Alternative conversion coatings to chromate for the protection of magnesium alloys

Corrosion Science 84 (2014) 135–146

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Alternative conversion coatings to chromate for the protection of magnesium alloys Sébastien Pommiers, Jérôme Frayret ⇑, Alain Castetbon, Martine Potin-Gautier Université de Pau et des pays de l’Adour/CNRS, Laboratoire de Chimie Analytique Bio-Inorganique et Environnement (LCABIE), UMR 5254 IPREM, Hélioparc, 2 Avenue du Président Angot, 64053 Pau Cedex 9, France

a r t i c l e

i n f o

Article history: Received 29 July 2013 Accepted 13 March 2014 Available online 20 March 2014 Keywords: A. Magnesium C. Oxidation B. cyclic voltammetry C. Oxides coatings

a b s t r a c t Chromium Conversion Coatings (CCCs) are widely used for the protection of magnesium alloys against the corrosion but this approach is limited due to the toxicity of hexavalent chromium. Alternative conversion coatings are based on solutions containing chromium (III), permanganate, phosphates, Rare Earth Elements (REEs) or vanadium. This review details the deposition and protection mechanisms of CCC technology and other promising processes. Permanganate/phosphate-based coating presents a corrosion resistance equivalent to CCC and REEs have self-healing properties but no coating present all the properties of CCCs. The development of new non-toxic conversion coatings remains a priority. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Magnesium and its alloys are used in several applications due to their advantageous high strength/weight ratios, low densities and biomedical material properties [1–6]. Unfortunately, these materials are also very vulnerable to corrosion, do not resist wear and are highly chemically reactive [1]. Other metals, such as aluminum and lithium, have been added to pure magnesium to decrease its shortcomings [7,8], which limit the use of magnesium and its alloys for computer parts and in the aerospace and the automotive industries [9]. The poor corrosion resistance of magnesium is the consequence of its standard potential (E° = 2.363 V/SHE (Standard Hydrogen Electrode)), which makes it extremely susceptible to galvanic corrosion [9]. Conversion coatings are used widely in the metal processing industry because they are the most effective means to improve the corrosion resistance of magnesium alloys [10]. These surface treatments consist of a series of baths including the treatment bath and one or two rinse baths of deionized water. Conversion coatings for magnesium alloys are generally solutions of hexavalent chromium compounds [1,11,12]. Nonetheless, hexavalent chromium is extremely irritating, toxic, caustic and harmful to the environment [1]. Since 2006, several environmental laws have limited the use of Cr(VI) in the European Union. Despite these health considerations, chromium coatings are still used. However, Cr(VI) compounds will be completely forbidden by 2017 in the European Community. The Registration, Evaluation, Authorization and Restriction of ⇑ Corresponding author. Tel.: +33 5 59 40 76 68; fax: +33 5 59 40 76 74. E-mail address: [email protected] (J. Frayret). http://dx.doi.org/10.1016/j.corsci.2014.03.021 0010-938X/Ó 2014 Elsevier Ltd. All rights reserved.

Chemicals (REACH) regulation, which came into effect on June 1, 2007 in Europe, limits the use of hazardous and toxic chemicals. REACH contains a list of Substances of Very High Concern (SVHC), and this includes chromium hexavalent compounds, the use of which is currently limited. Thus, there is now a real necessity to implement alternative conversion coatings [13]. In 2012, all existing alternative methods have been listed by H. Hornberger, S. Virtanen and A.R. Boccaccini: passivation, anodizing, chemical conversion, ion implantation, cathodic electrodeposition, sol–gel coatings, dipping and immersion and spraying [3]. Among them, conversion coatings remain the cheapest and the easiest to operate [10]. These ‘‘greener’’ processes, developed by scientists during the past decade, are based on several compounds such as trivalent chromium, phosphate, zinc phosphate, permanganate, phosphate/ permanganate, REE (Rare Earth Elements), vanadium or combinations of these compounds. However, no study has yet compared the performance of these surface treatments, and many of their mechanisms have not been fully elucidated. This review aims to investigate the Chromium Conversion Coatings (CCCs) process and all the alternative methods used currently. For each process, the assumed mechanisms of deposition and protection are given. The advantages and disadvantages of the new methods are listed, and their performance properties are compared to the CCC process. 2. Materials and methods 2.1. Nature and properties of alloys The two types of alloys discussed in this review are Mg–Al and Mg–Li alloys, which are used as substrates for the study of

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conversion coatings. Both of these materials are binary alloys, and their microstructures are similar. Grain boundaries rich in minor elements surround the grains of magnesium. Mg–Al alloys, commercially known as AZ91D, AZ61A and AZ80A, are composed mainly of magnesium (balance) and aluminum, but they can also contain small amounts of zinc and manganese (Table 1). Other elements, such as beryllium, silicon, copper, iron and nickel, can be found at concentrations below 0.001%. The presence of these trace elements does not affect the deposition of coatings on the alloy surface. These alloys are used primarily because of their good castability and because they are composed of two phases: an a phase that consists of a Mg matrix containing a solid solution of aluminum and an Mg17Al12 eutectic (a + b) phase surrounding the a phase as shown for the AZ91D alloy in Fig. 1 [16,20,21]. During immersion in conversion baths, an internal galvanic coupling occurs at the surface of Mg–Al alloys [9]. This local cell effect leads to the dissolution of the a phase due to its lower potential [16]. During the initial stage, magnesium dissolves from the a phase (anode) while bath oxidants are reduced by the b phase (cathode) [22]. The addition of lithium improves the plasticity of Mg–Li alloys [23]. When the lithium content is less than 5 wt%, the alloy consists of a solid solution of lithium in magnesium. Alloys containing more than 11 mass% are in a b phase. Between 5 and 11 wt%, a solid solution of magnesium contains both a and b phases (Fig. 2) [23]. Lithium (E0 = 3.045 V/SHE) is more chemically active than magnesium (E0 = 2.363 V/SHE), which means that Mg–Li alloys must be protected from galvanic corrosion [23]. When the magnesium–lithium alloy is immersed in the conversion solution, a local cell effect occurs between the a and the b phases. A micro-current forms at the substrate surface due to the potential difference between the two phases of the alloy [24]. During the early stage of immersion, the b phase (anode) corrodes preferentially and the a phase (cathode) is protected [24]. 2.2. Corrosion tests methods To assess the corrosion resistance of the coating different methods could be used: electrochemical measurements (polarization curves and electrochemical impedance), accelerated atmospheric corrosion tests or salt-spray tests [8]. 0 The corrosion current density (jcorr ) and the corrosion potential (Ecorr) are measured by electrochemical techniques. This test is carried out with a three-electrode cell system in which a platinum sheet is used as the counter electrode and an Ag/AgCl electrode or a saturated calomel electrode acts as the reference electrode [24]. The protected alloy is the working electrode, and the exposed area of the testing specimens varies between 0.2 and 2.35 cm2 in corrosion experiments. The three electrode system is immersed at room temperature in an aqueous solution of H2SO4 or NaCl with a concentration range between 0.01 and 1.0 mol/L [23]. A computer-controlled potentiostat measures the variation of the intensity (i) in the potential range from 0.3 V to 0.5 V with a scanning rate between 1 and 50 mV/s. Once the current density j (i/s with i:

Fig. 1. Structure of a Mg–Al alloy: AZ91D [9].

Fig. 2. Structure of Mg–Li alloy: Mg–8.8(wt%)Li [25].

intensity and s: surface of the working electrode) is drawn according to the potential E variations. Ecorr can be obtained easily with the log scaling for the j values as shown on Fig. 3 [8]. The first part of the curve (a) represents the reduction of the electrolyte and the second part (b) of the curve represents the oxidation of the element with the lowest corrosion potential in the alloy. The corrosion current density (jcorr) is the j measurement when E = Ecorr on the experimental curves [8]. An increase in the corrosion potential or/and a decrease in the corrosion current density imply an increase in the protection of the alloy. The annual corrosion (e), expressed in m/year, can be calculated from Eq. (1): 0

e¼

j Mt nF q

where M (g/mol) is the molar weight, j0 (A/m2) is the current density, n is the number of electrons (2 in the case of magnesium), S

Table 1 Composition of Mg–Al alloys. Alloy

AZ31 AZ61D AZ80A AZ91D AM60 a

Alloy composition (wt%a)

Refs.

Mg

Al (%)

Zn (%)

Mn (%)

Balance Balance Balance Balance Balance

3.0 6.3 7.8 9.0 6.0

1.0 0.7 0.4 0.7 –

0.2 – – – 0.3

Weight percentage.

ð1Þ

[14,15] [16] [16] [17] [18,19] Fig. 3. Determination of Ecorr and jcorr from an j = f(E) curve [8].

S. Pommiers et al. / Corrosion Science 84 (2014) 135–146

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(m2) is the test surface and q (g/m3) is the relative density. F is the Faraday Constant, and its value is 96 500 C/mol. During a salt-spray test, the protected alloy is submitted to an extremely corrosive medium: a 5 wt.% NaCl spray at 40 °C. The appearance of corrosion marks on the surface determines the life span of the alloy and is used to evaluate the protection provided by the coating.

because there are Cr(VI) species trapped in the protective film [13]. Several articles have detailed the structure, the composition or the formation mechanism of CCCs on magnesium alloys, and more information can be found about CCCs on aluminum and zinc alloys [12,13,32–35]. The deposition and protective mechanisms of CCCs on zinc and aluminum alloys complements the information gathered for CCCs on magnesium alloys.

3. Magnesium corrosion mechanisms

4.1. Protective property

The exposure of magnesium alloy to the environment is responsible for its corrosion. In contact with an aqueous environment, the oxidation of the magnesium and the reduction of the water occur simultaneously. This results in the formation of Mg(OH)2(s) [26]. The involvement of the intermediate species Mg+ in the magnesium dissolution has been speculated [27]. Magnesium is first oxidized into the intermediate species Mg+, then the intermediate species reacts with water to produce hydrogen and Mg2+. However, the corrosion of the magnesium can take different forms depending on the nature of the alloy and on the environment:

The exposure of magnesium alloys to the dichromate bath (K2Cr2O7 or Na2Cr2O7) leads to the oxidation of magnesium and the simultaneous reduction of Cr(VI) to form an adherent precipitate of trivalent chromium species (chromium (III) hydroxide Cr(OH)3(s) and chromium (III) oxide Cr2O3(s)) as detailed in Eqs. (2) and (3) [32–34]:

 Intergranular corrosion Magnesium alloys are rarely subject to intergranular corrosion because the grain boundary usually acts as the cathode and is not dissolved [28].  The galvanic corrosion or contact corrosion This type of corrosion is due to the direct contact of the magnesium and another metal in the presence of an electrolyte. The magnesium will acts as a sacrificial anode [29]. In order to limit the galvanic corrosion, it is possible to minimize the potential differential between the magnesium/magnesium alloys and another metal [29].  The pitting corrosion Pitting corrosion is a form of localized corrosion, whereby it results in the formation of pits in the corroded metal. In the case of Mg–Al alloys, selective attack along the Mg17Al12 network results in pitting. Air contaminated with salt also tends to break down the protective film leading to the formation of pits [28].  The stress corrosion cracking (SCC) Unlike pure magnesium, magnesium alloys are sensitive to SCC in atmospheric and aqueous environments. Two kind of mechanisms involved SCC [29,30]. Magnesium alloys containing alloying elements such as aluminum and zinc are considered the most susceptible to SCC in air, distilled water and chloride containing solutions [31]. In most cases, SCC in magnesium alloys is transgranular and is triggered by hydrogen embrittlement. However, when alloys are subjected to heat treatments, the change in material’s grain size and the precipitation of phases at grain boundaries can result in intergranular SCC [28]. In order to prevent any forms of corrosion, the magnesium alloy is dipped in a conversion bath to deposit a protective anticorrosive film. The most used coating is the CCC and the mechanisms of deposition and protection are detailed below. 4. Chromate conversion coating (CCC) CCCs are highly efficient coatings for the protection of magnesium alloys and they have the ability to ‘‘regenerate’’ themselves

þ 2þ 3Mg þ Cr2 O2 þ 2CrðOHÞ3 ðsÞ þ H2 O 7 þ 8Haq ¡ 3Mg

ð2Þ

þ 2þ 3Mg þ Cr2 O2 þ Cr2 O3 ðsÞ þ 4H2 O 7 þ 8Haq ¡ 3Mg

ð3Þ

The precipitation of trivalent chromium on the metal surface is assisted by the local pH increase that occurs when the hydronium ions are consumed during the reactions (2) and (3) [33]. For zinc alloys, the composition of the coating varies from one study to another. Kulinich et al. stated that the coating contains 40% undefined species of Cr(VI), 40% Cr(OH)3(s) and 20% Cr2O3(s) [13]. The partition of Cr(VI) in the coating is not homogeneous. The outermost layer of the coating is enriched with Cr(VI), whereas the inner layer contains little or no Cr(VI) [12,13,32]. During the early stage of CCC formation, a small amount of Cr(VI) is present in the film, but the increase of the coating thickness leads to less metallic dissolution and less reduction of Cr(VI). Consequently, the concentration of the hexavalent chromium increases with the thickness of the CCC [13]. The formation of the coating is controlled by the diffusion of the chromium (VI) species in the protective film. Moreover, Cr(VI) is incorporated into the hydrated Cr(III) oxide layer, creating a covalent Cr(VI)–O–Cr(III) liaison and resulting in a Cr(VI)/Cr(III) mixed oxide [33,34,36]. The thickness of the coating varies from 400 nm to 11 lm [1,11]. An example of a CCC is shown in Fig. 4a). All the detailed experimental procedures to obtain the coatings presented on Fig. 4 are reported in the respective tables. To promote the coating growth, K3Fe(CN)6 can be added as a catalyst in the hexavalent chromium bath. Indeed, the Fe(III)/Fe(II) couple is believed to catalyze the coating formation where Fe(III) is reduced to Fe(II) by the oxidation of the metal. Then, Fe(II) is oxidized back to Fe(III) by the reduction of the chromate (reactions (4) and (5)) [13,33,36,41]. 4 2þ 2FeðCNÞ3 6 þ Mg ¡ 2FeðCNÞ6 þ Mg

ð4Þ

3 2 3þ þ 6FeðCNÞ4 þ 7H2 O 6 þ Cr2 O7 þ 14Haq ¡ 6FeðCNÞ6 þ 2Cr

ð5Þ

However, although those authors do not mention it, these reactions are not without danger. These reactions must be performed in baths where the pH is not too acidic in order to avoid the formation of very toxic cyanhydric acid. The protective properties of CCCs are due to the presence of Cr(OH)3(s) in the coating structure. The more the level of Cr(OH)3 (s) in the coating is high, the more the coating is protective against corrosion [12,33]. The influence of the soluble Cr(VI) species is more controversial. When these species are leached out of the CCC, the treated surface maintains a substantial part of its initial corrosion resistance [12]. According to other studies, the concentration of hexavalent chromium determines the effectiveness of the protection [13,34].

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Fig. 4. SEM (Scanning Electron Microscopy) images of coatings obtained with treatment baths containing (a) Cr (VI) [12], (b) Cr (III) [12], (c) zinc phosphate [10], (d) permanganate with HNO3 [20], (e) permanganate with HF [20], (f) permanganate with HCl [20], (g) phosphate permanganate [37], (h) cerium [38], (i) Lanthanum [39] and (j) vanadium [40].

4.2. «Self-healing» property It is well know that the efficiency of the self healing ability depends on the quantity, the mobility and the diffusion capacity of the Cr(VI) trapped in the coating, but its chemical form has not been determined [33,42]. A study explained that the trapped Cr(VI) is soluble and can be released at defect sites to repassivate the surface by producing Cr(OH)3(s) and Cr2O3(s) [34]. Kendig et al. assumed that the CCC retains a certain quantity of Cr(VI) due to the incompleteness of reactions (2) and (3) [32]. Another study suggested that the polar oxo-Cr(VI) adsorbed on the coating prevents the adsorption of depassivating anions, such as chloride ions [12]. Table 2 summarizes the different baths and characteristics of the CCCs listed in the literature. There are wide ranges of Cr(VI) concentrations used in these baths (from 5 g/L to 200 g/L) and immersion times (from 60 s to 300 s). For the first two baths and coatings, the Ecorr values of the protected alloys were equivalent. This could be explained by the fact that these processes were already optimized to provide the best protection. However, the different alloy compositions could cause the gap (55.1 lA/m2) 0 between the two corrosion current densities (jcorr ). The two

AZ91D alloys were coated with two rather similar baths, and their corrosion potentials were identical. The main difference in their corrosion current density implies that an increase of the protection of the alloy can be obtained with the DOW 1 bath. The DOW 1 is an industrial process that was developed for stainless steel and aluminum alloys. The last three CCCs were deposited on three different AZ magnesium alloys. AZ91D contains more Al and AZ61 than the other two alloys, but all three alloys have similar Ecorr, 0 and AZ80D has the lowest value of jcorr (15.9 lA/m2). It can be deduced that there is no relationship between the Al content of 0 the alloy and the corrosion current density (jcorr ) or the corrosion potential (Ecorr). There is a gap of 0.4 V between the Ecorr values of the coated Mg–Al and Mg–Li alloys. This difference can be explained by the coating composition and structure. The protection offered by the CCCs on the Mg–Li alloys remains greater than on the Mg–Al alloys. Due to the recent limitations and the future interdiction of the use of chromium (VI), new alternative conversion coatings have been proposed to replace CCCs. In this paper, the mechanisms of the different baths used to treat Mg and its alloys are proposed by analogy with those treatments for aluminum and zinc alloys described more frequently in the literature.

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S. Pommiers et al. / Corrosion Science 84 (2014) 135–146 Table 2 Composition of the CCC (Chromate Conversion Coating) baths and their respective coatings compositions and properties. Bath characteristics

a b

Coatings characteristics

Alloy

Compounds

Concentration

Immersion time (s)

pH

Composition

jcorr(lA/cm2)

Ecorr(V)

Thickness(nm)

Ref.

Zn

Na2Cr2O7 H2SO4

200 g/L 10 g/L

60

1.2

40% of Cr (VI) 20% of Cr(III)

0.9

1.0 VNHEb

400

[12]

Mg–8.5Li

Na2Cr2O7 NaF CrO3

5 g/L 1 g/L 5 g/L

300

n.d

n.d

56

1.1 VSCEa

n.d

[11]

AZ91D

DOW 1: Na2Cr2O7 HNO3

10s–120

n.d

n.d

691.8

1.5 VSCEa

n.d

[9]

15% 25%

AZ91D

Na2Cr2O7 HNO3

180 g/L 261 mL/L

n.d

n.d

n.d

33.3 103

1.5 VSCEa

n.d

[16]

AZ80D

Na2Cr2O7 HNO3

180 g/L 261 mL/L

n.d

n.d

n.d

15.9  103

1.5 VSCEa

n.d

[16]

AZ61A

Na2Cr2O7 HNO3

180 g/L 261 mL/L

n.d

n.d

n.d

39.1  103

1.49 VSCEa

n.d

[16]

VSCE (SCE: Saturated Calomel Electrode). VNHE (NHE: Normal Hydrogen Electrode).

NO3 þ Mg þ 2Hþaq ¡ NO2 þ Mg2þ þ H2 O

ð6Þ

coatings than CCCs [1,12]. This can be attributed to the difference in thickness of the two films and to the presence of mobile Cr(VI) species in CCCs that allow the repassivation of flaws and corrosion pits [1,12]. Thus, Cr(VI) forms a passivating layer that prevents chloride anions from attacking the surface. It appears that Zn alloys are equally protected by Cr(VI) and Cr(III) coatings, but it is not certain whether this behavior will be observed on magnesium alloys. The only advantage of Cr(III) based coating is their resistance to thermal shocks. CCCs lose their corrosion resistance if heated to 70 °C, whereas Cr(III) coatings do not [1].

H2 O2 þ Mg þ 2Hþaq ¡ 2H2 O þ Mg2þ

ð7Þ

5.2. Phosphate based conversion coatings

5. Protective alternative treatments 5.1. Cr(III) based conversion coatings Cr(III) based coatings involve a redox reaction: the metal is oxidized by an oxidant added to the bath. This oxidant, NO 3 or H2O2, is simultaneously reduced as shown in reactions (6) and (7) [43,44].

Hydronium ions are consumed during this process, causing a local pH increase. This pH variation causes the precipitation of trivalent chromium as an insoluble hydroxide and oxide (reactions (8) and (9)).

Cr3þ þ 3H2 O ¡ CrðOHÞ3 ðsÞ þ 3Hþaq

ð8Þ

2Cr3þ þ 3H2 O ¡ Cr2 O3 ðsÞ þ 6Hþaq

ð9Þ

Table 3 includes chrome III baths and coatings characteristics. These treatments have been tested on aluminum and zinc alloys but not on magnesium alloys. Zhang et al. reported that the Cr(III) treatment produces a 90 nm coating made of 60% Cr(OH)3(s) and 40% Cr2O3(s) (Fig. 4b)) [12]. The film presents a smooth and continuous structure with no cracks [45]. It has already been demonstrated that the corrosion resistance is less effective with Cr(III)

Phosphate coatings are more environmentally friendly than CCCs, and many scientists have tested films deposited from phosphate solutions. Currently, this process is one of the most studied alternatives to CCCs on magnesium alloys. Phosphate coating on zinc, steel and aluminum is already a well-known process [19]. In contrast to Cr(VI), phosphate cannot oxidize the surface of an alloy. Phosphate can only passivate alloy surfaces. Without the presence of an oxidant, the oxidation of magnesium only occurs due to the reduction of water according to reaction (10).

Mg þ 2H2 O ¡ Mg2þ þ 2OH þ H2 ðgÞ

ð10Þ

To create an efficient film, it is necessary to add an oxidant to the bath to increase the rate of magnesium alloy oxidation [46]. The main oxidants used are NO 3 or H2O2 [46]. Nitrites can be added with nitrates to further accelerate oxidation.

Table 3 Composition of Chrome III baths and their respective coatings compositions and properties. Bath characteristics

a b

Coating characteristics

Alloy

Compounds

Concentration (mol/L)

Immersion time (s)

pH

Composition

jcorr (lA/cm2)

Ecorr (V)

e (m/year)

Thickness (nm)

Ref.

Al

Cr2(SO4)3

0.01

300

n.d

Cr(OH)3(s) Cr2O3(s) Al2O3(s) ZrO2(s) Zr(OH)4(s) AlF3(s) ZrF4(s)

0.4

0.5VSCEa

5.106

n.d

[45]

Zn

Permapass Immunox™ 3K

n.d

60

1.8

60% of Cr(OH)3(s) 40% of Cr2O3(s)

0.4

1.0 VNHEb

n.d

90 nm

[12]

VSCE (SCE: Saturated Calomel Electrode). VNHE (NHE: Normal Hydrogen Electrode).

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S. Pommiers et al. / Corrosion Science 84 (2014) 135–146

lower than the Cr2O3 (5.22 g/cm3) [49]. This property could be an hypothesis to explain the lower corrosion resistance of these coatings in comparison to CCCs (Table 4). If other divalent cations, such as Ca2+, are added to the bath and hydroxide ions are present at the alloy surface, then HPO2 and 4 PO3 preferentially bond to form insoluble CaHPO4,2H2O(s) as 4 shown in reactions (16) and (17) [23,50].

In the phosphate bath, an equilibrium exists among all forms of the orthophosphoric acid that can dissociate by successively liberating protons. The dominant form of orthophosphoric acid depends on the solution pH. Phosphate baths are mainly around pH 3.0 [23,47,48]. In these baths, H3PO4 and H2PO 4 are the primary species according to their pKa values: pKa1 = 2.15, pKa2 = 7.20, pKa3 = 12.35 [49]. The reactions 6, 7, and 10 lead to a local pH increase at the alloy surface and, consequently, to the modification of the phosphate 3 3 species to HPO2 4 and PO4 . The phosphate ions PO4 react with Mg2+ to form Mg3(PO4)2(s) as shown in reaction (11) [23].

3Mg2þ þ 2PO3 4 ¡ Mg3 ðPO4 Þ2 ðsÞ

Another possibility for the formation of Mg3PO4(s) has been considered [19]. The chemical reactions between the oxidized base metal Mg and the phosphate can be described by reactions (12)–(14):

ð12Þ

MgðH2 PO4 Þ2 ðsÞ ¡ MgHPO4 ðsÞ þ H3 PO4

ð13Þ

3MgHPO4 ðsÞ ¡ Mg3 ðPO4 Þ2 ðsÞ þ H3 PO4

ð15Þ

3Ca2þ þ 2PO3 4 ¡ Ca3 ðPO4 Þ2 ðsÞ

ð16Þ

In the case of Mg–Li alloys, the deposition occurs mainly on the b phase where the redox reactions 6, 7, and 10 occur. A conversion film forms as shown in reactions (11)–(16) [23,25]. The film grows until an equilibrium is established between film dissolution and formation [25]. To strengthen the corrosion resistance of the coating, Zn(II) salts are added to create an insoluble layer of Zn3(PO4)2(s) called hopeite. Sometimes ZnO is added to the bath because the addition of a metallic oxide can influence the microstructure of a phosphate coating and make the coating denser and thinner [47]. Zinc phosphate is one of the numerous solutions tested because it improves the corrosion resistance relative to a simple phosphate coating [18,19,47,48,51]. The density of this compound is 3.998 g/ cm3 [49]. The presence of this compound in the coating of magnesium phosphate increases the density of the layer and could be responsible of the better corrosion resistance. The reaction mechanisms are similar to the simple phosphatization process: metal alloys are oxidized while water is reduced from the surface, leading to the increase of pH [19]. The main difference appears during the formation of the coating.

ð11Þ

þ Mg þ 2PO3 4 þ 6Haq ¡ MgðH2 PO4 Þ2 ðsÞ þ H2 ðgÞ

Ca2þ þ HPO2 4 þ 2H2 O ¡ CaHPO4  2H2 OðsÞ

ð14Þ 0

The value of the thermodynamic function H for Mg3PO4(s) and Mg(OH)2(s) is 3780.6 and 924.2 kJ/mol, respectively. Consequently, Mg2+ preferentially bonds with PO3 4 , which explains the absence of Mg(OH)2(s) in the coating [23]. Mg3PO4(s) is so the main compound of the coating and the responsible for the corrosion protection in these kinds of coatings. Its density (2.74 g/cm3) is two times

Table 4 Composition of phosphate baths and their respective coatings compositions and properties. Bath characteristics

a

Coating characteristics

Alloy

Compounds

Concentration

Immersion time (s)

pH

Composition

jcorr (lA/cm2)

Ecorr (VSCEa)

Thickness (nm)

Ref.

Mg–8.8Li

NH4H2PO4 Ca(NO3)2

25 g/L 25 g/L

300

3.0

CaHPO4.2H2O(s) Ca3(PO4)2(s) Mg3(PO4)2(s)

0.2

1.58

n.d

[23]

AM60

H3PO4/Na2HPO4NaNO3/NaNO2 Zn(NO3)2 NaF

7.4 mL/20 g/L 3 g/L/1.84 g/L 1.84 g/L 1 g/L

600

3.0

n.d

10 000

0.9

n.d

[19]

AZ91D

H3PO4 ZnO NaF NaNO3/NaNO2 C4H4O6 Amine

17.5 g/L 3.2 g/L 1.7 g/L 0.17 g/L/0.83 g/L 2.2 g/L 0.18 g/L

n.d

3.0

Zn3(PO4)24H2O(s) Zn AlPO4(s) ‘‘MgZn2(PO4)2(s) Mg3(PO4)2(s)

AZ91D

H3PO4 ZnO Zn(NO3)2 NaF NaClO3 NH3 Amine

0.065 mol/L 0.0029 mol/L 0.102 mol/L 0.040 mol/L 0.0028 mol/L 0.0034 mol/L 0.007 mol/L

n.d

2.4

Zn3(PO4)24H2O(s) Zn(s)

n.d

1.17

n.d

[47]

AZ31

H3PO4/NH4H2PO4 NaNO3/NaNO2 Zn(NO3)2 NaF

7.4 mL/20 g/L 1.84 g/L/3 g/L 5 g/L 1 g/L

n.d

3

Zn3(PO4)2xH2O(s)

n.d

1.37

n.d

[51]

AZ31

H3PO4/NH4H2PO4 NaNO3 NaNO2 Zn(NO3)2 NaF

0.10/0.034 mol/L 0.021 mol/L 0.042 mol/L 0.068 mol/L 0.024 mol/L

n.d

3.07

Zn3(PO4)24H2O(s) MgHPO43H2O(s), Mg3(PO4)2(s), AlPO4(s), Al2O3(s) Al(OH)3(s), MgO(s) and/or Mg(OH)2 (s)

0.54  105

n.d

n.d

[14]

VSCE (SCE: Saturated Calomel Electrode).

[48]

S. Pommiers et al. / Corrosion Science 84 (2014) 135–146

The local increase of pH also allows the precipitation of Mg3(PO4)2(s) and especially relatively insoluble Zn3(PO4)2,4H2O(s) (Ks = 1035.3 [49]) as shown in Eqs. (11) and (17), respectively [14,19].

3Zn2þ þ 2Mg þ 2H2 PO4 þ 4H2 O ¡ Zn3 ðPO4 Þ2  4H2 OðsÞ þ 2Mg2þ þ 2H2 ðgÞ

ð17Þ

Two contradictory theories were developed for the deposition mechanisms of zinc phosphate coatings, and they are described by Kouisni et al. [18,19]. Wulfson et al. in 1937 and Cupr and Pelikan in 1965 asserted that phosphate precipitation occurs on anodic areas. The resulting amorphous layer consists of mixed phosphates of zinc and alloy metal, in this case, magnesium (reaction (18)):

Mg2þ þ 2ZnPO4 ¡ MgZn2 ðPO4 Þ2 ðsÞ

ð18Þ

This first layer is the base for the development of crystal nuclei of zinc phosphate Zn3(PO4)24H2O(s). This theory is based on the existence of ZnPO 4 resulting from the formation (reaction (19)) and then the dissociation (Eq. (20)) of Zn(H2PO4)22H2O in the bath [48]:

ZnðIIÞ þ 2H3 PO4 þ H2 O ¡ ZnðH2 PO4 Þ2  2H2 O

ð19Þ

ZnðH2 PO4 Þ2  2H2 O ¡ ZnPO4 þ H2 PO4 þ 2Hþaq þ 2H2 O

ð20Þ

In contrast, authors such as Machu in 1943 and Saison in 1962 put forward their hypothesis of the precipitation of zinc and magnesium phosphate on cathodic sites due to pH + gradients at the metal surface (Eq. (21)) [19].

3Zn2þ þ 3Mg þ 4H2 PO4 þ 4H2 O ¡ Zn3 ðPO4 Þ2 ðsÞ  4H2 OðsÞ þ Mg3 ðPO4 Þ2 ðsÞ þ 2Hþaq þ 3H2 ðgÞ

ð21Þ

According to Li et al. [47], the reaction of Zn(II) in the bath is due to the difference between the standard potentials of zinc (0.76 V/ SHE) and magnesium (2.36 V/SHE). The redox reaction between these two elements leads to the oxidation of Mg and the deposition of Zn at the alloy surface as shown in Eq. (22):

Zn2þ þ Mg ¡ Zn þ Mg2þ

ð22Þ

However, this hypothesis is controversial because the oxidation 0 strength of zinc is lower than that of Hþ aq (E = 0 V/SHE). It seems more likely that the hydronium ion is involved predominantly in the redox reaction. Thus, a complementary study of the deposition mechanism of zinc phosphate coatings is necessary. Furthermore, aluminum, which is present in several alloys, slows down the formation of the phosphate films on magnesium surfaces. Fluoride ions can be added to release Al complexes AlF3 6 from the cathodic sites. This complexion influences the zinc phosphate film formation by increasing the number of nuclei and allowing the formation of a more compact layer [19]. Sodium fluoride NaF precipitates aluminum ions in the solution to form Na3AlF6(s) [19]. The coating (Fig. 4c)) has a double-layer structure, an inner amorphous layer and a crystal outer layer. The inner layer consists of various compounds, such as Zn(H2PO4)2, MgHPO43H2O(s), Mg3(PO4)2(s), AlPO4(s), Al2O3(s) and/or Al(OH)3(s), and MgO(s) and/or Mg(OH)2(s). The outer layer contains large amounts of P and Zn consisting of (Zn3(PO4)24H2O(s)) [14]. Despite the unresolved questions about the deposition mechanisms, the nature of the coating has been studied by numerous authors. The doublelayer composition of phosphate coatings is responsible for the effective protection of the alloy. An amorphous dense inner layer is made of MgZn2(PO4)2(s), Mg(OH)2(s) and/or MgO(s) and some small amounts of Al(OH)3(s) or Al2O3(s). The outer porous crystal

141

layer is composed of crystal hopeite, Zn3(PO4)2(s) [14]. This layer is generally porous due to the presence of cracks and flaws [52]. The corrosion resistance of phosphate and zinc phosphate coatings can be evaluated by the difference between the Ecorr values for the bare (1.76 V/SCE (Saturated Calomel Electrode)) and the coated (1.58 V/SCE to 0.9 V/SCE) alloys given in Table 4. Higher corrosion potentials are obtained with zinc phosphate baths. The protection provided by phosphate alone is not equivalent to a CCC due to the formation of a low dense layer of Mg3(PO4)3. On the other hand, the zinc phosphate coating seems to provide better corrosion resistance than the CCC. Indeed, the corrosion potential is from 1.37 V/SCE to 0.9 V/SCE for this coating, and it was around 1.5 V/SCE with the Cr(VI) coating. The compositions of all the zinc phosphate baths are similar: a buffer made with orthophosphoric acid H3PO4, dihydrogen   phosphate H2PO 4 and NO3 with NO2 as an accelerating agent. The difference is the addition of a zinc salt. 5.3. Permanganate based conversion coatings (PCC) The coatings based on permanganate solutions are named PCCs (permanganate conversion coatings). These solutions contain Mn(VII) species acting as oxidant agents on the alloy. The reduced Mn(IV) species coat and passivate the substrate as shown in Eqs. (23) and (24) [53].

2MnO4 þ 3Mg þ 8Hþaq ¡ 2MnO2 ðsÞ þ 3Mg2þ þ 4H2 O

ð23Þ

2MnO4 þ 4Mg þ 10Hþaq ¡ Mn2 O3 ðsÞ þ 4Mg2þ þ 5H2 O

ð24Þ

It has been reported that PCCs and CCCs provide similar levels of corrosion protection on magnesium alloys [53]. The protective species are MnO2 or Mn2O3 and have similar or superior densities (respectively 5.0 g/cm3 and 9.6 g/cm3) than Cr2O3 formed in the case of CCC [49]. It could explain the similar corrosion resistance properties. PCC baths possess another advantage because they do not need to be heated [53]. Moreover, PCC coatings seems to be less affected by heat than CCCs and are more convenient for painting [53]. Table 5 groups the PCC baths that are usually tested on magnesium alloys. These baths contain KMnO4 associated with different strong acids [20,53]. The nature and the concentration of the acid affects the structure and the composition of the coating [20]. Fig. 4d–f show the different morphologies of PCCs. The addition of HNO3 affects the surface of the coating, forming clusters of particles (Fig. 4d) [20]. With the addition of HF (Fig. 4e), the coating has an amorphous structure. This film is thinner than the coating deposited in a bath containing HNO3, and the deposition rate is slower [20]. The thinner film can be explained by the reaction of fluorine ions with magnesium to produce insoluble magnesium fluoride MgF2(s), creating a passivating layer that prevents any further dissolution of magnesium [20]. Coatings dipped in the HF and the KMnO4 solutions are composed mainly of MgF2(s) and manganese oxides (MnO2(s), Mn2O3(s), Mn3O4(s)), while the coatings formed in the HNO3/KMnO4 bath contain Mg and Mn oxi-hydroxides. More cracks in the coating can be observed when HCl is present in the solution (Fig. 4f). The role of an addition of an acid to a permanganate bath on the protective properties of the final coating has not been quantified but the coatings characteristics (thinner film with HF, cracks with HCl and presence of clusters with HNO3) may decrease the corrosion resistance of the coating. Permanganate coatings provide a good alternative to CCC because they provide good corrosion resistance. The major problem that prevents PCCs from replacing CCCs is the stability of the bath pH. Important pH variations occur when PCC baths are used and the alloys are dipped in the solution. The attack of MnO 4 on Mg

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Table 5 Composition of permanganate baths and their respective coatings compositions and properties. Bath characteristics

Coating characteristics 3

Alloy

Compounds

Concentration mol/m

Composition

Ref.

AZ91D

KMnO4 HNO3

20 20–200

MnO2(s) Mn2O3(s) Mn3O4(s) Mg oxide/hydroxide

[20]

AZ91D

KMnO4 HCl Na2B4O7

20 20–200 Unknown

Mn oxide/hydroxide B oxide/hydroxide Mg oxide/hydroxide

[20]

AZ91D

KMnO4 HF

20 20–200

MnO2(s) Mn2O3(s) Mn3O4(s) MgF2(s) Mg oxide/hydroxide

[20]

alloy surfaces consumes Hþ aq ions (reactions (23) and (24)), increasing the pH of the solution. The addition of Na2B4O7 to an HCl bath has a buffering effect on these two compounds. The bath is steadier, and a minimal pH increase occurs over time [20,53]. However, the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) added tetraborate products to the list of Substances of Very High Concern (SVHC), and their use is now limited and will be completely forbidden in a few years. Other compounds must be found to stabilize the pH of PCC baths. 5.4. Phosphate/permanganate based conversion coatings The combination of phosphate and permanganate has been considered as a solution to avoid the pH increase in baths. In a potassium permanganate bath, monopotassium dihydrogeno-phosphate (KH2PO4) or manganese hydrogenophosphate (MnHPO4) is added as a buffer [22]. The reactions of the deposit are the same as discussed previously: the oxidation of magnesium and the reduction of Mn(VII) consume the Hþ aq responsible for the pH increase, allowing the phosphate species to precipitate at the metal surface (reactions 11, 23, and 24) [37,54]. According to Zhao et al., the coating thicknesses are between 7 and 10 lm [9]. Fig. 5 presents the variation of film thicknesses as a function of bath pH. Three baths (KH2PO4 + 40 g/L KMnO4) were tested with different concentrations of KH2PO4. The conversion coating thickness decreased gradually with the increase of the pH value and the concentration of KH2PO4 in the bath. Below pH = 3, a non-compact surface layer is formed because manganese is under the soluble Mn2+ form and not the protective MnO2. Around pH = 5, the formation of the coating is too slow due to

the quick main precipitation of Mg3(PO4)2 [9,55]. The conditions are optimum when the concentration of KH2PO4 is maximum (150 g/L) and the pH fixed between 3.0 and 3.5 to form a coating made essentially with MnO2 and Mg3(PO4)2. Several authors have tested phosphate/permanganate coatings by adding MnHPO4 to the bath and processing Mg–Al alloys, such as AZ91D, AZ61A and AZ80A [9,16]. For all Mg–Al alloys, MnHPO4 (s) tends to dissociate to Mn2+ and HPO2 as shown in reaction 4 (25).

MnHPO4 ðsÞ ¡ Mn2þ þ HPO2 4

ð25Þ

The grain boundaries act as cathodes, and grains function as anodes, forming local cell effects. Meanwhile, hydrogen and phosphate ions are consumed at the substrate/solution interface, causing a pH increase. This phenomenon favors the formation of Mg3(PO4)2(s) deposits. Mg(OH)2(s), MgO(s), MnO2(s) and Mn2O3(s) can be found in the coatings (Fig. 4g) and the presence of Al in the alloys leads to the formation of Al(OH)3(s), Al2O3(s) and MgAl2O4(s) as shown in reactions (26)–(28) [16].

Mg2þ þ O2 ðproduction in pickling bathÞ ¡ MgOðsÞ 3þ

2Al

þ 3O2 ðproduction in pickling bathÞ ¡ Al2 O3 ðsÞ

MgOðsÞ þ Al2 O3 ðsÞ ¡ MgAl2 O4 ðsÞ

ð26Þ ð27Þ ð28Þ

To evaluate the corrosion resistance of these conversion coatings, immersion tests were performed [24]. After 300 s of immersion, several corrosion pits were observed on the surfaces of the bare alloys. However, there was no trace of corrosion, even after 10 h of immersion, on the alloy protected by a phosphate/permanganate conversion coating. The presence of aluminum in the final coating does not disturb its corrosion resistance properties. Salt fog spray tests were performed with the same objective. The phosphate/permanganate and the chromate coatings had the same resistance during spray tests on Mg–Al alloys. The Ecorr values of phosphate/permanganate coatings, listed in Table 6, and of CCCs are similar, 1.45 V/SCE and 1.46 V/SCE, respectively [9]. The corrosion rate confirms the same performance: 13.20 mm/year for the phosphate–permanganate coating and 15.63 mm/year for the CCC. These results clearly indicate that phosphate–permanganate coatings are comparable to CCCs for the protection of magnesium alloys due to the dense MnO2 layer mixed with Mg3(PO4)2(s). But unlike CCCs, phosphate–permanganate coatings do not show any ability to regenerate. 5.5. Rare earth based coatings (REE)

Fig. 5. Conversion coating thickness as a function of the pH value and the bath concentration [9].

Among the seventeen REEs, cerium and lanthanum are the most commonly used for conversion coating. The corrosion resistance of

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magnesium is improved by adding a small amount of a REE, although an excessive addition of a REE detracts from the corrosion resistance. The optimum REE content is between approximately 0.3 wt% and 0.5 wt% of the alloy [57]. When the alloy is dipped in one of the conversion solutions (listed in Table 7), the preformed hydroxide film on the substrate surface immediately dissolves. After that, the primary anodic dissolution reaction of magnesium occurs simultaneously with the reduction of hydronium ions [11]. The addition of oxidants, such as NO 3 or H2O2, can favor oxidation [44,58]. At the same time, the formation of OH increases the pH at the interface between the substrate and the solution. For cerium baths, if the interfacial pH value is high enough, then Ce3+ precipitates on cathodic sites according to reaction (29) [44,58]:

Ce3þ þ 3OH ¡ CeðOHÞ3 ðsÞ

ð29Þ

Coating formation is fast, and it is mainly controlled by the production of OH at the surface of the alloy. As coating formation proceeds, the surface is covered gradually with cerium hydroxide and then oxide. The rate of coating formation gradually slows down. A dense Ce-based conversion coating is obtained on the surface of magnesium alloys. The conversion coating consists of a mixture of trivalent and tetravalent cerium oxides. Exposure to the atmosphere causes the oxidation of Ce(OH)3(s) to Ce(OH)4(s) and the dehydration of the hydroxides to oxides. Consequently, the Ce(IV) content is higher at the coating surface than inside the coating [57]. The conversion coating (Fig. 4h) consists of a mixture of CeO2(s) and Ce2O3(s) [11,39]. The inside and surface layers have different morphologies. It has been calculated that approximately 61% of the cerium species in the coating surface and 45% of the cerium species in the inside area exist in a tetravalent state [57]. When lanthanum (La) solutions are used, they contain La(NO3)3 with between 2 and 16.3 g/L [39,59,60]. The deposition mechanisms of these coating are similar to these of the cerium coatings. The conversion films (Fig. 4i) consist of mixtures of La(OH)3(s), La2O3(s), Mg(OH)2(s), MgO(s) and Al2O3(s) [11,39]. Despite

numerous studies on Ce coatings, the greatest corrosion resistance was obtained with La, although the reproducibility of those results was poor [59]. One of the important effects of the REEs on corrosion resistance is the ‘‘scavenger effect’’. Indeed, REEs create intermetallic compounds with impurities, cancelling the influence of some minor elements, such as Cl and Fe, on the corrosion resistance [59]. Indeed, the corrosion potential of these coatings varies between 1.50 V/SCE and 1.30 V/SCE as shown in Table 7, which is comparable to the values found for CCCs (between 1.10 V/SCE and 1.50 V/SCE) [57]. To improve the corrosion resistance, the presence of the same REE in the alloy and in the conversion bath is advised [59]. The corrosion resistance of the coated alloy is improved if the REE content in the alloy equals 0.3 wt%. Ce or is less than 0.1 wt% La [59]. REE coatings are the only baths that provide this ‘‘scavenger effect’’ with properties as interesting as the self-healing ability of CCCs. However, an adhesive weakness of a cerium conversion coating on AZ31 alloy was noted where the surface layer was easily peeled off with adhesive tape [57]. The adhesion between the surface layer and the inside layer of the coating was much weaker than between the inside layer and the metallic substrate. 5.6. Vanadium based coatings The corrosion protection performance of vanadium coatings on magnesium was also investigated [40,61–63]. Self healing properties similar to the CCCs can be obtained by adding vanadium based oxyanions to the coating. Vanadium solutions are used as corrosion inhibitors in many paints or pigments [61]. The effect of the concentration of the vanadium solution (10, 30 and 50 g/L) and their pH was investigated [40,61,62]. The coating obtained with a vanadium solution of 50 g/L provided corrosion resistance superior to all the conversion baths with lower concentrations [61–63]. The vanadium species responsible for the corrosion resistance are vanadium oxides [40]. The protective effects

Table 6 Composition of phosphate permanganate baths and their respective coatings compositions and properties. Bath characteristics

a

Coating characteristics

Alloy

Compounds

Concentration g/L

Immersion time (s)

pH

Composition

jcorr (lA/cm2)

Ecorr (VSCEa)

e (mm/year)

Thickness (nm)

Ref.

Mg alloy (10%Li 1%Zn)

KMnO4 KH2PO4

40 50

1200

n.d

Mg(OH)2(s) MgO(s) Mn2O3(s) MnO2(s) K and P detected

n.d

1.57

n.d

n.d

[24]

AZ91D

KMnO4 KH2PO4 H3PO4

40 150

600

3–6

n.d

585.8

1.4

13.2

7000–10000

[9]

AZ61A

KMnO4 MnHPO4

20 60

600

n.d

MgO(s) Mg(OH)2(s) MnO2(s) Mn2O3(s) MgAl2O4(s) Al2O3(s) Al(OH)3(s)

18  103

1.48

n.d

n.d

[16]

AZ80A

KMnO4 MnHPO4

20 60

600

n.d

MgO(s) Mg(OH)2(s) MnO2(s) Mn2O3(s) MgAl2O4(s) Al2O(s)3 Al(OH)3(s)

18  103

1.49

n.d

n.d

[16]

AZ91D

KMnO4 MnHPO4

20 60

600

n.d

MgO(s) Mg(OH)2(s) MnO2(s) Mn2O3(s) MgAl2O4(s) Al2O3(s) Al(OH)3(s)

18

1.5

n.d

n.d

[16]

AZ91D

MnHPO4.2H2O

n.d

1800

n.d

Mn, O, P, Mg, Al

n.d

n.d

n.d

10 000

[22]

AM60B

NH4H2PO4 KMnO4 H3PO4

100 20

1200

3.5

n.d

n.d

n.d

n.d

n.d

[56]

VSCE (SCE: Saturated Calomel Electrode).

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Table 7 Composition of REE (Rare Earth based coating) baths and their respective coatings compositions and properties. Bath characteristics

a

Coating characteristics

Alloy

Compounds

Concentration

Immersion time(s)

pH

Composition

jcorr (lA/cm2)

Ecorr (VSCEa)

Ref.

Mg–8.5Li

Ce(NO3)3 La(NO3)3 KMnO4

2 g/L 2 g/L 2 g/L

300

4.0

La2O3(s) CeO2(s) Mn2O3(s) MnO2(s)

n.d

1.5

[11]

Mg–8Li

La(NO3)3

5 g/L

1200

5.0

La(OH)3(s)

1.3

[39]

AZ63

CeCl3 H2O2

10 mg/L 50 mL/L

6  30

n.d

n.d

1.49

[38]

AZ31

Ce(NO3)3

0.05 mol/L

n.d

3.6

CeO2(s)

n.d

n.d

[57]

WE43

Ce(NO3)3

0.05 mol/L

300

3.6

n.d

n.d

n.d

[60]

WE43

La(NO3)3

0.05 mol/L

300

3.6

n.d

n.d

n.d

[60]

WE43

Pr(NO3)3

0.05 mol/L

300

3.6

n.d

n.d

n.d

[60]

AZ31

Ce(NO3)3 H2O2

10 g/L 20 mL/L

300

n.d

CeO2(s) CeO(s) Ce2O3(s) MgO(s) Mg(OH)2(s) Al2(s)

n.d

n.d

[44]

1.05  103

VSCE (SCE: Saturated Calomel Electrode).

Table 8 Summary of the advantages/disadvantages for each coating in comparison to the CCC (Chromate Conversion Coating). Coating

Advantages

Disadvantages

CCC Cr (III) Phosphate Zinc phosphate PCC Phosphate/permanganate REE Vanadium

Best corrosion resistance Resistance to heating superior than CCC Less affected by heat than CCC Corrosion resistance equal to CCC No bath heating is necessary, less affected by heat than CCC Corrosion resistance equal to CCC Scavenger effect Self-healing

High toxicity of its main compound Less corrosion resistance than CCC/thinner layer Small improvement of corrosion Problem of adhesion of the coating to the substrate Need to stabilize the pH of PCC No regeneration ability Expensive Sensitive to alloy composition

of vanadium coatings were examined after 7 days of immersion in 3.5% NaCl solution [63]. The average number and the size of the pitting attack decreased sharply for the samples coated with vanadium. The average number of pits on the surface of the polished samples was 50 pits/cm2. The number of pits decreased to less than 15.7 and 2 pits/cm2 for the samples treated in 10, 30 and 50 g/L solution, respectively. The increased corrosion resistance of the coated samples at 50 g/L and pH = 7 has been explained by the almost perfect selfhealing action that blocked the pitted areas from corrosive attack and other surface defects [61]. It seems that the formation of a vanadium oxide layer plays a distinct role in healing cracks in coating surfaces and repairing pits, and hence improving the overall localized corrosion resistance. Coatings formed in a 50 g/L vanadium solution are more effective than the other treatments with lower vanadium concentrations in reducing the number of pits due to the self-healing ability of the films and the ‘‘buffer effect’’ of the vanadium rich oxide layers that reject the corrosive chloride ions from the surface of the magnesium substrate [61]. The influence of the pH bath has been studied [61]. The increase of a 50 g/L vanadium solution pH from 7 to 9 had a negative effect on the corrosion protection performance of the coating. The alloy surface was severely corroded and covered with pits, and a vanadium layer loosely adhered to the substrate. A uniformly distributed compact coating (Fig. 4j) was obtained with a thickness from 1.5 to 2.5 lm for the 50 g/L vanadium solution [61]. Coatings that result from these processes are composed mostly of unidentified vanadium oxides. Vanadium coating seems to be one of the most promising options for the replacement of CCC. The self-healing ability of vanadium coatings due to their rich oxide layer is an important advantage. However, vanadium coatings have been responsible

for worsening the corrosion of the magnesium alloy EL21 [64]. This unexpected behavior has been explained by the formation of multi-oxide layers of vanadium in addition to the alloying elements Zr, Nd and Zn at the surface, resulting in heterogeneous coatings [64]. The effectiveness of the coating directly depends on the composition of the alloy. This phenomenon has been only observed for vanadium coatings and not for the other alternatives to CCCs.

6. Conclusion Advantages and drawbacks of each of the conversion coatings presented in this review are summarised on Table 8. The CCC is the reference for the protection of magnesium alloys, and its only disadvantage is the toxicity of its main compound. The description of its mechanism shows that the species responsible for the protection of the alloy is trivalent chromium oxide Cr2O3. So naturally, Cr(III) could be considered as the replacement for CCC. However, less corrosion resistance has been observed for Cr(III). This is due to the deposition of a thinner layer and the absence of hexavalent species in the coating that are responsible for the ‘‘self- healing’’ effect. The phosphatization of metals is a well-known process, and zinc can be added to the bath to increase the protection of magnesium to form mixed Mg3(PO4)2 and Zn3(PO4)2 layers. Zinc phosphate has superior corrosion resistance, but it presents no evidence of having the ability to ‘‘self heal’’. Permanganate can be used alone or with phosphate to create an efficient protective coating made essentially by a MnO2 layer with Mg3(PO4)2 when phosphate has been added in the solution. But the instability of the bath pH is the main problem with this approach, and it does not show a ‘‘self healing’’ ability. REE coatings could be considered as a solution, but only alloys that contain REE could benefit from

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protection similar to CCCs. REE and vanadium based coatings possess a ‘‘scavenger’’ and a ‘‘self healing’’ effect, respectively, that makes them comparable to the CCC. However, REE salts are expensive, and the efficiency of vanadium coating is highly dependent on the substrate composition. Today, the use of hexavalent chromium compounds remains the most efficient way to improve the corrosion resistance of magnesium alloys as well as aluminum and zinc alloys. However, as several laws recently limited the use of CCC because of the high toxicity of Cr(VI), it is now necessary to develop new effective alternative processes to protect magnesium alloys. The mechanisms of deposition of CCCs on aluminum and zinc alloys have been clearly identified, but few tests have been performed on magnesium alloys. Moreover, a number of mechanisms for the alternative methods proposed in the literature are unclear. Some of these mechanisms are controversial. To provide solutions, a more complete knowledge is needed of the mechanisms of CCCs (for example, to identify the exact mixed Cr(VI)/Cr(III) oxide) and other methods to enable the development of new conversion coatings that are even more effective than CCCs.

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