The current research processes of electroplating and electroless Ni-P alloy plating on magnesium alloys were reviewed. Theoretically, the reason for difficulties in electroplating and electroless plating on magnesium alloys was given. The zinc immersion, copper immersion, direct electroless Ni-P alloy plating and electroplating and electroless plating on magnesium alloys prepared by chemical conversion coating were presented in detail. Especially, the research development of magnesium alloy AZ91 and AZ31 was discussed briefly. Based on the analysis, the existing problems and future research directions were then given.

The corrosion mechanism of AZ31 magnesium alloy used as automobile components and the influence of the concentration of Cl⁻ ion in simulated acid rain (SAR) were studied by electrochemical tests and SEM. The results show that pitting corrosion happens around the AlMn phases locating at the grain boundary. The corrosion of AZ31 magnesium alloy in SAR is controlled by the rate of anodic dissolution and hydrogen evolution, and the corrosion rate of AZ31 increases with increasing concentration of Cl⁻ ion. However, the Cl⁻ ion in SAR is not the main influencing factor inducing the pitting corrosion.

An organic-magnesium complex conversion (OMCC) coating on AZ91D magnesium alloy was obtained by treating in a solution containing organic compounds. SEM, FESEM and XPS were used to examine the surface morphology, thickness and structure of the conversion coatings. The results show that the continuous and uniform conversion coating is deposited on AZ91D alloy and the main component of the coatings is organic compound containing benzene ring, which forms a chemical bond with magnesium. The polarization measurement and salt spray test show that the corrosion resistance of the conversion coating is much higher than that of traditional chromate conversion coating.

A Ca-P coating consisting of biodegradable β-tricalcium phosphate [β-TCP, β-Ca₃(PO₄)₂] accepted for medical application was coated on a biodegradable AZ31 alloy by chemical deposition to improve the corrosion resistance. The good bonding strength of the coating is obtained. The results show that the corrosion potential of the Ca-P coated AZ31 alloy increases significantly, and MG63 cells show good adherence, proliferation and differentiation on the surface of the coated alloy. The Ca-P coating might be an effective way to improve the surface bioactivity of magnesium alloys.

The calcium phosphate coatings were prepared by virtue of electrochemical deposition in order to improve the corrosion resistance of Mg-1.0Ca alloys in simulated body fluids. The chemical compositions, structures and morphologies of the coatings were investigated by energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD) and scanning electron microscopy (SEM), respectively. The potentiodynamic electrochemical technique was employed to investigate the bio-degradation behavior of Mg-1.0Ca alloys with Ca-P coatings in Hank’s solutions. The experimental results show that the deposited coatings predominately consist of flake-shape brushite (DCPD, CaHPO₄·2H₂O) crystallites. The corrosion resistance of the substrates with coatings is improved in Hank’s solutions significantly.

Anodic coatings were obtained by micro-arc oxidation on AZ91HP magnesium alloys in a solution containing 10 g/L NaOH and 8 g/L phytic acid. The effects of electric parameters including frequency, final voltage, duty cycle and current density on the corrosion resistance of anodic coatings formed on the magnesium alloys were investigated by using an orthogonal experiment of four factors with three levels. The results show that the final voltage plays a main role on the coating properties. The orders of affecting corrosion resistance and coating thickness are separately ranked from high to low as, final voltage>duty cycle>current density>frequency and final voltage>current density>frequency>duty cycle. The final voltage influences the corrosion resistance of the anodized samples mainly by changing the surface morphology and coating thickness.

An Al₂O₃ protective coating on magnesium alloy AZ31 was prepared by a repeated direct sol-gel process annealing at  300 ˚C and a composite coating was also deposited using Al₂O₃ particles dispersed sol followed by phosphating treatment and annealing at 300 ˚C. The morphologies, structures and critical adhesive loads as well as corrosion properties of the coatings were comparatively investigated by scanning electron microscopy (SEM), X-ray diffractometry (XRD), nanoscratch test and electrochemical measurement. The results show that the composite coating has a more uniform, crack-free layer and improved adhesion to the substrate as compared with that of the repeated direct sol-gel coating owing to its lower heat strain. The main phases in both coatings consist of γ-Al₂O₃ and δ-Al₂O₃ derived from annealed alumina sol, and the composite coating has an anticorrosion performance which is superior to that of the repeated direct sol-gel coating.

A novel Ni-P-SiC composite coating was prepared by electroless plating in order to improve the corrosion capacity and wear resistance of AZ91D magnesium alloy. The influence of pH values on deposition rates and properties of the coatings was studied. The microstructure and phase structure of the Ni-P-SiC coatings were analyzed by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The corrosion and wear resistance performances of the coatings were also investigated through electrochemical technique and pin-on-disk tribometer, respectively. The results indicate that the composite coating is composed of Ni, P and SiC. It exhibits an amorphous structure and good adhesion to the substrate. The coatings have higher open circuit potential than that of the substrate. The composite coating obtained at pH value of 5.2 possesses optimal integrated properties, which shows similar corrosion resistance and ascendant wear resistance properties to the substrate.

In order to investigate the microstructure of TiN and TiAlN coatings and their effect on the wear resistance of Mg alloy, TiN and TiAlN coatings were deposited on AZ91 magnesium alloy by multi-arc ion plating technology. TiN and Ti₇₀Al₃₀N coatings were prepared on the substrate, respectively, which exhibited dark golden color and compact microstructure. The microstructures of TiN and Ti₇₀Al₃₀N coatings were investigated by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The micro-hardness and wear resistance of TiN and Ti₇₀Al₃₀N coatings were investigated in comparison with the uncoated AZ91 alloy. The XRD peaks assigned to TiN and TiAlN phases are found. The hardness of TiN coatings is two times as high as that of AZ91 alloy, and Ti₇₀Al₃₀N coating exhibits the highest hardness. The wear resistance of the hard coatings increases obviously as result of their high hardness.

Anodic coatings were prepared by micro-arc oxidation on AZ91HP magnesium alloys in a base solution containing 10 g/L NaOH and 12 g/L phytic acid with addition of 0−8 g/L sodium tungstate. The effects of sodium tungstate on the coating thickness, mass gain, surface morphology and corrosion resistance were studied by eddy current instrument, electronic scales, scanning electron microscope and immersion tester. With the addition of sodium tungstate, the electrolytic conductivity increases and the final voltage decreases. The sodium tungstate has a minor effect on the coating thickness, but lightens the coating color. With increasing sodium tungstate concentration, the size of micropores on the coatings is enlarged and the corrosion resistance of the anodized samples decreases.

Oxide coatings were prepared on magnesium alloys in electrolyte solution of Na₂SiO₃ at different current densities (3, 4 and 5 A/cm²) with micro-arc oxidation process. X-ray diffractometry (XRD) results show that the oxide coatings formed on magnesium alloys are mainly composed of MgO and MgAl₂O₄ phases; in addition, the content of MgO increases with increasing the current density. The morphology and surface roughness of the coatings were characterized by confocal laser scanning microscopy (CLSM). The results show that the surface roughness (R_a) decreases with increasing the current density. Moreover, the electrochemical corrosion results prove that the MgO coating produced in the electrolyte Na₂SiO₃ at current density of 5 A/cm² shows the best corrosion resistance.

A novel composite coating was fabricated on AZ91 magnesium alloy by applying a composite surface treatment which combined the methods of plasma electrolytic oxidation (PEO) pre-treatment, electroless copper and benzotriazole (BTA) passivation. The cross-section microstructures and chemical compositions of coating were examined using scanning electron microscopy (SEM) equipped with energy dispersive analysis of X-rays (EDX). Potentiodynamic polarization curves and salt spray tests were employed to evaluate corrosion protection of the coating to substrate in 5% NaCl solution. It is indicated that electroless copper produces a rough interface between the electroless copper layer and the ceramic layer. The corrosion potential shifts to the positive direction significantly and the current density decreases by more than one order of magnitude. There is no visible galvanic corrosion pits on the surface of the composite coating combination of PEO and electroless copper after 168 h neutral salt spray testing. The color of copper after BTA immersion could be held more than 60 d.

AZ80 magnesium alloys were deformed at 200, 250, 300, 350 and 400 ˚C with different deformation degree of 50%, 75%, 83%, 87% and 90%, respectively. The corrosion properties of different deformed AZ80 samples were studied by galvanic test in 3.5% NaCl solution. The results show that plastic deformation could improve the corrosion resistance of AZ80 alloy; and the corrosion rate of AZ80 deformed at 250 ˚C with the deformation degree of 83% was the lowest, which was 33% of the as-cast AZ80 alloy. Further studies of the microstructure show that the refined grain size and continuously distribution of β phase around the grain boundary did have a positive effect on the improvement of corrosion resistance of AZ80 alloys. For AZ80 alloys, the smaller the grain size is, the more homogeneous the structure is, and the better the corrosion resistance is.

To obtain the refined electrodeposited nickel layer on AZ91D magnesium alloy, ultrasonic technology was applied in the processes of pre-treatment and electrodeposition. The phases of pre-treatment layer and the nickel coating were analyzed by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS), and the microstructure was observed by scanning electron microscopy (SEM). Then, the effects of ultrasonic dispersion on the microstructure of pre-treatment layer and the grain refinement of electrodeposited nickel layer were discussed. The results showed that the pre-treatment electrodeposited Cu-Sn layer with compact microstructure could be synthesized in alkaline copper-tin liquid with ultrasonic agitation, as a result, smooth and refined nickel coating formed on AZ91D magnesium alloy. On the other hand, preferred orientation in the coating decreased because of the refined grains.

To improve the surface properties, lining of magnesium alloys with hard powders by shot peening was carried out in order. The hard powders were tried to bond to the workpiece surface due to the collision of many shots. In order to fix the hard powders to the surface of the workpiece, the powders were set on an uneven surface. To easily facilitate fixing of powders, lining of the workpiece with the powder sandwiched between two aluminum foil sheets was also attempted. In this experiment, a centrifugal shot peening machine with an electrical heater was employed. The workpieces were magnesium alloys AZ31B and AZ91D, and the hard powders were commercial cemented carbide, alumina, and zirconia. The joinability of hard powders near the lined surface was observed by a optical microscope. The wear resistance was also evaluated by a wear test. The hard powders were successfully bonded to the surface of workpieces by the shot lining process. The results show that the present method is effective in wear resistance of the magnesium alloys.

Works on exploring an environmentally clean method for producing an Mg,Al-hydrotalcite (Mg₆Al₂(OH)₁₆CO₃·4H₂O) layer and/or calcium carbonate (CaCO₃) layer on Mg alloy in a carbonic acid solution system (aqueous HCO₃⁻/CO₃²⁻ or Ca²⁺/HCO₃⁻) at 50 ˚C were reviewed. Conversion treatment for the Mg,Al-hydrotalcite conversion coating was as follows. Mg alloy was treated  first in acidic HCO₃⁻/CO₃²⁻ aqueous for precursor layer formation on Mg alloy surface and then in alkaline HCO₃⁻/CO₃²⁻ aqueous to form a crystallized Mg,Al-hydrotalcite coating. Duration of an Mg,Al-hydrotalcite coating on Mg alloy surface was reduced from 12 h to 4 h by the conversion treatment. On the other hand, for reducing the formation time of CaCO₃ coating on Mg alloy, the aqueous Ca²⁺/HCO₃⁻ with a saturated Ca²⁺ content was employed for developing a CaCO₃ coating on Mg alloy. A dense CaCO₃ coating could yield on Mg alloy surface in 2 h. Corrosion rate (corrosion current density, J_corr) of the Mg,Al-hydrotalcite-coated sample and CaCO₃-coated AZ91D sample was 7−10 μA/cm², roughly two orders less than the J_corr of the as-diecast sample (about 200 μA/cm²). No corrosion spot on the Mg,Al-hydrotalcite-coated sample and CaCO₃-coated sample was observed after 72 h and 192 h salt spray test, respectively.

Isothermal and isochronal annealing was conducted to study the thermal stability of the nanocrystalline in the surface layer of Mg alloy AZ91D induced by high-energy shot peening (HESP). Field emission scanning electron microscope (FESEM) and X-ray diffractometer were used to characterize the microstructure. Results showed that nanocrystalline produced by HESP on the surface layer of the magnesium alloy AZ91D was 60−70 nm on average. The nanocrystalline could remain stable at about 100 ˚C, and grew up slowly between 100 ˚C and 200 ˚C. When the annealing temperature reached 300 ˚C, the growth rate of the nanocrystalline increased significantly. The kinetic coefficient n of the nanocrystalline growth was calculated to be 2−3 and the grain growth activation energy Q=39.7 kJ/mol, far less than the self-diffusion activation energy of magnesium atoms in the coarse polycrystalline material.

The effects of two kinds of heat treatments T4 (solution treatment) and T6 (aging treatment) on the corrosion behaviors of Mg-3Zn magnesium alloy were studied by electrochemical measurements and scanning electron microscopy (SEM). It is found that zinc element enriches along grain boundaries to exhibit a network microstructure for both T4- and T6-treated alloy. For T6 treatment, larger MgZn particles form mainly on grain boundary and fine MgZn particles precipitate on matrix. Compared with cast alloy, T4 treatment could decrease the amounts of MgZn particles, and decrease the zinc content of zinc-rich net-segregation. Electrochemical measurements show that T4 treatment increases the corrosion resistance while T6 treatment decreases the corrosion resistance of Mn-3Zn alloy.

To improve the wear and corrosion properties of AZ91D magnesium alloys, Cu-based amorphous composite coatings were fabricated on AZ91D magnesium alloy by laser cladding using mixed powders of Cu₄₇Ti₃₄Zr₁₁Ni₈ and SiC. The wear and corrosion behaviours of the coatings were investigated. The wear resistance of the coatings was evaluated under dry sliding wear condition at room temperature. The corrosion resistance of the coatings was tested in 3.5% (mass fraction) NaCl solution. The coatings exhibit excellent wear resistance due to the recombined action of amorphous phase and different intermetallic compounds. The main wear mechanisms of the coatings and the AZ91D sample are different. The former is abrasive wear and the latter is adhesive wear. The coatings compared with AZ91D magnesium alloy also exhibit good corrosion resistance because of the presence of the amorphous phase in the coatings.

A new surface protection process was developed to magnesium alloy against corrosion in aggressive environments. Firstly, a phosphate coating was formed on rinsed magnesium alloy. Then, powder painting was carried out on the phosphated magnesium alloy. Surface morphologies and phase compositions of the phosphate coating were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that the phosphate coatings formed in bath containing earth additives at room temperature have dense and fine microstructure. The phosphate coating provides excellent paint adhesion to the magnesium alloy. Salt spray tests indicate that the corrosion resistance of the phosphate coating plus paint could meet the demand of magnesium alloy automobile components in aggressive environments.