Phosphate chemical conversion coatings for magnesium alloys: a review

Abstract

Phosphate chemical conversion (PCC) coatings have been investigated for improving surface protection of magnesium alloys in aerospace, automobile, electronics, sports goods, and biomedical applications. Zinc, calcium, zinc–calcium, manganese, magnesium, molybdate, and dihydrogen manganese polyphosphate conversion coatings are widely used for improving appearance, bonding strength, corrosion resistance, wear resistance, biocompatibility, and biodegradability of Mg and its alloys. As an overview, several main types of PCC coatings for Mg alloys, their properties, and behavior in different environments particularly for biomedical applications have been discussed. The pre-surface treatments, deposition mechanism, as well as process parameters, i.e., bath compositions, temperature, time, and pH, are also elaborated in a separate section. Additionally, the main types of PCC coatings applied on Mg and its alloys, their microstructural features, and their biological performance are briefly described. Finally, applicable characterization techniques to evaluate the properties of PCC-coated Mg alloys were also discussed.

Introduction

In the recent years, utilization of Mg and its alloys has increased considerably in light mass engineering applications such as automobile industries, computer parts, mobile phones, aerospace components, sports goods, handheld tools, and household equipment.1 Mg and its alloys have also found applications in biomedical fields such as orthopedic implants for skeletal and dental disorders.2,3,4 Its success in such a broad spectrum of applications and industries is due to the many desirable properties Mg and its alloys have. For instance, Mg alloys have low density, high-strength-to-weight ratio, good castability, easy machinability, weldability, recyclability, damping capacity, thermal conductivity, excellent electromagnetic properties, attractive biocompatibility, osseointegration, and biodegradation characteristics.5,6,7 However, some characteristics of Mg and its alloy such as low corrosion resistance, wear resistance, and poor biodegradability are the existing challenges that still require intensive research for improvement.8,9,10

For this purpose, attempts to enhance these properties have been made either through developing new alloys or through surface modification. Surface treatments and coatings include chemical conversion coatings, electrochemical conversion coatings, organic coating, polymer coatings, composite coatings, electroplating, electroless plating, thermal spraying, anodic oxidation, magnetron sputtering, ion implantation, and laser processing.11,12 Conversion coatings are most widely used among various oxide, phosphate, chromate, and oxalate coatings to produce a barrier against corrosion, improve wear resistance, facilitate better paint adhesion, enhance oil absorption, form an electrically insulated surface, and upgrade biocompatibility and bioactivity.13,14 Phosphate chemical conversion (PCC) is a surface treatment in which soft, insoluble, crystalline-protective phosphate surface film is developed by the reaction of Mg with phosphating agents in the bath.15 Particularly, phosphate conversion coatings are a feasible, efficient, cost-effective, easily applicable way of protecting Mg alloys and help in getting rid of chromate-based coating technologies that may certainly have serious health concerns.16,17 Nitrates, nitrites, chlorates, peroxides, perborate, hydrogen peroxide, and nitro-guanidine are used in phosphating baths as accelerating agents. Some of these agents, like nitrous oxide, are hazardous for the environment.18,19,20,21 To reduce the ecological load, new phosphating techniques with little environmental impact are continuously being developed.9

In the last decade, the phosphating of Mg alloys has been carried out by using environmentally friendly additives such as NaClO₃,17 silicate,22 ethanolamine,23 Zn²⁺,8 sodium meta-nitrobenzene sulfonate,24 and Na₂MoO₄.25 Several investigations have been conducted on microstructure,20 electrochemical properties,26,27,28,29,30 hydrogen evolution reactions,31 weight gain,14 surface properties,32 pretreatments and optimization, as well as biomedical properties 33,34 of PCC coatings in the past years. This report describes not only the aforementioned properties, but also the deposition methods, chemical compositions, phosphating parameters, and types of conventional and newly developed PCC coatings. The objective of this review is to explain the various aspects of conventional and newly developed PCC coatings. After “Introduction” section, chemical reactions of PCC coatings including method, chemical compositions, phosphating parameters, and reactions are reviewed in “Chemical reactions of phosphate chemical conversion coatings” section. “Chemical reactions of phosphate chemical conversion coatings” section provides details of conventional and newly developed types of PCC coatings. The characterization techniques for the analysis of PCC coatings are discussed in “Characterization of PCC coatings” section. A comparison of the process variables will also be made to optimize the coating process and to improve the service performance of the PCC coatings in this section. A few highlights of this review and future recommendations will also be proposed in “Characterization of PCC coatings” section. The rapidly growing interest in the use of Mg alloys for energy, portable electronic devices, biomedical, aerospace, and transportation industries and application of PCC coatings for corrosion protection is evident in the literature. This review will serve as a collection of information on the current status of the PCC coating technology, specifically for Mg alloys to the concerned manufacturing industries, researchers, and end users in this field.

Chemical reactions of phosphate chemical conversion coatings

PCC coatings on Mg alloys involve several stages, i.e., cleaning, degreasing, pickling, phosphating, and sealing. The general sequence of the steps involved in the phosphating process is schematically presented in Fig. 1.

Fig. 1

Process flow diagram that describes the various steps involved in the application of PCC coatings

The integrity of the conversion layer depends highly on the contact between the metal surface and the phosphating solution. Thus, surface cleaning is an integral step to ensure effective phosphating and to enhance the metal/electrolyte interfacial characteristics. Both mechanical and chemical cleaning methods are used to great effect.24 Additionally, grinding and polishing of the surface are carried out to remove the top surface layer, resulting in a minimum of attached impurities, and to produce smooth surface finish.10,11,22 This is followed by rinsing in water, ethanol, and/or acetone to remove surface debris from the polished surface. To remove oil, grease, or any lubricant present on the surface, the degreasing is usually performed by dipping the Mg substrate in 3–10 wt% NaOH or KOH alkaline solution at 50–95°C for 5–10 min.17,22,24 The substrate is subsequently rinsed in cold water to remove degreasing products.

For surface activation and to remove surface impurities, pickling is performed in acidic solution before phosphating. Pickling is typically done using nitric acid, hydrofluoric acid, or sulfuric acid.35,36 After surface preparation and cleaning processes are complete, the phosphating of Mg alloys is performed by immersing the substrate in the bath. Various bath compositions and phosphating parameters have been reported in the literature as given in Table 1. After phosphating, the material is immersed in lubricant to seal the coating and to isolate the substrate from the environment.

Table 1 Bath compositions and process parameters for the deposition of PCC coatings on Mg alloys

When the Mg alloy is dipped in the phosphating solution, Mg²⁺ ions at microanodic sites are released according to reaction (1). The hydrogen ions available in the phosphating bath are reduced and formed nascent hydrogen via reaction (2). Specific adsorption of the nascent hydrogen at the active sites could hinder the dissolution of Mg at the microanodic sites until two hydrogen atoms combine to form hydrogen bubble via reaction (3). However, the intermediate oxidizing species O present of the surface could also interact with the hydrogen atoms according to reaction (4), which accelerates the phosphating process.8,18

$${\text{Mg}} \to {\text{Mg}}^{2 + } + 2e^{-} \left( {{\text{Anodic}}\,{\text{Reaction}}} \right)$$

(1)

$$2{\text{H}}^{ + } + 2e^{-} \to 2{\text{H }}\left( {{\text{Cathodic}}\,{\text{Reaction}}} \right)$$

(2)

$${\text{H}} + {\text{H}} \to {\text{H}}_{2}$$

(3)

$$2{\text{H}} + \left[ {\text{O}} \right] \to {\text{H}}_{2} {\text{O}} .$$

(4)

Zinc phosphate coating

With the immersion of Mg alloy in zinc phosphating bath, the reactions including Mg dissolution: reaction (1), hydrogen evolution: reaction (2), and Zn²⁺ reduction would occur simultaneously on the Mg surface. Consequently, Mg dissolution and the generation of hydrogen gas could slightly increase the pH of the solution at the substrate/solution interface. This increase in the interfacial pH would facilitate the precipitation of hopeite (Zn₃(PO₄)₂·4H₂O) via reaction (5), and these species may deposit on the surface of Mg. Furthermore, the controlled amount of NO₃⁻ species in the zinc phosphate coating bath acts as an accelerator, which promotes the Mg oxidation possibly via reaction (6), thus facilitating the formation of hopeite on the surface.8 Similarly, in the presence of Zn²⁺ species in the solution, hopeite formation on the Mg surface could also suppress the H₂ evolution.

$$3{\text{Zn}}^{2 + } + 2{\text{H}}_{2} {\text{PO}}_{4}^{ - } + 4{\text{H}}_{2} {\text{O}} + 4e^{-} \to {\text{Zn}}_{3} \left( {{\text{PO}}_{4} } \right)_{2} \cdot 4{\text{H}}_{2} {\text{O}}\left( {\text{s}} \right) + 2{\text{H}}_{2}$$

(5)

$${\text{Mg }} + {\text{NO}}_{3}^{-} + 2{\text{H}}^{ + } \to {\text{Mg}}^{2 + } + {\text{NO}}_{2}^{-} + {\text{H}}_{2} {\text{O}} .$$

(6)

Calcium phosphate coating

Initially, cathodic and anodic polarization reactions occur at various local sites on the metal surface and result in the formation of hydrogen bubbles according to reaction (7). Local pH and Mg²⁺ concentration also increase at the metal/solution interface. The presence of Mg²⁺ ions promotes the formation of berryite phase (MgHPO₄·3H₂O) by reacting with the phosphate ions present in the bath solution (reaction 8). After that, Ca²⁺ ions react with the available phosphate ions to form brushite (CaHPO₄·2H₂O) as per reaction (9). The simultaneous progress of these intermediate reactions on the surface of the Mg substrate results in the formation of stable whitlockite structure (Ca₉Mg(HPO₄)(PO₄)₆) as indicated by reaction (10).14

$${\text{Mg }} + 2{\text{H}}_{2} {\text{O}} \to {\text{Mg}}^{2 + } + 2{\text{OH}}^{-} + {\text{H}}_{2}$$

(7)

$${\text{Mg}}^{2 + } + {\text{H}}_{2} {\text{PO}}_{4}^{-} + {\text{OH}}^{-} + 2{\text{H}}_{2} {\text{O}} \to {\text{MgHPO}}_{4} \cdot 3{\text{H}}_{2} {\text{O}}$$

(8)

$${\text{Ca}}^{2 + } + {\text{H}}_{2} {\text{PO}}_{4}^{-} + {\text{OH}}^{-} + {\text{H}}_{2} {\text{O}} \to {\text{CaHPO}}_{4} \cdot 2{\text{H}}_{2} {\text{O}}$$

(9)

$$9{\text{Ca}}^{2 + } + {\text{Mg}}^{2 + } + 7{\text{H}}_{2} {\text{PO}}_{4}^{-} + 13{\text{OH}}^{-} \to {\text{Ca}}_{9} {\text{Mg}}\left( {{\text{HPO}}_{4} } \right)\left( {{\text{PO}}_{4} } \right)_{6} +\,13{\text{H}}_{2} {\text{O}} .$$

(10)

Zinc–calcium–phosphate coating

Transformation of Zn²⁺ ions into Zn(OH)₂ is highly pH dependant according to reaction (11), and this species is transformed into ZnO at elevated temperature via reaction (12). At pH 3, phosphoric acid decomposes into phosphoric ions, which reacts further to form hydrogen gas, magnesium phosphate, and zinc phosphate (13–15). Ca₃(PO₄)₂ is deposited before the deposition of Zn₃(PO₄)₂ via reaction (16). Due to the similar structure of Ca₃(PO₄)₂ and Zn₃(PO₄)₂, a doped CaZn₂(PO₄)₂ coating is deposited on the substrate (reaction 17).6

$${\text{Zn}}^{2 + } + 2{\text{H}}_{2} {\text{O}} \to {\text{Zn}}\left( {\text{OH}} \right)_{2} +\,2{\text{H}}^{ + }$$

(11)

$${\text{Zn}}\left( {\text{OH}} \right)_{2} \to {\text{ZnO}} + {\text{H}}_{2} {\text{O}}$$

(12)

$$3{\text{Mg}} + 2{\text{H}}^{ + } + 2{\text{H}}_{2} {\text{PO}}_{4}^{-} \rightleftharpoons {\text{Mg}}_{3} \left( {{\text{PO}}_{4} } \right)_{2} +\,3{\text{H}}_{2}$$

(13)

$$3{\text{Mg}} + 3{\text{Zn}}^{2 + } + 4{\text{H}}_{2} {\text{PO}}_{4}^{-} + 4{\text{H}}_{2} {\text{O}} \to {\text{Zn}}_{3} \left( {{\text{PO}}_{4} } \right)_{2} \cdot 4{\text{H}}_{2} {\text{O}} + 3{\text{H}}_{2} + {\text{Mg}}_{3} \left( {{\text{PO}}_{4} } \right)_{2} +\,2{\text{H}}^{ + }$$

(14)

$${\text{ZnO}} + 2{\text{H}}_{2} {\text{PO}}_{4}^{-} + 2{\text{H}}^{ + } \to {\text{Zn}}_{3} \left( {{\text{PO}}_{4} } \right)_{2} +\,3{\text{H}}_{2} {\text{O}}$$

(15)

$$3{\text{Ca}}^{2 + } + 2{\text{PO}}_{4}^{3 - } \to {\text{Ca}}_{3} \left( {{\text{PO}}_{4} } \right)_{2}$$

(16)

$${\text{Ca}}^{2 + } + 2{\text{Zn}}^{2 + } + 2{\text{H}}_{2} {\text{PO}}_{4}^{ - } + 2{\text{H}}_{2} {\text{O}} \to {\text{CaZn}}_{2} \left( {{\text{PO}}_{4} } \right) \cdot 4{\text{H}}_{2} {\text{O}} + 4{\text{H}}^{ + } .$$

(17)

Manganese phosphate coating

Deposition of manganese phosphate coating is accomplished by three stages including dissolution of metal, deposition of Mg(OH)₂ film, and finally the formation of (Mg/Mn)₃(PO₄)₂ coating. Right after immersion of the Mg substrate in the phosphating bath, reactions (18) and (19) take place, resulting in an increase in solution pH near the metal surface due to the formation of OH⁻ and Mg²⁺ ions. Extra Mg²⁺ and OH⁻ in comparison with Mn²⁺ and PO^3-₄ are deposited as an intermediate layer of Mg(OH)₂ on the metal surface (20). The formation of manganese phosphate species (reaction 21) depends on the bath temperature and pH.38

$${\text{MgO}} + 2{\text{H}}^{ + } \to {\text{Mg}}^{2 + } + {\text{H}}_{2} {\text{O}}$$

(18)

$${\text{Mg}} + 2{\text{H}}_{2} {\text{O}} \to {\text{Mg}}^{2 + } + 2{\text{OH}}^{-} + 2{\text{H}}_{2}$$

(19)

$${\text{Mg}}^{2 + } + 2{\text{OH}}^{-} \to {\text{Mg}}\left( {\text{OH}} \right)_{2}$$

(20)

$$4{\text{Mn}}^{2 + } + 3{\text{H}}_{2} {\text{PO}}_{4}^{ - } \to {\text{MnHPO}}_{4} + {\text{Mn}}_{3} \left( {{\text{PO}}_{4} } \right)_{2} +\,5{\text{H}}^{ + } .$$

(21)

Magnesium phosphate coating

Magnesium phosphate film deposition involves ionization of Mg and NH₄H₂PO₄ in water via reactions (1) and (22), respectively. Dissolution of Mg alloys produces hydrogen gas and increases the solution pH as the substrate/solution contact occurs; Mg²⁺ ions are formed with a varying concentration in solution near the substrate surface. This increase in Mg²⁺ concentration produces MgHPO₄·3H₂O and MgNH₄PO₄·6H₂O phases within a pH limit of 3–9.5 according to reactions (23) and (24).47

$${\text{NH}}_{4} {\text{H}}_{2} {\text{PO}}_{4} \to {\text{NH}}_{4}^{ + } + {\text{H}}_{2} {\text{PO}}_{4}^{ - }$$

(22)

$${\text{Mg}}^{2 + } + {\text{H}}_{2} {\text{PO}}_{4}^{ - } + 3{\text{H}}_{2} {\text{O}} \to {\text{MgHPO}}_{4} \cdot 3{\text{H}}_{2} {\text{O}} + {\text{H}}^{ + }$$

(23)

$${\text{Mg}}^{2 + } + {\text{H}}_{2} {\text{PO}}_{4}^{-} + {\text{NH}}_{4}^{ + } + 6{\text{H}}_{2} {\text{O}} \to {\text{MgNH}}_{4} {\text{PO}}_{4} \cdot 6{\text{H}}_{2} {\text{O}} + 2{\text{H}}^{ + } .$$

(24)

Types of phosphate chemical conversion coatings

PCC coatings are formed by transforming the surface of the substrate into a nonmetallic crystalline layer. The conversion reaction takes place in an acidic bath containing phosphate ions. Hydrogen evolution at the Mg substrate/solution interface increases the local pH of the bath and thus promotes the nucleation of phosphate crystals at the surface. PCC coatings have been categorized into several main types. These primary PCC coatings presented in Fig. 2 are discussed in the following sections.

Fig. 2

Major types of PCC coatings applied on Mg alloys

Zinc phosphate coatings

Zinc phosphate (Zn–P) conversion coating is a cheap, effective, and environmentally friendly coating system, commonly used as a surface treatment for improving adhesion of paint on carbon steels,48 stainless steels,49 and aluminum alloys.50 It is also used as a cheaper alternative to chromate conversion coatings on Mg alloys. Zn–P-coated Mg alloys exhibit improved corrosion resistance, bonding strength, and wear resistance.8,9,24 Hopeite (Zn₃(PO₄)₂·4H₂O) is the main constituent phase in the microstructure of Zn–P coatings.7,8,9,17,18,23,24 Two phases of hopeite: orthorhombic hopeite and para hopeite, have been identified in the Zn–P coatings. For orthodontic applications, these phases play an important role in Zn–P coatings as bone cement and as a joint material between the tooth root and crown.36,44,51,52 Various types of accelerating agents such as sodium meta-nitrobenzene sulfonate (SMBS),24 sodium chlorate (NaClO₃),17 and monoethanolamine (MEA)23 in the Zn–P bath are added to produce refined, homogenized, and denser coatings to improve the corrosion resistance of Mg alloys. Phuong et al.8 reported that the pH and Zn²⁺ concentration in the phosphating bath have a significant influence on the properties of Zn–P conversion coatings. According to Liang et al.1 immersion, time and temperature of phosphating bath are major factors in the production of homogeneous deposit of Zn–P conversion coating on Mg alloy. Phuong et al.9 investigated the effect of heat treatment on the microstructure of Zn–P coatings. It was found that in bare AZ91D alloy, heat treatment transformed the β-phase into the α-Mg phase and may catalyze hydrogen evolution during deposition. As a result, smaller β-phase fractions produced lower coating weight, larger crystal size, and better corrosion resistance as illustrated in Fig. 3.

Fig. 3

The cross-sectional image of Zn–P-coated AZ91 alloy; the immersion time was 10 min.9 Reprinted with permission from Elsevier™

Calcium phosphate coatings

Calcium phosphate (Ca–P) conversion coatings are extensively used for upgrading biocompatibility and biodegradability of Mg alloys currently used in orthopedic implants and other biomedical applications. Their attractive osteoconductive and biodegradable characteristics are the main reasons behind their popularity. Ca–P coatings are well suited for biomedical applications, as 70% of natural bone tissues are composed of Ca–P.10,13,22 Two constituent phases, hydroxyapatite (HA)22 or brushite,5,10 have been observed in the microstructure of Ca–P conversion coatings. HA has a hexagonal crystal structure53 and low solubility,54 while brushite has a monoclinic crystal structure and has a comparably higher solubility than other Ca–P coatings.55 Previous work demonstrated that increasing bath temperature significantly promoted nucleation, growth, coverage rate, crystallinity, and corrosion resistance of Ca–P coatings on the AZ60 Mg substrate.13 Additionally, doping of silicate in the Ca–P coatings considerably decreased the degradation rate and enhanced cell attachment.22 Xu et al.5 and Xiao et al.10 worked on the implantation of Ca–P-coated Mg alloys in rabbit femora as shown in Fig. 4. Cell culturing analysis was performed by using L929 cell lines in a PRMI 1640 media having the composition of 100 IU/mL penicillin, 100 mg/mL streptomycin sulfate, 2 mM glutamine, and 10% (vol/vol) fetal bovine serum. The experiment was performed in 48-well plates placed in a humidified incubator at 37°C and filled with air containing 5% CO₂.

Fig. 4

Mg–Mn–Zn alloy (left) and calcium phosphate-coated (right) rod implanted in rabbit femora5 reprinted with permission from Elsevier™

Zinc–calcium phosphate coatings

Zn²⁺ ion substitution in calcium phosphate solution during the deposition process encourages the formation of a zinc–calcium phosphate (Zn–Ca–P) coating. This coating has refined microstructure containing hopeite with trace amounts of Zn and Ca. Zn–Ca–P coatings, also known as scholzite, exhibit monoclinic crystal structure and stability greater than HA or hopeite.56 Zn–Ca–P coatings provide nucleating sites for hydroxyl carbonate apatite.57 The presence of Zn and Ca remarkably improves the crystallinity and corrosion resistance of Mg alloys.14,35 Corrosion resistance is also highly sensitive to the bath temperature and increases with an increase in bath temperature.6 Zeng et al.11 reported that the addition of cerium in the phosphating bath produced a more compact and highly corrosion resistant Zn–Ca–Ce–P coating compared to Zn–Ca–P coatings. Figure 5 exhibits the SEM images and EDS spectra of both coatings having Zn, P, Mg, F, and Ca elements in their composition. EDS spectra presented high fluoride concentration in the noncrystalline regions, i.e., point A (4.75%) and point C (19.73%) in Zn–Ca–Ce–P coating as shown in Fig. 5a. The fluoride content in Zn–Ca–P coating was found to be 20.16% as depicted in Fig. 5b. On the other hand, crystalline phases have negligible fluoride contents in both coatings. The high fluoride contents in the noncrystalline phases indicate the heterogeneous distribution of MgF₂, CeF₃, and CaF₂, while Zn and Ca elements in both coatings exist as Zn(NO₃)₂ and Ca(NO₃)₂, respectively. The percentages of Zn and P elements in the Zn–Ca–Ce–P coating are greater than that of Zn–Ca–P coating. The high concentration of Zn in the Zn–Ca–Ce–P coating has presented excellent crystallinity and greater volume fractions of hopeite in the coating.

Fig. 5

SEM images of (a) Zn–Ca–Ce–P and (b) Zn–Ca–P coatings (b) with corresponding EDS spectra of (c) Zn–Ca–Ce–P (point C) and (d) Zn–Ca–P coatings (point B)11 reprinted with permission from Elsevier™

Manganese phosphate coatings

Manganese phosphate (Mn–P) coatings are cost-effective and environmentally friendly and are commonly used for pistons51 and high-strength steel applications.52,58 Due to inferior thermal expansion properties, high porosity, and defective microstructures of Zn–P coatings, Mn–P coatings are becoming more popular as an alternative. Recently, Mn–P coatings have also been used for the surface improvement of Mg alloys. Chen et al.38 produced double-layered Mn–P coatings consisting of Mg(OH)₂ intermediate and (Mg/Mn)₃(PO₄)₂ top layers on AZ91D alloy. It was reported that a higher temperature accelerates coating growth and improves corrosion resistance with a negligible corroded area (> 1%) after 48-h salt spray test in 5 wt% NaCl solution. Cui et al.39 applied a three-stage mechanism to develop a double-layered Mn–P conversion coating on AZ31 Mg alloy. The first stage involved the dissolution of the loose oxide layer and the co-deposition of AlPO₄ in addition to Mg₃(PO₄)₂. In the second stage, the development of a composite intermediate layer containing Mg₃(PO₄)₂, AlPO₄, and Mg(OH)₂ is produced which promoted the formation of phosphate nuclei. This is followed by the growth and uniform coverage of the surface with phosphate-enriched coating in the third stage as shown in Fig. 6.

Fig. 6

SEM images of Mn–P coating produced in bath 2 (at 90°C and at pH 2.6; adjusted by the addition of 2 M NaOH and 1 g L⁻¹ MEA solution). The immersion time was varied from 1 to 60 min.52 Reprinted with permission from Elsevier™

Magnesium phosphate coatings

Magnesium phosphate (Mg–P) conversion coatings have primarily been utilized for the corrosion protection of steels.36,59,60,61 However, they can serve the same purpose when coated onto Mg alloys.40,47 The availability of Mg²⁺ ions from the substrate results in a faster growth rate of Mg–P coatings. The microstructure of Mg–P coatings contains farringtonite (Mg₃(PO₄)₂), berryite (MgHPO₄.3H₂O),62,63,64 and struvite (MgNH₄PO₄·6H₂O) phases.63,65,66,67,68 The uniformity and density of the microstructure and corrosion resistance of Mg–P coatings have been observed to be highly sensitive to the pH of the phosphating bath as shown in Fig. 7.40 Ishizaki et al.47 developed a single-phase Mg–P coating having an orthorhombic berryite phase on AZ31 alloy. It was reported that this Mg–P coating considerably improved the corrosion and scratch resistance of AZ31 alloy. Phuong et al.41 fabricated Mg–P coating on AZ31 alloy by immersion in a solution containing Mg²⁺ and PO₄³⁻ ions. It was reported that a coating of uniform thickness having microcracks was developed. Similarly, this Mg–P coating significantly improved the corrosion resistance and other surface properties of the AZ31 alloy. Ishizaki et al.42 also developed an anticorrosive composite coating having semicrystalline Mg–P and crystalline magnesium hydroxide compounds on Mg alloy. A combination of chemical conversion and steam curing processes was used for this purpose. In this case, a composite coating of 30 μm was developed exhibiting much-improved corrosion resistance compared to uncoated Mg alloy.

Fig. 7

Micrographs of Mg–P-coated AZ31 alloy produced in solutions of (a, b) pH 4.5, (c, d) pH 5.5, (e, f) pH 6.5, and (g, h) pH 7.540 reprinted with permission from Elsevier™

Dihydrogen manganese polyphosphate coatings

Dihydrogen manganese polyphosphate coatings are considered environmentally friendly and are produced by chemical phosphating in a bath free of toxic elements such as chromate, fluoride, and nitrate. It is a gray-black, dense, and crack-free coating having a thickness in the range of 12–15 µm containing the MnHPO₄·2.25H₂O phase in the microstructure.43 Xue-jun et al.43 rigorously studied the effect of bath temperature and free acid concentration on the properties of dihydrogen manganese polyphosphate coatings. Both of these parameters were found to have a great impact on the compactness and corrosion resistance properties of this coating. Although increasing bath temperature improved the density and corrosion resistance of the coating, it adds residue/undissolved particles in the phosphating solution which can be controlled by the addition of free acids. The limited anodic dissolution and restricted hydrogen evolution on this coating resulted in the increase in overall corrosion resistance of AZ31 Mg alloy. SEM micrographs and the corresponding EDS spectrum of these coatings are illustrated in Fig. 8. Gray-black, dense, crack-free coating was produced having large crystallite within the coatings of a few microns thick (Figs. 8a, 8b). The formation of the surface coating and its bonding with the substrate can be observed in the cross-sectional image in Fig. 8c. This coating was mainly composed of O, P, and Mn elements, while Mg and Al peaks originated from the AZ31 alloy substrate (Fig. 8d). The absence of toxic Cr, F, and N elements in the EDS spectrum validated the formation of environmentally friendly characteristics of phosphate coatings.

Fig. 8

Dihydrogen manganese polyphosphate coating produced on AZ31 alloy. (a, b) morphology, (c) cross-section of the metal/coating interface, (d) EDS line spectra showing the distribution of elements as given in (c)43 (reprinted with permission from Elsevier™)

Molybdate phosphate composite conversion coatings

To enhance the corrosion resistance of ferrous and zinc alloys, molybdate phosphate (Mo–P) conversion coatings are in common use. These coatings can also be produced on the Mg alloys to lower their dissolution tendency in the biological environment. Yong et al.46 found that molybdate phosphate coating was a composite of metaphosphate and oxide of molybdate phases having an alveolate crystal structure. This coating exhibited much better corrosion resistance than molybdate and chromate conversion coatings as reported in this study.

Characterization of PCC coatings

Microstructure analysis by SEM

The scanning electron imaging is the most important and widely used tool to examine the microstructural features, i.e., grain structure, orientation, morphology including microstructural defects, microcracks, and pores in addition to chemical composition. Figure 9 exhibits the surface morphology of Zn–P-coated Mg alloy under various phosphating times ranging from 1.5 to 180 s. Figure 9a demonstrates several small-size (0.2–0.3 μm) crystal nuclei nucleated on the surface of the substrate at 1.5 s of phosphating. Such a large number of crystal nuclei (2.1 × 10⁵ crystal nucleus-cm⁻²) validated their uniform distribution of crystal nuclei on both α- and β-phases (Mg₁₇Al₁₂) of the substrate. The growth processes of Zn–P coating at 5, 10, and 30 s of phosphating are illustrated in Figs. 9b–9d. For 5–30 s of phosphating, some particles grew very rapidly to develop large-size (5–10 μm) white flower-shaped hopeite phase. On the other hand, 60 s of phosphating produced slab-shaped phosphate crystals (Fig. 9e), while 180 s of phosphating resulted in a surface fully covered with slab-shaped phosphate crystals (Fig. 9f).21 Increasing phosphating time and Zn²⁺ concentration caused gradual growth of a small crystal nucleus into flakes and subsequently into a flower-shaped structure with the increase in phosphating time and Zn²⁺ concentration.8,24 The addition of sodium meta-nitrobenzene sulfonate (SMBS) in the phosphating bath significantly eliminated microcracks, reduced impurity particles, and refined the morphology of the coating.24 It was also found that SMBS decreased the coating compactness, reduced the microdefects and porosity, and enhanced the surface roughness.23 In addition, a higher concentration of Zn²⁺ improved the coverage rate of Zn–P coatings.8

Fig. 9

SEM micrographs of the Zn–P coating on the AZ91D Mg alloy obtained after (a) 1.5 s, (b) 5 s, (c) 10 s, (d) 30 s, (e) 60 s, and (f) 180 s of phosphating time17 reprinted with permission from Elsevier™

Ca–P coatings were observed to have a porous and net-shaped structure with microcracks.5 Su et al.13 investigated the effect of bath temperature on the morphology and elemental composition of Ca–P coating as illustrated in Fig. 10. At 15°C, a relatively slow hydrogen evolution reaction results in a moderate coating coverage rate and low thickness was observed (Fig. 10a). Increasing bath temperature to 37°C increased the coating nucleation and growth rate and produced denser coating with a high coverage rate (Fig. 10b). Further increase in bath temperature to 60°C causes vigorous hydrogen evolution reaction, producing bubbles which impede the nucleation of the coating. At this temperature, large-size coating flakes were obtained indicating sufficient space for crystals to grow (Fig. 10c). Therefore, few crystals of phosphate were nucleated at 60°C as evidenced by reduced Ca and P contents (Fig. 10d). Small-size needle-shaped flakes were replaced by large-size flakes exhibiting excellent crystallinity as evidenced by greater Ca–P ratio (Fig. 10d).

Fig. 10

Surface morphologies of Ca-P conversion coatings (a–c) and (d) the elemental composition of Ca and P obtained from EDS analysis. The coatings were produced at different bath temperatures (a) 15°C, (b) 37°C, and (c) 60°C (pH 2.8; time 20 min)13 reprinted with permission from Elsevier™

Additionally, 40 min of phosphating lowered the coverage rate and compactness, while slightly increasing the length of the flake. It has been reported that Zn–Ca–P coatings also exhibited a flower-shaped structure that is highly sensitive to bath temperature.6,14 Zeng et al.6 reported that with the increase in bath temperature from 40 to 60°C, microstructural variations such as homogeneous nucleation and growth of the coating resulted in the formation of heterogeneously distributed less dense flower-shaped crystallites having cracks in the microstructure. The low temperature also improved the growth and homogeneous distribution of hopeite with higher coverage rate and compactness containing minute pores and cracks. As a result of its morphology, the addition of Ce in Zn−Ca–Ce−P also played a major role in deposition and improvement in the coating properties. SEM micrographs of Ce-doped Zn−Ca–Ce−P and Zn−Ca−P coatings are illustrated in Fig. 11. Figure 11a exhibits crystalline structure having long ridges of thickness 1–2 μm and short flake-shaped crystalline precipitates. The cross-sectional images of Zn−Ca–Ce−P coating validated the presence of double-layered coating. This double layer consists of an outer layer having ridge-shaped crystalline structure and an inner layer having a compact structure with microcracks. No such cracks were observed within the Zn−Ca–Ce−P coating (Fig. 11b). On the other hand, the microstructure of Zn–Ca–P coating comprised of refined flower-shaped structure and the existing cracks within the flower-shaped structure (Fig. 11c) cause relatively low compactness and adhesion than that of Zn−Ca–Ce−P coating as shown in (Fig. 11d).11

Fig. 11

SEM micrographs of (a, b) Zn–Ca–Ce–P coating and (c, d) Zn–Ca–P coating11 reprinted with permission from Elsevier™

Phase analysis by XRD

X-ray diffraction (XRD) is a powerful method of analyzing the crystal structure, phases, texture, average grain size, crystallinity, strain, and crystal defects in the crystalline materials. Diffraction peaks produced by constructive inference between monochromatic X-ray beams and the solid sample provide a pattern of periodic atomic arrangements in a given material.5

The XRD pattern of Ca–P coatings revealed that specific diffraction peaks associated with the (020) habit plane of brushite phase along with Mg whitlockite (MWH, Ca₉Mg(HPO₄)(PO₄)₆) at 10.9°, 13.9°, and 17.1° and Mg hydrogen phosphate trihydrate (MHPT, MgHPO₄·3H₂O) at 17.5^o were present as shown in Fig. 12.5,13

Fig. 12

XRD patterns of (I) bare Mg alloy and (II) coated Mg alloy immersed in a chemical conversion bath for 20 min, at 37°C having pH 2.813 reprinted with permission from Elsevier™

XRD analyses of Zn–P coatings produced in a phosphating bath containing different concentrations of sodium meta-nitrobenzene sulfonate at 40–45°C temperature and 2.2–2.4 pH are shown in Fig. 13. The coating shows that it is mainly comprised of the hopeite phase growing along the (011) habit plane. The addition of sodium meta-nitrobenzene sulfonate (SMBS) in the phosphating bath altered the growth orientation to (002) and reduced the diffraction intensity of the Zn-enriched phase.24

Fig. 13

XRD patterns of Zn–P coatings, produced in phosphating bath having various concentrations of sodium meta-nitrobenzene sulfonate; (a) 0 g L⁻¹, (b) 2.0 g L⁻¹, (c) 4.0 g L⁻¹, (d) 8.0 g L⁻¹24 reprinted with permission from Elsevier™

The intensity of the (011) plane of hopeite increased approximately linearly, overtaking the intensity of the (241) plane.23 Heat treatment of the substrate also significantly influenced the phases of the Zn–P coating. β-Mg₁₇Al₁₂ phase present in bare AZ91 alloy decreased with the increase in heat treatment time and finally disappeared by dissolving into the α-phase.9 Phuong et al.8 reported that XRD peaks exhibited the presence of hopeite in the coatings together with the α-Mg of the substrate. Various diffraction peaks associated with (311) (011) hopeite planes are evident in addition to the diffraction peak of the α-Mg indexing (101) plane. With decreasing Zn²⁺ concentration, the average diffraction intensity of the (311) and (010) hopeite planes was decreased, whereas the dominant diffraction peak indicating the existence of α-Mg appeared.

XRD pattern of Mn–P coating showed the presence of inner Mg(OH)₂ and outer (Mg/Mn)₃(PO₄)₂ phases in Mn–P-coated AZ91D alloy.38 Jayaraj et al.40 reported that major peaks in the XRD pattern of uncoated AZ31 alloy were attributed to the α-Mg phase, which decreased with the application of Mg–P coating specifically at pH 4.5. Along with these sharp α-Mg diffraction peaks, the small diffracted peaks associated with struvite phase (MgNH₄PO₄·6H₂O) and berryite phase (MgHPO₄·3H₂O) were also observed. On the other hand, dihydrogen phosphate manganese coating exhibited only a single peak of the MnHPO₄·2.25H₂O phase.43 Yong et al.46 observed both α-phase (Mg) and β-phase (Mg₁₇Al₁₂) in both bare and coated Mg alloys. It was found that Mo coatings were amorphous, while Mo/P conversion coating had a crystalline structure. The Mo–P coating is comprised of complex phases such as Me_x(PO₄)_y, CaMoO₄, MgAl₂O₄, MgO, Al₂O₃, MnO, and net-shaped oxide MoO₃. The different phases identified by XRD of PCC coatings on Mg alloys are reported in Table 2.

Table 2 Dominant phases that could form in the microstructures of various types of PCC coatings applied on Mg alloys as reported in the literature

Elemental analysis of PCC

Energy-dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS) are widely accepted techniques to determine the surface composition of the coated samples both qualitatively and quantitatively.33

Through EDS analysis, it was revealed that the top layer of Zn–P coating contained Zn, P, and O elements, indicating the existence of hopeite phase, while the inner layer contained Mg, O, P, and Al elements, suggesting the possibility of MgHPO₄.3H₂O, Mg₃(PO₄)₂, AlPO₄, Al₂O₃, Al(OH)₃, MgO, and/or Mg(OH)₂ phase formation.8 Additionally, it was found that Ca–P coatings are comprised of O, P, Ca, and Mg elements with minute fractions of Zn in varying percentages. The XPS spectrum of Mg–Zn coating presented the existence of oxidized Zn²⁺ and Mg²⁺ species on the surface.5 Fractions of Ca and P contents in Ca–P coatings were observed to be highly sensitive to phosphating temperature and showed a large fluctuation at different bath temperatures. In the case of Ca–P coating, a peak was found at 347.0 eV binding energy corresponding to Mg whitlockite (MWH, Ca₉Mg(HPO₄)(PO₄)₆ which has a similar whitlockite structure with β-TCP.13 Zeng et al.14 compared Zn–P and Zn–Ca–P coatings and detected signatures of Mg 1s, O 1s, P 2p_3/2, and Zn 2p_3/2 elements in the Zn–Ca–P coatings as shown in Fig. 14. The splitting of the resolved peaks associated with binding energy values suggested the formation of various compounds on the surface. For instance, the doublet peaks at 131.8 eV and 132.7 eV are associated with PO₄³⁻ and HPO₄²⁻ species (Fig. 14a). Similarly, as shown in Fig. 14b, the formation of Zn₃(PO₄)₂·4H₂O and ZnO species was elucidated from the splitting Zn2p_3/2 and Zn2p_1/2 peaks, respectively.

Fig. 14

The high resoultion XPS spectra of Zn–Ca–P-coated Mg alloy, (a) P 2p_3/2, (b) Zn 2p, (c) Mg 1s, and (d) O 1s14 reprinted with permission from Elsevier™

It has also been reported that both Zn–Ca–P and Zn–Ca–Ce–P coatings contained O, Zn, P, Mg, F, Al, and Ca elements. Mg contents in Zn–Ca–Ce–P coatings were greater than in Zn–Ca–P, while greater contents of F indicated the presence of MgF₂, CeF₃, and CaF₂ phases.11 XPS verified the existence of Mg, O, P, and Mn at the surface of Mn–P coatings. It was discovered that O atomic concentration reduced from 62.8 to 7.8% and the Mg content increased from 26.9 to 61.9% along with the coating depth. Mn and P concentrations detected in the outer layer decreased gradually from 3.4 and 6.6 to 0.9 and 2.4%, respectively. High-resolution scanning revealed the presence of inner Mg(OH)₂ and outer (Mg/Mn)₃(PO₄)₂ layers in double-layered Mn–P coatings on AZ91D alloy.38 Ye et al.45 found that sodium dihydrogen phosphate (SDHP) coating mainly consisted Mg, P, C, and O elements. The detection of phosphorous on the surface confirmed the successful deposition of the coating. Figure 15 illustrates the XPS spectra of Mo and Mo–P conversion coatings 1/2 MoO₄²⁻/H₂PO₄^– molar ratio. Major elements, i.e., Ca, O, N, Al, Mo, and Mg, were detected in the Mo coating, while two major elements Mn and P were detected in Mo–P coating. Significant reduction in the intensity of Mg, Mo, Ca, and N peaks was found in Mo coating compared to the considerable increase in the P, Mn, and O peaks in Mo–P. XPS also confirmed that metaphosphate played a very important role in the conversion mechanism of coating deposition.46

Fig. 15

XPS survey spectra of (a) Mo coating and (b) Mo–P coatings produced in the solution containing 1/2 molar ratio of MoO₄^2–/H₂PO₄^– species after immersion and pretreatment for 5 min46 reprinted with permission from Elsevier™

Validation of deposition by Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy (FTIR) has been used to verify the deposition of PCC coatings. An FTIR spectrometer simultaneously collects high spectral resolution data over a wide range of wavelengths. The spectrum obtained by FTIR represents molecular absorption and transmission, creating a molecular fingerprint of the sample and thus verifying the deposition of PCC coatings.26 Singh et al.22 utilized this technique for the verification of Ca–P coating deposition and observed the effect of silicate ions on the Ca and P bonding over the coated AZ31 and Mg–4Y alloys in the phosphating solutions. It was reported that bending bands of hydroxyl corresponding to hydroxyapatite phase were evident in the IR spectra of allocated samples, suggesting the formation of tricalcium phosphate (β-TCP) or a mixture of HA and β-TCP phases in the microstructure. Peaks in the range of 570 and 605 cm⁻¹ are due to the bending of O–P–O bonds, and the prominent band in the range of 960–1088 cm⁻¹ was attributed to the P–O stretching vibration as illustrated in Fig. 16.

Fig. 16

Fourier transform infrared spectra of different phosphating bath (PB) (iv) β-TCP-, (iii) PB1, (ii) PB2, and (i) PB3 coated samples of (a) AZ31 and (b) Mg–4Y. FTIR spectra of (iv) uncoated, (iii) PB1, (ii) PB2, and (i) PB3 samples after incubation for 1 week in Dulbecco’s Modified Eagles’ Medium on (c) AZ31 and (d) Mg–4Y22 reprinted with permission from Elsevier™

Electrochemical analysis

One of the major characteristics of phosphate conversion coatings is their corrosion resistance.31 The corrosion resistance of PCC has been verified using a wide range of techniques such as open-circuit potential,13,38 Tafel scan,1 potentiodynamic polarization,70,71 electrochemical impedance spectroscopy,37,38 immersion test,40 weight loss test,10 and hydrogen evolution test.6,11,35 The corrosion behavior was examined in various aqueous environments, i.e., 3.5% NaCl,1,14 5% NaCl,17 borate buffer,10,23 0.5 M NaCl solution, simulated body fluid,13 and Hank’s solution.6 Specifically, EIS can provide instantaneous data on the impedance of a surface subjected to polarization, which is directly proportional to the corrosion resistance.38,55 By the application of Zn–P coating on an AZ91D alloy, corrosion potential shifted 350 mV to a positive potential with a significant decrease in corrosion current density (0.02 mA cm⁻²) compared to the bare alloy which indicated the improved corrosion resistance after coating.17 The Zn–P coating offered sufficient corrosion protection that can be further improved with the addition of MEA in the bath solution during the PCC deposition process. In particular, 1.2 g L⁻¹ concentration of MEA in the phosphating bath provided maximum corrosion protection.23 The corrosion protection efficiency of Zn–P coatings was observed to be independent of Zn²⁺ concentration in the bath solution.8 Ca–P coatings exhibited considerable corrosion resistance and survive for a relatively long period when implanted in the rabbit femora.10 Su et al.13 found that the temperature and pH of the phosphating bath have a significant influence on the porosity in the microstructure and precise control of these parameters could significantly improve the corrosion protection of AZ60 alloy. Zn–Ca–P coatings produced using a 60°C bath temperature provided a minimal corrosion current density (5.7 μA cm⁻²) when compared to a bare substrate (27 μA cm⁻²). The decrease in the corrosion current density is in good agreement with hydrogen evolution rates that may appreciably affect the coating uniformity.6,14 Comparison of Zn–Ca–P and Zn–Ca–Ce–P coating showed that the addition of cerium caused uniform segregation of calcium and improved the compactness of Zn–Ca–Ce–P coatings, which resulted in comparatively improved corrosion resistance when compared to the former coatings.11 Although the corrosion protection efficiency of Mg–P coating is significant, it can still be further improved by increasing the pH of the phosphating bath.40 Figure 17a shows potentiodynamic polarization scans which indicate that corrosion current density of double-layered manganese phosphate (DLMP) coating decreased significantly compared to bare AZ91D. The corrosion current of the bared sample was 18 μA cm⁻², whereas the DLMP coating developed at room temperature (pH 4) was 4.5 μA cm⁻², indicating four times lower corrosion current presented by DLMP coating. The corrosion current further decreased from 4.1 to 1.1 μA cm⁻² as the temperature of the bath was increased from 40 to 80°C, respectively. Chen et al.38 explained that it is due to the rapid formation of intermediate Mg(OH)₂ film and top (Mg/Mn)₃(PO₄)₂ film in Mn–P coating developed at pH 4 and bath temperature of 80°C, which provided maximum corrosion resistance to AZ91D. It was further explained that at a constant temperature of 80°C, the increase in bath pH was from 2 to 6 and no significant change in the corrosion current was observed (changed from 11.5 to 11.9 μA cm⁻²) as shown in Fig. 17b. Figures 17c and 17d show the EIS spectra that supported the potentiodynamic polarization results. The coating resistance (R_c) and charge transfer resistance (R_ct) were calculated by fitting the experimental impedance spectra by simulating with an equivalent electrical circuit model as shown in Fig. 17f. Particularly, the R_c increased from 2.95 to 6.11 kohm cm² as the bath temperature was increased from 40 to 80°C at pH 4. On the other hand, at near-neutral conditions (pH 6), the R_c decreased to 1.79 kohm cm² that was developed in acidic solution (pH 2). The 3D plot Fig. 17e shows the relationship between temperature, pH, and corrosion current of DLMP coating developed at pH 4 and 80°C under optimized bath composition.

Fig. 17

(a, b) Potentiodynamic polarization plots of AZ91D in 0.1 M NaCl, (c, d) Nyquist plots, (e) 3D plot of i_corr vs. temperature and pH, and (f) the equivalent electrical circuit model used for the simulation of impedance spectra38 reprinted with permission from Elsevier™

Ye et al.45 studied the rate of hydrogen evolution on bare and on the sodium dihydrogen phosphate (SDHP)-coated WE43 alloy. It was found that the hydrogen evolution rate was considerably higher on the bare sample compared to the SDHP-coated sample. Finally, the corrosion potential of a bare alloy shifted from − 1.58 to − 1.42 V after the application of phosphate coating. Also, the current density of WE43 was reduced from 1.583 × 10⁻⁶ to 1.125 × 10⁻⁴ A cm⁻², respectively, after SDHP coating.

The thickness and weight gain measurement

The thickness and weight of PCC coatings play an important role in the performance of phosphatized Mg alloys. The thickness of the PCC coating can be measured by observing the cross section through SEM.14,22,40 Typically, PCC coatings having a thickness in the range of 2–48 μm are deposited on Mg alloys.14,17,22,38,40 It has been reported that an increasing concentration of Zn²⁺ ion in the phosphating bath increases the coating thickness. However, at extremely high concentrations of Zn²⁺ ions, the pH of the phosphating bath decreases and reduces the coating thickness. This pH effect on the coating thickness is attributed to the accelerated etching rate of the substrate.8 The pH of the bath is a key factor in determining the coating thickness, weight, and other properties. Jayaraj et al.40 found that a pH of 4.5 caused deposition of thicker Mg–P coatings on AZ31 alloys (48 μm). A further increase in pH to 5.5, 6.5, and 7.5 caused a reduction in the coating thickness. Chen et al.38 developed a thin (2.8 μm) Mn–P coating at pH 2 on AZ91 alloy that is attributed to the development of structural defects in the coating due to significant hydrogen evolution. The effect of heat treatment on the weight of Zn–P coating was studied by Phuong et al.9 They reported that an increase in the heat treatment time from 0 to 4 h could significantly increase the weight of the coating. The reduction in the coating weight was observed when heat treated for a longer period (> 4 h) and become constant at 24 h of heat treatment. Li et al.17 observed that the initial 30 s of zinc phosphating on AZ91D alloy caused the fastest coating growth and the highest gain in the coating weight (9.8 gm⁻²), while a further increase in immersion time does not affect the growth rate and coating weight due to the surface coverage by a barrier film.

Biocompatibility and bioactivity analysis

Biocompatibility refers to the material’s compatibility with living tissues or systems without inducing any toxic, physiologically reactive, or injurious effects. An appropriate biocompatible material does not cause immunological rejection. The bioactivity generally describes the response of living tissues when they interact with the foreign body e.g. metallic implant or a drug. These effects could either be beneficial or adverse, depending on the immune system response of any human body towards the foreign matter.34 It has been reported that Ca–P coatings efficiently protect Mg alloys by restricting the interaction of body fluids.72 Ca–P coatings contain Ca(H₂PO₄)₂ phase which hydrolyzes into brushite. This brushite then nucleates onto the surface of the Mg alloy and improves its biocompatibility. Micro-CT imaging, a noninvasive small animal imaging technology, has rapidly advanced over the past decades.25 The superiority of this method in evaluating biocompatible materials implanted into bone enables 3D reconstruction. Micro-CT imaging of the Ca–P coated on Mg alloy revealed that 3D reconstruction on these coatings exhibited much-improved biocompatibility.10

Biocompatibility was estimated by using MC3T3-E1 murine osteoblasts, and attachment of cells with bare and Ca–P-coated AZ31 and Mg–4Y alloys was investigated. Ca–P-coated AZ31 alloy exhibited the best cell attachment in the osteogenic environment after 3 days, while uncoated AZ31 alloy undergoes fewer cell attachments during the same period as illustrated in Fig. 18. On the other hand, after 3 days, bare Mg–4Y alloy showed the best cell attachment in the osteogenic environment, while coated Mg–4Y alloy had fewer cell attachments. Thus, uncoated Mg–4Y alloy was found to be more compatible than bare AZ31 alloy under similar conditions.22

Fig. 18

Images of (a) bare AZ31 and AZ31 immersed in Phosphating bath (PB) (b) PB1, (c) PB2, and (d) PB3 as well as uncoated (e) Mg–4Y and Mg–4Y immersed in (f) PB1, (g) PB2, and (h) PB3 after 3 days of culture are also shown to compare the surface activity toward cell proliferation22 reprinted with permission from Elsevier™

The pathological examination of the tissues surrounding the Ca–P-coated Mg alloy implant was conducted by Xu et al.5 who observed zero lymphocytic infiltration in the first week followed by reconstruction of bone matrix and thinner connective tissues in the second week. In the third and fourth weeks, the growth of interconnected bone trabecular and bone matrix was developed in addition to the formation of new bone as illustrated in Fig. 19. They concluded that Ca–P-coated Mg alloy offered much better biocompatibility than the bare alloy. Ye et al.45 reported that all the phosphatized samples of WE43 alloys exhibited 80% cell viability (the ability of a cell to survive and function properly). This can be attributed to the excellent biocompatibility of phytic acid and fluorine. However, the uncoated alloys exhibit 30% cell viability, showing the superior properties of PCC coatings on WE43 alloys. The hemolysis rate of the coated WE43 alloy was found to be reduced from 9.27% to approximately 3.57%, highlighting the significant improvement in the blood compatibility of the coated WE43 Mg alloy.68,73

Fig. 19

Images of the implant-bone interface, transformation growth factor beta 1, after 1, 2, 3, and 4 weeks of implantation5 reprinted with permission from Elsevier™

Wang et al.74 investigated the degradation behavior and bone response of Mg–Zn–Ca Mg alloy coated with calcium-deficient hydroxyapatite (Cadef HA). They reported that in vivo valid life of Cadef HA-coated substrate in rabbit femora is almost 8 weeks. After this period, degradation of coating increases, which is attributed to its reaction with body fluid and substitutional Mg²⁺ ions in Cadef HA. Cadef HA coating also has excellent osteoconductivity and promotes new bone formation on the substrate surface. Zou et al.74 investigated the in vitro blood biocompatibility of Zn–Ca–P-coated Mg–1.33Li–0.6Ca alloys and reported that Zn–Ca–P-coated Mg–1.33Li–0.6Ca alloy has tremendous blood compatibility, which is according to the biomedical requirements.75 Kavitha et al.34 worked on corrosion resistance and bioactivity of strontium phosphate (Sr–P) coating and reported that Sr–P-coated Mg alloy exhibited tremendous bioactivity in simulated body fluid immersed for 240 h due to the formation of apatite phase in the coating.

Conclusions and outlook

This review paper outlines the application of phosphate chemical conversion coatings on Mg and its alloys. The chemical reactions involved during the chemical conversion process and effect of operational parameters are rigorously discussed. The microstructural features, surface morphology, phase identification, elemental composition, corrosion behavior, biocompatibility, and bioactivity analyses of the coated Mg alloys via PCC processes have also been reviewed and discussed in detail. The PCC coatings have been utilized as a protective and bioactive coatings for Mg alloys for many years. But in the last two decades, the efforts have been more devoted to the development of new PCC coatings than in the improvement of existing processes to enhance the performance characteristics of the conventional PCC coatings in the biological environment. The rigorous research in the process optimization and on the development of highly biocompatible and corrosion resistant PCC coatings for Mg alloys is still challenging and requires more attention. Many studies describe the application of PCC coatings on pure Mg and its alloys, i.e., AZ91D, AZ91, AZ31, AZ60, AM60, AM30, WE43, Mg–Li–Ca, Mg–Mn–Zn, and Mg–4Y. However, the process parameters and PCC coating application procedures for Mg alloys manufactured via powder metallurgy and/or selective laser melting have not been given much attention. For instance, the improvement in the adhesion of the coatings on Mg alloys is still a challenge and needs further research. In many studies the PCC coatings have been developed by the addition of various environmentally friendly additives such as rare earth salts. For example, cerium nitrate and lanthanum nitrate that can be beneficial for further enhance the biocompatibility and electrochemical properties of the PCC coated Mg and its alloys. Mostly, the PCC coatings have been deposited in a bath having a pH between 2 and 9.3 and at temperature ranging from 15 to 95°C. The effective immersion time for the formation of PCC coating (thickness in the range of 2–48 μm) was found to be within 0.03–4320 min. Due to the remarkable properties of the PCC-coated Mg alloys, these have received a large attention in the wide range of applications. But only a few PCC coatings are being used for biomedical applications, which still demand more precise research at both micro and nanoscale levels. The deposition of biocompatible PCC coatings and their scaffolding properties for efficient bone growth is another area of interest and can be studied. At last, based on the information given in the literature, the process optimization and development of novel multilayered PCC coating processes are proposed. It is expected that these processes could significantly address the existing challenges with the Mg and Mg alloys i.e. low corrosion resistance, rigorous hydrogen evolution in biological media and poor biocompatibility issues.