Impressive strides in amelioration of corrosion and wear behaviors of Mg alloys using applied polymer coatings on PEO porous coatings: A review

Arash Fattah-alhosseini, Razieh Chaharmahali, Kazem Babaei

Department of Materials Engineering, Bu-Ali Sina University, Hamedan 65178-38695, Iran

Abstracts

Keywords: Composite coating; Mg alloys; PEO; Polymer; Corrosion properties; Wear resistance.

1.Introduction

Mg and alloys of magnesium are among the lightest metals used in engineering.Low density, good electromagnetic shielding, tremendous dimensional durability, high machinability, and higher damping capacity are only a few of the advantages of Mg [1–6].As a result, Mg and its alloys have a wide range of applications in industries such as airplanes,automobiles, and communications [7,8].Also, because of its biodegradability, great biocompatibility, high bioactivity, and comparable mechanical properties to the bone, Mg and its alloys are gaining a lot of attention as potential biodegradable implants [9–12].However, it is associated with a number of drawbacks,including low corrosion resistance and high chemical activity, some weaknesses over tribological features and wear resistance, poor plasticity [4,13-15].

Magnesium alloys have been subjected to extensive research, involving microstructural and surface modification techniques, to combat corrosion and wear.A basic strategy is microstructural alteration, which includes alloying with elements that are relatively noble (such as Zn, Al, Mn, Ca, Li,rare-earth elements, and Sc) [16–22].However, using the alloying methods, it is still difficult to produce Mg alloys with acceptable properties.Surface modification utilizing various processes such as electroless [23,24], electrochemical plating[25,26], conversion coating [27,28], physical vapor deposition(PVD) [29,30], plasma spray [31], anodizing [32], chemicalvapor deposition (CVD) [33,34], and PEO [35–39]is one of the most common ways to increase Mg characteristics.The method of PEO has been demonstrated to be effective in formation of protective films on Mg alloys.PEO coatings, with their high hardness, outstanding corrosion performance, and metallurgical adhesion to the substrate, are a viable solution for Mg alloy corrosion in a wide variety of technical applications [40–43].Its features are widely used as a transitional coating for subsequent surface modifications [44].

Although PEO coatings have a variety of desired qualities, the ceramic-like coating has many micropores and microcracks as a result of the continuous and intense sparking and gas evolution on the substrate’s surface.Aggressive species can permeate these defects and drastically impair the substrate’s corrosion resistance [45–47].Several approaches have been developed to reduce the defects of PEO layers on Mg alloys to improve their protective properties.One of the most effective strategies for sealing microdefects and preventing aggressive species from getting into the substrate is to form an organic film on a PEO-layer surface.It is worth noting that the PEO process produces layers with a rough and convoluted surface structure [48,49], allowing them to be used as a foundation for creating composite coatings of polymer-containing [50].Sealing components’ impact on enhancing the corrosion capabilities of PEO layers by sealing cracks and pores has also been studied [50,51].Phosphate,sol-gel, and metals electroless deposition are some of the technologies that were utilized to seal the PEO-layers pores[52–56].Sealing the pores with polymeric films is also commonly.

To rise the corrosion performance of PEO coatings, Wang et al [57].employed a low molecular weight polymer to fill the pores in the oxide coating structure.PEO coating, on the other hand, is porous and rough, which can help improve the adhesion of organic coatings to metallic surfaces.Some research has recently been done on the utility of the PEO technique as a pre-treatment method for organic coating.Although the polymer coating cannot be properly bound to the Mg substrate, the PEO coating can provide porous surface or physical bonding sites that aid in the bonding of the polymer coating.Arrabal et al [52]., looked into the expected influence of PEO pre-treatment on improving polymeric coating adhesion and corrosion resistance on AZ31.Indeed, sealed samples outperformed untreated PEO samples in terms of protection.Nevertheless,the corrosive electrolyte penetration into the coating over a long period of time results in the formation of a significant number of corrosion products that finally results in the destruction and delamination of the epoxy coating[51,58-60].

Given the importance of sealing PEO coatings, this review paper presents the latest developments in polymer coatings for magnesium and its alloys.In this study, the structure and properties of coatings containing polymers such as Polytetrafluoroethylene (PTFE), polylactic acid (PLA), Polypropylene (PP) Polycaprolactone (PCL), Polyethyleneimine (PEI),chitosan (CS) were investigated Fig.1.shows the chemical structure of polymers used on Mg alloys.

Fig.1.Chemical structure of polymeric coatings on alloys of magnesium.

2.Mg and its alloy

Magnesium and its alloys have a strong potential for usage as a lightweight structural material in aerospace and automotive applications due to their high specific strength and excellent technical features [61–65].Unfortunately, Mg and its alloys have a number of drawbacks that prohibit them from being widely used in a variety of industries.One of the factors prohibiting the use of Mg and its alloys is their low corrosion performance, particularly in acidic and salty environments [66,67].Magnesium and its alloys have low corrosion performance due to two factors: (i) the very negative electromotive force,and(ii)the poor protective characteristics of the surface coating produced on magnesium, which easily leads to galvanic corrosion, pitting, stress corrosion cracking,and other corrosion forms.In a specific environment, however, Mg and its alloys may experience a variety of corrosion types [68–70].

Surface modification is a good approach to make Mg alloys more corrosion resistant[71,72].Mg and its alloys can be protected using a variety of coating processes, such as PVD[73,74], thermal spray [75,76], CVD [77], sol-gel [78,79],hydrothermal [80,81], electrophoretic deposition (EPD) [82],PEO [83–86].A lot of coating approaches have been utilized and are also commonly in practice to reach the pleasant corrosion properties of Mg and its alloys.Among numerous possible and available coating techniques to boost the Mg alloys corrosion resistance, PEO is a favorite method due to its environmental friendliness and high efficiency [83].Due to the in-situ creation of dense ceramic layers, PEO technology was also widely applied in the field of anticorrosion coating for metal corrosive protection and wear [87,88].

3.PEO process

PEO is one of the new kinds of surface modification technology to produce ceramic coatings on metals like aluminum[89–94], Mg [46,49,95-101], zirconium [102–107], titanium[108–111], and niobium [112]as well as their alloys.Good wear resistance, high hardness, mild corrosion resistance, di-electric properties, and better thermal stability are all characteristics of PEO coatings [113–117].This method is uncommon for other types of metals since, during the coating process, more current density and arc voltage are required than for light metal substrates, resulting in considerable electrical energy consumption.The PEO process begins with sample preparation, which includes cleaning, surface polishing, and pre-treatments, followed by the application of an appropriate electrolyte bath, which is determined by the desired qualities.A pair of electrodes, containing an anode (the workpieces)and cathode (usually steal electrode), are required to provide a good electrochemical set-up.Following the initial setup, the correct current and potential are employed to generate PEO coating.Different current modes, such as bipolar or unipolar,could be used to carry out the PEO process [118,119].

Many elements, such as post-treatments and PEO parameters,have been discovered to alter the quality of PEO coating.The morphology and other characteristics of PEO coatings on magnesium alloys will be affected by changes in PEO parameters, which will change the discharge behavior during PEO.The properties of PEO coatings are affected by several conditions including substrate [120], electrolyte composition [92,121-123], electrical parameters [124–128], additive[129–133]the alloy nature, electrolyte temperature, and time[134,135].

3.1.Fundamentals and mechanism of the PEO process

Many theories exist to explain how PEO coatings deposit on various surfaces [136,137].PEO’s oxide coating production mechanism is complex, involving multiple thermalchemical, plasma-chemical, and electrochemical reactions[138–140].The competing and primary reactions include the dielectric breakdown, formation of the oxide coating, dissolving of the pre-existing layer, and anodic gas evolution.The dominance of any of these reactions is determined by the type of electrolytes,the alloy composition,the applied current density, and the concentration of various constituents used [141].The following are the normal reactions that happen within the PEO coating in magnesium alloys [44,142]:

Fig.2.Illustration of formation mechanism within PEO [83].

For the PEO process, anodizing, arc discharges, micro-arc discharges,spark discharges are the four key processes for depositing an oxide film, according to Rakoch et al [143]..According to the scheme in Fig.2, during the anodizing stage,an extremely porous layer is generated on substrate in addition to a notable increase in anode voltage.A discharge occurs in spark discharges because of using a voltage that is greater in comparison to the oxide layer dielectric capacity.Micro-arc discharge and arc discharge steps happen at the same time, with differing geometrical discharge sizes and intensities, resulting in deposition growth.Because of the high released energy of discharges, micro-arcs are employed to the solution instead of coating at the end of the arc discharge process.As a result, defects of local micro-sized occurred inside the coatings that are undesirable since they reduce corrosion resistance [83,144].

3.2.Microstructure of PEO coating

The PEO coating on the magnesium alloy substrate is made up of two layers [145]: an exterior porous layer with a micrometer-scale thickness and a compact interior layer on the Mg alloy substrate.Due to the sintering process in PEO,the outer loose and rough layers are made up of irregularly dispersed integrated grains of various sizes [146].In addition,the PEO coatings’ exterior layer contains large cavities, deep pores, and microcracks.The inner layer is denser, with fewer large cavities and smaller micropores.The development of many microchannels caused by the continuous and powerful gas bubbles and sparking discharge on the coating surface,some of which do not solidify in time throughout the PEO technique, results in micropores and cavities.

When the coated surface meets the electrolytic solution,the thermal stress imparted during the quick solidification of the molten oxide causes microcracks [147–150].Because PEO coating is produced on the substrate through chemical conversion, there is a strong bond between the magnesium alloy substrate and the dense layer [151].

In comparison to standard anodizing treatments, the PEO technique may consistently generate an oxide film that strongly adheres to a substrate, making it an efficient way to improve the corrosion performance of magnesium alloys or,as a pre-treatment, to improve the adherence for post coatings [152].PEO coatings have a number of benefits over other coatings.PEO layers are extremely durable and stable,allowing them to be employed at high temperatures.The surface characteristics of Mg alloys can be greatly improved by PEO treatment.PEO coatings, for example, outperform other chemical conversion layers in terms of wear and corrosion resistance [52].Furthermore, the cracks and pores created in PEO coatings during micro-arc discharges can assist relieve the coating’s residual stress[153,154].The adhesive–substrate interface of the joints would have a greater binding strength if the outer film of PEO coatings had a porous structure [155–158].However, high-porosity PEO coatings may have negative consequences.The porous nature of PEO layers increases surface area and, as a result, the coatings’ corrosion rate.As a result, corrosive solutions are more likely to be adsorbed in pores and penetrate into the inner film, and then to the magnesium alloy.

Much research has been done on the sealing of PEO pores,which improves the properties of the coating [159].Zhu et al [160].sealed the pores created in the PEO coating process.They created composite coatings at different times using the hydrothermal process.The results showed that sealing the porosity of the coating by hydrothermal method has improved corrosion and abrasion resistance.Merino et al [161].developed an effective multilayer system to increase the corrosion resistance of AZ31 Mg alloy using PEO process and solgel.After deposition of the silica sol-gel coating, prepared from TEOS, GPTMS, colloidal nanoparticles of SiO2and 1-Methylimidazole (MI) as an epoxy ring opener, the pores of the oxide layer were sealed.The deposition of the hybrid coating blocked the penetration of corrosive electrolytes through the pores and increased the corrosion resistance.

Porosity, pores configuration, and pores distribution in the interface between the substrate and the PEO inner layer are all essential aspects that influence PEO coatings’ corrosion resistance [54].

4.Composite coatings with PEO on Mg alloys

Generally, when the surface of magnesium alloys is coated with single-layer coatings, two problems arise: First, if conversion coatings are applied in order to protect alloys of magnesium, they are insufficient and cannot provide corrosion protection for a long time, causing magnesium alloys to degrade over a short period of time [4].Second, applying coatings that have a substrate contact will not provide long-term protection.The adherence of the applied coatings is insufficient owing to deposits of oxide-hydroxide-carbonate generated on the magnesium surface.As a result, corrosive chemicals penetrate the top coating and reach the coating/substrate interface, causing fast delamination.The corrosion products volume generated beneath the coating is so large that it separates it from the magnesium substrate [162,163].As a result,it is proposed that single-layer coatings are not an effective solution for protecting magnesium alloys.The manner of demolition and disbandment of the protective coating caused by corrosive ions reaching the magnesium substrate is depicted schematically in Fig.3.Inadequate coating-to-substrate adhesion, as well as a large volume of corrosive deposits, are the most typical reasons for the coating being swiftly removed from the substrate [137].

By creating multi-layered and complicated systems, the composite coating is a viable method for preventing corrosion of magnesium alloys in corrosive conditions.Chiu et al[164]., used hot pressing, arc spray, and anodizing to create a high corrosion performance coating for the AZ31.To protect the AZ31, Chen et al [165].employed Ni electroless plating and polymer coating methods.The results revealed that fabricating multilayer coatings could considerably improve the corrosion performance of AZ31.Nonetheless,there are certain limitations to conversion coatings, such as environmental restrictions, the difficulty of creating silane coatings, and nitrate and chromium ion carcinogenicity.As a result, using green and environmentally friendly pre-treatment processes to replace traditional pre-treatments is highly desirable [166,167].Because of its high adhesion to the substrate and porous character, PEO conversion coating is utilized as a basis in composite coatings [168].Using composite coating in producing PEO coating can improve its corrosion resistance.

One of the most efficient methods to increase the protective effect of PEO coating is to modify it.PEO/organic coating,DLC/ PEO, PVD/ PEO, PEO/sol-gel, and PEO/electroless Ni plating are some of the combined methods that were boosted to increase the protective properties of oxide coating in this case [154,169-171].Arrabal et al [52]., studied the expected PEO pre-treatment effect on improving polymeric coating adhesion and corrosion resistance on AZ31.

Polymer coatings have the advantage of being simple to apply, as a simple dipping-drying procedure can produce thick,protective coatings with minimum energy use.The use of Mg alloys for these coatings allows for the addition of pigments to control the coating color, which is a significant factor for the esthetic look of the coated object, particularly for commercial components.The high electric resistance of polymer coatings is another fascinating application.Non-conductive polymers are insulators with capacitances as low as 10–11 nF·cm-2[172].According to Scharnagl et al [173]., this results in exceptionally high impedances since the substrate is electrically insulated from the environment.Dense polymer coatings have stronger impedances and longer stability in electrochemical tests than PEO and conversion coating techniques.

In the realm of biomedicine, the issue is to develop coatings that give good corrosion protection in terms of controlled deterioration and biocompatibility, allowing Mg implants to be commercialized.These coatings should give excellent corrosion protection for the first two to three months and then degrade slowly after that.Polymer coatings are particularly interesting in this field since some polymers may be surface modified to allow the attachment of bioorganic moleculessuch as lipids and proteins, significantly increasing the coating’s biocompatibility.Overall, the PEO/polymer composite coating technique has a wide range of applications Fig.4.depicts the projections for Mg alloys in several sectors utilizing this composite coating technique [174].

Fig.3.Illustration of corrosive agent’s penetration to interface of single-layer/ substrate coating and the coating separation from the surface [137].(With permission from Ref [137].; License Number: 5172,540,919,618, Oct 19, 2021).

Creating a thin polymer layer on a PEO coating is one of the most attractive ways which has been considered by many researchers.These thin layers improve the corrosion and tribological properties by covering the cracks and porosity(Fig.5).A variety of synthetic and natural polymers were used in the composite coatings on PEO layers on Mg alloy.The properties of these polymers along with the method of their application are summarized in Table 1 [58,175-186].

Table 1Various polymer/PEO composite coatings and their effects on the properties of Mg alloys.

5.Properties of composite coating (PEO/Polymer) on Mg alloys

5.1.Microstructure and thickness of the composite coatings

The formation of electrical discharge sparks on the electrode leads to a porous surface structure and the presence of cavities and small cracks is inevitable [187,188].Researchers have used different polymers in different ways on PEO coatings to fill cavities and defects.Finally, after the application of silane films, the morphology of the PEO coating surface has changed, and a film of silane covers the whole surface of the PEO coating.

Low friction coefficient, chemical stability, agelessness,electrical insulating qualities, and performance throughout a wide range of low and high temperatures are all characteristics of fluoropolymers.Fluoropolymers are hydrophobic compounds that can aid in the creation of a hydrophobic surface with a water angle of 120° to 150° or higher.As a result, two separate studies utilizing various types of fluoropolymers were done to evaluate the benefit of generating protecting composite layers according to the PEO coating by super-dispersed polytetrafluoroethylene (PTFE) [189].Similar properties can be achieved with a layer made of a telomeric solution of TFE.TFE telomeric solution is utilized for the production of a protecting layer over an oxide coating using various methods,including brushing on the surface,immersing,or spraying[190,191].

Imshinetsky et al [192].employed a PEO coating combined with SPTFE.The polymer coating was immersed on the PEO layer and after drying, it was placed in the oven at 310 °C for 10 min Fig.6.shows the SEM image analysis of the surface layers.As can be seen, the morphology of the surface depends on the method of sample processing.The surface of the PEO coating has pores and defects caused by the evolution of gas during the plasma oxidation and depletion process (Fig.6a).The SPTFE polymer film is uniformly formed on the PEO coating surface and it is seen that it reduces the porosity and defects of the coating and creates a perfectly smooth and uniform surface (Fig.6b).

Cui et al [182].developed PEO coatings in a solution of sodium hydroxide and phytic acid.In the following, the coated specimens were immersed in NaOH solution for 1 h due to the formation of hydroxide groups and improved bonding of the polymer layer.Finally, the samples were immersed in a polymethyltrimethoxysilane (PMTMS) polymer solution.Heat treatment was then performed to remove immersioninduced H2O molecules, followed by PEO/ PMTMS compos-ite coating.The porous(volcano-and island-like)morphology of the as-prepared PEO coating is typical, with microcracks and micropores [193,194].The number of microcracks and micropores decreases after the NaOH treatment.Mg(OH)2is formed when NaOH reacts chemically with the Mg substrate [195].The bigger pores are still alive.The coating of PEO/PMTMS,on the other hand,possesses a flat surface having a few spherical particles.These findings indicate that the majority of MTMS molecules in the solution could have a reaction with groups of hydroxyl from the PEO coating by hydrolysis condensation procedures,while the remaining MTMS molecules inside the electrolyte polymerize into microspheres by self-condensation.

Fig.4.Application of PEO/polymer duplex coatings in various industries [174].(With permission from Ref [174].; License Number: 5207,691,325,164, Dec 14, 2021).

Fig.5.Illustration of the PEO/polymer double-layer and its proper protecting treatment [137].(With permission from Ref [137].; License Number:5172,540,919,618, Oct 19, 2021).

Fig.6.SEM photos of (a) PEO coating and (b) composite coating obtained on the MA8 alloy [192].(With permission from Ref [192].; License Number:5172,541,255,935, Oct 19, 2021).

Fig.7.Photos of SEM for (a) the specimens surface with PEO-coating and(b)with composite coatings formed by applying the TFE telomeric solution in EA, (c) Freon 113, and (d) PFCB on the PEO layer [185].(With permission from Ref [185].; License Number: 5172,550,222,019, Oct 19, 2021).

Gnedenkov et al [185].investigated composite coatings using various TFE solvents on PEO layers.After performing the PEO coating process, TFE polymer in different solvents such as ethyl acetate (EA), pentafluorochlorobenzene (PFCB), trifluorotrichlorethane (Freon 113) solved, and then the polymer layer was created by immersion method on PEO coatings.The specimen surface morphology for composite coating(CC)differs relying on the telomeric electrolyte utilized at creation,as demonstrated in SEM images (Fig.7).Having a solution of TFE telomers within EA, the polymer is dispersed atop the PEO film with no intensive embedment inside its pores to form an electrolyte coating (Fig.7b).Simultaneously, apparent porosity is reduced to 2.69%, compared to 3.89% for the standard PEO coating(Fig.7a).When the fluoropolymer from an electrolyte solution in Freon 113 is applied to the PEO film for generating the CC-2, the morphology of surface changes dramatically.As some of microdefects and pores reduce, the surface will get steadier (Fig.7c).In compared to the basic PEO coating, apparent porosity will be declined 5.9-fold(from 3.89% to 0.66%).CC-3 composite coatings producedusing a TFE telomeric solution in PFCB had the most uniform polymer-containing film.On the surface of this coating,the fewest number of microdefects (microcracks, pores) is identified (Fig.7d).The apparent porosity is declined using over 8-fold while comparing to the basic PEO layer (from 3.89% to 0.47%).Furthermore, the produced coatings surface is more uniform and less boosted than the base PEO film(Fig.7a) and the coatings of CC-1 and CC-2 (Fig.7b, c).

Fig.8.SEM images of the composite coating obtained on the MA8 alloy with (a) singleand (b) fourfold SPTFE treatment [197].(With permission from Ref [197].; License Number: 5172,550,465,860, Oct 19, 2021).

As mentioned, the formation of a polymer film on the PEO coating results in the closing of pores and cracks.In this way, it will improve the wear and corrosion properties.Researchers have found that the number of polymer layers created can also play a role in sealing defects.In this regard,Mashtalyar et al [180].investigated the effect of the number of SPTFE polymer layers on PEO coating.In this study,polymer layers were sprayed once, twice, and three times on PEO coating.The results showed that PEO coating contains porosity on the surface.The morphology changes dramatically and the surface becomes homogeneous after just one application of the polymer coating.By applying two layers and three layers of polymer, the amount of surface defects is considerably decreased, and the surface created after applying three layers of the polymer has the most uniform morphology among other samples and its surface is completely smooth and almost flawless.The same trend and results were examined in another study by Nadaraia et al [196]..

In another study, Gnedenkov et al [197].showed that the protective behavior of a composite coating depends on the number of polymer layers.In this research, single-layer and four-layer polymer coatings have been investigated.SPTFE polymer has a very high protection property due to its high chemical excitability.The results obtained from the composite coating surface in Fig.8 show that the operation of the polymer monolayer did not lead to the formation of a homogeneous layer, and porosity is observed on the surface, but by applying four layers of polymer, homogeneous and flawless coatings are observed.It can be concluded that in addition to the polymer type, the type of solvent used in sealing the porosity and defects of the PEO coating, the application of the polymer and the number of suitable layers increase the protective properties several times.Of course, it should be noted that the number of layers should not be large because it will lead to cracks and scaling of the polymer coating.

The use of additional surface treatments can create multiple layers able to seal the micro-defects of PEO layers [198,199]and introduce new capabilities to the device.The presence of an exterior porosity layer and an inner compact sublayer in the coating is indicated by the PEO film cross-section.The existence of pores and other microdefects in the exterior porous layer morphology is the result of intense micro discharges at the metal interface/coating/electrolyte and rapid cooling of the micro discharge zone after it has attenuated [197,200-202].The corrosion characteristics of the PEO-coating are harmed by the existence of such pores.The presence of a thin polymeric layer is shown by the cross-section of the composite coating: the thickness of surface film does not surpass 1–2 μm.It can be concluded that the pores in the PEO coating were sealed by the polymer since it was a single-phase polymer embedded into the porous region.Due to this technique,a composite coating was produced, in which a PEO film serves as a matrix filled with a polymeric material.Because particles of polymer material are introduced into the coating pores,in comparison to the thickness of base PEO film, the total thickness of the polymer-containing coating changes insignificantly.

Toorani et al [176].created a three-layer coating of PEO/silane/epoxy for the AZ31B alloy to improve adhesion strength and corrosion resistance.After forming the PEO coating within a phosphate-based solution with Ce(NO3)3·5H2O,a silane film having varying volume ratios ofγ-amino propyltriethoxysilane (APTES) and tetraethoxysilane (TEOS) was deposited on the PEO specimen via dipping it in silane electrolyte for 30 s.At last, as a final layer, the epoxy resin was applied Fig.9.shows SEM pictures of PEO/Silane duplex coatings in cross-section.In the pictures, the silane films are given.The silane films on the PEO coatings’ surface are around 850 nm-1.60 μm thickness, as can be shown.Crosssectional images clearly show the silane film’s penetration into the PEO coating pores.When comparing the structures of TEOS and APTES silanes, it can be concluded that the APTES silane has three functional groups, which allows it to precipitate during the condensation step of the layer produc-tion process and hence cannot form thick layers.TEOS, on the other hand, contains four functional groups that enable it to create a more dense network.

Fig.9.SEM photos of the duplex specimen in cross-section at various magnifications [176].(With permission from Ref [176].; License Number:5172,550,672,546, Oct 19, 2021).

Fig.10.Cross-section and surface morphologies of (a) ME3, (b) PE1, (c)PE3, and (d) the thicknesses of pretreated and epoxy layers of different samples [203].

Yang et al [203].used epoxy-based polymers as a sealing agent on porous coatings.The thickness and microstructure of the epoxy layers were investigated under different immersion conditions.After immersing the samples in epoxy polymer solution at 150 °C within 90 min, a homogeneous top layer is seen over all epoxy-coated specimens.Mg substrate was obtained by immersion three times in epoxy solution (ME3),and samples containing PEO coating and epoxy layer were obtained by dipping once in solution (PE1) and three times immersion in solution (PE3).

Fig.10 shows the surface and cross-section images of ME3, PE1, and PE3.Pre-treated surfaces are rarely visible from the top view for all specimens, implying a rather thick epoxy film on all pre-treated substrates of magnesium.The roughness values are in an agreement with these results.Epoxy formulation (e.g., composition and viscosity), surface conditions(e.g.,composition or porosity),and post-cure treatment all have a synergistic effect on the epoxy layer’s qualities.Furthermore, open pores are evident on the PE1 surface created with a single dipping in the formulation of epoxy,however, as the repetition of dipping in the formulation of epoxy increases to three times, the number of open pores is significantly reduced (PE3).Because of the considerable variation in surface morphologies between PE3 and PE1, a multi-dipping method appears to improve pore sealing, particularly large ones, and hence increases the compactness of hybrid coatings.The photos of cross-section morphology back up this claim.As illustrated,epoxy films of uniform thickness form on the pre-treated surfaces, and the epoxy polymer fills cavities and discharge channels inside the PEO coatings due to the epoxy formulation’s penetration during the immersion time.On the surfaces of pre-treated, epoxy films of uniform thickness are created, as indicated, and the top epoxy film of ME3 has a thickness of 16.7 ± 0.3 μm that is seemingly thicker in comparison to the top epoxy film of the PE specimen (PE3: 15 ± 1.6 μm and PE1: 13.5 ± 0.4 μm).These discrepancies show that porous anodized coatings consume the epoxy component because of penetration in the pores and that this process is facilitated by increasing the dipping repetition.

5.2.Corrosion behavior

Many studies have shown that using polymeric-based coatings improves corrosion resistance.Because of their high chemicals resistance, outstanding adhesive strength, and excellent protective properties, these coatings have been widely used [51,204,205].As a result, composite coatings with reasonably strong corrosion resistance have been widely utilized to protect magnesium alloys.These coatings are popular because of their simplicity, good corrosion resistance,cost-effectiveness, and high coating ability, even on complex parts [4,162].PEO as a pre-treatment increased the corrosion performance of organic coatings on magnesium alloys, according to studies.This porous and rough microstructure can aid in the adherence of organic coatings to the surface of magnesium alloys.Consequently, utilizing the PEO technique, a pre-treatment for organic coatings generates double-film coatings for magnesium alloys in a severe corrosive environment could be a potential technique to provide superior corrosion protection for magnesium alloys.Various research has looked into the combination of polymeric and PEO coatings, which has resulted in an enhancement in the protective qualities of top coatings placed on the magnesium surface.

Gnedenkov et al [197].utilized fluoropolymer to increase the protection behavior of the PEO layer on MA8 alloy.Impedance spectroscopy was used to assess the influence of fluoropolymer(CC-1x,CC-4x)use on the PEO film with later thermal treatment on the state of the electrolyte/ composite film interface.In Bode (Fig.11a, b) and Nyquist (Fig.11c)plots,the impedance spectra of the studied samples are shown.The Bode plots (Fig.11a, b) illustrate the feature of changes in morphological and electrochemical behaviors, as well as sample heterogeneity, as a result of the production of diverse composite films on the surfaces [189,206].The Nyquist plotfor an uncoated specimen depicts a capacitive semicircle and an inductive loop (Fig.11c).

Fig.11.(a, b) Bode and (c) Nyquist plots for specimens made of MA8 alloy within 3 wt.% NaCl electrolyte with no coating, with PEO-coating,with composite coating gained with a single use of SPTFE (CC-1x) and with composite coating gained with fourfold use of SPTFE (CC-4x) [197].(With permission from Ref [197].; License Number: 5172,550,465,860, Oct 19, 2021).

Fig.12.Models of coating structure and relevant EECs for fitting experimental impedance data [197].(With permission from Ref [197].; License Number: 5172,550,465,860, Oct 19, 2021).

The spectrum of none coated specimen can be fitted by a simplified equivalent electrical circuit (EEC) with one R2–CPE2-circuit (Fig.12a), where R2shows the charge transfer resistance, and CPE2reveals the double-layer capacitance.There are two semicircles in PEO-coating in the Nyquist plot:one at high frequencies and the other at intermediate frequencies (Fig.12b).Both semicircles exhibit capacitive property and are linked to PEO coatings’ two-layered structure.The existence of the outer porous film causes the high-frequency loop, whereas the presence of the inner nonporous sub-layer causes the intermediate-frequency loop.An EEC with a couple of R–CPE circuits, as depicted in Fig.12b, can fit these spectra.The R1–CPE1elements in this EEC represent the porous section of the oxide film, whereas the R2–CPE2elements represent the nonporous film [207].EEC with two R–CPE circuits characterizes the bulk behavior of the coating,according to the utilized models of the PEO layer structure.Via fitting the EIS spectra by EEC using circuits of three R–CPE, the quantitative characteristics of the EEC elements that characterize the composite films generated due to the SPTFE treatment (CC-1x, CC-4x) of the base PEO coating, were determined (Fig.12c) [207].The third time constant (R3-CPE3)is connected to the SPTFE-induced sealing of pores due to utilizing a polymer and later thermal treatment.In the base PEO coating, a closed gap was produced between the pore bottom and the polymer plug (Fig.12c).The bulk properties of the system under investigation are described by the three R-CPE circuit EEC given.This result indicates the coating’s high homogeneity and the absence of cracks and defects in its structure.

In order to cover the pores inside the oxide layer employed to AM60B, Bestetti et al [208].used four kinds of polyester,epoxy, polyester-epoxy, and polyester + anti-corrosion coatings.The PEO coating increased the organic coating’s adherence and, as a result, the capabilities of anti-corrosion in epoxy and polyester + anti-corrosion coatings.

As a nontoxic and environmentally acceptable polymeric material,polypropylene(PP)has good water and corrosion resistance.Chen et al [209].employed PP layer to increase the PEO layer’s protective qualities on AZ31.According to the results of the polarization experiment, when compared to the PEO coated specimen, the specimen corrosion current density that was sealed by PP, was declined using more than three times.These findings indicate that corrosive species (Cl-)transferred between the metal and the solution was considerably reduced.As a result, cathode reactions and anodic dissolution reactions were considerably reduced,most likely due to the blocking and barrier influence of PP on the oxide layer,as well as the filling of microdefects on the oxide coating layer’s surface with PP.

Ballam et al [210].investigated the corrosion behavior of PEO coatings and polymer powder coatings as a protective top layer.Based on the results, composite coating is the most promising way to improve the corrosion resistance of Mg alloy in chloride-containing medium.This is due to the ability of the polymer coating to seal all imperfections in the PEO coating.Toorani et al [211].investigated the effect of one mineral inhibitor (cerium nitrate) and four organic inhibitors(DDTC, MBO, HQ-8, I3C) on PEO coatings to improve the protective properties of PEO/Silane/Epoxy coatings.The corrosion results showed that the presence of inhibitors in the coating systems has led to the creation of active corrosion protection properties for the coating system.The presence of HQ-8 organic inhibitor provides optimal performance for coating systems.

Srinivasan et al [169].improved the corrosion resistance of AZ31 by using a combination of polyetherimide and PEO coatings.Immersion in a polymer bath solution was used toapply the polymeric coating and then drying it for four hours inside a vacuum oven at 40 °C.Testing of corrosion revealed that the corrosion performance of the duplex coating (polymer+PEO)employed on the AZ31 in 0.1 M NaCl was identical.Leakage of the solution via the electrolyte ions penetration into the layer caused damage to the single-layer polymer coating, resulting in the initiation of corrosion/hydrogen reduction/production of magnesium hydroxide at the substrate interface [212].The polymer coating can fill the PEO coating pores in a dual-layer system, therefore the combination of the PEO and polymer coating increases the corrosion resistance of the magnesium substrate within immersion of prolonged.Similarly, after 1000 h of immersion, the surface optical micrograph of the dual-layer specimens shows no visible degradation to the surface.In contrast, monolayer polymer coating or the PEO coating only lasted about 50 h in water.Plots of Bode for the duplex coating specimen and polymer coating specimen employed on the magnesium show the benefit of PEO coating like an internal film in boosting polymer coating adhesion strength (Fig.13).In the salt spray test, a good synergistic influence of double-film coatings on corrosion resistance was seen, with the single-layer PEO coated specimen deteriorating after 48 h, but the dual-layer specimen showing no signs of degradation up to 300 h.According to the findings,the polymeric coating’s adherence improves in the presence of a PEO film as a pre-treatment, and the polymer’s impact of sealing cavities and pores in PEO structures provides superior corrosion performance for the magnesium specimen.

Fig.13.EIS plots of (a) Polymer coating, (b) PEO + Polymer coating made of AZ31 alloy in 0.1 M NaCl solution at various immersion times [169].

Cui et al [60].investigated the protective properties of duplex coatings on AZ91D, such as polymer and PEO coatings including a solvent, barium sulfate, and polyurethane resin.The spray method was used to apply the cured coating to the surface.The results indicated that a polymer coating having a thickness of nearly 92 μm forms rather securely on the PEO ceramic film, resulting in good mechanical self-locking all over the interface that is rough.The manufactured doublelayer coating corrosion current density is 2–3 times lower than that of the PEO single-layer coating, and no corrosion is observed after 500 h of immersion in 3.5 percent NaCl.After 168 h of immersion, severe localized corrosion was detected on the specimen with only PEO covering.

The hydrogen evolution plots of the AZ31, PEO, and PEO/PMTMS coatings are presented in Fig.14 [182].The volume of hydrogen evolution (Fig.14a) can be rated as follows in ascending order: the PEO/PMTMS coating, the PEO coating, and the AZ31 substrate, suggesting that the composite coating has superior corrosion resistance.In addition,Fig.14b shows the hydrogen evolution rate (HER) of the AZ31 substrate, PEO, and PEO/PMTMS coatings with respect to time of immersion.The dissolving of the AZ31 and the generation of Mg(OH)2precipitate correlates to the primary decline in the curve of HER in the AZ31.The small rise in HER for the PEO-coated specimens was attributable to coating deterioration, whereas the HER of the PEO /PMTMS coating did not change significantly.As a result, the HERs for the specimens is sorted in ascending order as follows:coating of PEO/PMTMS, PEO coating, PEO coating, and alloy substrate.These data show the coating of PEO/PMTMS composite has proper long-term corrosion performance.

Mashtalyar et al [186].evaluated the effect of three Fluoroparaffin polymers with melting temperatures of 90,110, and 180 °C The electrochemical behavior of the coating was evaluated using the polarization method.The obtained results show the improvement of the protective behavior of composite coatings with Fluoroparaffin compared to the base PEO film.For all composite specimens, the corrosion current density decreased and the resistance of polarization and corrosion potential increased relative to the PEO coating.For all composite coatings, there is an increase in thickness.The greater the melting point of the Fluoroparaffin employed in this circumstance, the greater the thickness and subsequent filling of the PEO pores.According to the obtained results,composite coating obtained with use Fluoroparaffin with melting temperature of 180 °C has the highest protecting behavior and this coating current density has been reduced twice compared to the PEO layer.

Gnedenkov et al [189].utilized the SPTFE to increase the protection behavior of the PEO film on MA8 alloy.Evaluation of the effect of the SPTFE (CC-1x, CC-5x) application on the PEO layer.The findings of electrochemical tests led to the conclusion that PEO layers treated with SPTFE had a favorable influence on the inhibitive properties of the produced composite coatings.The form of the polarization plots is illus-trated in Fig.15, and the corrosion parameters estimated from them clearly show that specimens with composite coatings at the surface have better protective behavior than the specimens without coatings and the base PEO film.In comparison to the same systems, specimens with composite coatings had nobler open-circuit potentials.Even a single treatment of the PEO film with SPTFE improves the inhibitive capabilities of the generated coatings to some extent.In terms of the five-fold SPTFE treatment, it provides the best protection for the specimen of all the ones tested.

Fig.14.(a) Hydrogen evolution volume and (b) HERs as a function of immersion time in 3.5 wt.% NaCl for 248 h: (I) AZ31 substrate, (II) PEO coating,and (III) PEO/PMTMS coating [182].(With permission from Ref [182].; License Number: 5172,551,489,449, Oct 19, 2021).

Fig.15.Polarization plots for specimens made of MA8 alloy obtained in 3%NaCl electrolyte [189].(With permission from Ref [189].; License Number:5172,560,131,533, Oct 19, 2021).

Table 2 summarizes the research on the duplex coating of PEO/Polymer and its findings [175,176,180,182-186,189,196]..Toorani et al [176].investigated a PEO/Epoxy coating potential as a saving bin used for corrosion inhibitors by using a PEO/Epoxy coating with a La(NO3)3addition like an inhibitor within the PEO coating [213].The existence of inhibitors in the PEO coating causes the development of insoluble La(OH)3that inhibits the electrolytes penetration in PEO film, according to their findings (Fig.16).The corrosive ion penetration resistance of the double-layer coating without inhibitor was only 3 weeks, while the coating with the inhibitor was not even damaged after 6 weeks.The fundamental cause for this behavior was the reduction in corrosion products volume at the interface of PEO/Epoxy due to the obstacle role of La(OH)3.

Table 2Summary of results of research conducted on PEO/Polymer coating system.

Exposure of the coating to corrosive electrolytes leads to the penetration of corrosive ions and weakens the performance of the coating.Therefore, it accelerates the corrosion of the coating/substrate interface surface and thus reduces the adhesion of the coating.Reducing the adhesion of the coating causes it delamination.Due to the fact that polymer coatings are used on PEO coatings, so the surface bonds between PEO coatings and polymer coatings play an important role.Adhesion strength depends on the surface properties of the PEO coating.Therefore, the morphology and chemistry of the surface before applying the polymer coatings is very important and affects the adhesion of the interface.The adhesion of the polymer coating is affected by two mechanisms.

First, the surface morphology of PEO coating samples which number and pore size affect the adhesion of the polymer coating.The surface with better uniformity makes better contact with the polymer coating.The second effect is on the bonds created with the polymer coating [133].In this regard, Turani et al [176].investigated the adhesion strength of aγ-amino propyltriethoxysilane (APTES) silane coating.Important parameters such as coating structure and functional groups were effective in improving the adhesion to the PEO coating.According to the obtained results, the presence of APTES with NH2groups improved the adhesion of the epoxy layer, significantly reduced the water contact angle and increased the hydrophilic properties on the PEO coating.Also,the reactants (NH2groups) in the silane structure formed a covalent bond with the hydroxy metal groups on the surface of the PEO coating, thus increasing the adhesion of the epoxy coating on the PEO surface.

5.3.Wear properties

In addition to improving corrosion behavior, polymer coatings reduce the coefficient of friction.Which has been researched in this field.Gnedenkov et al [189].studied the in-fluence of SPTFE on the protective behavior of PEO coatings on MA8.Due to the presence of SPTFE on its surface, the composite coating has the best tribological properties.The friction coefficient is lowest here, with an average value of roughly 0.04, more than 8-fold lower than the first coating created in the PEO technique.Maximum corrosion protection is provided by the composite coating with totally sealed pores of the base PEO film as well as a reduction in friction coefficient for details made of Mg alloys.The low value of this parameter implies that the coating antifriction behavior have significantly improved.SPTFE is used like a lubricant in the present case.The chemical interaction of the steel with coating material and variations in the contacting surface areas are absent in this situation due to chemical inertness, ensuring consistent friction coefficient values.

Fig.16.Illustration of corrosion behavior of PEO/Epoxy double-layer coating with and with no La(NO3)3 [176].(With permission from Ref [176].; License Number: 5172,560,326,688, Oct 19, 2021).

Fig.17.The friction coefficient vs.number of cycles for (1) specimens with PEO coating, and with composite coatings got having distinct Fluoroparaffin:(2) PAP-90, (3) PAP-110, (4) PAP-180 [186].(With permission from Ref[186].; License Number: 5172,560,494,214, Oct 19, 2021).

Mashtalyar et al [186].investigated the effect of Fluoroparaffin polymer at three melting temperatures of 90, 110,and 180°C.The tribological experiments demonstrated a considerable Fluoroparaffin influence on the composite coating wear time increase, as shown in Fig.17.In compared to the basal PEO-coating, the time of abrasion increases by 5.8 to 16.9 times.The Fluoroparaffin film acts as a dry lubricant, which reduces the base PEO-sublayer wear (Fig.17).At the end of the test, monotonous abrasion is detected for all composite coating specimens, with the friction coefficient rising.The friction coefficient of specimens having CC(PAP-1110) and CC(PAP-180) has a nearly identical dependence on the cycles number.Within the wear experiment, the friction coefficient gradually rises in the range of 0.1–0.2, thenrises sharply towards the test end to 0.4.Composite coatings made with PAP-180 and PAP-110 Fluoroparaffin possess the highest wear resistance.These coatings wear is nearly a couple of orders of magnitude less in comparison to that of a PEO film.

Fig.18.SEM images of the wear trace of specimens with composite coatings at various steps [197].(With permission from Ref [197].; License Number:5172,550,465,860, Oct 19, 2021).

Also, the wear-resistance properties of the produced coating were evaluated inside tribological experiments.For the examined coatings, the friction coefficient is shown to be dependent on the corundum ball rotation cycles.The experimental results allows us to conclude that the present fluoropolymer in the composition of composite coating has a substantial impact on sample wear.Comparing with the first PEO coating,a single SPTFE treatment increases the coating wear time by more than 40 times, while a fourfold treatment rises it by over 55 times.

The wear in coatings of composite polymer-containing can be divided into three stages based on the evolution of the friction coefficient within the experiment and the surface image after the test (Fig.18).The abrasion and dry sliding of the SPTFE-containing surface film happened in the first stage (Fig.18a).The friction coefficient increased dramatically in the second step (abrasion of PEO-coating) (Fig.18b).While comparing to the base PEO film, the development of the polymer-containing layer reduces wear via two orders of magnitude for the single treatment of SPTFE and four orders of magnitude related to the treatment of fourfold polymer.Obtained results point to the possibility of significantly increasing the wear resistance of Mg alloys by forming coatings of composite polymer-containing on their surfaces.Because of the efficient use of Mg alloy, it is possible to not only extend the material’s lifetime but also to greatly reduce the entire mechanism specific weight.This will eventually lead to a reduction in the cost of its manufacturing, operation, and maintenance [197].Table 3 summarizes the research conducted on composite coatings [185,186,189,197,214].

Table 3The details of wear tests.

Fig.19.Schematic of the wear mechanism of composite coatings during dry sliding wear [203].

Due to the parameters influencing the mechanical behaviors of the material and the microstructure of the surface,Yang et al [203].proposed the wear mechanism of the composite coatings, which is schematically shown in Fig.19.In this study, the effect of different immersion conditions on abrasion behavior has been investigated.First, the sample coated with PEO was obtained by immersing it once in solution (PE1) and three times immersing it in solution (PE3).Owing to its unstable microstructure that includes cracks, micropores as well as several defects, a tiny quantity of coating debris might be simply formed on the porous PEO coating under each load.The wear attack of abrasive on the steel ball and anodized coatings is induced by the hard, microscopic ceramic particles present inside the contact pair, particularly as the anodized film is fully stripped from the magnesium surface (Fig.19(a)).Damage to the coating may begin in the open pores of PE1 (Fig.19(b)), where the concentration of stress is frequently located throughout the edges.Despite this,the dry sliding experiment demonstrated less defective morphology of surface and the least measured loss of volume for PE1, confirming the perk of the soft top epoxy film.PE3(Fig.19(c)) that is manufactured by triple dipping in the formulation of epoxy,effectively avoids the damage and collapse behavior seen in PE1 and PEO.Due to greater penetration of the epoxy formulation into the pores in the PEO coatings,this significant improvement in wear resistance can be attributed mainly to adequately sealed surface and the reinforced microstructure of the PEO coatings Fig.20.shows the effect of different types of polymers on corrosion and abrasion behavior.

Fig.20.Effect of polymer type on corrosion and abrasion behavior.

In conclusion, dip-coating porous coatings of PEO having durable epoxy polymer have been shown to improve PEO coating wear resistance under dry sliding wear circumstances.Sealing PEO layers provide constant support against applied loads in the optimized system of hybrid coating, whereas the top epoxy film resists wear attack via plastic deformation.This improvement in performance is due to both the sealing agent saturation in the porous coatings and the mechanical properties.

6.Conclusions

Due to the various industrial applications of Mg alloys,much research has been done to improve its properties.Many studies have shown that the PEO process is a likely strategy to improve the tribological and corrosion resistance behavior of Mg alloys.Irregular microcracks and pores appear inside the oxide film during the coating process, thus creating pathways for corrosive species to penetrate.Research has led to several strategies for improving PEO coating defects.Optimization of coating process parameters such as electrolyte concentration and composition, current density and voltage in addition to frequency have been investigated to enhance the PEO coating characteristics.The production of thicker and denser PEO coatings has been done at optimal factors.However, these coatings require to be enhanced.Composite coatings can be expected to be a common way to amend the Mg alloys properties in the near future.

PEO / Polymer based two-layer coatings can overcome problems with proper sealing of defects.Composite coatings can offer lower wear rates, longer service life, higher corrosion resistance and many other useful properties.PEO coating adhesion to the substrate in addition to the polymer coating high density is the principal causes for appropriate protection of this two-layer system.Filling the pores in the structure of PEO results in the slight penetration of corrosive ions into the surface.Improved corrosion protection using polymer coatings has been depicted in a lot of investigations.These coatings have been highly utilized due to their high resistance to chemicals and protecting properties as well as great adhesion strength.The PEO procedure is also a surface process technology that looks to have the capability of creating the right environment for bonding top coatings.Investigations indicate that as a pretreatment, PEO improves the organic coatings corrosion protection on Mg alloys.Porous and rough and microstructure can be helpful for better adhesion of the organic coating on the surface of Mg.Consequently, using the PEO procedure as a pretreatment of organic coatings and therefore,making a bilayer coating could be a possible way to obtain great corrosion protection for alloys of Mg in highly corrosive environments.A set of strategies, including increasing the thickness of the PEO layer and providing multiple functions through PEO / Polymer bilayer coatings, offers better properties in Mg alloys.Therefore, more efforts are required to strengthen Mg alloys in diverse uses through these hybrid approaches.

Declaration of Competing Interest

None.