutions for 30s and pulled out of the solution at a speed of 2 mm/s. Then the Mg/P/Z/F composite materials were prepared. After heating process of the Mg/P/Z/F materials at 50 o C for 30min, the Mg/P/Z/F/H materials were gained. Table 1 showed the layers of five groups of the samples (Bare, Mg/P, Mg/P/Z, Mg/P/Z/F and Mg/P/Z/F/H).

Surface characterization and property tests
Surface characterization experiments were performed via scanning electron microscopy and energy dispersive spectrometry at 10 kV (VEGA3, Tescan China Ltd., China). At ambient temperature the water contact angle were measured using an optical contact angle meter (Dataphysics OCA 15EC, Germany) with a 5μL water drop. Fourier transform infrared (FTIR, NEXUSFT-870, USA) spectra were recorded in the range of 400-4000cm −1 . Electrochemical tests were performed by electrochemical workstation (CorrTest CS350).

Wetting behaviors
The wetting behaviors of four groups (Mg/P, Mg/P/Z, Mg/P/Z/F and Mg/P/Z/F/H) were tested in this study. The resulting surfaces showed different hydrophobicities of Mg/P, Mg/P/Z, Mg/P/Z/F and Mg/P/Z/F/H composite materials (Fig 1). In Fig 1a, we could see that the ecofriendly and degradable PCL coating was hydrophilic and the CA of Mg/P materials is 65.5±1.1 o (Fig 1a). With the addition of ZnO powders, because of the ZnO could construct the micro-nano rough structure the CA of the Mg/P/Z was from 65.5±1.1 o to 98.6±1.5 o (Fig 1b). Then by mixing PFDTES at room temperature the Mg/P/Z/F materials were prepared. However, the CA of Mg/P/Z/F was similar as that of Mg/P/Z. In room temperature condition, mixing PFDTES did not change the wetting behaviors evidently and the CA of Mg/P/Z/F was 96.5±1.8 o (Fig 1c). After 30 minutes' heating process at 50 o C of the Mg/P/Z/F, we gained the Mg/P/Z/F/H and the CA of Mg/P/Z/F/H surface was 159.0±1.

Surface characteristics
The SEM and EDS surface morphology of M/P/Z, M/P/Z/F and M/P/Z/F/H composite materials were shown in Fig 2. The Mg/P/Z exhibits a heterogeneous surface with micro-scaled roughness and high porosity (Fig 2a and 2d). In Fig 2b and 2e, the Mg/P/Z/F materials were prepared with low porosity and the ZnO particles were bonded evenly together by the PCL. However, we could observe that there were many gully defects on the surface of M/P/Z/F composite materials. After 30 minutes , heating process at 50 o C the gully defects were repaired and the surface of the M/P/Z/F/H became superhydrophobic (Fig 2c and 2f). In the former study, after heating process the Mg/P/Z/F at 35 o C and 45 o C for 30minutes, the CA of the composite materials' surface did not change obviously and the CA was about 98 o . In addition, the EDS of Mg/P/Z (Fig 2g), Mg/P/Z/F (Fig 2h) and Mg/P/Z/F/H (Fig 2i) illustrate that without heating process the contents of PFDTES was tested low on the surface of Mg/P/Z/F (Si: 0.28 wt.%; F: 2.14 wt.%). However, after heating process at   [43,44] At 832 cm −1 for Zn-O stretching for Mg/P/Z/F/H could explain that after heating process the PCL transfer occurs on ZnO. [45] The FTIR tests were consistent with the EDS results and after heating process the ZnO powders were exposed. Here we propose the molecules rearrangement mechanism to explain the hydrophobic and superhydrophobic change process at softening temperature of PCL (50 o C). The molecules rearrangement mechanism was as shown in Fig 4. At the softening temperature (50 o C, about 9 o C lower than the melting point of PCL (Mw: 80,000)) the PCL molecules could move more violently and freely. At the same time, the incarceration ability of PCL is weakened to the PFDTES. After 30minutes' heating process, the PFDTES and PCL molecules cause molecules rearrangement and the PFDTES molecules are exposed on the surface of the composite M/P/Z/F/H materials to form the superhydrophobic surface (Fig 4). Furthermore, frequent molecular motion could repair the gully defects of the surface of the M/P/Z/F/H materials (Fig 2c and f).  With a three-electrode system in 300 mL 3.5 wt.% NaCl electrochemical impedance spectra (EIS) were carried out. Samples were exposed with a surface area of 1 cm 2 . A saturated calomel and a platinum mesh electrode were used as the reference and the counter electrode, respectively. In Fig 5 the EIS plots of the five groups (Bare, Mg/P, Mg/P/Z, Mg/P/Z/F and Mg/P/Z/F/H) were reflected in 3.5 wt.% NaCl after a proper stabilization time (about 40min to 60min). In the Nyquist plane the impedance value of the working electrode is represented by the diameter of the capacitive loop. [25] As shown in  Besides, corrosionpotential (Ecorr), corrosioncurrentdensity (icorr) and corrosionrate (CR) were calculated by using Tafel extrapolation methods (Fig 6 and Table 2). The polarization carves could be a typical indication of the materials' stability. The substrate corrosion current density of the composite materials decreased significantlywith the repair the gully defectsrepaired and molecular rearrangement after heating process. By the Cassie-Baxter state, the air trapped of the micro/nanostructured surface can improve the hydrophobicity and corrosion resistance of Mg/P/Z/F/H materials. [40,41] The Mg/P/Z/F/H materials had the best corrosion resistance and the corrosion rate of Mg/P/Z/F/H was 1.9152×10 -3 mm/y. Here we found that with the addition of ZnO the corrosion resistance of Mg/P/Z sample decreased obviously because the ZnO powders destroyed the continuity of PCL layer (Fig 2d). And with the mixing of PFDTES the layer's discontinuity was modified (Fig 2e).

Stability and adhesion
For the practical application of materials, the stability is a highly important parameter. So the mechanical and superhydrophobic stability of materials' surfaces should be considered. In wet atmosphere at room temperature the stability of superhydrophobic was tested as is shown in Fig 8a. And the results illustrated that the superhydrophobic M/P/Z/F/H surface was stable and the CA of M/P/Z/F/H surface was over 155 o after 168h at wet atmosphere (Fig 8b, the variation ranges of CA were between +1.8 o and -1.6 o ). The mechanical adhesion stability of the surface was evaluated by using Scribe-Grid Test (ASTM D 3359-78). [25] Fig 9a and b shows the optical images of the superhydrophobic M/P/Z/F/H surface before (Fig 9a) and after (Fig 9b) the tape test. At the edges and within the square lattice there is no detachment or delamination of the film in  3.6. Self-cleaning test As an essential part of self-cleaning properties the ash adhesion resistance of the coating is tested by the chalk dust. Further, the self-cleaning ability of the superhydrophobic Mg/P/Z/F/H composite materials was shown in Fig 10. The Mg/P/Z/F/H samples were kept at an inclination of 10°. Each time 10μL water droplet was dropped from 20mm height to the red specified area. Simultaneously, the specified area was completely exposed after 10 water droplets and the water droplets took the chalk dust easily.

Conclusion
In this study, environmentally degradable superhydrophobic Mg/P/Z/F/H composite materials with less heavy metals were prepared successfully. And in 3.5 wt.% NaCl the electrochemical studies manifested that the Mg/P/Z/F/H had the outstanding corrosion resistance. About process of the superhydrophobic Mg/P/Z/F/H at 50 o C for 30 minutes, we proposed the molecules rearrangement mechanism to explain the hydrophobic and superhydrophobic change process. What's more, the Mg/P/Z/F/H materials have well self-cleaning properties, good adhesion strength and stability in wet atmosphere and provide a feasible scheme in engineering technology and medical engineering application.