The electrochemical conversion coating produces a protective layer on the Mg surface through electrochemical reactions (Luan et al., 2011; Liu et al., 2020). The added advantages of this method include low processing temperature, ease of controlling coating thickness, and coating complex shapes. The surface modification by electrochemical conversion of Mg-based materials falls into one of the following categories: anodization, electrodeposition (ED), electrophoretic deposition (EPD), and plasma electrolytic oxidation (PEO).

Acid treatment is a cost-effective method to achieve a controlled degradation rate by treating the sample in acid solutions. The surface etching done by acid helps remove the surface contamination that occurred during the processing of metallic alloys. These contaminants, if not removed, cause micro-galvanic cells and promote corrosion on the substrate (Nwaogu et al., 2010). In addition, the thin films formed on the surface also help in passivation and thereby reduce the degradation. The treatment also helps in improving the surface energy favorable for cell adhesion and proliferation (Gawlik et al., 2019; Rahim et al., 2020). Mao et al. (2013) treated Mg–Nd–Zn–Zr alloy in 40% (vol.) hydrofluoric acid for 12 h to develop a protective magnesium fluoride (MgF₂) layer on the surface. Although the conversion coating improved degradation resistance, there was no significant improvement in cytocompatibility, and the hemolysis ratio was brought down from 52.0 to 10.1% only. Liu et al. (2019) obtained better corrosion resistance and biocompatibility for Mg-Sr alloy deposited with Ca ions and phytic acid treatment under a controlled pH level. The samples exhibited improvement in the in vitro behavior due to phosphoryl groups and Ca²⁺ ions on the coating, which supported HA formation. Similarly, Rahim et al. (2021a) reported improvement in biomineralization of Mg-Ca alloys by treating the samples with phosphoric acid and nitric acid. The formation of magnesium-phosphate and magnesium-hydroxide layers helps by providing a stable platform for nucleation and growth of HA on the sample surfaces. However, the stability of conversion coatings formed during acid treatment is a significant concern during long-term applications. Hence, acid treatment is preferable as a surface pre-treatment to improve the adhesion of subsequent deposition coatings.

Like acid treatment, alkali treatment is also a simple chemical conversion coating method to produce a Mg(OH)₂ layer on the surface by treating it with a suitable alkaline medium (Saxena and Raman, 2021). Gu et al. (2009) compared the effect of different alkaline mediums, namely, disodium phosphate, sodium bicarbonate, and sodium carbonate, on Mg-Ca alloy. When treated with the alkaline mediums, the film formed on Mg-Ca alloy improved the degradation resistance without inducing any toxicity in L-929 cells. Among the alkaline mediums used, sodium bicarbonate showed the slowest degradation rate and maximum calcium deposition. Alkali treatment with different treatment times confirms that the film’s thickness increases with an increase in treatment time (Tang et al., 2017). Wang et al. (2019b) reported improvement in corrosion resistance due to removing secondary phases from the surface by alkali-fluoride treatment on Mg-Zn-Y-Nd alloy. The in vivo test on the rat model substantiates uniform corrosion without any subcutaneous gas cavity formations. Li et al. (2020b) reported that the excess hydroxyl ions formed on the AZ31 alloys while treating them with NaOH acted as a corrosion barrier and promoted the reaction with vanillic acid treatment. However, like acid treatment, the stability of the conversion coating by alkali treatment is a crucial concern, and hence, they are better suggested as a pretreatment process.

Hydrothermal treatment is a simple, cost-effective coating technique to develop a coating on a metallic substrate. Xie et al. (2019) obtained a thin film of Mg₅(CO₃)₄(OH)₂·4H₂O particles with spherical flower-like morphology after 0.5 h of hydrothermal treatment in 0.5 M NaHCO₃. The film thickness increased with treatment time and finally obtained an irregular film of granular MgCO₃ and Mg(OH)₂ deposits after 3 h of treatment. However, hydrothermal treatment for 2 h showed better adhesion, and during the electrochemical corrosion test, the E_corr shifted to the nobler side by 343 mV and I_corr was reduced by one order of magnitude. Ali et al. (2021) prepared CaP coating on AZ31-3Ca alloy by single-step hydrothermal deposition using calcium nitrate (1 M) and diammonium hydrogen phosphate (0.6 M). The treatment was done at 100°C for 3 h and using two different pH values (4 and 7). Figure 4 shows that the coated samples exhibited thick irregular-shaped plate-like depositions covering the substrate surface without any cracks. After immersion, unlike the bare samples, the treated samples developed thick depositions and protected the surface beneath. The coated samples exhibited a 60% reduction in degradation rate and a 65% reduction in H₂ evolution after 28 days of immersion. The coated samples retained more than 85% of the compressive strength after 14 days of immersion, while no measurable sample was left for the uncoated sample after 3 days to do the compression test. The cell culture study on MC3T3-E1 showed better cell attachment and cytocompatibility for both the coated samples. A similar approach was attempted before by Wang et al. (2019c), wherein Ca ions in HA were partially substituted with Sr ions. The addition of Sr resulted in forming a thin HA nanorod, and its microstructure changed from flower-like to a network-like structure. The compact morphology of Sr-doped coating can strongly hinder the contact between Mg substrate and the corrosive medium. Hence, Sr addition enhanced degradation resistance with improvement in cell adhesion and proliferation. Zhou et al. (2020) also obtained a high crystalline coating of nHA/ZnO on Mg bulk metallic glass that reduced the E_corr by more than 500 mV and the I_corr to a low value of 12.35 μA/cm². The presence of coated samples also produced an anti-bacterial rate of S. aureus and colon bacillus close to 100% due to the presence of ZnO. Similarly, the anti-bacterial effect was also reported by Li et al. (2021a), who developed a super hydrophobic HA/stearic acid composite coating on AZ31 alloy by hydrothermal treatment followed by treatment with stearic acid. The low surface energy inhibited the adhesion and growth of E. coli and S. aureus by entrapping the air layer in the nanostructured coating to reduce the contact between the surface and bacteria when exposed to the bacterial culture medium. The hydrothermal treatment was also reported as a pre-treatment to improve the adhesion of other surface coatings (Dilshad et al., 2019; Xi et al., 2020).

The conversion coating techniques are easy to execute and help in fine-tuning the surface microstructure. However, in cases such as acid or alkali treatment, the developed layers may not withstand the aggressive environment. The key findings from the conversion coating studies are listed in Table 1.

The dip-coating method is suitable for coating complex shapes and relatively larger surfaces. The method allows the formation of functional polymeric or ceramic layers to act as a stable scaffold for promoting biomineralization and controlling the degradation (Guo et al., 2020; Rahim et al., 2021b). Abdal-Hay et al. (2012) dip coated poly vinyl acetate (PVAc) on AM50 alloy using dichloromethane, tetrahydrofuran, and N, N-Dimethylformamide as solvents. The coating developed using dichloromethane showed the highest adhesive strength (9.8 MPa), which was attributed to electrochemical bonding between PVAc and the substrate. The coating not only reduced the weight loss from 36.7 to 4.2 mg but also increased osteoblast viability by 30% on the first day and 50% on the third day of cell culture. Wang et al. (2019a) dip coated pure Mg rods with polyurethane (PU) functionalized with polyethylene glycol (PEG). The coating increased the degradation resistance, and the E_corr was reduced by 430 mV and the I_corr was reduced by two orders of magnitude. The coated samples also showed better resistance to E. coli bacterial adhesion due to the hydration barriers formed by the PEG chain. However, the use of the 2,2-dimethoxy-2-phenyl acetophenone catalyst reduced the L2 cell viability of PU coating from 85% to below 50% after PEG functionalization. Nikbakht et al. (2021) evaluated the performance of hydrofluoric acid pre-treatment followed by HA dispersed silane coatings on AZ31 alloy. The nano HA particles (size >200 nm) dispersed in silane coating enhanced degradation resistance by up to three orders of magnitude and improved cell proliferation. However, higher HA dispersion beyond 1,000 mg/L in the silane medium caused particle agglomeration to accelerate the degradation rate, as shown in Figure 6. Yang et al. (2021) coated Mg with a hybrid composite layer with a thickness of 1–2 µm using iron chloride hexahydrate dissolved in 20 ml de-ionized water. This method combined the conversion and the deposition process in a single step. The coating consisted of Fe along with Mg(OH)₂/MgO on pure Mg. The electrochemical measurement monitored continuously for 8 h in DMEM solution reported a large oscillation in E_corr between 200 and 1,200 mV due to the self-healing behavior after dip coating. Although the dip coating process is easy, controlling thickness and achieving the required coating adhesion are a major concern (Muñoz et al., 2020; Singh et al., 2021b). It is suggested that a proper surface pre-treatment and close control of process parameters are necessary to overcome the limitations.

Spin coating appears to be a better method to deposit the biodegradable polymers due to its uniform coating thickness and better adhesion than dip coating. Spin coating biopolymers showed good adhesion onto the substrate with improved degradation resistance and cytocompatibility (Mousa et al., 2021; Zhou et al., 2021). Kozina et al. (2021) spin coated chitosan/water glass (CS/WG) on CaP-coated Mg-20Zn substrate. The CS-based coating hindered the corrosion for a longer immersion duration in Hank’s solution. The degradation rate significantly decreased from 3.2 to 1.22 mm/year with a corresponding reduction in H₂ gas evolution. Yang et al. (2019b) spin coated polyether imide (PEI) on di ammonium hydrogen phosphate (DAHP). The coating reduced corrosion potential by more than 900 mV and corrosion current by five orders of magnitude. Unlike the typical cracked morphology observed for the bare samples, the composite spin coated samples exhibited a stable, smooth surface suitable for cell attachment, as shown in Figure 7. The cell distribution assessed by Vybrant Dil staining for pre-treated and spin coated samples showed higher density and better cell viability.

Zhao et al. (2018) developed multilayer polyvinylpyrrolidone/polyacrylic acid polymer coating using a novel layer by layer spin coating method and compared the performance with a similar dip coated multilayer sample. The coatings reduced corrosion current values from 1.63 × 10 − 5 to 3.11 × 10 − 6   and 9.89 × 10 − 7 A/cm², respectively, for dip coated and spin coated samples. Additionally, after immersion in HBSS, the spin coated samples had minor and smoother cracks with more Ca and P deposits on the surface. Kim et al. (2017) spin coated PLLA polymer on micro-patterned HA (by photolithography) to obtain a micro-textured HA/PLLA coating. The unique micro-texture obtained consisted of a periodic arrangement of crystallographic orientated needle-like HA crystals surrounded by PLLA. The coated samples exhibited excellent degradation resistance, biocompatibility, and high stability during mechanical strain. However, the spin coating method has limitations for larger surface areas.

The surface modification by sol-gel coating generates a thin layer of gel on the Mg surface dipped in a colloidal solution through polycondensation. Sol-gel dip coating is favorable for orthopedic applications as it utilizes low cost and low processing temperature to coat complex shapes using a wide range of materials (Figueira, 2020). Rojaee et al. (2013) prepared sol-gel–derived nHA coating using calcium nitrate tetrahydrate and phosphorous pentoxide as the precursors in ethanol. The coated AZ91 alloy exhibited a 100-mV anodic shift and reduction of I_corr from 22.14 to 2.83 μA/cm², as well as about three times increase in charge transfer resistance (R_ct). During the immersion test in modified SBF solution, the coating significantly reduced Mg ion release and maintained the pH value below 7.6 throughout the test. It may be noted that the concentration of the particles dispersed in the sol-gel is a critical parameter. Hernan et al. (2021) investigated different concentrations of functionalized graphene nanoplatelets (GNP) added to silica sol-gel via dip coating. The lowest GNP concentrations of 0.005–0.05 wt.% produced no visible defects or cracks. However, higher concentrations caused an increment in nanocharges of the sol-gel and changed the morphology. As a consequence of nanocharges, the viscosity was affected, and agglomeration of GNPs led to the formation of cracks resulting in an irregular coating morphology. Although the higher GNP concentration will not affect cell proliferation, the cracked coatings may increase the degradation rate.

Omar et al. (2020) synthesized 58S and 68S bioactive silica glasses using the sol-gel method and dip coated on the AZ91D substrate. Although cracks were formed on the coatings, both coated samples produced excellent apatite formation from the third day of immersion. The coatings imparted enhanced corrosion resistance in Hank’s medium, and the samples gradually degraded with time. The cell culture studies showed that the cells were well adhered, spread, and elongated for the coated samples. Su et al. (2018) reported HA and fluorine-doped HA sol-gel to seal porous CaP coating on AZ60 alloy by spin coating. The improvement in electrochemical corrosion resistance of the alloy after CaP coating further improved with sol-gel spin coating. During the immersion test in SBF, the H₂ gas evolution was reduced from 0.4 to 0.32 ml/cm² by CaP coating and was further reduced to 0.1 ml/cm² after the sol-gel spin coating. The coated sample also exhibited a reduction in pH from 10.5 to 8.5 after 7 days of immersion. (Kania et al. (2020) compared TiO₂ deposition via sol-gel spin and sputter coating. Compared with the bare sample, the spin coated substrate showed more than four times the reduction in H₂ gas evolution while sputter coating showed only a marginal reduction.

Electrospinning utilizes electrical force to draw thin polymeric fibers onto the surface of the substrate and can be used to develop a coating on the surfaces which have a morphology similar to that of the extracellular matrix (Hanas et al., 2016; Rezk et al., 2019; Panahi et al., 2020). Panahi et al. (2020) electrospun PCL and PCL with nano BG on nitric acid pre-treated AZ91 alloy. After immersion in SBF, irregular deposits were observed for uncoated samples, while coated samples exhibited characteristic cauliflower-like morphology. Hanas et al. (2018) prepared PCL/nHA composite coating on nitric acid pre-treated AZ31 alloy by electrospinning. The results indicate that the extracellular morphology of electrospun fibrous PCL/nHA coating acted as a stable scaffold for accelerated biomineralization. The degradation rate of the coated samples was less than half of that of the uncoated samples throughout the immersion test. Furthermore, the L6 rat cells showed excellent cytocompatibility with significant improvement in percentage cell viability by more than 50%. Additionally, the uncoated sample exhibited poor cell adhesion, while the cells flattened and expanded on the composite coated sample due to the characteristic coating morphology, as shown in Figure 8. Perumal et al. (2020) prepared a bilayer coating of PCL/nHA by dip coating followed by electrospinning on an AZ31 cage implant and studied the in vivo behavior of New Zealand white rabbits. The in vivo test results showed enhanced bone healing and controlled biodegradation of the implant at the diseased site.

Karthega et al. (2021) electrospun coated PCL and PCL/nano TiO₂ with different TiO₂ compositions (2, 4, and 6 wt.%) on AM50 alloy. A higher concentration of nano TiO₂ increased the fiber diameter due to the increased viscosity of the spinning solution that favors the degradation resistance. The composite coating with 6 wt.% nano TiO₂ reduced E_corr from 1.45 V to well below 0.5 V and I_corr by more than four orders of magnitude. The distribution of L929 cells was also more uniform for the composite nano-fiber coating. The electrospinning PLLA incorporated with GO and Ag-NP on Mg-1Ca-3Zn alloy also improved the degradation resistance and bioactivity of Mg alloys (Bakhsheshi-Rad et al., 2020). The anti-bacterial resistance of bare and PLLA coating was significantly enhanced after the addition of GO. The inhibition zone for E. coli and S. aureus growth was doubled after the addition of Ag-NP. The oxygenated groups in GO facilitate the homogeneous release of Ag + ions to cause bacterial destruction. Bakhsheshi-Rad et al. (2019) also reported excellent improvement in degradation resistance and anti-bacterial property for PCL/MgO-Ag electrospun on tantalum pentoxide sputter coated Mg-Ca-Zn alloy. However, incorporating a higher amount of particles can reduce the degradation resistance as the electrospun fibers become rough with reduced diameter.

PVD is a coating technique that physically deposits atoms or ions onto the substrate to yield a highly adhesive and uniform coating. Sputtering, thermal evaporation, and ion plating are the different types of PVD.

During the CVD process, volatile precursors can react with the Mg surface to obtain thin films and coatings. Although CVD coating demands high initial investments, it offers a controllable deposition rate to obtain uniform thickness for complex geometries. Chen et al. (2019b) deposited MgO/MgF₂ composite coating on porous Mg samples and obtained more than 50% reduction in H₂ gas evolution. The electrochemical test also significantly reduced E_corr by more than 300 mV and I_corr by more than one order of magnitude. Since the composite coating has limited solubility in SBF, they acted as a barrier and blocked contact with the substrate to prevent rapid corrosion. However, with the increase in immersion time, SBF passes through the coating’s pinhole defects, leading to substrate corrosion. Yang et al. (2019a) prepared carbon nitride film on AZ31B alloys with urea as the precursor at three different temperatures. The corrosion potential of coated samples shifted to the noble side by more than 275 mV, and charge transfer resistance improved by two orders of magnitude. Apart from improving electrochemical corrosion resistance, during the early stage of immersion in SBF, the coated samples only had a little H₂ evolution while the uncoated sample exhibited uncontrolled release. Even though coating deposited at different temperatures yielded similar degradation resistance, biocompatibility studies suggest 450°C as the preferred processing temperature for carbon nitride coating on AZ31B alloys. However, delamination and contamination of the CVD deposits should be carefully evaluated to propose this technology for biomaterials.

Besides the methods mentioned above, biodegradation and bioactivity of Mg-based materials are modified by coating metallic and non-metallic materials by different thermal spray processes. Mardali et al. (2019) deposited micro-HA particles on AZ61 alloys using the high-velocity oxygen fuel (HVOF) spray method. Compared to conventional thermal spray, the HVOF method produced micro-cracks on the coating to obtain an open-pore structure for improving cell adhesion. The higher quantity of HA crystalline phases decreased H₂ evolution by more than 85% and increased charge transfer resistance by more than one order of magnitude. However, a gradual loss in degradation resistance occurred over time due to the leakage of SBF solution into the substrate through micro-cracks. Singh et al. (2019) attempted plasma spray coatings using HA, Nb, and HA/Nb to improve the degradation resistance and biocompatibility of ZK60 alloys. The coatings caused slight shifting of corrosion potential to the nobler side with three to four times reduction in the corrosion current density. However, only HA coating and Nb-reinforced HA coatings showed hydrophilicity and a hemolysis ratio of less than 5%. Bansal et al. (2020), Bansal et al. (2021) reported that plasma spray coating of HA with Sr and ZnO improved the degradation resistance of Mg alloys. Incorporating reinforcing particles into the HA increased the coating microhardness significantly to prevent rapid deterioration and impart load-bearing capacity during implant applications. The characteristic nanostructure obtained by sealing the pores of the PEO layer by ZrO₂ plasma spraying reduced E_corr by more than 600 mV and I_corr by more than three orders of magnitude (Daroonparvar et al., 2018). Also, the anti-bacterial study in E. coli bacteria shows an increase in the inhibition zone diameter from 1.2 to 2.5 mm after coating due to the accumulation of nano ZrO₂ in the bacterial membrane and cytoplasmic areas (Gowri et al., 2014). Daroonparvar et al. (2020) obtained improved wear and degradation resistance with excellent bioactivity during cold spraying of hard Ti and Ta/Ti coating on AZ31 alloys. Ti coating improved Rct by two orders of magnitude, whereas Ta/Ti coating improved R_ct by four orders of magnitude. In addition to the passivating nature of the coatings, the rough island-like coated surface acted as nucleation sites for CaP formation in HBSS.

The physical deposition coating methods help in providing the required composition and morphology for the surface. However, in many cases, a proper pre-treatment is often necessary before the deposition coating to ensure proper adhesion between the substrate and the coating. The key finds from deposition methods are included in Table 2 for easy reference.

The method utilizes a PEB that rapidly melts and then quenches the Mg-substrate to create metallurgical modification on the surface and subsurface region while the bulk of the substrate material is not affected by the treatment. In addition to controlling the degradation rate, PEB can significantly improve the surface mechanical properties. Gao et al. (2007) reported the formation of a protective nano-grained MgO layer on AZ91 alloy. Beneath the MgO layer, a molten layer of Al over super-saturated α-Mg was formed by solute trapping. The modified layer favored passive film formation and reduced the degradation rate by two times. However, the rapid melting and solidification of the AZ91 alloy surface produced crater defects with secondary phase eruption. Lee et al. (2020) also observed selective removal of pre-existing precipitates in the PEB modified layer that reduced H₂ evolution from 5.8 to 2.8 ml/cm² and weight loss from 12 to 6.5 mg/cm². In addition to forming a protective layer, removing impurities and grain refinement can also promote degradation resistance (Lee et al., 2019). Morini et al. (2021) surface-treated AZ91 and AM60 alloys using low-energy high-current PEB through different acceleration voltages (15–30 kV) and a high number of pulses (up to 100). Although the process resulted in Al enrichment on the layer modified by PEB increasing the Vickers microhardness value by more than two times, there was no significant improvement in the electrochemical corrosion test using 3.5 wt.% NaCl solution. Furthermore, detailed in vitro degradation and cytotoxicity investigations are necessary to understand PEB-treated Mg-based materials’ degradation behavior for implant applications.

Like electron beam treatment, LSM uses laser heat to modify the Mg surface, while the latter has a better controllable nature and excellent repeatability. LSM creates intense microstructural changes with fine dendritic grain formation to a controllable level of depth without significant porosity. The extent of the modified layer depends on the LSM process parameters such as scan speed, laser power, and spot size. Wu et al. (2021) explored LSM on microstructure and surface texture evolution in AZ31B alloy. The high thermal gradient during the treatment led to predominant prismatic crystallographic texture along the Z direction. Concurrently, grain growth in the same direction led to the formation of a cellular/dendritic microstructure with uniform distribution of Mg₁₇Al₁₂ phase along the grain boundaries. The LSM also produced rougher surfaces to enhance surface energy. The fine grain boundaries acted as nucleation sites for HA formation, and the Ca/P ratio of biomineralization product was 1.68. The characteristic microstructure also exhibited uniform corrosion due to the homogeneous distribution of the intermetallic phase that acted as a corrosion barrier (Wu et al., 2017). Zhang et al. (2019a) investigated the effect of melting and surface texturing during LSM on degradation of Mg-Gd-Ca alloys. The periodic surface textures enhanced mechanical properties and the degradation resistance. The cytocompatibility of untreated samples was significantly improved due to the attachment and proliferation of cells along the direction of LSM-induced nanotexture. Chopra et al. (2021) reported that the nanotexturing achieved on the implant surface could act like a “bed of nails” against bacterial growth while promoting bioactivity. However, at higher scanning rates, small cracks are formed due to the thermal stress during rapid cooling of the laser-melted zones (Guan et al., 2009). Lu et al. (2019) also observed grain refinement leading to an increase in secondary phase distribution on the Mg-substrate and consequently improving the wettability and degradation resistance. Guo et al. (2021) obtained improved cell viability and cell–cell connectivity for laser-treated surfaces due to their low degradation and higher surface energy. Rakesh et al., 2019 obtained improved degradation resistance with controlled pH increase for higher laser power and energy density. Manne et al. (2018) performed LSM on Mg-2.2Zn alloy using different laser powers and scanning speeds. The most refined microstructure corresponds to a power of 125 W and a scanning speed of 30 mm/s. They reported a decrease of more than 40% in the corrosion rate in HBSS and improved biomineralization due to surface energy enhancement for the LSM-treated substrate. Optimized LSM process parameters are essential to control the microstructure for improving cytocompatibility and degradation resistance. Furthermore, in vivo and in vitro study reports on LSM are limited to making a conclusive statement.

SMAT is a severe plastic deformation process to introduce compressive residual stress into the Mg surface without affecting the microstructure of the bulk. The repeated impact of the substrate surface during the process with flying balls from different directions produces a hard nanocrystalline layer due to twinning and dynamic recrystallization and sub-grain formation (Showron et al., 2020; Singh et al., 2021a). Laleh and Kargar (2011) found enhancement in ultra-fine grain refinement and the microhardness value for AZ91D alloy irrespective of the ball size, which varied from 2 to 5 mm in diameter. However, the corrosion rate and depth of the nanocrystalline layer increased with the ball diameter due to the increased defect density. Li et al. (2014) obtained an extraordinary increase in H₂ evolution and a higher pH value for pure Mg and Mg- 1Ca alloys after SMAT. This decline in degradation resistance is because of the increased crystalline defect density after ultra-fine grain refinement. In a similar study, Chen et al. (2019a) also observed that SMAT decreases the degradation resistance of AZ31 alloy due to an unusual increase in the surface roughness. After SMAT, the H₂ evolution, weight loss, and the corresponding corrosion rate doubled. The electrochemical corrosion test also showed similar results, with larger balls having the highest corrosion rate. Additionally, the degradation resistance of SMAT samples deteriorates due to surface contamination arising from the attrition balls and the processing chamber (Showron et al., 2020). Thus, it can be seen that the use of SMAT for biodegradable applications of Mg-based alloys is less effective. Hence, it is suggested that SMAT may be considered as a pre-treatment technique and needs to be combined with a proper surface coating technique.

Shot peening also introduces compressive residual stress into the Mg surface using a similar principle to SMAT. However, shot peening uses comparatively smaller balls to project into a fixed substrate with a higher velocity. Mhaede et al. (2014) observed that shot peening was an effective pre-treatment method to improve micro-hardness and degradation resistance. However, shot peeing unaided by coating deteriorated degradation resistance with a considerable drop in the polarization resistance. Yao et al. (2021) obtained excellent results for shot peening AZ91 alloys after zinc coating. The shot peening as post-processing improved the compactness of Zn coating with excellent improvement in microhardness and corrosion resistance. Compared to the bare sample, the H₂ evolution and weight loss were reduced by three times and ten times for the shot-peened Zn-coated samples. Peral et al. (2020) reported that higher surface roughness during shot peening contributes to the rapid degradation. Also, the detailed study focusing on the biological performance of different types of shot-peening did not yield satisfactory results. Bagherifard et al. (2018) observed higher I_corr values with no significant improvement in cell viability for different shot-peened samples. The above discussions suggest that shot peening can be considered a promising surface treatment method when used with deposition coatings as a pretreatment or post-treatment process.

Surface treatment using the laser shock peening (LSP) method could overcome the limitations associated with shot peening. Guo et al. (2019) tuned the surface morphology and compressive residual stresses by controlling the laser power density. The increase in surface roughness did not deteriorate the degradation rate due to the predominant effect of compressive residual stress. Rather, the grain refinement and compressive residual stress promoted the formation of a dense passive film and reduced the degradation rate by more than 45–52%. Ge et al. (2017) reported that two-sided LSP of AZ31B alloy reduced corrosion current and the stress corrosion cracking (SCC) susceptibility index by about 85 and 47%, respectively. The improvement in degradation resistance was attributed to the fine grain refinement from 17.5 µm to 15.7 nm during LSP. A more significant number of grain boundaries acted as corrosion barriers by accelerating the passivation layer formation and reducing micro-galvanic couples between the interior of grains and grain boundaries. Zhang et al. (2018) also obtained improved mechanical properties and wear resistance without significantly improving degradation resistance and cytocompatibility. Xiong et al. (2020) investigated the effect of LSP, MAO, and LSP followed by MAO treatment on the stress corrosion cracking of AZ80 alloy in SBF. The favorable basal texture reconstructed along the (0002) plane during LSP/MAO treatment reduced E_corr by more than 300 mV and I_corr by more than two orders of magnitude.

Unlike conversion or deposition coating techniques, the surface metallurgical modifications techniques tailor the surface and subsurface microstructure. The disadvantages due to low coating adhesion and instability of the layer formed on the surface in the case of coating methods can be overcome by these techniques. The key findings from the surface metallurgical modification methods are included in Table 3.