The Impact of Pre- and Post-Processing Processes on Corrosion Resistance of MAO Coatings on Mg and Its Alloys: A Systematic Review

1. Introduction

Magnesium alloys exhibit broad application prospects in aerospace, automotive, and biomedical fields due to their light weight, high specific strength, and excellent vibration damping properties, as shown in Figure 1, Mg alloys have different properties and are widely used in various fields including aerospace, military, and orthopedic fixation for fractures[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. However, Mg is a highly reducing metal with a standard electrode potential of -2.37V, which is more negative than that of commonly used metals such as iron (-0.45V), titanium (-1.63V), and aluminum (-1.66V). In the air, pure Mg readily forms a brittle and porous MgO film, while in humid environments or electrolytes, it tends to generate a protective Mg(OH)₂ layer with low solubility. Nevertheless, the Mg(OH)₂ layer is prone to detachment, re-exposing the Mg matrix and accelerating corrosion. Mg alloys undergo various forms of corrosion, and their corrosion mechanism involves galvanic corrosion between the second phase in the matrix (e.g., Mg₁₇Al₁₂ phase in Mg-Al systems [28, 29] ) or impurities and the matrix, forming unstable hydroxide films, thereby leading to the dissolution of matrix Mg. Consequently, the inferior corrosion resistance of Mg and its alloys limits their range of applications [30,31,32]. Currently, the primary methods to enhance the corrosion resistance of Mg and its alloys include alloying[33,34,35] and applying surface protective coatings[36, 37].

For pure Mg, the addition of elements such as Zr, As, and Li can enhance its corrosion resistance. In the case of Mg alloys, the incorporation of elements like Al[42], Mn[43], Zn[42, 43], Ca[34, 42, 43], and Y[44]can improve corrosion resistance to varying degrees. This improvement can be attributed to two possible mechanisms: firstly, alloying elements can inhibit cathodic reactions or galvanic corrosion, thereby enhancing the corrosion resistance of the matrix; secondly, they can form a dense passivation film on the surface of the matrix, effectively blocking the erosion of corrosive media. Figure 2[45] illustrates the effects of different alloying elements and their contents on the corrosion potential and corrosion current density of pure Mg. Notably, the Mg-0.2Sb alloy exhibits slightly higher corrosion resistance than pure Mg, whereas the Mg-0.3Sb alloy shows slightly lower corrosion resistance. Therefore, alloying can indeed enhance the corrosion resistance of Mg alloys to a certain extent, underscoring the importance of selecting the appropriate alloy element content. Secondly, alloying is primarily employed to improve the mechanical properties of Mg alloys, with limited enhancement in corrosion resistance[34, 35].

The preparation of corrosion-resistant protective coatings on the surface of Mg alloys commonly involves a number of different methods, including conversion coating[36, 46–49], sol-gel method[50,51,52], electroless plating[53,54,55], electroplating[56], anodic oxidation[57], and micro-arc oxidation (MAO) [58,59,60]. MAO technology has gained widespread attention for its simplicity, high efficiency, and environmentally friendly nature. It is capable of producing composite oxide coatings with good adhesion and high thickness on valve metals and their alloys including aluminium, Mg, and titanium[61,62,63,64,65,66,67,68,69,70,71]. By utilizing plasma generated by instantaneous high-voltage discharges, it enables the in-situ growth of thicker and more corrosion-resistant protective coatings on the surface of Mg alloys. Table 1 presents the corrosion potentials and corrosion current densities of MAO coatings prepared on the surface of Mg and its alloys (Ahmad Keyvani et al.). [72] A ceramic coating of composite hydroxyapatite nanoparticles was prepared on the surface of AZ31 Mg alloy using MA. The study revealed that the electrochemical corrosion potential (E_corr) of the coating was -1.156 V, significantly higher than that of the AZ31 matrix, which exhibited an E_corr of -1.536 V. In a related study, Y. Vangölü et al. [58] developed a Zn-doped MAO coating on AZ31 Mg alloy, resulting in an enhanced electrochemical corrosion potential of -0.71 V. It can be inferred that the MAO technique may effectively improve the corrosion resistance of Mg alloys to some extent.

As illustrated in Figure 3(a), the MAO apparatus is configured to use the sample and stainless steel container as electrodes through a dual-polarity pulse power supply. The positive pole of the power supply is connected to the sample, and the negative pole is connected to the stainless steel container. In addition, a refrigeration system is typically incorporated to regulate the temperature of the electrolyte throughout the oxidation process. The addition of a stirrer to the electrolyte enhances the uniformity of the coating composition. Furthermore, incorporating a stirrer helps maintain the coating preparation at a constant temperature. Figure 3(b) illustrates the MAO coating preparation process, where the voltage changes over time can be roughly divided into three stages: (Ⅰ) the formation of the passivation film and the anodic oxidation stage; (Ⅱ) the spark discharge stage; (Ⅲ) the MAO stage. When the voltage is less than the breakdown voltage, the sample undergoes the formation of the passivation film and the anodic oxidation process; when the voltage exceeds the breakdown voltage, the insulating oxide film is disrupted, causing numerous tiny white sparks to appear on the material surface. This stage is referred to as the spark discharge stage. Upon further voltage increase, large red arc points are observed on the surface, exhibiting movement. After a designated period of time, the red arc points grow in size and depth, eventually forming an orange-red spark. Figure 3(c) shows the schematic diagram of the MAO coating preparation process for Mg alloy, where the basic process of MAO includes: passivation stage, anodic oxidation stage, spark discharge stage, and MAO stage. Prior to reaching the critical breakdown potential, the material surface undergoes ordinary passivation and ordinary anodic oxidation, resulting in the formation of a thin insulating oxide film.

Based on the operational principle of MAO technology, it is inevitable that inherent defects such as micro-pores and microcracks will be formed within the MAO coatings, as illustrated in Figure 4 [71], which can result in the infiltration of corrosive media into the Mg substrate, thereby compromising its ability to offer long-term corrosion protection in challenging environments. The formation of micropores and cracks can be attributed to a number of factors, including: Firstly the rapid solidification of molten oxides in the electrolyte can induce thermal stresses, thereby generating microcracks in the coating structure [82]. Secondly, continuous micro-arc discharge creating micro-channels on the coating surface; and thirdly, a Pilling-Bedworth ratio (PBR) value of 0.81 < 1 for MgO, which is the primary component of the MAO layer and determines its porous structure[83]. The presence of pores and microcracks within the coatings serves as fast pathways for corrosive ions to diffuse through, thus diminishing their effective barrier effect against corrosive chemical media and consequently diminishing their corrosion resistance. In light of this understanding, researchers both domestically and internationally are inclined towards enhancing the corrosion resistance of Mg alloys by either pre-processing the substrate or post-processing the MAO coating.

As shown in Figure 5, there are numerous factors that affect the performance of MAO coatings, including the substrate, electrolyte, electrical parameters, additives, pre-processing. The content and types of alloy elements in different Mg alloys can influence the phase composition, microstructure, and properties of MAO coatings. For instance, the Mg₁₇Al₁₂ intermetallic compound in AZ91D alloy can lead to the formation of vertical pores, while AZ31 alloy tends to form spherical pores. Different power supply operating modes also have an impact: MAO films formed under constant voltage mode exhibit the greatest roughness, thickness, and uniform corrosion resistance; whereas films obtained under constant power mode have the lowest surface roughness, are dense, and possess the best pitting corrosion resistance[84, 85]. Electrical parameters play a crucial role in the corrosion resistance of Mg alloy MAO coatings. For example, applying a negative voltage in constant voltage mode can enhance the coating's density, thereby increasing its corrosion resistance. The termination voltage is closely related to the coating's thickness and density[86]; as the termination voltage increases, the coating's roughness increases but its density decreases. An increase in frequency results in a reduction in energy per breakdown event, which in turn leads to a decrease in the size of the discharge pores in the coating, an increase in film density, and an enhancement in corrosion resistance. [87, 88] The electrolyte is one of the most direct factors influencing the MAO layer of Mg alloys. Here, we categorize electrolytes into common electrolytes and electrolytes with additives. Among the most commonly used electrolytes, silicates and phosphates have been demonstrated to significantly enhance corrosion resistance. The conductivity and concentration of the electrolyte affect the arc initiation voltage, which in turn affects the size of the micropores in the film and subsequently the film's structure and corrosion resistance. An excessively high pH value of the electrolyte can affect the dissolution of the MAO coating on Mg alloys, reducing its corrosion resistance. Pre-processing enhances the corrosion resistance of MAO coatings by improving the interfacial bonding strength between the coating and the substrate and reducing defects such as porosity and microcracks on the surface of the MAO coating [89, 90]. Post-processing improves the density of the MAO coating on Mg alloys by sealing pores or forming composite coatings, thereby enhancing corrosion resistance. Among the factors influencing MAO coatings on Mg alloys, other factors have been extensively studied over the past decade[91,92,93]. In recent years, there has been a surge in research on the impact of pre-processing and post-processing on the corrosion resistance of MAO coatings on Mg alloys. Therefore, this review focuses on the influence of pre-processing and post-processing on the corrosion resistance of MAO coatings on Mg alloys.

The prevalent methodologies for assessing the corrosion resistance of MAO coatings on Mg alloys encompass salt spray testing, electrochemical testing techniques, and immersion tests. The salt spray test involves artificially creating a saline environment to induce metal corrosion, with regular monitoring of the degree of corrosion to evaluate metallic durability against corrosive elements. The underlying corrosion mechanism is attributed to the electrochemical reaction between chloride ions penetrating through the protective layer and the substrate metal; furthermore, the equipment requirements for this test are relatively straightforward. While it effectively simulates most real-world corrosive environments, its duration can be extensive and susceptible to environmental influences. Conversely, immersion tests entail submerging Mg alloy samples in corrosive media at specified temperatures and concentrations (e.g., NaCl solution), evaluating corrosion resistance via mass loss or changes in metal ion concentration within the solution.Although these experimental conditions are uncomplicated and capable of mimicking various corrosive scenarios, they also require prolonged exposure periods that complicate rapid assessments of coating performance.Commonly employed electrochemical testing methods include Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP).Corrosion resistance is assessed by analyzing obtained values such as E_corr and I_corr; this approach offers high sensitivity for identifying defects and elucidating corrosion mechanisms while providing swift results due to shorter testing durations.Consequently, EIS and PDP have been selected as measurement techniques for evaluating corrosion resistance in this study.

This paper presents a comprehensive review of the impact of pre-processing (e.g., laser processing, magnetron sputtering (MS), cold spraying (CS), etc.) and post-processing (e.g., impregnation, sol-gel treatment, hydrothermal treatment (HT), etc.) on the composition, structure, and corrosion resistance of MAO coatings on Mg alloys. The focus is to elucidate the role and mechanism of pre-processing in optimizing the interfacial bonding between MAO coatings and substrates, as well as post-processing in enhancing the densification of MAO coatings. Furthermore, it outlines current challenges in this field and anticipates future development prospects.

2. Effect of Pre-Processing on the Corrosion Resistance of MAO Coatings on Mg Alloys

The pre-processing process involves the implementation of a targeted treatment regimen on the Mg alloy substrate in advance of the MAO procedure. This is undertaken with the objective of optimizing the surface microstructure and overall structure. The utilization of an appropriate pre-processing method has the potential to enhance the interfacial bonding strength between the coating and substrate, as well as reduce surface porosity, microcracks, and other defects in the MAO coating, thereby improving its corrosion resistance. Commonly employed pre-processing techniques for Mg alloy MAO include laser treatment[94,95,96,97,98], MS[99, 100], CS[101, 102], etc. Table 2 presents a summary of the ways in which these pre-processing methods contribute to enhancing the corrosion resistance of Mg alloy MAO coatings.

As depicted in Figure 6, the pre-processing stage encompasses a diverse array of methodologies, primarily laser treatment, MS and Shot peening(SP) among others. These techniques are primarily employed to enhance the adhesive strength of the subsequent coating, which is achieved by altering the microstructural characteristics of the Mg alloy surface. In the case of MS, a thin coating layer is deposited onto the Mg alloy surface, serving as a foundational layer upon which the MAO coating is subsequently formed.

2.1. Surface Laser Process

Laser processing technology involves the use of high-power laser beams to heat, melt, and evaporate materials from the surface, resulting in changes to the grain size and chemical composition of the substrate surface layer due to high thermal gradient and rapid cooling rates [112,113]. This technique has garnered significant attention in the field of material preparation due to its high efficiency and precision. Laser treatment of Mg alloys not only homogenizes the surface microstructure but also dissolves the second phase in the matrix [114], creating favorable conditions for the preparation of MAO coatings with excellent comprehensive performance.

Laser Shock Processing (LSP), also known as laser peening (LP), generates intense shock waves on the surface using high-power pulsed laser beams. This process can induce compressive residual stresses in the material[114], thereby enhancing the surface structure of Mg alloys and positively impacting the generation of MAO coatings[94, 95]. As shown in Figure 7, during the LSP process, the sample surface is covered with an absorbing layer and a limiting layer, and the pulsed laser beam is focused on the absorbing layer, which absorbs the laser energy and generates a high-temperature and high-pressure plasma. Various physical processes that occur during the generation and expansion of nanosecond laser-generated plasma into the ambient gaseous medium, due to the presence of a limiting layer, the outward diffusion of the plasma is limited, resulting in a shock wave transmitted to the target material. When the shock pressure exceeds the Hugoniot elastic limit of the material, plastic deformation occurs, creating deep compressive residual stress in the surface area. Defects such as twins and dislocations are formed. These defective structures enhance the stability of residual stress through pinning effect. In addition, the crystal structure within the material is refined.Xiong et al. [96] further demonstrated that the combination of LSP pre-processing and MAO technology significantly improves the corrosion resistance of AZ80 Mg alloys. This enhancement is primarily attributed to LSP pre-processing refining grain size and forming a nanocrystalline film, which effectively reduces the adsorption of Cl- ions on the surface and promotes the formation of a MgO passivation layer, as a result of LSP pre-processing. Upon penetration of the MAO coating by a Simulated Body Fluid (SBF) solution, a dense passivation film is immediately formed on damaged portions in the vicinity of the exposed surface of the nanocrystalline layer, thereby acting as a self-healing agent.

In addition to LSP, the corrosion resistance of MAO coatings on Mg alloys can be further enhanced through Laser Surface Melting (LSM). The LSM technique allows for precise control over the microstructure of the material, thereby optimizing the surface structure and grain size of Mg alloys[97, 98]. Research by Wang [109] and Liu[110] demonstrated that LSM pre-processing significantly improved the corrosion resistance of MAO coatings on AZ91D alloys. Figure 8 illustrates the cross-sectional morphology of unpretreated and pretreated MAO coatings, revealing a significant reduction in micropores in the pretreated coatings. Furthermore, Chen et al.[116] successfully produced a denser MAO coating on AZ91D alloy surfaces using LSM pre-processing, resulting in a substantial improvement in corrosion resistance. The dissolution and redistribution of β-phase (Mg₁₇Al₁₂) induced by LSM led to a reduction in its content, effectively mitigating galvanic coupling corrosion between α-Mg matrix and β-phase. This reduction in galvanic coupling corrosion prevents concentrated corrosion on large areas of α-Mg matrix, thus averting severe localized corrosion. However, the effect of laser treatment technology on the enhancement of the corrosion resistance of Mg alloy MAO coatings is usually not very significant (I_corr is reduced by about 1 order of magnitude), most critically, due to the low elastic limit of Mg, even at low laser intensities can lead to a large intrinsic deformation of the material, which can affect the fatigue life of the material.

2.2. Magnetron Sputtering

MS offers a significant advantage as a pre-processing process for Mg alloys, resulting in a relatively dense, uniform, and stable surface for the MAO process. This leads to improved coating quality and ensures even distribution throughout the Mg alloy surface with minimal defects or weak areas. Furthermore, this technology enhances the bonding force between the coating and substrate, effectively extending the service life of the coating. Additionally, by adjusting the composition and structure of the sputtered layer, it is possible to optimize the performance of the MAO coating to meet diverse application requirements[99, 100]. As shown in Figure 9, The principle of MS is that the potential difference in the deposition chamber causes argon ions (plasma) to ionize, accelerate and Orient to the target. These plasma ions can move the target atom or molecule by directly colliding or triggering a "collision cascade" that causes the atom to emit, which then condenses into a film on the substrate (anode) . Wei et al. [99] demonstrated that MS of a pure aluminum layer prior to MAO on AZ31 Mg alloy can produce coatings with varying morphologies and phase compositions. The coating prepared with aluminates as the MAO electrolyte primarily consists of irregular nodules and fine pores of α-Al₂O₃ and γ-Al₂O₃, lacking a pancake-like structure. In contrast, the coating prepared with silicates as the MAO electrolyte is characterized by a porous and pancake-like structure dominated by γ-Al₂O₃. Furthermore, compared to direct MAO on the AZ31 substrate, MAO carried out on a MS Al layer results in the formation of denser γ-Al₂O₃, leading to improved compactness. The oxidation process yields a more dense Al₂O₃ (PBR of 1.28[118]), indicating that MS can enhance the corrosion resistance of Mg alloy MAO coatings. Hu et al. [100] demonstrated that the initial deposition of a dense pure aluminium layer approximately 11 μm thick via MS onto the surface of an AZ31 substrate followed by obtaining an oxide layer dominated by γ-Al₂O₃ through MAO resulted in significantly improved corrosion resistance for the MAO coating, as evidenced by an I_corr value of only 3.93×10⁻⁶A·cm⁻² after 5 minutes, in comparison to 2.54×10^-4A·cm⁻² for the substrate.

2.3. Cold Spray

Figure 11 illustrates the operating mechanism of the CS technology. Initially, a compression device pressurizes the gas, which then flows through a heater to increase its temperature. At the nozzle outlet, the gas undergoes rapid expansion, reaching velocities beyond the speed of sound. By utilizing the high-pressure and heated gas at the front of the spray gun, metal particles are accelerated to supersonic speeds and ultimately impact and adhere to the surface of the substrate material. The CS method, powered by gas, achieves plastic deformation and particle deposition through the high-speed collision of micron-sized solid particles with the substrate. This process leads to the solidification of particles, resulting in a coating formed through mechanical interlocking and local metallurgical bonding between the particles[120, 121], thereby enhancing the corrosion resistance of the material. In the realm of Mg alloy corrosion protection, CS has garnered widespread attention due to its exceptional corrosion resistance. CS is capable of depositing a layer of metal or alloy with similar physicochemical properties as the substrate onto the surface of Mg alloys, such as Cu and Al layers. This deposited layer not only closely integrates with the substrate but also plays a pivotal role in subsequent MAO processes by improving bonding between the substrate and MAO coatings, thus enhancing adhesion and durability. Simultaneously, the metal deposition layer itself may possess properties such as corrosion resistance and wear resistance, which are preserved and further enhanced during the MAO process, thereby significantly improving the overall performance of the Mg alloy. As depicted in Figure 10, Rao et al. [101] fabricated a composite coating on AZ31 Mg alloy through the application of CS and MAO techniques; notably, the coating in the CS pretreated group exhibited greater density than that in the unpretreated group, effectively obstructing the intrusion of corrosive media. Following CS pre-processing, the E_corr of MAO increased from -1.453 V to -1.153 V, indicating that CS pre-processing substantially bolstered the corrosion resistance of Mg alloy after MAO treatment. This not only validates the efficacy of CS technology in safeguarding Mg alloys against corrosion but also offers novel insights and strategies for developing high-performance protective coatings for Mg alloys.

2.4. Other Treatments

In the context of pre-processing methods, SP and UCFT are both effective in inducing plastic deformation of the substrate's surface. This manipulation accomplishes grain refinement and establishes the foundation for a high-quality substrate that is conducive to MAO processes[103, 122, 123]. Research conducted by Daniel et al. [106] has demonstrated that Mg alloys treated with SP are capable of generating denser coatings during MAO. The micro-pits and imperfections introduced by SP serve as nucleation sites for the coating material, leading to a more dense structure. Moreover, a meticulously controlled SP regimen enhances the substrate's surface reactivity, resulting in a proliferation of sites with favorable thermodynamic properties. This manipulation ultimately enhances the overall performance and efficiency of the system, thereby creating propitious conditions for the formation of MAO coating nuclei. Compared to MAO without pre-processing, SP followed by MAO resulted in a decrease in the I_corr of the coating from 7.11×10^-7A.cm⁻² to 2.14×10^-7A.cm⁻². Chen et al. [107] demonstrated that UCFT pre-processing combined with MAO significantly enhanced the corrosion resistance of AZ31B Mg alloy. As depicted in Figure 12, the surface structure of the samples treated with UCFT and MAO was notably improved, and the grain size was refined to 30-80 nm. The corrosion resistance of the samples treated with the combination of UCFT and MAO was enhanced compared to those treated with single MAO due to a variety of phases present in the coatings, such as Mg₃(PO₄)₂、tertiary calcium phosphate （TCP）、CaHPO₄·2H₂O（DCPD）、HA（Ca₁₀(PO₄)₆(OH)₂）、Mg and MgAl₂O₄, among other compounds, which contributed to an enhanced chemical stability and corrosion resistance.

The method of chemical conversion coatings plays a crucial role as a pre-processing prior to the MAO of Mg alloys, achieving the effect of cleaning and activation through the reaction or adsorption of active ingredients in the solution with the surface of Mg alloys. This process may lead to the formation of a chemical conversion film on the substrate surface[108]. The study by Hariprasad et al. [108] has fully demonstrated the advantage of utilizing the chemical conversion coatings method in pre-processing Mg alloy MAO. The corrosion current (I_corr) of the composite coating, formed by combining cerium conversion (CC) and MAO on the surface of AZ31 Mg alloy, is 2 orders of magnitude lower than that of a coating prepared by MAO alone. This significant enhancement can be primarily attributed to the high thickness and densification of the composite coatings. In conclusion, employing chemical conversion coatings as a pre-processing for MAO can markedly enhance corrosion resistance in coatings.

2.4. Conclusions

The pre-processing process plays a crucial role in the preparation of MAO coatings, as it has the capability to modify the microstructure of the alloy, enhance the bonding force between the coating and the substrate, and improve the coating performance to meet the requirements of complex application environments. Therefore, when selecting the pre-processing process, consideration should be given to alloy composition, application scenarios, performance requirements and cost-effectiveness in order to optimize coating performance[96–100, 109, 110, 116].

3. Effect of Post-Processing on the Corrosion Resistance of MAO Coatings on Mg Alloys

The enhancement of corrosion resistance in Mg alloy MAO coatings is currently a key focus of research, with post-processing technology being a significant area of interest[124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140]. Post-processing techniques serves to effectively seal surface defects such as micropores and microcracks, thereby forming a composite coating with the underlying MAO coating. Commonly employed post-processing methods include impregnation[59], sol-gel technology[140], electrophoretic deposition(EPD)[59, 141, 142], and HT[143,144,145]. The schematic diagram of different post-processing processes is shown in Figure 13, and the coatings produced by the different post-processing are described in detail below.The effects of different post-processing techniques on the corrosion potential and corrosion current density of Mg alloy MAO coatings are summarized in Table 3. The results demonstrate that post-processing can indeed contribute to an improvement in the corrosion resistance of MAO coatings to some extent.

3.1. Impregnation Treatment

Following the implementation of impregnation, hydroxides or other compounds may form on the surface of Mg alloy MAO coatings. These compounds serve to fill the micropores and microcracks in the coatings, effectively preventing the penetration of corrosive media. Simultaneously, impregnation strengthens the adhesion between the coating and the substrate, resulting in an overall enhancement of coating performance[124,125,126,127,128,129,130].

Phosphate-based buffers are commonly utilized under acidic conditions and are frequently employed in impregnation due to their unique chemical properties. It has been observed that while phosphate-based solutions may result in partial coating dissolution, precise control of the impregnation time can effectively manage the degree of coating dissolution and deposition, thereby achieving optimal coating performance[124, 125]. This is evidenced by the work of Qian et al.[124], who immersed Mg samples post MAO into phosphate solutions containing various cations, leading to a notable reduction in micropore size on the coating surface and an enhancement in densification. Figure 14 visually depicts the corrosion mechanism of MAO coatings following impregnation with NaH₂PO₄ solution, wherein HPO₄^2- from H₂PO^4- ionization reacts with Mg²⁺ to form a deposit layer of MgHPO₄, which effectively seals the pores of the coating and improves its corrosion resistance. Qian et al. [125] demonstrated that the impregnation with a solution containing copper salts and phosphate salts substantially improved the corrosion resistance of MAO coatings on AZ91D Mg alloy. The presence of MgHPO₄ crystalline phase in the coating after sealing treatment in a solution with added NaH₂PO₄ resulted in a considerable reduction in the depth of corrosion pores and the number of corrosion products in the severely corroded area, compared to unimpregnated MAO coatings, after 384 hours of immersion in 3.5 ωt% NaCl. In a similar study, Qian et al.[126] conducted phosphate impregnation on AZ91D Mg alloy MAO coatings and observed that the surface became smoother and pore size decreased post-processing. Furthermore, upon sealing for 15 minutes, the coating obtained a dense structure with MgHPO₄ as its main composition. The I_corr value for phosphate phosphate-impregnated MAO was approximately one order of magnitude lower than that for unimpregnated treatment.

Rare earth elements, renowned for their exceptional physicochemical properties, exhibit significant potential in enhancing the overall performance of Mg alloy MAO coatings. Recently, researchers have focused specifically on studying how incorporating rare earth elements into the sealing treatment affects the corrosion resistance of Mg alloy MAO coatings. Mohedano et al.[127] observed that a Ce-rich layer could be formed on the surface of MAO coatings on Mg-Y-Zn alloy substrates following treatment with Ce salts, effectively reducing coating pores. This is due to the oxidation of Ce³⁺ ions by H₂O₂ in the sealing solution (Eq. (1)) and partial dissolution of MgO and Mg2SiO4 in acid, resulting in local alkalinity and the formation of insoluble CeO₂ (Eqs. (2)-(4)). The enhanced corrosion resistance is attributed to pore blockage by the accumulation of CeO₂ within the coating structure.”

$$2{Ce}^{3+}\left(aq\right)+H_2{O}_2+2{OH}^{-}\left(aq\right)\to 2{{Ce\left(OH\right)}_2}^{2+}$$

(1)

$$MgO\left(s\right)+2H^{+}\left(aq\right)\to {Mg}^{2+}\left(aq\right)+H_{2}O\left(l\right)$$

(2)

$${Mg}_2{SiO}_4\left(s\right)+4H^{+}\left(aq\right)\to 2{Mg}^{2+}\left(aq\right)+{SiO}_2\left(aq\right)+H_{2}O\left(l\right)$$

(3)

$${{Ce\left(OH\right)}_2}^{2+}\left(aq\right)+2{OH}^{-}\left(aq\right)\to {Ce\left(OH\right)}_4\left(s\right)\to {CeO}_2\left(s\right)+2H_{2}O\left(l\right)$$

(4)

Mingo et al. [129] conducted a study that further validated the beneficial impact of Ce-based sealing on the corrosion resistance of Mg alloy MAO coatings. The researchers observed that Ce salt treatment induced a dissolution/precipitation reaction on the MAO surface, leading to the formation of Ce-rich compounds (CeO₂). These compounds effectively obstructed the ingress of corrosive media into the coating, thereby augmenting its corrosion resistance. Additionally, Sun et al. [130] demonstrated a significant reduction in I_corr for MAO-Ce(NO₃)₃ coatings treated with impregnation, compared to unimpregnated MAO coatings, when exposed to 3.5 ωt% NaCl solution. This improvement can be primarily attributed to the insoluble cerium oxide formed during the MAO process, which efficiently sealed micropores in the MAO coating and prevented penetration of corrosive solutions into the substrate region, consequently enhancing its corrosion resistance. Phuong et al. [131] conducted an investigation on the impact of sealing AZ31 Mg alloys coated with MAO coatings using Ce and phosphate solutions. The study results demonstrated that the I_corr of the samples after sealing treatment with cerium salt decreased by one order of magnitude compared to the I_corr of MAO 125.9 × 10⁻⁸A cm^-2, leading to a significant enhancement in corrosion resistance. Pezzato et al. [128] assessed the influence of Nd salt sealing post-processing on the corrosion resistance of AZ91-coated Mg alloy samples through electrochemical tests. The findings indicated that the Nd salt sealing treatment notably improved the coatings' corrosion resistance, primarily attributed to the effective physical barrier provided by the sealing layer, which effectively obstructed penetration by corrosive substances.

The preference for organic coatings among researchers is based on their exceptional chemical resistance and advanced protective properties. These coatings have become a prevalent post-processing method for enhancing the corrosion resistance of Mg and its alloy MAO coatings. In comparison to traditional inorganic electrolytes, organic electrolytes are environmentally friendly and meet the criteria of the requirements of clean production[132,133,134,135]. Liu et al. [132] successfully fabricated MAO-(SA+Ce) coatings for AZ31 Mg alloys by impregnating them with stearic acid, hydrogen peroxide, and cerium nitrate dissolved in ethanol. The self-healing mechanism of MAO-(SA+Ce) composite coatings was characterized in Figure 15, depicting the cross-sectional morphology consisting of a MAO layer (≈4.0μm) and a newly formed Ce conversion film (≈2.5μm) on the exterior. Additionally, the I_corr of the MAO-(SA+Ce) composite coating was reduced by 3-4 orders of magnitude compared to direct MAO coating. Dou et al. [133] conducted a study on the impact of chitosan (CS) on the corrosion resistance of Mg alloy MAO coatings, and observed that the sealing film effectively closed the micropores of the MAO coatings, thereby enhancing their corrosion resistance. Štrbák et al. [134], on the other hand, investigated the effect of aqueous-based preservative containing corrosion inhibitors on the corrosion behavior of MAO coatings and found that the sealing treatment was capable of retaining water-based corrosion inhibitor-containing preservatives in MAO porous coatings, thus effectively preventing corrosion in aqueous solutions with varying aggressive chloride ions. Pak et al. [135] utilized phytic acid/3-aminopropyltrimethoxysilane as an effective corrosion inhibitor to prevent the corrosion of MAO coatings. Through aminopropyltrimethoxysilane hybridization for surface treatment of Mg alloys, composite coatings were obtained exhibiting excellent corrosion resistance with an E_corr value of -1.566 V in 3.5% NaCl solution. Gnedenkov et al.[136] treated MA alloys using 8-hydroxyquinoline (8-HQ) solution to modify the MAO coating, resulting in significantly enhanced protective properties of the coating. The corrosion current value of the treated coating was 86 nA/cm², which is one order of magnitude lower than that of the MAO layer (810 nA/cm²), indicating an enhanced corrosion resistance after post-processing. Xue et al. [137] developed a MAO/CIP-PMTMS composite coating, leveraging the hydrophobicity and barrier effect of organics to achieve an I_corr of 7.13×10^-8A/cm², significantly lower than that of the MAO coating (3.54×10^-7A/cm²). Toorani et al.[138] further improved the corrosion resistance of Mg alloy MAO coatings by treating them with a three-layer composite coating of silane and epoxy, resulting in coatings with higher densification and thickness.

3.2. Sol-Gel Treatment

The sol-gel method, when used as a post-processing, effectively seals the holes in the MAO coating of Mg alloys, offering advantages such as strong coating adhesion, good barrier effect, and environmental protection[139]. For instance, Li et al. [140] demonstrated the preparation of HDTMS/SiO₂ composite coatings on AZ91 Mg alloy surfaces pretreated with MAO through hydrophobicity and sol-gel processes. The study revealed that the interface between the MAO coating and the gel hydrophobic layer was not distinct, indicating that the overall coating treated by the sol-gel process was relatively dense and significantly improved the corrosion resistance of the samples. This improvement can be attributed to two possible reasons: firstly, by replacing hydrogen in hydroxyl groups with Si-(CH₂)₁₅CH₃ the capillary pressure within silica networks is increased by transforming Si-OH into Si-O chains, effectively preventing gel coating cracking and reducing substrate corrosion tendency; secondly, grafting a long carbon chain (-(CH₂)₁₅CH₃) onto silica forms a hydrophobic layer, enhancing the ability of the sol-gel layer to inhibit corrosive ion penetration. Furthermore, Li et al. [153] utilized a combination of MAO technology and sol-gel method to enhance corrosion resistance in Mg-lithium alloys. The findings indicated that the composite coatings, prepared using MAO technology and the sol-gel method, exhibited favorable densification, minimal defects, and significantly enhanced corrosion resistance in Mg alloys. Malayoglu et al.[154] employed sol-gel treatment as a post-processing technique to develop composite coatings for Mg alloys with micro-oxidation coatings, aiming to improve their corrosion resistance.The results demonstrated that the sol-gel post-processing effectively sealed the MAO coatings, resulting in enhanced corrosion resistance.

3.3. Hydrothermal Treatment

HT, which is recognized as an environmentally sustainable and cost-effective method, has attracted significant attention from both domestic and international researchers and scholars [143, 155]. Malekkhouyan et al. [155] conducted the synthesis of LDH coatings on AZ31 Mg alloy based on MAO coatings through HT. Their salt spray experiments revealed that no substantial changes were observed on the sample surfaces after 7 days of MAO treatment followed by HT, while the coating of a single MAO sample had been completely deteriorated.”. Consequently, it can be concluded that HT effectively enhances the corrosion resistance of Mg alloy MAO coatings. Zhang et al. [100] employed HT for the post-processing of Mg alloy MAO samples. The findings revealed a six-order increase in the R_CT value (a measure of corrosion resistance) of the hydrothermally treated Mg alloy MAO coating compared to that of the direct MAO coating without HT. Dai et al. [144] successfully prepared a new MgAlY Layered Double Hydroxides-LDHs film containing salicylate on the surface of AZ31 alloy MAO coating by using one-step hydrothermal method and intercalation method. The reaction process for in situ growth of MgAlY-LDHs/salicylate film on AZ31 alloy was analyzed as depicted in Eqs. (5)-(11):

$$2Mg\left(s\right)+O_2\to 2MgO\left(s\right)$$

(5)

$$3{Mg}^{2+}\left(aq\right)+2{PO}_4^{3-}\left(aq\right)+22H_{2}O\left(aq\right)\to {{Mg}_3\left({PO}_4\right)}_2•22H_{2}O\left(s\right)$$

(6)

$$4Al\left(s\right)+3O_2\left(g\right)\to 2{Al}_2{O}_3\left(s\right)$$

(7)

$$MgO\left(s\right)+H_{2}O\left(aq\right)\to {Mg\left(OH\right)}_2\left(s\right)\downarrow$$

(8)

$${Al}_2{O}_3\left(s\right)+3H_{2}O\left(aq\right)+2{OH}^{-}\left(aq\right)\to 2{{Al\left(OH\right)}_4}^{-}\left(aq\right)$$

(9)

$$Mg\left(s\right)+2Al\left(s\right)+8H_{2}O\left(aq\right)\to {Mg\left(OH\right)}_2\left(s\right)\downarrow +2{Al\left(OH\right)}_3\left(s\right)\downarrow +4H_2\uparrow$$

(10)

$${Al\left(OH\right)}_3\left(s\right)+{Al}^{3+}+5{OH}^{-}\left(aq\right)\to 2{Al\left(OH\right)}^{-}\left(aq\right)$$

(11)

The MLYS specimen (salicylic acid embedded + HT + MAO) exhibited a significantly high film resistance value of 1.83 ×10⁶ Ω·cm². The MgAlY-LDHs/salicylic acid films grown on MAO and post-treated Mg alloys demonstrated excellent corrosion resistance, attributed to the enhanced densification and self-healing properties of the coatings, thereby ensuring the corrosion protection of the Mg alloys. Wang et al.[145] employed a hydrothermal growth method for in situ formation of MgAl-LDHs film on the surface of MAO-coated Mg alloy. The composite film of MAO-HT demonstrated superior short-term and long-term protective properties compared to the MAO coating. The R_ct value of the composite film increased by one order of magnitude in comparison to that of the MAO film, which can be attributed to the sealing effect of LDHs on the micropores of the MAO film. Consequently, HT has been shown to enhance the corrosion resistance of Mg alloy MAO coatings. In addition to sealing micropores, HT exhibits ion exchange properties between LDH layers, enabling attraction or adsorption of corrosive ions such as Cl^-, thereby further mitigating their corrosive impact on Mg alloy substrates.

3.4. Spraying and Electrodeposition

The coating method involves the utilization of electrodeposition, Air Plasma Sprayed (APS), and other techniques to create a protective layer on the surface of a metal or other material. This is primarily employed to enhance the material corrosion resistance, abrasion resistance, electrical conductivity, and other properties. Farshid et al.[152] utilized MAO and electrodeposition on AZ91 Mg alloy substrate to fabricate MAO/PDA composite coatings with self-healing capabilities, improved corrosion resistance, and bioactivity. A uniform, rough, dense, and hydrophilic MAO-PDA coating was electrodeposited at a dopamine concentration of 1 mg/ml (120 T-1C) with an I_corr of 4.31×10^-10A/cm², which was significantly lower than that of the MAO sample (3.53×10^-8A/cm²). Nadaraia et al.[46] combined MAO and fluorinated polymer spraying to prepare coatings on Mg-Mn-Ce alloys. The application of SPTFE resulted in a significant reduction in coating porosity from 18% to 2% after three applications. The incorporation of fluorine into the MAO layer validates the effectiveness of the fluoropolymer in sealing micro-defects within the MAO coating, indicating that the composite method can greatly enhance the corrosion resistance of the alloy[156]. Additionally, Mohammadreza et al. [156] utilized APS as a post-processing after MAO for improving the corrosion resistance and antimicrobial activity of Mg alloys.The APS-treated coating comprises an inner barrier layer, an outer porous layer, and a nano-structured zirconium dioxide layer. Furthermore, it was observed that molten zirconium dioxide particles infiltrated into the outer porous layer of the MAO coating, effectively expelling air present in micropores and thereby sealing the MAO coating, thus improving its corrosion resistance.

3.5. Other Treatments

Laser surface processing as a post-processing can effectively address the surface treatment challenges of various materials. Wang et al. [132] demonstrated that laser processing can significantly reduce the number and size of pores on the surface of MAO coatings, thereby enhancing the corrosion resistance of the coatings. EPD is an electrochemical method used to deposit charged particles onto the electrode surface to form a coating[157]. The porous structure of MAO coatings necessitates additional polymer deposition at the bottom of the pores, and electrophoresis embeds particles into the pores to create a robust composite coating. Studies have shown that EPD of ZnO nanoparticles [59], Polytetrafluoroethylene (PTFE)[141], and Graphene oxide (GO)[142] in MAO coatings has resulted in pore sealing effects. The findings indicate that ZnO nanoparticles can be embedded in coating pores to form a dual-layered coating through physical interlocking, thereby improving corrosion resistance[59]. Polytetrafluoroethylene was utilized to seal the pores, thereby increasing the coating thickness and barrier property to enhance corrosion resistance[141].Graphene oxide effectively seals MAO micropores and microcracks, leading to a more dense coating and improved corrosion resistance[142]. MS, a widely employed method of physical vapor deposition (PVD), offers numerous advantages including high film purity, efficient deposition, uniformity of deposited film, and uniform and effective coverage of the sample[158].

3.6. Conclusions

Suitable post-processing processes can effectively fill the surface micropores and microcracks of the MAO coatings, thereby enhancing their densification and corrosion resistance. The sol-gel technique has been shown to significantly improve both the densification and bonding strength of the coating, leading to enhanced long-term corrosion resistance. HT and plating methods are effective in improving the corrosion resistance of MAO coatings because they provide superior sealing and can achieve substantial coating thickness. Hence, when selecting appropriate methods, it is essential to comprehensively consider these factors along with application requirements in order to optimize both the corrosion resistance and overall performance of the coating.

4. Summary and Prospect

A In conclusion, the combination of Mg alloy MAO treatment with pre-processing or post-processing can significantly enhance the corrosion resistance of Mg alloys in various corrosive environments. Firstly, pre-processing (laser treatment, SP, UCFT) optimizes the microstructure of the Mg alloy surface and improves coating adhesion and densification, thereby enhancing corrosion resistance. Secondly, pre-processing (MS, CS) deposits a film layer (such as Al layer) on the Mg alloy surface to modify the composition of the MAO film layer and form a more dense substance for improved corrosion resistance. Thirdly, post-processing techniques (impregnation, sol-gel, EPD, MS and HT) seal micropores and microcracks on the MAO coating surface to regulate its structure and improve corrosion resistance. Some post-processing techniques also generate a denser layer on the MAO coating surface to provide an additional corrosion barrier.

As this research field progresses, it is anticipated that this technique will be adopted more widely in industries where corrosion resistance is a critical requirement. Nevertheless, there remains scope for further investigation and optimization in the long-term efficacy of Mg alloy MAO and the combined application of pre-processing and post-processing coatings. This can be achieved by maximizing the utilization of the composite treatment method through strategic pre-processing timing and methods, as well as post-processing agents, techniques, and duration. Currently, post-processing agents primarily consist of inorganic compounds; however, due to the intricate nature and diverse performance of organic matter, further comprehensive exploration is warranted for its potential use as a post-processing agent for Mg alloys. There is a paucity of research examining the impact of factors such as temperature and stress for long-term corrosion resistance. It is of the utmost importance to address these issues in order to establish an industrial basis for experimental use. It is imperative to select environmentally friendly post-processing agents that meet experimental requirements. The development of eco-friendly post-processing techniques or utilization of natural compounds will not only contribute to sustainable coating preparation but also significantly positively impact the industrialization process.