Research Progress on Corrosion Resistance of Magnesium Alloys with Bio-inspired Water-repellent Properties: A Review

Thanks to its excellent mechanical properties, magnesium alloys have many potential applications in the aerospace and other fields. However, failure to adequately solve corrosion problems of magnesium alloy becomes one of the factors restricting its wide use in many industrial fields. Inspired by nature, researchers designed and fabricated bio-inspired water-repellent (superhydrophobic and slippery liquid-infused porous surface) surfaces with special wetting properties by exploring the surface microstructures of plants and animals such as lotus leaf and nepenthes pitcher, exhibiting excellent corrosion-resistant performance. This article summarizes the research progress on corrosion resistance of magnesium alloys with bio-inspired water-repellent properties in recent years. It mainly introduces the corrosion reasons, types of corrosion of magnesium alloys, and the preparation of magnesium alloys with bio-inspired water-repellent properties to improve corrosion resistance. In particular, it is widely used and effective to construct water-repellent and anti-corrosion coating on the surface of magnesium alloy by surface treatment. It is hoped that the research in this review can broaden the application range of magnesium alloys and provide a powerful reference for the future research on corrosion resistance of magnesium alloys.


Introduction
Magnesium alloy has the advantages of light weight, high specific strength and specific rigidity, good shock absorption performance, excellent electrical and thermal conductivity, and is easy to cut and renewable [1][2][3][4][5][6] . It is recognized as the most promising lightweight and green engineering material, and has many potential applications and increasingly positive development prospects in the fields of aerospace, transportation, electronic devices, biomedicine, and our daily life [7][8][9][10][11] . Magnesium and its alloys are expected to become alternative materials for other traditional metals by virtue of abundant resources and many excellent properties [12][13][14][15] .
The positive prospect of magnesium alloy as a structural material, however, is in distinct contrast to the situation it has been facing nowadays [16] . One of the reasons is the corrosion problem of magnesium [17,18] . As a matter of fact, the chemical properties of magnesium are very active, and its standard electrode potential is −2.37 V, which is lower than that of Fe, Zn, Al and other metal elements [19,20] . It is very easy to form galvanic corrosion with impurity elements or the second phase [21] . Moreover, magnesium alloy products are also easily oxidized during processing and use [22][23][24] , and thus the loose and porous surface oxide film is difficult to form stable and effective protection for the alloy [25,26] . It is obvious that poor corrosion resistance has become a bottleneck restricting the potential of magnesium alloys [27] . Therefore, improving the corrosion resistance of magnesium alloys is not only extremely important in practical applications but can offer significant economic returns [28,29] .
At present, the main methods to improve the corrosion resistance and extend the service life of magnesium alloys include micro-alloying [30][31][32] , microstructure control [33,34] , surface treatment [35,36] and preparation of functional coatings [37] . The widely used and effective method is to construct a coating with anti-corrosion performance on the surface of magnesium alloy through surface treatment [38,39] . Bio-inspired water-repellent surfaces with its unique interface characteristics and advantages have become a new idea to solve the corrosion problems of metal materials [40,41] . The bio-inspired water-repellent surfaces introduced in this article are divided into superhydrophobic surfaces and Slippery Liquid-Infused Porous Surfaces (SLIPSs) [42,43] . The fabrication of magnesium-based bio-inspired water-repellent surface helps to construct a function on the substrate of magnesium alloy, improving the corrosion resistance of magnesium alloy in an effective manner [44][45][46] . The fabricated bio-inspired water-repellent surfaces can effectively cut off the direct contact of corrosion medium such as humid air and erosion solution to the magnesium alloy substrate and reduce the corrosion [47][48][49] .
This article summarizes the research progress of corrosion resistance of magnesium alloys in five parts covering factors which affect corrosion and different types of corrosion, basic principle of wettability and practical applications of the bio-inspired water-repellent surface in corrosion resistance of magnesium alloys and highlights the development of corrosion resistance of bio-inspired water-repellent magnesium alloys as well as scientific problems.

Corrosion of magnesium alloy 2.1 Influencing factors of magnesium alloy corrosion
Magnesium alloys are prone to electrochemical reactions under the corrosion environment of simulated seawater. The reaction process is as follows [ where the electrolyte is a NaCl aqueous solution, Cl − is a corrosive medium, and the Mg 2+ produced by the anode reaction will react with Cl − . The reaction equation is: In practical applications, there are many factors that affect the corrosion of magnesium alloys: alloy elements [52][53][54] , secondary phase size and distribution [55,56] , grain size [57][58][59] , crystal orientation and texture strength [60,61] , crystal defects [62,63] , and environmental factors [64][65][66] (Fig. 1). The corrosion of magnesium alloys is mostly galvanic corrosion, and the α-Mg substrate is mainly corroded. Therefore, the corrosion resistance of the substrate is the key to determining the corrosion behavior of the entire alloy. Researchers analyzed the factors affecting the corrosion of magnesium alloy, and improved the corrosion resistance of magnesium alloy by a number of measures including adding alloy elements [67][68][69][70][71] , reducing the size of the second phase and making the distribution uniform [72][73][74][75][76] , refining the grains [77][78][79][80][81] , changing the crystal orientation and texture strength [82][83][84][85][86] as well as reducing crystal defects [87][88][89] (Fig. 2). Table 1 summarizes some of factors which affect the corrosion resistance of magnesium alloy.
Adding alloy elements may change the chemical components of magnesium alloy directly or indirectly and drive change in its form of organization, and the second phase size and distribution, consequently improving the corrosion resistance behavior of magnesium alloy [90,91] . On the other hand, grain size of magnesium alloy can be reduced by alloying [92] and plastic deformation [93] , so that mechanical properties of alloys can be improved effectively. Thus, fine grain strengthening is one of the key ways to improve the mechanical properties of magnesium alloy, and changing the grain size will create an important impact on corrosion resistance of magnesium alloy [94,95] . The presence of alloying elements will lead to the appearance of a second phase in the magnesium alloy. The second phase is usually a compound of Mg and Al [96,97] , Zn [98] , rare earth and other metal elements [99,100] , and its electrochemical stability is higher than that of the matrix phase. Therefore, the corrosion behavior of magnesium alloys is mainly   [73] . Casting and hot-rolling AM60 alloy AM60+1In alloy has the strongest corrosion resistance Total corrosion [73] Casting and extrusion Mg-Sm-Zn-Zr alloy The corrosion resistance is increased by 3 times. Galvanic corrosion [75] Grain size Hot-rolling Mg-1Ca alloy Refinement of the structure significantly reduces the corrosion rate Local corrosion [78] Rolling and annealing AZ61 alloy Reduced grain size and improved corrosion resistance Galvanic corrosion [80] Hot-rolling Mg-4Li-1Ca alloy Improved alloy strength and corrosion resistance Total corrosion [81] Crystal orientation and texture strength

Rolling
Mg-5Li-1Al alloy The corrosion resistance of the alloy is improved. Hydrogen induced cracking [82] Cut AZ31 Mg alloy Improve the corrosion resistance. Hydrogen induced cracking [85] Directional solidification Mg-4wt% Zn alloy Improved alloy corrosion resistance. Pitting corrosion [86] Crystal defect Cast and extrusion AZ91D magnesium alloy Improved alloy corrosion resistance. Galvanic corrosion [87] Compressive deformation Mg-Y alloy Improved the electrochemical corrosion performance.
Pitting corrosion [88] Pre-stretch AZ31 alloy Improved corrosion resistance Hydrogen induced cracking [89] micro-galvanic corrosion because of the potential difference between the second phase and the substrate [101,102] . Although the second phase is generally not corroded, the type, content, morphology and distribution of the second phase can affect the corrosion of the magnesium matrix, so the second phase plays a vital role in the corrosion of magnesium alloys [103,104] . However, there are also a few opinions that the increase of grain boundary area will reduce the corrosion resistance of magnesium alloy, and a small number of crystal defects inside the magnesium alloy can provide strong driving force for corrosion reaction, and hence make the surface form a thin and dense Mg(OH) 2 corrosion protection film, thus delaying the corrosion process and improving the corrosion resistance [105] .

Corrosion types of magnesium alloy
The corrosion process of magnesium alloy can be classified from different perspectives. According to different corrosion environments, it can be divided into natural corrosion and industrial environment medium corrosion. By the type of corrosive media, it is divided into atmospheric corrosion, water corrosion and soil corrosion. When it comes to the mechanism of corrosion process, corrosion occurs by chemical corrosion and electrochemical corrosion. According to the type of corrosion morphology, it can be divided into total corrosion and local corrosion. Local corrosion also includes galvanic corrosion [106,107] , pitting corrosion [108] , stress corrosion [109][110][111] , fatigue corrosion, intergranular corrosion and hydrogen induced cracking (Fig. 3) [112] .
Magnesium alloy is widely used as a kind of light metal structure material while poor corrosion resistance becomes its worst disadvantage. Corrosion often occurs in the hidden parts which are not easy to be detected, which will reduce the strength, plasticity and toughness of magnesium alloy, and then lead to the failure of structural materials. For instance, "catastrophic corrosion", such as bridge fracture, oil and gas pipeline explosion, will bring huge economic loss and even personal harm. It is of great significance to study the factors affecting the corrosion of magnesium alloy for controlling the corrosion behavior of alloy and effectively avoiding or reducing the occurrence of corrosion.
3 Basic principle of wettability 3.1 Theoretical study on wettability of superhydro phobic surfaces Neinhuis [113] and Barthlott [114] carried out a large amount of research on water-repellent plant and found that there were micron-grade raised rough structures and wax layer on the surface of the plant. Feng et al. [115] researched the "Lotus-Effect" and explored that the true secret of the superhydrophobic effect of the lotus leaf surface was the micro-nano composite structure on the lotus leaf surface. The bumps of the two sizes were compounded with each other, so that the surface of the lotus leaf had good hydrophobic property [116] . Researchers fabricated bio-inspired water-repellent surface with superhydrophobic property by mimicking the micro-nano papilla structure of lotus leave (Fig. 4). In addition, they defined the superhydrophobic surface and identified two key factors to obtain the surface: one is to construct a complex micro/nano rough structure, and the other is to modify the surface with low surface energy materials [117,118] .
For the qualitative analysis of the wettability of liquid drops on solid surfaces, the size of contact angle (CA) of water drops is an important indicator to determine the wettability of the surface. Wettability can be divided into four types according to the CA: superhydrophilic surfaces (CA ≤ 5˚), hydrophilic surfaces (5˚ ≤ CA ≤ 90˚), and hydrophobic surfaces (90˚ ≤ CA ≤ 150˚), and superhydrophobic surfaces (CA ≥ 150˚) [119][120][121] .
The Wenzel model and Cassie-Baxter model are commonly used to analyze and explain the mechanism of solid superhydrophobic surfaces with different adhesion behaviors [122][123][124] , as shown in Fig. 5a. Wenzel introduced the γ dimensionless surface roughness factor as a modification of the Young's equation [125] : where γ is the ratio of the actual surface area to the apparent area. As the value of γ is greater than 1, the surface roughness structure has a strengthening effect on the wettability. The Cassie-Baxter model is similar to the wetting state of water droplets on the surface of lotus leaves in nature [126] , and exhibits a surface characteristic of low adhesion, as shown in Fig. 5b. When the surface composite contact reaches equilibrium, the applicable solid surface wetting equation is deduced from the thermodynamic angle [127,128] : where ƒ 1 and ƒ 2 represent the area fraction of solid-liquid and liquid-gas interface contact at the solid interface, θ 1 and θ 2 represent the intrinsic contact angles of the solid-liquid and liquid-gas interface. For liquid-gas-solid three-phase compound interface equilibrium, that is, θ 2 = 180˚, ƒ 1 +ƒ 2 =1, substituting into Eq. (4) can deduce:   [117] .
When the solid surface roughness increases, it is beneficial to increase the contact area between the liquid and the air film at the liquid-solid contact interface. Some scholars have found that during the preparation of superhydrophobic surfaces of magnesium alloys, the corrosion resistance of the Cassie state (low adhesion) on the superhydrophobic surface is better than that of the Wenzel state (high adhesion) on the superhydrophobic surface. High-adhesion superhydrophobic surfaces have a larger contact area with liquids than low-adhesion superhydrophobic surfaces [129] . A large amount of air present on the low-adhesion superhydrophobic surfaces (Cassie state) acts as an air cushion, which can prevent the corrosion solution from directly eroding the magnesium alloy [130] (Fig. 6).

Theoretical study on wettability of SLIPSs
Inspired by pitcher plants in nature (Figs. 7a-7e), bio-inspired slippery surface means the surface with certain lubrication effect achieved by lubricating liquid filling [131] . The new bio-inspired surface has emerged in recent years and exhibited special surface wettability, so it is a derivative exploration of superhydrophobic surfaces. Wong et al. [132] first proposed "SLIPS" in 2011, and gave three criteria for designing SLIPS: first, lubricating oil can penetrate into the rough structure of solid-phase substrate to wet the solid-phase substrate, and realize the solid combination of the two. Second, in order not to be replaced by other liquids, the solid phase substrate should be wetted by lubricating oil preferentially. Third, the lubricating oil and the tested liquid must not be mutually soluble (Fig. 7f).
Aiming to meet the second principle, the lubricant and solid-phase substrate must be matched in physical and chemical properties so as to form a solid working system. Moreover, the lubricating oil is not compatible with the test liquid. The surface energy of the solid-liquid interface is E a when the test liquid thoroughly wets the solid substrate. When the test liquid floats on the top and the lubricant completely wets the solid phase substrate, the surface energy of the solid-liquid interface is E α . When no test liquid floats on the top and the lubricant thoroughly wets the solid substrate, the surface energy of the solid-liquid interface is E β . In order to ensure that the solid phase substrate is preferentially wetted by the lubricating oil, and the lubricating oil stored in the microstructure is not replaced by the test liquid, ∆E α = E a − E α > 0 and ∆E β = E a − E β > 0 must be met.
Preston et al. [133] and Anand et al. [134] focused on the first and second principles and analyzed the characteristics of porous substrates filled with lubricating oil with different surface energies. In summing up the failed design cases, it was found that there were five types of failures caused by the interaction between the oil layer and the test liquid ( Fig. 8): first, the surface energy of the oil layer was too low, the "cloaks" phenomenon of the  [130] . Fig. 7 (a -e) nepenthes pitcher and peristome morphology [131] ; (f) schematic illustration of fabricating the SLIPS [132] . wrapped test liquid occurred, resulting in the gradual loss of oil layer. Second, the surface energy of the lubricating oil was high, and the test liquid could not condense into droplets and slide down. Third, the oil layer failed to completely wet the rough substrate  [133] .
surface. Fourth, part of the oil layer was replaced by the test liquid. Fifth, the oil layer was miscible with the test liquid. In short, the SLIPS design must satisfy the following 5 formulas: The oil layer will not "cloak" the test liquid: The test liquid cannot be completely spread on the surface of the oil layer: The oil layer can completely wet the substrate: The oil layer can still spread on the surface of the substrate in the test liquid environment: The oil layer and the test liquid are not miscible: where S is the spreading coefficient, γ is the surface tension, s, o, v, l are the solid substrate, lubricating oil, gas environment and test liquid respectively, R is the roughness index.

Preparation technology of magnesium-based bio-inspired superhydrophobic surface
Bio-inspired superhydrophobic surfaces have gradually become a new idea to solve the problem of poor corrosion resistance of metal materials due to their unique interface characteristics and advantages [135][136][137] .

Hydrothermal method
Hydrothermal treatment is achieved by placing the precursor in an autoclave and making it react at high temperature and pressure conditions. The equipment required for this treatment is simple and easy to operate, and the energy-efficient and low-cost process can occur at high temperature and pressure conditions and be used in a wide range of applications. Besides, the nanoscale materials treated by this process exhibit high purity, crystallinity and dispersibility and are controllable in morphology [192] . Li et al. [193] prepared a 146˚-150˚ - [190] AZ31B magnesium alloy Micro-/nano petal-like structure 151˚± 0.5˚ 4˚± 0.5˚ [191] CA = contact angle, SA = sliding angle, -indicates no mention.
superhydrophobic coating with chemical stability and durability on the surface of AZ31 magnesium alloy by hydrothermal synthesis method. The static water contact angle was 156.7˚, and the superhydrophobicity could be maintained for more than one year when exposed to air. In addition, the superhydrophobic coating in 3.5 wt% NaCl solution had good corrosion resistance. Zhang et al. [194] prepared a Mg(OH) 2 /Mg-Al composite coating on the AZ31 alloy substrate by co-deposition and hydrothermal methods. The surface was modified by stearic acid, and the maximum static contact angle was 153.5˚, and the superhydrophobic surface showed good stability in electrochemical test, hydrogen evolution test and immersion test, significantly improving the corrosion resistance of AZ31 alloy (Fig. 10). But even so, this technique requires to be carried out at high temperature and pressure conditions, and therefore equipment must meet a number of strict requirements, which inhibits the development and application of the hydrothermal method in more fields. Moreover, it is not feasible for mass production due to poor preparation conditions and technical difficulties [195,196] .

Electrochemical deposition method
Electrochemical deposition is a simple and efficient method to prepare superhydrophobic surfaces on magnesium alloy, which relies on the reduction reaction of cathode to deposit metal or composite layer on the surface of material [197][198][199] . Cui et al. [200] prepared  [194] . superhydrophobic micro-arc oxidation/zinc stearate (MAO/ZnSA) coating with micro plate-like structure on Mg-4Li-1Ca alloy by electrochemical deposition method, and the static contact angle was 153.5˚ ± 0.5˚. The superhydrophobicity of the MAO/ZnSA composite coating effectively sealed the surface of MAO, and hence prevented the contact between the corrosion solution and the substrate, significantly enhancing the corrosion resistance of the Mg-4Li-1Ca alloy (Fig. 11). Li and Kang [201] prepared superhydrophobic a coating on AZ31 magnesium alloy by electrochemical deposition and surface modification. The static contact angle and sliding angle were 156.2˚ ± 0.6˚ and 1.0˚. Superhydrophobic coatings showed excellent corrosion resistance and chemical stability when immersed in 3.5 wt% NaCl solution and corrosive liquids. After 900 mm and 1100 mm mechanical wear tests, the coating maintained superhydrophobic property and corrosion resistance.
The process can be achieved by employing compact equipment and simple process flow in a short cycle with high metal deposition rate, and is feasible for mass production because it is a low-cost, energy-efficient and easy-to-control method. On the other hand, the film obtained by this process shows weak cohesion strength with the substrate, and in addition to this, heavy metal pollution and adverse manufacturing conditions are unavoidable problems during processing [202,203] .

Micro-arc oxidation
Micro-arc oxidation (MAO), also known as micro-plasma oxidation or anodic activation deposition, is a surface modification technology that produces ceramic coatings on metal surfaces. By controlling the micro-arc oxidation electrical parameters and the electrolyte system, coatings with different morphologies and structures could be prepared. The prepared ceramic coating had  [200] .
the advantages of high hardness, good wear resistance and strong corrosion resistance [204,205] . Zhang et al. [206] prepared a superhydrophobic coating on the surface of Mg-1Li-1Ca alloy by MAO and stearic acid modification, and the static contact angle was 155.5˚ (Fig. 12a).
In the potentiodynamic polarization, EIS, and 3.5 wt% NaCl solution immersion tests, the MAO/SA-7h coating showed excellent corrosion resistance and the corrosion current density was significantly reduced (Figs.12b -12e). Liu and Xu [207] prepared an AZ31 magnesium alloy superhydrophobic coating in a stearic acid ethanol solution using a two-step method of MAO and superhydrophobic treatment. The static contact angle of the surface was 156.96˚. Compared with the AZ31 alloy substrate, the corrosion current density of the superhydrophobic AZ31 alloy was reduced by several orders of magnitude, the amount of hydrogen evolution was greatly reduced, and the corrosion resistance was obviously improved.

Spraying method
Spraying method is a technology that the coating particles impact the alloy substrate at high speed and then deposit on the alloy surface by aerodynamic force. Li et al. [208] sprayed a fluorine-free suspension on a magnesium alloy substrate to prepare a strong superhydrophobic coating, and the contact angle and sliding angle were 159.5˚ and 3.8˚ (Fig. 13a). After a series  [206] . of mechanical damage tests and exposure to harsh environmental conditions, the coating still maintained good superhydrophobicity. In addition, the coating exhibited excellent self-cleaning performance and corrosion resistance in air and oil (Fig. 13b), and self-healing resistance to O 2 plasma etching (Figs. 13c and 13d). Shi et al. [209] prepared a polyphenylene sulfide-polytetrafluoroethylene/SiO 2 (PPS-PTFE/SiO 2 ) coating on the AZ31 magnesium alloy by spraying. The morphology, composition, contact angle, abrasion behavior and corrosion performance of the composite coating were tested by scanning electron microscopy, infrared spectroscopy, contact angle test, abrasive paper wear, and electrochemical tests. The static contact angle of PPS-PTFE/SiO 2 coating was in the range of (152˚-145.5˚) ± 0.3˚, and the sliding angle was less than 5˚. PPS-PTFE/SiO 2 coating had good abrasion resistance and excellent corrosion resistance. This process is an environmentally friendly and easy-to-use means of preparing large area coating on different types of substrates with low cost. However, uneven coating occurs during the spraying process, and hazards exist that pose a potential danger to operators' health [210,211] .

Dipping method
The superhydrophobic surface can be obtained directly by immersing the magnesium alloy into the solution, so the surface that used this method does not need to be modified with low surface energy materials again, which is beneficial to the rapid and large-scale production of superhydrophobic surfaces [212] . Xun et al. [213] prepared a coating with low adhesion and superhydrophobic properties on the surface of AZ31B alloy by a two-step in-situ dipping method. The coating had good mechanical stability and ultra-low water adhesion, which gave AZ31B alloy excellent corrosion resistance (Fig. 14). In addition, compared with the AZ31B substrate, the coating had anti-bioadhesion properties, which greatly reduced the adhesion of biomolecules (proteins, bacteria and cells). Ishizaki et al. [214] prepared myristic acid modified micro/nano structure on the surface of AZ31 alloy by one-step dipping method, and the   [213] . static contact angle of the surface was more than 150˚. The superhydrophobic samples were immersed into solutions of pH 4, 7 and 10 for 12 h, and the average static contact angles were 90˚ ± 2˚, 119˚ ± 2˚, and 138˚ ± 2˚, indicating that superhydrophobic coatings had certain chemical stability. Besides, the superhydrophobic coatings showed good corrosion resistance in electrochemical test.

Chemical etching
Chemical etching method is a manufacturing process that uses strong acid, strong base or concentrated salt solution to remove materials from magnesium alloys to produce rough micro/nano structures [215,216] . Feng et al. [217] prepared a superhydrophobic coating on the surface of AZ91 magnesium alloy by using sulfuric acid etching, AgNO 3 treatment, and dodecyl mercaptan modification. The water contact angle and sliding angle were 154˚ and 5˚. Electrochemical experiments showed that superhydrophobic surfaces had good corrosion resistance. Wang et al. [218] used chemical etching to generate nano-scale three-dimensional porous structures on the surface of AZ31 magnesium alloy (Fig. 15e), and made the surface superhydrophobic by oleic acid modification to obtain a coating with a static water contact angle of 155˚ (Figs. 15b and 15d). The air cushion effect in the superhydrophobic coating effectively isolated the contact between the corrosive medium and the substrate (Fig. 15c), resulting in improved corrosion resistance of the magnesium alloy substrate. After 6 months of storage in the air, it still demonstrated a good superhydrophobicity (Fig. 15f). Advantages of this process are that it is easy and simple and doesn't require complicated equipment for etching, and capable of producing surface textures with good controllability without applying current and voltage. However, waste stream generated from the process poses dangers to the environment and witnesses high chemical disposal costs [219,220] .

Wire electrical discharge machining
Wire Electrical Discharge Machining (WEDM) is one of the nontraditional machining processes for removing the material from the workpiece surface by using a continuous moving wire electrode which is usually made of copper or molybdenum as a machining tool electrode to establish a discharge channel via the application of impulse voltage between positive and negative electrodes of the workpiece and the wire electrode backed by the pulsed power supply of the machine tool. Making use of the instantaneous high temperature induced in the discharge channel by collisions of charged particles, the material on the surface melts and vaporizes, resulting in removal of material from the workpiece [221,222] . Xu et al. [190] studied the influence of the number of power tubes on the performance of a workpiece surface by machining an AZ91D magnesium alloy using a high-speed wire electrical discharge machine (WEDM-HS). The results showed that the surface treated by WEDM-HS was covered with a carbon layer and exhibited high hydrophobicity (the contact angle range between 146˚ and 150˚). Moreover, an increase in the number of power tubes caused the micro-pits and cracks on the surface of the magnesium alloy to be reduced and resulted in improved corrosion resistance. Qiu et al. [191] fabricated a superhydrophobic surface with micro-nano petal-like structure on an AZ31B magnesium alloy by combining WEDM-HS process and surface modification method, with a contact angle and a sliding angle of 151˚ ± 0.5˚ and 4˚ ± 0.5˚, respectively (Fig. 16). The experimental results indicated that the superhydrophobic surface exhibited excellent corrosion resistance and wear resistance, and the corrosion current density decreased by almost an order of magnitude. During the friction-wear test, the superhydrophobic surface demonstrated a lower coefficient of friction. WEDM is a low-cost and highly efficient option for manufacturing as well as has high efficiency in removing electro-corrosion products while violent vibration of the electrode wire, low accuracy of the machined surface and serious material waste are problems to be addressed [223,224] .

Composite method
The composite method is essentially a combination of two or more processing methods to obtain a micro-nano structure superhydrophobic coating on the surface of the magnesium alloy [225][226][227] . The two or more methods are used together in order to combine best properties and make the prepared coatings easy to control and exhibit stronger superhydrophobicity and durability than those fabricated by a single processing method. Zang et al. [228] prepared a bio-inspired lotus seed-like superhydrophobic coating on the surface of AZ91D alloy by a combination of hydrothermal synthesis and sonication assisted electroless plating. The static water contact angle was 153.9˚ ± 2.7˚, and the sliding angle was less than 5˚. The superhydrophobic coating had good corrosion resistance, which could effectively isolate the corrosion solution and protect the 749 Fig. 16 Scanning electron microscope morphologies and water contact angle (inset) of (a) the bare magnesium alloy, (b -d) the high-speed wire electrical discharge machining (HS-WEDM) surface, and (e -h) the HS-WEDM/stearic acid composite surface [191] . magnesium alloy substrate. Superhydrophobic coatings showed thermally induced reversible wetting transitions between superhydrophilic and superhydrophobic states, and had excellent fatigue resistance (Fig. 17). Ding et al. [229] prepared an anti-corrosion coating with superhydrophobic and self-repairing on the surface of AZ31B magnesium alloy by hydrothermal synthesis and spraying. The static water contact angle of the composite coating was 163˚. The chemical test, immersion method and scanning vibrating electrode method were used to study the corrosion resistance of the coating. Compared with Layer Double Hydroxides (LDHs) coatings, superhydrophobic composite coatings had good corrosion resistance, durability and self-healing properties, which is of great significance for expanding the potential applications of magnesium alloys.

Preparation technology of magnesium-based SLIPS
In practical applications, it is found that the superhydrophobic property of superhydrophobic coating is not stable, which will lead to the failure of superhydrophobic coating under high temperature, high pressure or surface damage [230] . Based on the bio-inspired principle, the researchers studied the special surface properties of the pitcher's mouth area, and synthesized a SLIPS by injecting a low-surface-energy lubricant into the micro/nano-structured substrate [231][232][233] . SLIPS lubricants can effectively isolate the corrosion of the substrate by the corrosive medium. Especially in the liquid, the lubricants and the aqueous solution are incompatible with each other, and can be stored for a long time, which has a long-term protection potential for the substrate to reduce corrosion [234][235][236] . Table 3 summarizes the innovative technologies applied to the surface processing of magnesium alloys.
SLIPS repels liquids based on the lubricating fluid layer and solid substrate with special microstructure. The solid substrate with special microstructure firmly locks the lubricating liquid layer, and the low-surface-energy lubricating liquid layer has a repellent effect on the liquids. The liquids have a larger contact angle and smaller contact angle hysteresis on the SLIPS [245] .
method and hydrothermal method to generate MgAl-LDH coating on the surface of AZ91D alloy, and chemically modified and injected lubricant to prepare PEO-LDH-SLIPS. In the immersion test and electrochemical test, the PEO-LDH-SLIPS showed long-term water-repellency and self-healing ability of surface damage. The water-repellency and self-healing properties made the AZ91D magnesium alloy have excellent corrosion resistance (Fig. 17). Zhang et al. [239] used a hydrothermal method to form a barrier layer on the surface of AZ31B alloy, chemically modified and injected lubricant to prepare a double-layer anti-icing and corrosion resistant SLIPS. Compared with superhydrophobic coatings, SLIPSs had smaller rolling angles, long-lasting corrosion resistance and anti-icing performance. Zhang et al. [242] anodized the magnesium alloy in the choline chloride-ethylene glycol based deep eutectic solvent, changed the external anode current density, and formed a conversion film with porous network and jagged nanorod arrays on the surface of the magnesium alloy. After surface modification and injection of lubricating oil, superhydrophobic surface and SLIPSs were obtained. In the electrochemical test, superhydrophobic surface and SLIPSs exhibited better corrosion resistance.

Summary and outlook
The corrosion resistance of magnesium alloy is extremely poor, which severely restricts its application range in different fields. Improving the corrosion resistance of magnesium alloy has become an urgent problem that needs to be solved. This article summarizes the research progress on corrosion resistance of magnesium alloys with bio-inspired water-repellent properties in recent years. By analyzing the factors and types of corrosion affecting magnesium alloys, based on the bio-inspired principle, the superhydrophobic surface and SLIPS are prepared on the surface of magnesium alloy to improve the corrosion resistance of magnesium alloy. Nevertheless, during the preparation and application of the magnesium-based bio-inspired water-repellent surfaces, there are still many problems to be solved: (1) The bio-inspired superhydrophobic surface of magnesium alloy has gradually become a new idea to solve the problem of poor corrosion resistance of magnesium alloy due to its unique interface characteristics and advantages. As mentioned earlier, various processing technologies for constructing superhydrophobic coatings on the surface of magnesium alloys have been proposed, but these technologies still face some problems that need to be solved urgently. For example, the micro-arc oxidation method causes high power consumption, produces a porous surface, and requires a composite packaging treatment. The hydrothermal method has poor preparation conditions and is not suitable for large-scale production. Volatile organic solvents are used in spraying method, which is harmful to human body and creates uneven coating. The preparation process of the composite method is complex and unsuitable for mass production. In the electrochemical deposition method, the adhesion between the film and the substrate is weak and the heavy metal pollution will occur. The dipping method/chemical etching method/solution deposition/conversion coating all can pollute the environment and the cost of waste liquid treatment is high. Laser processing technology is costly and inefficient. The surface accuracy of WEDM is low and the material waste is serious. The anodized film is SLIPS failure Fig. 18 Schematic protection mechanism for the smart anticorrosion system on Mg alloy [238] .
brittle and porous, making it difficult to process complex workpieces.
(2) Superhydrophobic coating has a wide application in the fields of improving corrosion resistance of magnesium alloys due to its unique properties. On the other hand, some issues still need to be addressed such as high manufacturing cost and limited large-scale production. Furthermore, low-surface-energy modifiers used during manufacturing are expensive, and fluorine-containing substances such as fluoroalkyl silanes and fluoroacrylic copolymers pose significant risks to human health and the environment. In addition, it is found in the practical application of the superhydrophobic coating that its superhydrophobicity was not stable and failed at high temperatures and pressures, or on damaged surface. It is thus considered to be of great significance to develop environmentally friendly, cost-effective and efficient modifiers and to design simple but effective manufacturing process ensuring superhydrophobic coating with self-healing performance on the surface of magnesium alloys can be obtained.
(3) Compared with the superhydrophobic surface, SLIPS has more excellent corrosion resistance and durability, but there are still some problems in the prepa-ration process of SLIPS. The microstructure of the substrate surface is too simple to store lubricant adequately, and the processing technology is unsuitable for large-scale preparation because of its complex operations and long cycle time. Besides, problems such as expensive lubricant, volatile lubricant and poor durability still need to be solved. To solve this problem and extend the service life and create greater value of SLIPS in practical applications, researchers have tried to produce regular-shaped nanostructures by increasing the complexity of the surface microstructure, and have considered replacing liquid lubricants by solid ones. So far, there are few studies on the preparation of SLIPSs magnesium alloys, which needs to be further promoted and studied.
(4) Both superhydrophobic surface and SLIPS can effectively improve the corrosion resistance of magnesium alloy and extend its service life, making it possible to speed up production and use of magnesium alloy in various fields, but there are some problems that still have not been solved. Since the environment in practical applications is complex and diverse, the corrosion resistance of magnesium alloys is more demanding. Especially in biomedical and industrial production, the corrosion resistance and biocompatibility of magnesium alloys need to be considered comprehensively. Therefore, the application research and development technology of magnesium alloy with multi-functional needs to be urgently proposed. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

Acknowledgment
The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. [8] Zhao X, Gao   [62] Joshi S, Singh R C, Chaudhary R. Effect of rotational speed in friction stir processing on the microstructural and me-