Graphene-based coatings for magnesium alloys: exploring the correlation between coating architecture, deposition methods, corrosion resistance and materials selection

2.1.1 Graphene-based films

An important starting point to explain the interaction between graphene-based coatings and magnesium substrates is revealed in the report by Neupane et al. (2015). They proposed an innovative approach to produce a protective coating on pure magnesium substrates by coupling silane and GO coatings. Firstly, a silane layer was formed upon immersion of the magnesium substrate in a 3-aminopropyltriethoxysilane (APTES) hydrolyzed solution, subsequently cured in an oven. The silane layer acted as an intermediate bonding film to the top GO coating. Due to the chemical inertness of the graphene-based material, this is a must-attend issue to improve its interaction with the magnesium substrate. Otherwise, coating adhesion will be poor, and long-term protection will not be reached. After silanization, the GO coating was formed by dip coating, upon immersing the silanized sample on a GO suspension. They suggested that amine groups on the surface of the silane film triggered the formation of covalent bonds with functional groups of GO, giving rise to a stable film. Tong et al. (2016) adopted a similar approach to develop GO films on silanized Mg–5.7Zn–0.8Ca substrates. The GO films presented a rippled morphology that was attributed to oxygen-containing groups and to the inherent roughness of the magnesium alloy surface. According to the authors, coverage of the silanized surface by the GO sheets implied in the formation of a complex multilayer structure. The rippled surface would enable the release of interfacial stress between GO sheets and the silanized substrate, enhancing the adhesion strength. As a consequence, the corrosion resistance of the coated samples could be improved with respect to the pristine magnesium surface.

Maqsood et al. (2020) reported the formation of a GO coating directly on the surface of AZ31B magnesium alloy by electrophoretic deposition. The deposition mechanism was explained based on a sequence of chemical reactions that are originated from the negatively charged character of GO that tends to deposit on the positively charged anode surface. Thus, in the first step, carboxylate groups (negative charges) on the graphene structure will lose electrons as they reach the anode surface, being oxidized, and, subsequently, deposited on the anode surface. Deposition proceeds by the oxidative decarboxylation reaction, as the surface is fully coated by the GO film. Graphene sheets present unpaired electrons due to carboxylate groups interaction with the anode, and CO₂ is released. These graphene sheets with unpaired electrons are free to move in the graphene oxide structure, growing at the anode surface. By interacting with other unpaired electrons sheets, they form covalent bonds during film growth.

Fernández et al. (2019) combined electrochemical and chemical methods to deposit graphene oxide directly on the surface of pure magnesium samples. An initial pre-passivation potentiostatic treatment was conducted by exposing the samples to a 6 M KOH solution at room temperature, and applying −1.0 V_Ag/AgCl for 30 min. The best corrosion properties were obtained for the samples sequentially immersed in 0.5% bovine serum albumin (BSA) and an alkaline solution (pH = 11), containing GO. After drying, the samples were reduced in a dithionate solution. BSA enhanced adhesion between GO and the magnesium substrate, thus improving the corrosion resistance of the treated samples.

2.1.2 PEO-based composite films

Composite coatings based on PEO layers with addition of nanoscale fillers such as graphene-based materials are a viable way of improving the corrosion resistance of magnesium alloys. PEO, also known as micro-arc oxidation (MAO), is defined as a high voltage process, during which an oxide coating is formed on light metals, such as magnesium, titanium or aluminum alloys. In the present work, we will adopt the term PEO throughout the whole text. The main processing steps were described by Farshid and Kharaziha (2021), consisting of anodizing, spark discharge, micro-arc discharge and arc discharge, as schematically displayed in Figure 5. Initially, in the anodizing (electrolysis) step, a highly porous oxide coating is grown in the substrate due to the high voltage applied to it. Next, spark discharges are formed on the workpiece to the dielectric breakdown of the oxide layer. Finally, the last two steps (arc and micro-arc discharges) are simultaneous, and lead to film growth. Moreover, in the final step, excessive energy released by the micro-arcs discharges in the electrolyte may lead to micro-defects formation in the PEO layer which, in turn, deteriorates its corrosion protection ability (Rakoch et al. 2013). Particle addition to the electrolyte can seal the PEO layer defects, thus improving its corrosion resistance. Details on particle incorporation in PEO can be found in the excellent review by Lu et al. (2016). Although there are many different parameters affecting the corrosion properties of PEO coatings on magnesium alloys, such as electrolyte composition, applied voltage and type of substrate, the general reactions can be summarized as follows:

Figure 5:

Illustration of the PEO deposition process of magnesium alloys. Reproduced with permission from Elsevier (Farshid and Kharaziha 2021).

Zhao et al. (2017) prepared a PEO coating using GO as an additive in the electrolyte bath on an AZ31 magnesium alloy substrate. The PEO process was carried out using a DC pulse power supply, in two steps. Firstly, the samples were oxidized at a constant current density of 1 mA cm⁻² in an electrolyte containing phosphate ions and potassium hydroxide. Next, these samples were further oxidized at a constant voltage of 400 V in a different electrolyte, containing the GO additive at concentrations up to 3 g L⁻¹. The compactness of the PEO layer was improved by adding GO to the electrolyte. Pore area of the outer band was reduced, as well as the pore band next to the substrate was thinner for the GO-containing PEO layer. The authors explained the compact-enhancing action of GO sheets based on their negative surface charge (negative zeta potential) that drives them toward the positively charged anode, thus enabling its incorporation during PEO layer growth. A similar effect of pore reduction was reported by Han et al. (2018) for a graphene/PEO composite coating formed on the AZ91 alloy. The micrographs in Figure 6 clearly display the pore-reducing action of graphene when incorporated into the PEO layer.

Figure 6:

SEM micrographs showing the cross-sections of PEO coatings with different concentrations of GO: (a) without GO; (b) 50 mg L⁻¹; (c) 150 mg L⁻¹; (d) 200 mg L⁻¹. Reproduced with permission (Han et al. 2018).

Qiu et al. (2015) have also developed a double-step deposition process, combining the PEO process and a GO-based film to protect the ZK60 magnesium alloy against corrosion. In the first step, the PEO coating was obtained using a DC pulse power supply. The electrolyte was a silicate-based bath at room temperature. The PEO-coated samples were, then, subjected to dip coating in GO-containing suspensions that were previously prepared to obtain the GO films based on a layer-by-layer (LBL) self-assembly process. In this case, the samples were repeatedly immersed in the GO suspensions, thus forming the LBL film. Coating morphology was characterized by the typical porous oxide layer for the PEO-coated samples, whereas the two-step process was effective at eliminating the pores of the coated surface, showing that GO improved the compactness of the pristine PEO coating.

Zhang et al. (2020) proposed a one-step process to obtain the PEO coating with GO additive on the AZ31B magnesium alloy substrate. Thus, the composite coating was obtained by dispersing GO in distilled water, and adding this suspension directly to the PEO electrolyte (silicate-based solution), under adequate ultrasonic stirring. Coating morphology was dependent on the GO content added to the PEO electrolyte. An optimum GO content was observed to produce the most compact film (20 mL L⁻¹). The compactness of the PEO layer was enhanced due to the consequent increase of the number of micro-arc discharges and decrease of energy of the sparks that lead to the formation of more blocked pores. The reduction of the surface porosity of the PEO layer after GO addition can be seen in the SEM micrographs displayed in Figure 7.

Figure 7:

SEM micrographs of the PEO/GO composite coatings obtained by Zhang et al. (2020) and surface porosity concentration for different GO contents: (a) without GO; (b) 5 mL L⁻¹; (c) 10 mL L⁻¹; (d) 20 mL L⁻¹; (e) 40 mL L⁻¹; (f) surface porosity percentage. Reproduced with permission from Elsevier.

Chen et al. (2021) employed PEO to prepare a GO/MgAl layered-double-hydroxide (MgAl-LDH) on the AZ31 magnesium alloy. LDHs are a class of anionic clays (Guo et al. 2018). They display a lamellar brucite-like structure, showing attractive features to protect metals against corrosion, such as anion-exchange capacity, self-healing ability (Wang et al. 2019). Details about different methods of preparing LDH coatings on magnesium alloys were described by Guo et al. (2018). In the work by Chen et al. (2021), the composite GO/MgAl LDH was obtained by the following route. Firstly, the PEO coating was formed on the magnesium alloy substrate, using an electrolyte consisting of a mixture of NaAlO₂ and NaOH. The next step was the formation of the LDH coating based on a hydrothermal method conducted at 418 K for 12 h. This process was carried out in an autoclave, containing a suspension with different GO concentrations. The deposition process of the GO/MgAl-LDH composite coating is illustrated in Figure 8. The morphology of the LDH coating without GO addition was characterized by the formation of vertically grown sheets. By adding GO, the surface morphology was altered to an ordered structure, whose compactness was dependent on the GO concentration. The composite coating exhibited morphological features of the LDH sheets, and also of the wrinkled aspect of GO nanosheets.

Figure 8:

Schematic illustration of the deposition process to prepare GO/MgAl-LDH coatings on the PEO-coated AZ31 magnesium alloy. Reproduced with permission from Elsevier (Chen et al. 2021).

One additional and important aspect in the literature on PEO-graphene composite coatings on Mg alloys is the effect of the PEO process on the graphene compound. The electrical conditions of the PEO process can affect structural characteristics of the graphene structure to some extent. Han et al. (2018) gave experimental evidence to such statement. They studied the effect of graphene additive on the corrosion resistance of PEO-coated AZ91 alloy. They reported a decrease of the corrosion current density to the formation of a less porous PEO layer with graphene addition. Using XPS, they observed that graphene was oxidized during PEO, increasing the relative amount of C–O and C=O bonds. Zhao et al. (2017), in turn, have reported that for a GO-PEO composite film deposited on the AZ31 alloy the relative amount of GO’s sp² bonds was partially recovered after PEO. Additionally, C–O bond relative concentration was decreased, as observed by XPS. Based on these results, the authors concluded that the high temperature and aqueous electrolyte of the PEO process led to a partial reduction of GO. Such structural alterations did not impair, though, the main effect of graphene-based compounds on blocking the intrinsic defects of the PEO layer.

2.1.3 Hydroxyapatite-based composite films containing graphene compounds

Calcium phosphate coatings are the standard materials to enhance the biocompatibility of metallic implants. Hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) is by far the most important calcium phosphate compound used in the biomedical field. It resembles the mineral part of the human bone, being extensively employed as a bioactive material (Rahman et al. 2020). HA-based coatings are reported as a viable way of retarding the corrosion rate of implantable magnesium-based biomedical devices, and also of improving their corrosion properties (Tomozawa and Hiromoto 2011). The aim of developing graphene-HA composite coatings relies on the several aspects. For instance, GO was reported to promote osteoblast adhesion and stimulate biomineralization in titanium substrates (Zancanela et al. 2016). Furthermore, it can act as a reinforcing agent, improving the mechanical stability of the HA coating (Rafiee et al. 2009). Yet, the corrosion resistance of HA films is also enhanced by graphene addition (Singh et al. 2020).

Peng et al. (2020) developed a bilayer coating on the AZ31 alloy, consisting of an inner HA layer and an outer GO layer obtained by spin coating. A thin GO film (123 ± 10 nm) was shown to be formed on the HA-coated substrate. The surface morphology was not altered with respect to the typical cauliflower aspect of the HA layer. Nonetheless, the top GO film acted as an impervious layer that improved the corrosion resistance of the metallic substrate.

A hybrid HA-GO coating was prepared on AZ91D alloy substrates by Gao et al. (2015). The deposition method consisted of a double-step procedure. In the initial step, a GO layer was dip-coated on the magnesium alloy substrate. Next, the GO-coated samples were immersed in a simulated body fluid (SBF) solution at 37.5 °C. The SBF solution contained phosphate and calcium ions to promote the formation of HA on the top surface. The morphology of the intermediate GO layer was characterized by ripples, as reported by Tong et al. (2016). The HA top film resembled flower-like clusters or flake-like films overlapping the GO layer, depending on the exposure time to the SBF solution.

Wu et al. (2019) employed a hydrothermal method to synthesize a mixed HA-rGO coating on the AZ31B alloy. The GO particles were added directly in the hydrothermal solution. Phase composition was assessed by X-ray diffraction, revealing the films consisted of dicalcium phosphate and HA, independently of the GO concentration in the hydrothermal solution. Coating morphology displayed a prismatic shape, associated with the presence of dicalcium phosphate in the synthesized layer. In the absence of GO, the deposited layer presented several holes that were not formed when GO was part of the hydrothermally deposited film. The formation mechanism of the mixed HA-rGO coating is illustrated in Figure 7. Graphene oxide and ions such as Ca²⁺, PO₄³⁻, HPO₄²⁻, OH⁻ and H₂PO₄⁻ surround the magnesium alloy substrate and react with oxygen-containing functional groups of the GO structure such as –COOH and –OH, as shown in Figure 9. Apatite is, then, grown in situ, according to reactions (7) and (8). During the hydrothermal treatment, GO is reduced to rGO in the high temperature and pressure environment of the autoclave, thus forming the mixed Apatite/rGO layer.

Figure 9:

Schematic illustration of the mixed HA-rGO film formation mechanism, as proposed by Wu et al. (2019). Reproduced with permission from Elsevier.

In another paper (Wu et al. 2020b), the same group proposed a different approach to obtain mixed HA/rGO coatings on the AZ31B alloy. Instead of directly depositing the HA/rGO film on the substrate by the hydrothermal method described in the previous paragraph, an oxide layer was initially formed by plasma electrolytic oxidation. The PEO electrolyte was comprised of an alkaline solution, containing silicate, calcium and phosphate ions. The PEO coated alloy was, next, subject to hydrothermal treatment in a Teflon lined autoclave, containing the solution with GO, and also apatite-forming compounds such as Ca(NO₃)₂·4H₂O and KH₂PO₄·3H₂O. The aim of the intermediate PEO layer was to increase the adhesion strength between the HA/rGO film and the magnesium alloy substrate. The formation mechanism of the HA/rGO was similar to that illustrated in Figure 9, with a prismatic shape of the HA grains.

Wen et al. (2017) have also developed a mixed HA/GO based on plasma electrolytic oxidation, but following a different route than the one used by Wu et al. (2020b). Instead of depositing the mixed HA/GO film on a previously PEO-coated substrate, deposition was made in a one-step process. Hence, a mixed HA/GO powder was prepared based on a sol-gel process. Then, this powder was added to the PEO electrolyte, consisting of a phosphate-based solution. Coating surface displayed a typical aspect of PEO-coated alloys in the absence of GO, being characterized by the presence of several circular pores. Conversely, the mixed HA/GO surface had an evident reduction of the porosity level. This was attributed to the partial pore sealing given by the strong interfacial bond between HA and graphene oxide.

A recent trend in the development of HA/graphene-based composites for biomedical magnesium alloys relies on ternary coatings. The interest of incorporating an additional component to the surface film is to enhance the adhesion strength between the HA-based film and the metallic substrate. This approach was recently employed by Ahangari et al. (2021) to prepare HA-carboxymethyl cellulose-graphene composite coatings on the AZ31 magnesium alloy using electrophoretic deposition (EPD). They mention the reinforcing effect of graphene on the HA structure. Moreover, carboxymethyl cellulose (CMC) is also described as a possible adhesion promoter between the ceramic HA phase and the metallic substrate. Hence, a solution consisting of HA nanosized powder, graphene and CMC was prepared and used as basis for the EPD electrolyte. After deposition, the coatings were sintered at 450 °C for 2 h. The surface morphology of the graphene-containing coatings was more compact than that of the HA or HA-CMC films. CMC had an obvious positive effect on increasing the adhesion strength of HA to the AZ31 substrate.

Askarnia et al. (2021) employed chitosan, another organic biocompatible compound, to decrease the porosity of HA/GO coatings prepared by EPD on AZ91D substrates. The first step was to prepare a suspension consisting of a mixture of HA powder, GO nanosheets and chitosan powder to the EPD process. Then, deposition occurred at 15 V for 20 min. The coating morphology was greatly affected by the presence of GO nanosheets in the EPD electrolyte. The film produced without adding GO to the suspension displayed several cracks, typical of the EPD process, formed post-sintering and drying of the film. One additional aspect highlighted by the authors is referred to the possible formation of cracks due to shrinkage of chitosan after evaporation of the solvent employed in the preparation of the EPD solution. By adding GO, crack formation was hindered due to mechanical interlocking with HA particles. The effect of chitosan on the compactness of the EPD layer was less marked than the GO nanosheets. Similar conclusions were drawn by Saadati et al. (2021) for HA-chitosan-GO coating deposited by EPD on a biodegradable 90.9%Mg–4%Zn–4%Sn–0.6%Ca–0.5%Mn alloy. They observed that GO addition reduced the porosity of the EPD coating. They explained the action of chitosan molecules during EPD. According to the authors, chitosan molecules are charged during the deposition process with positive charge. As the HA nanoparticles are added to the electrolyte, the positively charged chitosan molecules attract them, reaching electrostatic neutralization. Attachment of GO nanosheets to the chitosan-HA mixture occurs via chemical interaction with hydrophilic functional groups (–COOH and –OH). Negatively charged GO nanosheets undergo a self-assembly process with the protonated amine groups of the chitosan molecules, thus improving the adhesion strength of the EPD film.

2.1.4 Polymer matrix composite coatings

Composite coatings based on the architecture of dispersing graphene compounds into polymeric matrices have been developed by several authors, owing to the possibility of improving the corrosion resistance of metallic substrates (Hussain et al. 2019). However, it is well-known that the corrosion protection ability must be supported by an effective dispersion of the graphene-based nanosheets into the polymer matrix. Otherwise, defective coatings are obtained, triggering the corrosion processes of the coated metal, instead of providing adequate barrier properties.

Due to its relatively high reactivity arising from the oxygen-containing functional groups, GO is considered as a good candidate for being efficiently dispersed into polymeric matrices, thus forming corrosion resistant composite coatings. Based on this philosophy, Jin et al. (2019) developed an innovative acrylamide/acrylic acid functionalized GO composite coating on the AZ91D magnesium alloy. An in situ co-polymerization method was employed to obtain a poly(acrylamide-co-acrylic acid) in the presence of GO. The composite coating was applied on the metallic substrate using a wet film applicator. Coating morphology was mainly smooth, but presented small gaps and particles, associated with shrinkage and formation of polymer clusters during the drying process of the composite coating.

Nazeer et al. (2019) extended this approach by incorporating one additional inorganic phase into the polymeric matrix, besides graphene oxide nanosheets. They prepared a novel poly(butyl methacrylate)/GO/TiO₂ nanocomposite coating to protect the AZ31 magnesium alloy against corrosion. The GO ability to disperse inorganic nanoparticles into polymeric matrices, avoiding particle agglomeration is highlighted. This is ascribed to the strong π–π interactions and van der Waals forces between GO nanosheets (Xu et al. 2017). The corrosion protection ability of the nanocomposite coating can obviously benefit from these features. Initially, the poly(butyl methacrylate)/GO/TiO₂ blend was prepared by mixing each component in an organic solvent. The composite coating was deposited on the substrate by spin coating. The hydrophobic nature of poly(butyl methacrylate) and the compact structure of the deposited layer were considered as the main features to improve the corrosion resistance of the magnesium alloy substrate.

Ikhe et al. (2016) investigated the effect of perfluorinated polysiloxane (PPFS)/GO composite coating on the corrosion resistance of the AZ31 magnesium alloy. According to the authors, the structure of the develop film consists of super-hydrophobic polymer-anchored GO. The super-hydrophobic character is given by the perfluorinated side chains. They proposed an ingenious coating architecture based on the chemical interactions between PPFS and GO. The large surface area of the PPFS/GO composite favors complete coverage of the metallic substrate, thus forming an effective barrier against corrosive species. One further advantage is that PPFS is able to attach to both faces of graphene structure, increasing the amount of super-hydrophobic molecules. Yet, some hydroxyl groups of PPFS and oxygen functional groups of GO are able to interact with oxides/hydroxides on the substrate surface, enhancing coating adhesion. After preparation of the PPFS/GO composite, the mixture was dissolved in anhydrous ethanol and deposited on the substrate by spin coating. The coating morphology consisted of thin paper-like layers, resembling the morphology of pure GO, and showing no signs of particle aggregation. Using chemical analysis by EDS (energy dispersive X-ray spectroscopy) they showed that GO was evenly covered by PPFS. Strong hydrophobicity was achieved with respect to pure GO, as indicated by contact angle measurements. These features are seen in Figure 10.

Figure 10:

(a) SEM micrographs showing GO and PPFS/GO morphologies, along with photographs of drops on the surface of GO and PPFS/GO coatings on silica glass. The hydrophobic nature of the PPFS/GO composite coating is evident from the bottom right photograph. (b) EDS mapping for Si, F, O and C. Reproduced with permission from Elsevier (Ikhe et al. 2016).

An attractive route was explored by Catt et al. (2017). They prepared a composite coating based on a conductive polymer/GO mixture to control the corrosion rate of pure magnesium ribbons. The approach was based on the idea that when GO is coupled with a conductive polymer coating, the corrosion resistance of the underlying substrate may be improved. The GO nanosheets would act as immobile anionic dopants. Enhancement of the corrosion protection ability would arise from an electrical interaction between the components of the composite film. The conductive polymer backbone with a positive charge would be reduced by the electrons that are release during corrosion of the metallic substrate. The GO dopants would remain trapped in the coating layer during this process, leading to a negatively charged film, as reported for polypyrrole (PPy) having dodecyl-sulfate as dopant in its backbone (Ohtsuka 2012). Catt et al. (2017) employed poly(3,4-ethylenedioxythiophene) as the conductive polymer. The composite conductive polymer/GO film was obtained directly on the magnesium surface by electropolymerization.

Focusing applications of magnesium alloys as intravascular implant materials (stents), Gao et al. (2019a) produced a multilayer coating on AZ31B substrates. Coating architecture was based on a layer-by-layer deposition method, obtaining a composite chitosan/heparinized graphene oxide film. Due to its excellent blood compatibility, heparin can improve the surface properties of intravascular biomedical devices. Thus, GO functionalization with heparin is an attractive route to attain good biocompatibility for magnesium-based stents. Furthermore, an additional way of enhancing the biocompatibility of magnesium alloys, and control its degradation rate is by using chitosan as a coating material, as observed by other authors (Askarnia et al. 2021; Jiang et al. 2016). Based on this approach, Gao et al. (2019a) firstly prepared heparin functionalized GO (HGO). The AZ31B surfaces were modified by immersion in 16-phosphono-hexadecanoic acid and chitosan solutions. The chitosan-modified samples were, then, subject to layer-by-layer deposition to obtain the multilayered HGO/chitosan coating (Chi/HGO). A schematic representation of the deposition process is displayed in Figure 11. Surface morphology presented a dense, layered aspect.

Figure 11:

Schematic representation of the layer-by-layer deposition method employed to obtain chitosan/heparinized graphene oxide film. Reproduced with permission from Elsevier (Gao et al. 2019a).

Another class of graphene-based compounds, graphene quantum dots (GQDs) are zero dimensional materials whose attributes are derived from carbon dots and graphene (Sun et al. 2013). They consist of one or more layers of graphene, and can be considered as small fragments of graphene (Al Jahdaly et al. 2021). They have attracted attention as nanofillers for improving the corrosion resistance of polymeric coatings (Pourhashem and Rashidi 2019). This is due to a combination of useful characteristics such as the capacity of forming dispersions with high stability in water, good bonding interaction with coating materials or substrate due to the presence of edge groups (hydroxyl, carbonyl, epoxy), and possibility of chemically doping the GQD structure, tailoring specific properties (N-doping and S-doping) (Gao et al. 2019b; Pourhashem et al. 2018; Ramezanzadeh et al. 2019). Jiang et al. (2020) used this strategy to develop N-doped GQD/polymethyltrimethoxysilane (PMTMS) composite coatings to protect the AZ91D magnesium alloy against corrosion. The N-doped GQD were electrodeposited on the surface of the magnesium substrate. Hence, the functional groups of the N-doped GQDs promoted adhesion to PMTMS. The formation mechanism of the PMTMS coating was based on the hydrolysis reaction of the MTMS monomer, as shown in Equation (9), producing (HO)₃–Si–CH₃. Next, this species reacts with hydroxyl groups on the N-doped GQD film. A compact PMTMS layer is, then, formed via a hydrolysis condensation reaction.

A different route based on the combination of graphene oxide with organic-inorganic phases to obtain hybrid composite coatings on magnesium substrates was proposed by Al Zoubi et al. (2019). Coating design consisted of an initial PEO treatment of the AZ31 magnesium alloy in an alkaline solution. Next, PEO-treated samples were immersed in a 8-hydroxyquinolin ethanolic solution, obtaining a flower-like layer morphology by dip coating. The last step was to expose these samples to a GO solution for 13 h. The final hybrid organic-inorganic film was obtained after drying at 300 °C for 2 h. The hybrid film grows by a self-assembly process based on π–π stacking and hydrophobic interactions due to affinity between the organic 8-hydroxyquinolin layer and GO.

2.1.5 Other types of graphene-based composite coatings

In the present section, other approaches for improving the corrosion resistance of magnesium and its alloys by graphene-based coatings are presented. They rely on inorganic films that are not based on the PEO process and do not contain HA. An example is found in the report by Kavimani et al. (2017) who developed rGO/SiC nanocomposite coatings on pure magnesium strips by electrodeposition. Coating morphology was characterized by uniform distribution of the film over the substrate. In another publication (Kavimani et al. 2018), the same group has described the successful formation of rGO/TiO₂ hybrid composite coatings on pure magnesium substrates. The composite film was also obtained by electrodeposition by immersing the samples in an rGO solution, containing TiO₂ nanoparticles. The process was conducted at an applied potential of 12 V for times of up to 5 min.

Electrodeposition was also employed by Wu et al. (2015) to produce Mg(OH)₂/GO composite coatings on the AZ91D magnesium alloy. Co-deposition of Mg(OH)₂ and GO was accomplished in an electrolyte consisting of a mixture of magnesium nitrate and dispersed GO sheets at an applied potential of −1.9 V (vs. Ag/AgCl) for 1 h. In the absence of GO sheets (Mg(OH)₂ electrodeposited layer), the coating morphology presented several cracks, micropores and particle clustering. Conversely, the hybrid Mg(OH)₂/GO coating displayed a more compact surface, avoiding the formation of clusters.

Bakhsheshi-Rad et al. (2016) followed a different route to obtain hybrid nano-silica/GO films on a biodegradable Mg–Ca–Zn alloy. The hybrid GO/SiO₂ layer was obtained by a two-sped process. Firstly, the SiO₂ film was deposited on the substrates by magnetron sputtering. Next, the SiO₂-coated surfaces were dipped in a GO solution, being subsequently dried at 100 °C for 1 h. The last step was repeated three times to increase the GO content in the hybrid layer. The magnetron-sputtered SiO₂ film contained inherent cracks, forming a bubble-like structure. The hydrid GO/SiO₂ layer displayed an aspect of sheet-like morphology that was ascribed to the presence of oxygenated functional groups on the GO structure.

2.1.6 Common aspects of graphene-based coatings for magnesium alloys

After describing the most widespread approaches for producing graphene-based coatings on magnesium alloys in the previous sub-sections, some common aspects are discussed in the present section.

The most critical concern when graphene-based coatings are deposited directly on the magnesium substrate is the adhesion strength. Proper deposition routes must be employed to improve the connection between the metallic surface and the carbonaceous compounds. Authors report different intermediate layers between the substrate and the graphene-based film, improving the interaction between graphene and metal, such as silane molecules. In addition to surface modification based on chemical treatments, electrochemically deposited films are also reported. In this case, chemical reactions between the graphene compounds (especially GO) and the substrate (anode) are triggered by charged species generated during the electrochemically-based deposition.

When composite coatings are deposited, irrespective of the coating architecture, graphene-based compounds play a core role on the film compactness. The main challenge is to reduce coating porosity, improving the barrier properties of the matrix, either organic or inorganic. As a general guideline, graphene-based compounds may be envisaged as nanofillers that increase the diffusion length of corrosive species, making the composite layer impervious, thus enhancing the corrosion resistance of the underlying metallic substrate. This is the most common approach for composite coatings. Yet, specific chemical interactions between graphene oxide and organic molecules may be explored to promote the formation of coatings with enhanced hydrophobicity and adhesion to the substrate.