Met. Mater. Int., Vol. 19, No. 2 (2013), pp. 273~281 doi: 10.1007/s12540-013-2023-0

Zinc Phosphate Conversion Coatings on Magnesium Alloys: A Review 1,2

1,2

1

1

Nguyen Van Phuong , Kyuhwan Lee , Doyon Chang , Man Kim , 1 1,* Sangyeoul Lee , and Sungmo Moon 1

Materials Processing Division, Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 642-831, Korea 2 University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 305-350, Korea (received date: 22 December 2011 / accepted date: 29 March 2012) Phosphating is one of the most widely used surface treatments of steels and aluminum due to its low-cost, easy mass production, good corrosion resistance and good adhesion with paint. Many researchers have tried to expand applications of the phosphating process, especially to magnesium alloys for automobiles and aerospace applications. Recently, the coatings on magnesium alloys by zinc phosphate conversion coatings (Zn3(PO4)2.4H2O) have been intensively studied. This paper reviews the state-of-the-art of phosphate conversion coatings developed for magnesium alloys, in terms of coating properties, phosphate conversion coatings processes and compositions of phosphating bath. Keywords: metals, coating, corrosion

1. INTRODUCTION Magnesium is an alkaline earth metal and the eighth most abundant element on the earth, making up approximately 1.93 wt% of the earth’s crust and 0.13 wt% in the oceans [1]. Mg possesses several advantages including a high strength to weight ratio and is one of the lightest metals among struc−3 tural metals, with a density of 1.74 g/cm , which is only two-thirds of aluminum and one-fourth of iron. Additional properties that contribute to magnesium’s versatility in the automotive, electronic, and aerospace industries include a high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping characteristics, good machinability and easy recyclability [2]. These properties make magnesium alloys applicable to the automobile, computer parts, aerospace components, mobile phones, sporting goods, handheld tools and household equipment [1]. The use of Mg alloys in the automotive industry can significantly decrease the weight of automobiles, by which fuel consumption is decreased, without sacrificing structural strength. However, unfortunately, magnesium is a very reactive element, prone to a number of undesirable properties, including poor corrosion and wear resistance, poor creep resistance, and high chemical reactivity, that has hindered its widespread use in many applications. In air and humid air, the surface of magnesium and its alloys form *Corresponding author: [email protected] ©KIM and Springer, Published 10 March 2013

oxide, carbonate or/and hydroxide layers, which are easily destructible. Attempts have been made to improve the corrosion resistance of the Mg alloys by chemical treatment, anodizing, plating, metal coatings and organic coatings. Among them, one of the most effective and cheapest ways to prevent corrosion is by chemical conversion coatings of the base metals. Chemical conversion coatings can protect a substrate by providing a barrier between the metal and its environment through the corrosion inhibiting chemicals they contain [3]. This paper reviews the state-of-the-art of phosphate conversion coatings on magnesium alloys, which have been developed for industrial applications.

2. CHEMICAL CONVERSION COATINGS Conversion coatings are produced by chemical or electrochemical treatment of a metal surface to produce a protective film, which is less reactive in aggressive environments than the original metal surface. There are a number of different types of conversion coatings, including chromate conversion coatings, phosphate-permanganate conversion coatings, and fluorozirconate conversion coatings. The conventional conversion coatings are based on treatment solutions containing chromium compounds that have been shown to be highly toxic and carcinogens [4]. Phosphate conversion coatings have been regarded as one of the suitable alternatives to chromate conversion coatings due to their low toxicity and appropriate properties [5-6]. Phosphate conversion coatings on steels are classified into five types: zinc system, zinc cal-

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cium system, manganese system, manganese iron system and iron system coating [7-10]. However, not all of these coatings are suitable on Mg alloys. Till now, zinc, calcium, manganese, barium and molybdenum [3,11-13] phosphate conversion coatings have been reported to be applicable to Mg alloys. In aluminum alloys, zinc phosphate conversion coatings have been successfully used as a primer coating for automotive industries for many years because they enhance good adhesion of the paint and substrates [14-15]. For Mg alloys, zinc phosphate conversion coatings are being explored as an alternative to conventional chromate conversion coatings. Thus, recently zinc phosphate conversion coatings on magnesium alloys by zinc phosphate (Zn3(PO4)2.4H2O) have been intensively studied.

3. ZINC PHOSPHATE CONVERSION COATINGS 3.1 Chemical compositions of commercial Mg alloys In the natural environment, pure magnesium is highly reactive and corrodes rapidly even in humid air. Commercial Mg alloys include many alloying elements, such as Al, Sn, Mn, Zn, Zr and rare earth metals, that have been added to decrease chemical reactivity [16]. The chemical compositions of typical Mg alloys are listed in Table 1. Aluminum is the most commonly used alloying element and forms the basis of commercial Mg alloys. The addition of aluminum to

magnesium improves strength and corrosion resistance, reduces the melting point and increases the melting range [19]. The maximum solubility of aluminum in magnesium is 12.7 wt% under equilibrium conditions where α-Mg-rich and β-Mg17Al12 phases can be precipitated. Zinc is also one of the alloying elements commonly added in conjunction with Al. 3.2. Chemical compositions of zinc phosphate conversion coatings bath Zinc phosphating solutions are based on dilute phosphoric acid, and include accelerating agents, activating agents, coating agents, organic acids additive, special additives and ions for co-deposition. Several zinc phosphating recipes are listed in Table 2. The role of each component of the zinc phosphating solution is described below. 3.2.1. Phosphoric acid In the phosphating solution, phosphoric acid is one of the agents to control the free acid value (FA) and total acid value (TA), which refer to the free [H+] and total phosphate concentration present in the phosphating solution, respectively. FA, TA and their ratio (FA:TA) should be controlled to improve the coating quality [30]. FA affects the etching rate of the metals and hydrogen evolution rate, and normally pH is controlled to be around 3 [20-29]. During the phosphating process, the hydrogen evolution causes an increase of pH on

Table 1. Chemical compositions of typical Mg alloys [17,18] Mg Alloy AZ31 AZ31B AE44 AM50 AM60 AXJ530 AZ61 MRI153M AZ91 AZ91D

Al 2.8 3.0 4.3 5.0 6.0 5.0 6.0 8.0 9.0 9.0

Zn 0.9 1.0

Ca

Sr

Rare Earth

Zr

Mn 0.2 0.2

<0.04 1.0

<1.0 0.28 1.0 1.0

3.0

<1.0

1.0

<1.0

<1.0 0.13

0.68 1.0

Mg Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance

Table 2. Chemical compositions of some zinc phosphating baths NaNO2/ NaNO3 3/1.84 4.9

SI.No.

Unit

H3PO4

1 2

g/L g/L

12.43 27.2

3

mol/L

0.065

4

g/L

17.5

3.2

0/2.5

1.7

5

g/L

3.2

0-0.8

1.7

6

g/L

4.0/0

2.0

7

g/L

16.8 Adjusted pH 11.3

Na2HPO4 Zn(NO3)2 20.0

ZnO

5.0 6.8 0.102

10.0

0.029

6.0 12.5

2.2

NaF

NaClO3

1.0 1.2 0.04

2.3

0.028

Other Ethanolamine Organic amide, Ammonia Sodium molybdenum, tatric acid Tatric acid, SDS Adjusted pH 3.2 by H3PO4/ With or without Ca(NO3)2 H2O2 or alkyl phosphate

Ref. [20-22] [23] [24] [25,26] [27] [28] [29]

Zinc Phosphate Conversion Coatings on Magnesium Alloys: A Review

the metal surface, so that hopeite crystal (Zn3(PO4)2.4H2O) can be formed on the surface [20-29]. It was also reported that a uniform phosphate coating can be obtained when the phosphating bath pH is higher than 1.8, below which the surface of magnesium alloy is corroded [26]. On the other hand, some white sludge deposit was observed on the surface of the phosphate coating when the phosphating bath pH is more than 2.5 [26]. The TA and FA:TA ratio can change the ratio of Zn/P in the coatings, and also can affect the rates of formation and growth of hopeite crystal [20]. 3.2.2. Accelerators for the phosphating – – – Oxidizing agents such as NO3 , NO2 , ClO3 , C6H4(NO2) (SO3)Na (sodium metanitro benzene sulphonate - SMBS), C12H25SO4Na (sodium dodecyl sulfate - SDS) or a mixutre of them, have an influence on the depolarization of hydrogen evolution reaction by consuming the hydrogen ions at the cathodic sites [20-23]. Niu et al. [5] found that the addition of SMBS in phosphating bath resulted in not only an increase of the phosphate coating formation rate but also the formation of less porous – – coatings. If NO3 and NO2 ions are present in the zinc phosphating bath, nitrate ion is reduced to nitrite ion which results in an increase of the local pH on the metal surface. The increased local pH facilitates the precipitation of hopeite layer on the Mg alloy surface [20]. Amini et al. found that the rate of phosphate coating precipitation on Mg alloy is increased, and denser and corrosion-resistant phosphate coatings are produced when SDS is used as an accelerating agent instead of sodium nitrite in a phosphating bath. 3.2.3. Activators Fluorides such as NaF, HF were normally added as an activator in zinc phosphate coating solutions [20-29]. According to Kouisni et al. [20,22], the presence of aluminum in Mg alloys tends to inhibit the formation of zinc phosphate coatings by competing with the formation of aluminum phos3– phate. Adding fluorides is useful for the formation of AlF6 dissolvable into the bulk solution. Some researchers such as Cheng et al. [21], and Zeng et al. [28] also shared the same point of view with Kouisni. On the other hand, Lian et al. [25] and Niu et al. [26] suggested that fluoride ions can react with Mg to form a MgF2 film on the surface which reduces the etching rate of the Mg substrate. They concluded that good-quality phosphate coatings can be obtained only when the etching rate of the substrate is much lower than the formation rate of coatings in the phosphating solutions [25,26]. 3.2.4. Coating agents Literatures show that phosphate coatings mainly consist of hopeite (Zn3(PO4)2.4H2O) and some compounds such as Mg3(PO4)2, MgZn2(PO4)2 etc. [20-29]. Hopeite is one of the important coating compositions in the phosphate layer which has found widespread application in the corrosion protection

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of steels, aluminum alloys and magnesium alloys. Hopeite can be used as an alternative coating material to chromium conversion coating due to its non-toxic and anticorrosive properties. A well crystallized hopeite layer can be formed in zinc phosphate solution [20,23,28], whereby coating agents such as ZnO, Zn(NO3)2 are normally used. A wide range of zinc concentrations can be used to form the hopeite layer as can be seen in Table 2. 3.2.5. Other agents Hydroxy carboxylic acids such as tartaric, citric acid, tripolyphosphate, potassium tartrate, nitrobenzene sulphonate have been employed to reduce the amount of sludge produced in the phosphating bath. During the phosphating treatment, some insoluble phosphate may precipitate out as sludge, and this would influence the coating quality and lifetime of the phosphating bath. Organic acid can combine with some insoluble phosphate to form a solvable complex that will stabilize the phosphating bath [25-27]. Some other additives, such as ethylene diamine tetra acetic acid (EDTA), nitrilo triacetic acid (NTA), diethylene triamine pentaacetic acid (DTPA), gluconic acid, polycarboxy o-amino acid, and ethanolamine are commonly used as a chelating agent and corrosion inhibitor. They are highly effective in the formation of phosphate coatings at a low cost [24,25]. Li et al. [23] studied the effect of ethanolamine on the morphology of phosphate coatings and concluded that ethanol amine can modify the morphology and increase the corrosion potential of Mg alloys. Some metal ions, such as nickel [25], and calcium [28] can be added in the phosphating bath for incorporation of those metal ions into the zinc phosphate coating on Mg alloy, to increase its adhesion and corrosion resistance. Zeng et al. [28] added (CaNO3)2 to a zinc phosphating bath, and they 2+ 2+ found that both Zn and Ca can be incorporated into the phosphate coatings on Mg alloys, which then exhibit better corrosion resistance than zinc phosphate coatings alone. 3.3. Phosphating process details In general, the phosphating process includes six procedures as illustrated in the flow chart (Fig. 1). Some of their operations may be omitted or additional steps can be added, depending upon the surface condition and/or type of the base metal. 3.3.1. Cleaning The surface of Mg alloys is normally contaminated with oils, greases, waxes, oxide, hydroxide and carbonates. Various methods such as sand blasting, solvent degreasing, vapor degreasing, alkaline cleaning and pickling have been used to remove the contaminants. Sand blasting is an effective method of mechanical cleaning. However, it is relatively expensive and it is used if chemical

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magnesium and Mg alloys. Many researchers attempted to use the same recipe with different Mg alloys and they reported that coatings can be formed [20,21,37]. In the laboratory, all of the phosphating processes were carried out by immersion of the samples in the phosphating baths. Some parameters, such as stirring, bubbling, temperature and pH, play an important role in the quality of coatings. 3.4. Zinc phosphate conversion coatings reactions The reactions occurring on the surface of Mg alloys during the zinc phosphating process are rather complicated. Literatures [20,25,26,28,36] have explained some possible reactions on Mg alloys in zinc phosphate solution. When Mg alloys are immersed in acidic phosphating solution, magnesium is quickly ionized, generating magnesium ions and electrons on the surface according to reaction (1), and hydrogen is released by reaction (2). 2+

Mg → Mg + 2e Fig. 1. A flowchart of the operating sequence involved in the phosphating process.

treatments are not applicable. Organic solvents are widely used to remove organic contaminants from the magnesium surface. However, they are toxic and flammable, so they are not safe in use. Alkaline cleaning agents provide an economical and effective way as an alternative to organic solvents. Most researchers used 10 wt% KOH for the cleaning of Mg alloys with an immersion time from 3-5 min at room temperature [27] or up to 60 °C [20-26]. Some of the researchers used acids such as H3PO4, HNO3 and HF [32-34] to remove rust, oxide and hydroxide layer. Nwaogu et al. [35] studied the effects of organic acids pickling on the corrosion resistance of magnesium alloy AZ31. He found that organic acids cleaning reduced the surface impurities and enhanced corrosion resistance. Removal of at least 4 µm of the contaminated surface was required to reach corrosion rates less than 1 mm/year in a salt spray condition [35]. Among the three organic acids examined, acetic, oxalic and citric acids,, acetic acid was the best choice. In steels phosphate, some studies used colloidal titanium phosphate (Na4TiO(PO4)2.0-7H2O) [34] for the creation of a phosphating active center. However, this is not commonly used in Mg alloys phosphating [20]. 3.3.2. Zinc Phosphating According to Table 2, a wide variation of phosphating compositions is available. Although researchers reported different coating compositions, the main coating composition is hopeite [20,23]. The coating also can include AlPO4, Zn, Zn2Mg(PO4)2 [26,29,36], or MgO, Mg3PO4, MgF2, Zn, ZnO [28]. Phosphating can be effectively performed on both pure

+

–

(1)

–

2H + 2e → H2

(2)

The presence of an accelerator such as nitrate ion, can react with hydrogen ions and increases the surface pH via reaction (3). –

+

–

–

NO3 + 2H + 2e → NO2 + H2 O

(3)

In the solution, phosphoric acid can be dissociated according to reaction (4) : +

–x

H3 PO4 ⇔ xH + H3 – x PO4 (x = 0,…,3) −3

−8

(4) −13

Ka1=7.25×10 , Ka2=6.31×10 , Ka3=3.98×10 at 25 °C. At pH 3±0.2, H2PO4− is the major ion because of its low 2+ solubility, and Mg ions can precipitate as Mg3(PO4)2 on the magnesium surface by reaction (5): –

+

3Mg + 2H + 2H2 PO4 → Mg3 ( PO4 )2 + 3H2

(5)

The formation of hopeite is explained by reaction (6): –

2+

+

3Zn + 2H2 PO4 + 2H + 4H2 O + 6e

–

(6)

→ Zn3 ( PO )2 .4H2 O + 3H2

The main phosphating reaction is described by reaction (7): 2+

–

3Mg + 3Zn + 4H2 PO4 + 4H2 O → Zn3 ( PO4 )2 .4H2 O + 3H2 + Mg3 ( PO4 )2 + 2H

+

(7)

The increased surface pH will increase the formation rate of hopeite by reaction (7), so that nitrate ions can act as an accelerator for phosphating reactions. The formation of MgZn2(PO4)2 is explained by reaction (8): 2+

2+

3–

Mg + 2Zn + 2PO4 → MgZn2 ( PO4 )2 (8) The role of fluorides is to complex the Al3+ ions that

Zinc Phosphate Conversion Coatings on Magnesium Alloys: A Review

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release area for the cathodic reactions [20,22]: –

3–

Al + 6F → AlF6 + 3e

–

(9)

Fluoride ions might affect the coating properties by the formation of MgF2 as written by reactions (10,11) [28]: Mg + 2H2 O → Mg ( OH )2 + H2 +

(10)

–

Mg ( OH )2 + 2H + 2F → MgF2 + 2H2 O

(11)

In some studies, Zn and ZnO were found [26,28,36] and explained by reactions (12-14). 2+

–

Zn + 2e → Zn

(12)

Zn + 2H2 O → Zn ( OH )2 + H2

(13)

Zn ( OH )2 → ZnO + H2 O

(14) 2+

In the presence of co-deposition ions such as Ca , the formation of Zn-Ca coatings are reported to occur by reactions (15,16) [28]: 2+

2–

Ca + HPO4 → CaHPO4 2+

2+

(15)

– 4

Ca + 2Zn + 2H2 PO + 2H2 O → CaZn2 ( PO4 )2 .2H2 O + 4H

(16)

+

Several fundamental mechanisms for the formation of phosphate coatings have been proposed. Machu [38] has made an important contribution in clarifying the mechanism of phosphating of steel. He was the first to consider the phosphating process as electrochemical and topochemical reactions. The basics of his approach are to explain the kinetics of the process based upon oxidizers, reducing agents and accelerators. Saison [39] put forward the hypothesis of precipitation of metal phosphates at the micro cathode sites due to the pH gradient at the metal-electrolyte interface. Wulfson and Rabinovitch [40] supposed that the precipitation of insoluble phosphates is induced when the solubility is exceeded. This occurs at the anodic sites on the metal surface. An amorphous layer composed of mixed phosphates of zinc and base metal (such as Fe, Mg etc.) is formed. Cupr and Pelikan [41] proposed that the existence of the ion Zn(PO4)− participates in the formation of trimetallic phosphates at the anodic sites and cathodic sites according to the following reactions [17,18], respectively: –

Me + 2ZnPO4 → MeZn2 ( PO4 )2 + 2e –

Fig. 2. SEM micrographs of the phosphate coatings formed on AZ31 Mg alloy for (a) 1 min, (b) 5 min and (c) 15 min at 50 °C in zinc phosphate solution containing H3PO4 (12.43 g/L), Zn(NO3)2 (5.0 g/L), Na2HPO4 (20.0 g/L), NaNO2 (3.0 g/L), NaNO3 (1.84 g/L) and NaF (1.0 g/L).

–

2+

2Zn ( PO4 ) + Zn + 4H2 O → Zn3 ( PO4 )2 .4H2 O

(17) (18)

where Me is a substrate metal such as Fe, Mg, etc.. 3.5. Zinc phosphate conversion coating characteristics 3.5.1. Structure and composition Various structures and compositions of zinc phosphate

coatings on Mg alloys have been reported by many authors [20,21,23,27-29]. Kouisni et al. and Li et al. [20,23] found that many small crystals or clusters were formed on AM60 Mg alloy during immersion in the phosphating solution for 1-2 min. The crystals or clusters continue to grow with immersion time, contacting each other at the joint, and covering all the surface after 10 min [20,23]. Another report [28] showed that very small particles of hopeite crystals are formed randomly, distributed on a network film with microcracks. By fitting electrochemical impedance spectroscopy (EIS) data with equivalent electrical circuits, Li et al. [23] suggested that the zinc phosphate coatings include two layers, an inner flat amorphous layer and an outer porous crystal layer. The outer layer was composed of crystal clusters of hopeite (Zn3(PO4)2.4H2O). Figure 2 shows SEM micrographs of the phosphate coatings formed on AZ31 Mg alloy at 50 °C in a zinc phosphating bath after 1, 5 and 15 min. After 1 min of phosphating treatment, many randomly distributed small platelet crystals were formed and some of them started to grow. After 5 min, crystals covered all of the surface of the substrate, and formed joints where two or more crystals met. The joints were regarded as defect regions because the resistance of the crystal layer is lowered by their presence [23]. It was also observed that some cracks appeared when phosphating time goes up to 15 min (Fig. 2(c)). The structure and chemical composition of the zinc phosphate coatings are normally analyzed by X-Ray diffraction and energy-dispersive X-ray spectroscopy (EDS). X-Ray diffraction studies show that zinc phosphate coatings are mainly composed of hopeite Zn3(PO4)2.4H2O. Other compositions such as Zn, Mg3(PO4)2, AlPO4, MgZn2(PO4)2 and MgF2 of chemical conversion coatings have been reported in

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Fig. 3. XRD patterns of bare AZ31 (a) and phosphate coatings (b) formed on AZ31 for 10 min at 50 °C in zinc phosphate solution containing H3PO4 (12.43 g/L), Zn(NO3)2 (5.0 g/L), Na2HPO4 (20.0 g/L), NaNO2 (3.0 g/L), NaNO3 (1.84 g/L) and NaF (1.0 g/L).

several papers [17-21]. Figure 3 presents typical XRD patterns of bare substrate AZ31 (a) and zinc phosphate coatings (b). In this figure, only X-ray peaks of α-Mg were visible for the AZ31 substrate, while additional diffraction peaks, corresponding to the phase of the hopeite crystal, were observed for the phosphate-coated AZ31 sample. By EDS analysis, it was generally known that the chemical compositions of phosphate coatings normally consist of Mg, Zn, P, O and F. High fluorine contamination in the coatings were observed [28], and they explained that MgF2 is formed during the phosphating treatment by reaction (11). 3.5.2. Coating weight and coating thickness In magnesium phosphating treatment, coating thickness is 2 usually quantified in terms of weight per unit area (g/m ) which is commonly referred to as coating weight [25,26]. The ratio between coating weight and coating thickness of 2 the zinc phosphate coatings was reported to be about 1 g/m ≈ 1 µm [26]. The coating weight is calculated according to equation WP=(W1 – W2)/S, where WP is the phosphate coating 2 2 weight (g/m ), S is area of substrate (m ), W1 is the weight of sample after phosphating (g), and W2 (g) is the weight of sample after removing the coatings in the alkaline bath (100 g/L sodium hydroxide, 90 g/L sodium ethylene diamine tetracetate, 4.0 g/L triethanolamine at 65-70 °C and 15 min) [25,26]. Another factor which can affect the phosphate coating quality on Mg alloys is etching weight [25,26]. Etching weight reflects the reaction ability of the substrate or coatings with the phosphating solution, which is calculated by WE=(W02 W2)/S, where WE is the etching weight (g/m ) and W0 is the weight of the substrate before phosphating (g).

Fig. 4. The coating weight (a) and etching weight (b) of magnesium AZ31 by immersion time at 50 °C in zinc phosphate solution containing H3PO4 (12.43 g/L), Zn(NO3)2 (5.0 g/L), Na2HPO4 (20.0 g/L), NaNO2 (3.0 g/L), NaNO3 (1.84 g/L) and NaF (1.0 g/L).

The increase of coating weight depends on the compositions of the phosphating solution, substrate material etc., and is normally about 5-10 g/m2 after phosphating 10 min [12,17,25,26]. Figure 4 shows that both the coating and etching weights increased simultaneously with phosphating time. Etching weight is higher than phosphating weight, indicating that the phosphating coatings are formed and dissolved repeatedly. It is suggested that the phosphate itself is dissolvable in the phosphating solution. Coating weight reaches 10 g/m2 after 10 min of phosphating treatment. 3.5.3. Evaluation of corrosion performance Mg alloys are highly susceptible to corrosion, particularly in acid or salt-spray conditions. There are various kinds of corrosion of Mg alloys: galvanic corrosion, pitting corrosion, intergranular corrosion, filiform corrosion, crevice corrosion, and stress corrosion cracking, as reviewed by Zeng et al. [42] and Guo [43]. In aqueous environments, the overall corrosion reaction and passivation reactions of Mg alloys can be written as: +

2+

Mg + 2H → Mg + H2 (in acid) 2+

–

Mg + 2OH → Mg ( OH )2 (in alkali)

(19) (20)

The overall reactions are the sum of the following partial reactions: 2+

–

Mg → Mg + 2e (anodic reaction) +

–

2H + 2e → H2 (cathodic reaction in acid) –

(21) (22)

–

2H2 O + 2e → H2 + 2OH (cathodic reaction in alkali) (23) For evaluation of the corrosion resistance of Mg alloys and their coatings, several methods, such as electrochemical meth-

Zinc Phosphate Conversion Coatings on Magnesium Alloys: A Review

Fig. 5. Potentiodynamic polarization curves of bare AZ31 (a) and phosphate coatings (b) on AZ31 in a borate solution (0.93 g/L H3BO3 and 9.86 g/L Na2B4O7, pH= 9.2) at RT and a scan rate of 1 mV/s. The phosphate coatings were formed for 10 min 50 °C in zinc phosphate solution containing H3PO4 (12.43 g/L), Zn(NO3)2 (5.0 g/L), Na2HPO4 (20.0 g/L), NaNO2 (3.0 g/L), NaNO3 (1.84 g/L) and NaF (1.0 g/L).

ods, immersion test, electrochemical impedance spectroscopy (EIS) and salt spray test etc. are generally used. Potentiodynamic polarization test has been carried out in borate solution (0.93 g/L H3BO3 and 9.86 g/L Na2B4O7 at pH 9.2) [20,23], or 3.5% NaCl solution [21,27,37]. The corrosion test results are expected to be more accurate in a borate buffer solution because it can keep the pH at the substrate surface stable [23]. An example of the protectiveness of bare Mg AZ31 alloy surface and zinc phosphate coatings evaluated though potentiodynamic polarization techniques in the borate buffer solution is shown in Fig. 5. AZ31 alloy treated by Zn phosphating showed a higher corrosion potential (Ecorr) value, of −1.3 V vs. SCE than −1.66 V vs. SCE of Mg AZ31 alloy. This suggests that phosphating treated AZ31 has a greater protective property against corrosion than nontreated AZ31. Corrosion rates (icorr) can be obtained from the potentiodynamic polarization curves, and the corrosion rate of AZ31 Mg alloy was reduced by 1 order of magnitude after phosphating treatment. The EIS technique is a very powerful tool for understanding corrosion kinetics across the film and interface between electrolyte and metals. By using an equivalent electrical circuit model, it can provide lots of information, such as solution resistance (Rs), coating resistance (Rc), charge transfer resistance (Rct), coating capacitance (Cc), and double layer capacitance (Cdl). The corrosion behavior of phosphate coatings has been examined using EIS in borate [20,23], or NaCl 3.5% solution [21,27]. The changes in the coating parameters with time can give information on how the coating degradation occurs and help establish a mechanistic pathway.

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3.5.4. Adhesion of the phosphate coating The adhesion of the coatings is one of the most important properties for their practical application. Phosphate coatings are required to have good adhesion not only with the substrate but also with paint for industrial application. An adhesion test is normally conducted according to ISO 2409 standard [44], which is a test method for assessing the resistance of paint coatings against detachment from the substrates when the coatings are cut into right-angle lattice patterns and pulled out by an attached tape. The tape peel test method was carried out as a six-step classification test. The classification of “0” indicates the best adhesion result of the coatings, and that of “5” indicates the worst adhesion result of the coatings. Niu et al. [26,29] showed that the adhesion of paint on the zinc phosphate coatings is better than that of paint on chromate conversion coatings due to the microporous structure of the phosphate coatings.

4. OTHER PHOSPHATE CONVERSION COATINGS Several phosphate conversion coatings have been reported to be applicable to Mg alloys, such as the phosphate-permanganate system, calcium phosphate system, barium phosphate system and molybdenum/lanthanum phosphate system. Phosphate-permanganate treatments are regarded as a more environmentally friendly process and considered to be an alternative to chromium conversion coatings for improving the corrosion resistance of Mg alloys [11,45-50]. The bath for the phosphate-permanganate coatings normally contains KMnO4, phosphate ion-containing compounds such as NaH2PO4, KH2PO4, K2HPO4 and NH4H2PO4. Phosphoric acid is also added to control the pH of the solution [45-48]. The phosphate-permanganate coatings showed an increased corrosion resistance, and a micro-cracks structure which improves the adhesion of paints significantly. Phosphates and permanganate-containing coatings were obtained from the highly acidic baths, which are thick, and showed much higher polarization resistance than the uncoated alloy [45]. On the other hand, oxides and hydroxide-containing coatings were produced in low acidity baths, which are thin, and they showed polarization resistance comparable to the uncoated alloy [45]. Lin et al. [46] also investigated the formation of phosphate-permanganate coatings on AZ31 alloy. They found that the conversion coatings obtained after more than 2 min are composed of two layers: an inner porous layer contacting the substrate and an outer cellular overlay. Both the layers are comprised of magnesium, aluminum, manganese, oxygen, and phosphorus species. The coatings continued to grow at decreasing rate with continued immersion. Another study investigated the use of phosphate-permanganate treatments for AZ91D [48]. They found that the conversion coating was approximately 7 µm to 10 µm thick after 10

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min of immersion, and the best pH was between 3 and 5. Calcium phosphate conversion coatings have also shown promise as potential pretreatments for Mg alloys [51-54]. The films formed in calcium phosphate bath consisted of calcium hydrogen phosphate hydrate (CaHPO4.2H2O), tricalcium phosphate (Ca3(PO4)2) and magnesium phosphate (Mg3(PO4)2), which improved the corrosion resistance of Mg alloys. The bath compositions normally are composed of calcium ions such as Ca(NO3)2.4H2O, CaCl2 and phosphate ions such as NaH2PO4, KH2PO4, K2HPO4, NH4H2PO4. Inorganic acid such as H3PO4, HNO3 or base such as NH3 were used to control pH in the range between 3 and 6. The con3– centration of Ca2+ and PO4 are emphasized as being important to the formation of the conversion coatings. The high 3– Ca2+ and low PO4 concentration were found to attain good corrosion resistance. Barium phosphate conversion coatings have also been proposed [55,56]. The coatings formed in a bath containing Ba(NO3)2, Mn(NO3)2 and NH4H2PO4 are composed of Ba, P, O, Mg, Zn, Mn and Al, and showed better corrosion resistance than that of manganese phosphate [55]. Barium phosphate conversion coatings obtained from the bath containing H3PO4, Ba(H2PO4)2 and NaF [56] were amorphous and also improved the corrosion resistance. Molybdenum/lanthanum phosphate conversion coatings also significantly increased corrosion resistance and even showed protectivity against corrosion of Mg alloys almost comparable to the traditional chromate-based coating [13]. The conversion bath was composed of Na2MoO4, Ca(NO3)2, Mn(Ac)2, NaNO3, H3PO4 and NaH2PO4. XPS and XRD analyses indicated that the coating possibly contained composite phases which consisted of Mex(PO4)y (Me= Mg, Al, Ca, Mn), CaMoO4, MgAl2O4, MgO, Al2O3, MnO, as well as molybdate oxide with an ‘‘alveolate-crystallized’’ structure.

5. CONCLUSIONS This review outlines coating characteristics, processes and chemical compositions of phosphate conversion coatings on Mg alloys. Although the researches on the phosphating of Mg alloys have been conducted in laboratory scale, not in industrial production scale, some desirable properties such as improved corrosion resistance, wear resistance and paintability etc., have been achieved, as described in this review. Further requirements for the preparation of uniform and stable phosphate coatings are discussed, in terms of scalability, eco-friendly process, and low cost.

ACKNOWLEDGMENTS This research was supported by the research grant of KIMS (PNK2842).

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