Phosphate coatings on magnesium alloy AM60 part 1: Study of the formation and the growth of zinc phosphate films

Phosphate coatings on magnesium alloy AM60 part 1: study of the formation and the growth of zinc phosphate films

L. Kouisni ^a , M. Azzi ^a , M. Zertoubi ^a , F. Dalard ^b, *, S. Maximovitch ^b

a

Laboratoire d’Electrochimie et Chimie de l’Environnement, Faculte´ des Sciences Aı¨n Chock, Universite´ Hassan II, B.P. 5366 Maˆarif, Casablanca, Morocco

b

Laboratoire d’Electrochimie et de Physico-chimie des Mate´riaux et Interfaces, UMR 5631 INPG-CNRS, ENSEEG, BP 75, Saint – Martin-d’He`res Cedex 38402, France

Received 22 April 2003; accepted in revised form 23 October 2003 Available online

Abstract

The corrosion protection by zinc phosphate conversion coating on magnesium alloy AM60 is studied. Three phosphatation solutions containing phosphoric acid, phosphate ions, nitrates and nitrites added with zinc and fluorides were used. Therefore, the present investigation aims to study the role of the phosphating bath components on the phosphating process and to enhance the possibility of obtaining phosphate layers on magnesium AM60 alloy by immersion in the various phosphatation solutions. The morphology and the coating composition on the electrode surface were analysed (SEM, EDX, X-ray diffraction and Raman spectroscopy). The phosphate films formed are mainly composed of tetra-hydrated zinc phosphate, otherwise known as hopeite. A mechanism is proposed to explain the germination and the growth of the phosphate crystals on AM60 alloy.

D 2004 Published by Elsevier B.V.

Keywords: Phosphatation; Hopeite; Magnesium alloy; OCP; SEM

1. Introduction

In the automotive industry, the use of new materials other than iron will allow both vehicle weight reduction and fuel saving. Consequently, it takes into account the limitation of oil reserves and the environmental problems associated with fuel product emissions [1]. Nowadays, investigations are directed towards aluminium, magnesium and their alloys.

Indeed, magnesium alloys exhibit an attractive combination of low density and high strength/weight ratio [2], have a high thermal conductivity, very good electromagnetic fea- tures and are easily recycled. These properties make them ideal candidates for lightweight engineering applications, especially in the automotive industry, computer parts, the aerospace industry and cellular phones [1]. However, mag- nesium and its alloys are characterised by low corrosion resistance, which has limited their use [3,4]. Before painting

and in order to increase the corrosion resistance, one of the most effective methods is to form a conversion coating on the surface between the metal and its environment. Among the possible treatments, we chose zinc phosphatation. This is a chemical conversion coating where a zinc phosphate layer is formed on the metal surface.

The phosphatation of zinc, electro-galvanised steel, steel and aluminium are well-known processes [5 – 11] whereas phosphating treatment of magnesium and its alloys remains very rarely explored. Zhukov et al. [12] studied by second- ary-ion mass spectrometry, the chemical composition on the surface layer formed on the MA 21 magnesium – lithium alloy in the initial stage of phosphating. In general, for zinc, steel and aluminium it was shown that in phosphoric acid solutions, these metals dissolve and release metal ions. At the same time, hydrogen is evolved from the surface, which leads to a local increase in the pH and phosphate ion concentration, the phosphate ions combining with the metal ions to form an insoluble zinc phosphate layer [5 – 7]. At present, phosphatation processes are studied in relation to the sizeable expansion of new metal surface treatments but

0257-8972/$ - see front matter D 2004 Published by Elsevier B.V.

doi:10.1016/j.surfcoat.2003.10.061

* Corresponding author. Tel.: +33-476826591; fax: +33-476826777.

E-mail address: [email protected] (F. Dalard).

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various phosphatation mechanisms have been suggested in the literature [13 – 19].

In the work reported here, the phosphatation of mag- nesium alloy AM60 is studied by measuring the open circuit potential (OCP) and polarisation curves of the magnesium alloy. The effect of bath components is stud- ied, with or without zinc ions added, and with or without fluoride ions added. Surface analyses by scanning electron microscopy (SEM, EDX) were made in order to show the formation of zinc phosphate film on the magnesium alloy.

X-Ray diffraction was used to analyse the composition of this film and Raman spectroscopy was undertaken in order to clarify the nature of this film. A mechanism of germi- nation and growth of phosphate film on magnesium surface is proposed.

2. Experimental

2.1. Sample

Pure magnesium cannot be used for structural applica- tions due to its low corrosion resistance, so we chose magnesium alloy AM60 which contains 6 wt.% Al and 0.28 wt.% Mn. Aluminium is the additive most often used to improve the corrosion resistance and the mechanical prop- erties of magnesium alloy via the formation of a mixed oxide layer (MgO/Al

O

) which is more protective than that formed in the case of pure magnesium [20].

Samples (in the shape of discs with surface area of 0.28 cm

) were prepared from magnesium alloy AM60.

They were polished successively to a 1200 and 2400 grit finish. The samples intended for the surface analysis were polished to 3 Am then to 1 Am, being ultrasonically degreased in an acetone bath for 3 min between each polishing step. They were cleaned in ethanol and dried in a warm air stream. By similarity with the processes of galvanised steel and aluminium phosphatation, we pro- ceeded via an activating step (pre-phosphating treatment) using a colloidal solution of titanium phosphate, which allows the creation of active centres for zinc phosphate crystal growth [21].

2.2. Phosphatation baths

Three phosphatation baths were studied. Their composi- tion is summarised in Table 1. The bath contents were selected in order to evaluate the precise role of the different

components. At first, we prepared the bath 1 containing phosphoric acid, phosphate ions, nitrates and nitrites, the ions necessary for the magnesium oxidation. This bath contained neither zinc nor fluoride ions, its solution pH was approximately 3. These pH conditions support the magnesium corrosion. The accelerator agent (NO

/NO

) is a stronger oxidant than proton.

In bath 2, zinc ions in the form of zinc nitrate were added and the pH value was maintained at the same value as for bath 1. Under these pH conditions, zinc phosphate (ZnHPO

) is stabilised by phosphoric acid [7]. Zinc ions were added to favour the formation of a crystalline film of tertiary zinc phosphate (hopeite) on the treated metal [3,5,7,9]. In the third bath, fluoride ions were added as an activation agent [5] and the pH was maintained constant (pH c 3).

2.3. Electrochemical set up

Phosphatation is carried out in a thermostated electro- chemical cell, which is composed of a three-electrode Pyrex glass cell (Metrohm). The bath temperature was fixed at 45 F 2 j C. A saturated calomel electrode (SCE) was used as a reference electrode, and placed in an extension containing the phosphatation solution to avoid a chloride effect. A platinum electrode of approximately 1.5 cm

was used as an auxiliary electrode. In order to control the mass transport, a rotating disk electrode EDI 101 Radiometer at 1000 rev./min was used, the exposed surface area being 0.28 cm

. Potentiodynamic electro- chemical tests were carried out using a computer-moni- tored Solartron/Schlumberger Model 1287 potentiostat and the electrochemical software CWare for Windows, the results were analysed with the graphical and analysis Software CView for Windows. In potentiodynamic polar- isation tests, the working electrode was immersed in each bath and then polarised from the corrosion potential at a scan rate of 0.5 mV s

in the anodic and cathodic directions.

2.4. Surface analysis

The morphology and the film compositions were studied by scanning electron microscopy (SEM, EDX) PHILIPS XL 30. Observations were made after the immersion of the samples in the various solutions.

Surface analysis of the samples was performed using a diffractometer equipped with an X-ray generator, a cop-

Table 1

pH and composition of magnesium alloy AM60 phosphatation baths

pH Composition

Bath 1 3F02 Na

HPO

(20 g.l

)+H

PO

(7.4 ml)+NaNO

(3 g.l

)+NaNO

(1.84 g.l

)

Bath 2 3F02 Bath 1+Zn(NO

)

(5 g.l

)

Bath 3 3F02 Bath 2+NaF (1 g.l

)

per cathode (wavelength k=1.5406 A ˚ ) and a software Diffrac+ SOCABIM. In order to clarify the chemical structure of the hopeite crystal film, Raman spectroscopy was used. The Raman instrument (Dilor XY) was used with a source of light (Ar

laser, excitation wave- length=514 532 nm).

3. Results and discussions

3.1. Open circuit potential (OCP)

The formation of the phosphate layers was studied by following the open circuit potential (OCP) of the AM60 magnesium alloy in the three baths as a function of time.

In Fig. 1, the OCP is shown as a function of time for the AM60 samples immersed in the phosphatation baths 1 to 3. The OCP for AM60 initially increases and then stabilises at different values. These values depend strong- ly on the composition of the solutions. Two types of curve are obtained according to the range of potential (Fig. 1). The first one (curve a) corresponds to bath 1 with the lowest value of potential (approx. 1780 mV after 600 s immersion time). This OCP value is close to that for a magnesium electrode in acidic media. The second one corresponds to baths 2 (curve b) and 3 (curve c), and is characterised by higher potentials, an important potential jump which shifts the OCP close to the zinc potential is seen. When zinc ions are present (bath 2), the potential stabilised at approximately 1140 mV after 600 s immersion time. The addition of fluoride

ions (bath 3) increased the potential even more, the potential stabilising at approximately 1120 mV after the same immersion time. The jump in potential from 1780 mV (bath 1) to 1120 mV (bath 3) indicates that the presence of zinc and fluoride ions in the phospha- tation bath supports the formation of a layer which is different from that formed in bath 1.

The electrochemical reactions which could occur in these baths during AM60 magnesium alloy phosphatation are discussed in the following section.

3.1.1. Effect of the phosphoric acid and accelerator agents (bath 1)

The main reactions were the formation of a magnesium phosphate film on the metal surface and the release of hydrogen. Both reactions occur simultaneously:

Mg ! Mg ^2þ þ 2e ð1Þ

2H ^þ þ 2e ! H ₂ ð2Þ

According to the pH values, several acid – base equilibria occur.

At pH=3, H

PO

is the major ion, because of their low solubility, Mg

ions precipitate as Mg

(PO

)

on the magnesium surface:

3Mg ^2þ þ 2H 2 PO ₄ ! Mg ₃ ðPO 4 Þ ₂ þ 4H ^þ ð3Þ The complete reaction is as follows:

3Mg þ 2H ^þ þ 2H 2 PO ₄ ! Mg ₃ ðPO 4 Þ ₂ þ 3H 2 ð4Þ In the phosphating bath 1, the accelerator agents increase the rate of magnesium alloy oxidation. Here, the stronger oxidants are the nitrate ions. The nitrate reduction is:

NO ₃ þ 2H ^þ þ 2e ! NO ₂ þ H 2 O ð5Þ During this nitrate ion reduction, the local pH at the metal – solution interface can increase quickly and facilitate the precipitation of insoluble phosphate.

3.1.2. Effect of the Zn

ions (bath 2)

In the literature, two contradictory theories are proposed to explain the formation of the zinc phosphate (hopeite) layer. According to Wulfson et al. [22], Cupr and Pelikan [18] phosphate precipitation is induced when the solubility product is exceeded. This occurs on the anodic areas. An amorphous layer composed of mixed phosphates of zinc and base metal (iron) is formed. This layer constitutes the basis for the development of crystal nuclei of metallic phosphates. Their theory is based on the existence of [ZnPO

]

resulting from the dissociation of Zn(H

PO

)

.

Fig. 1. Open circuit potential of magnesium alloy AM60 as a function of

time in: (a)=bath 1, (b)=bath 2, (c)=bath 3, bath 1=Na

HPO

(20 g

l

)+H

PO

(7.4 ml)+NaNO

(3 g l

)+NaNO

(1.84 g l

), bath 2=Bath

1+Zn(NO

)

(5 g l

), bath 3=Bath 2+NaF (1 g l

), temperature=45 j C.

Development of the amorphous layer in the anodic areas is caused by the electrochemical reaction:

Me þ 2ZnPO ₄ ! MeZn 2 ðPO 4 Þ ₂ þ 2´e Me ¼ Fe ð6Þ On the anodic sites, there is a high concentration of metal cations resulting from attack of the metal (Me), the solubil- ity product Me

(PO

)

is achieved and could enable its precipitation.

However, authors such as Machu [23] and Saison [24]

put forward the hypothesis of precipitation of metal phos- phates on the microcathodic sites due to the pH gradient at the metal – electrolyte interface, caused by reactions (2) and (5).

A certain number of important questions on the theory of cathodic deposition might be raised [25]:

Why do the insoluble phosphate crystals that form at the interface (liquid/solid) become fixed there and do not precipitate in the form of sludge.

During phosphating treatments by spraying, where there is continuous renewal of the solution at pH between 2.5 and 3.2, how can the pH reach the local values of 5 – 6 necessary for phosphate precipitation to occur?

Metal phosphate precipitation by hydrolysis releases phosphoric acid. How can the pH be maintained at 5 for the time needed for the phosphate layer to form?

In the present study, it was assumed that phosphatation could be initiated on the cathodic sites. In the case of alloy AM60 for example, the phase b Mg

Al

[26] could constitute a cathodic area. Similarly, metal nuclei (titanium phosphate particles) deposited during the activation step would act as a nucleation agent [21]. Furthermore, the formation of zinc nuclei is thermodynamically possible, since in the first times when the electrode is immersed in the solution containing zinc ions, the OCP is considerably lower than the equilibrium potential of the zinc (Fig. 1).

The OCP reaches higher values (1140 mV/SCE) (Fig.

1) in the presence of zinc ions (Zn

). The reduction reactions (2 and 5) lead to an important local pH increase near the cathodic nuclei, and the cathodic reaction occurring on these nuclei is:

3Zn ^2þ þ 2H 2 PO ₄ þ 2H ^þ þ 4H 2 O

þ 6e ! Zn ₃ ðPO 4 Þ ₂ 4H 2 O þ 3H ₂ ð7Þ Thus, the addition of zinc ions to the solution support the formation of the crystals of zinc phosphate. The zinc ions combine with phosphate ions to form an insoluble zinc phosphate film.

In the presence of zinc ions the whole phosphatation reaction is:

3Mg þ 3Zn ^2þ þ 4H 2 PO ₄ þ 4H 2 O ! Zn 3 ðPO 4 Þ ₂

4H 2 O þ 3H ₂ þ Mg ₃ ðPO 4 Þ ₂ þ 2H ^þ ð8Þ

The OCP measurements in the baths give information about the phosphate layer formation since the only differ- ence is the composition of the phosphating baths. Conse- quently, the gap between the OCP with or without zinc ions is due to the formation of a new layer on the electrode surface in bath 2.

First, cathodic nuclei then zinc phosphate are formed on the magnesium surface due to the increase of pH on the cathodic sites. The formation of insoluble magnesium phos- phate is possible as well on the anodic area. These results could be confirmed by surface analysis.

3.1.3. Effect of the Zn

ions associated with fluorides (bath 3)

The comparison of the potential as a function of immer- sion time in bath 3 (Fig. 1) and bath 2 shows that the fluoride ions increased the potential (to approx. 1120 mV/

ECS). The presence of zinc and fluoride in the phosphating bath led to the formation of a better passive layer.

Aluminium, which is presence in the alloy AM60, slow down the formation of the phosphate film on the magnesium surface. Adding fluorides to the bath is a useful technique for removing aluminium ions, which eventually precipitate in the bath through the following reactions [8 – 10,21]:

Al þ 6F ! AlF ³ ₆ þ 3e ð9Þ

AlF ³ ₆ þ 3Na ^3þ ! Na ₃ AlF ₆ ð10Þ

The role of fluorides is thus to complex the Al

ions that release area for the cathodic sites (zinc). Fluoride ions might then influence the zinc phosphate film formation and increase the number of zinc nuclei.

3.2. Anodic and cathodic polarisation plots

The polarisation curves of AM60 in the three baths are shown in Fig. 2. Bath 1 favorize the amorphous phospha- tation whereas both baths 2 and 3 lead to the formation of the crystalline phosphatation.

The three different OCP according to the phosphating bath components are seen again. The lowest potential value is obtained for the bath 1 (1800 mV/SCE), the highest value is obtained with the third bath containing zinc associated to fluoride ions (1100 mV/SCE).

The cathodic curves are similar even if the hydrogen

discharge does not occur at the same potential, whereas

the anodic polarisation curves are different. In bath 1, a

current increasing with increasing of potential is ob-

served. The anodic current increases slowly with increas-

ing potential. In this case, (bath 1) the anodic reaction

could correspond to the formation of a magnesium

phosphate layer on the surface. After the addition of

zinc ions (bath 2), the OCP shifted to the higher region,

a small reduction in cathodic current density appeared

compared to that in the bath 1, and a peak is observed

at E

=0.7 V, i

=20 mA/cm

. The cathodic current in bath 2 remains unchanged compared to bath 1, even though the OCP is higher than that noted in bath 1. The observation of the peak and the variation of the current and the anodic potential occurred after zinc addition could be due to the protecting effect of the zinc phosphate layer that limits the available anodic surface.

After the addition of the fluoride ions (bath 3), the peak is enhanced, and is at E

=0.9 V, i

=10 mA/cm

. The anodic current is lower than that of bath 2. The cathodic current in the case of bath 3 remains equal to that of bath 2 even if the OCP is higher than that of bath 2.

The addition of fluoride ions in the case of bath 3 yields a larger protecting effect of the zinc phosphate layer on the magnesium surface. These ions involve the scouring action of the aluminium surface (reactions 9 and 10) (aluminium is present in the alloy) by supporting the formation of the more protective phosphate layers. The available anodic surface must have decreased.

These results are in agreement with the OCP measure- ments; they emphasise the role of the bath components in magnesium alloy AM60 phosphatation. The Zn

ions modify the phosphate layer formed. Two successive steps are proposed in order to explain the growth of the phosphate layer: (i) the activation of cathodic nuclei; (ii) the precipi- tation of zinc phosphate on these cathodic nuclei.

SEM images and EDX analysis were used to see whether the fluoride ions increase the number of zinc nuclei.

3.3. Morphology and composition of the coating

In order to confirm the various mechanisms of phos- phate film growth, we observed and studied the coating

formed on the magnesium surface by SEM and EDX.

SEM photographs (Fig. 3) of the coating, obtained after 600 s in the three solutions, show the evolution of the metal surface in the presence of each phosphating bath component.

In bath 1, containing only the phosphoric acid, the phosphate ions and the accelerator agents, no crystalline structure coatings are formed. In baths 2 and 3, contain- ing fluorides and/or zinc ions, the formation of a well- crystallised coating is observed. These photographs (Fig.

3) also show that the size reduction of the crystals and the formation of more compact layers are due to the presence of the fluoride ions (bath 3). The EDX analyses of the coating obtained in the baths 1 – 3 are also shown in Fig. 3. They indicate that the composition of the layer in the first bath is different from that in the baths 2 and 3. In bath 1, the coating contains magnesium phosphate and a larger amount of aluminium compared to its content in the alloy, which means that the selective alloy corrosion leads to a faster corrosion of aluminium compared to that of magnesium, so the surface is more enriched in aluminium. In baths 2 and 3 with zinc ions, the film appeared to be highly enriched in zinc and only a small amount of aluminium and magnesium was detected. This result confirms the role of the zinc and fluoride ions added in solutions 2 and 3. Zinc ions support the formation of the zinc phosphate crystals, and slow down de corrosion of magnesium alloy.

As we noted above, the oxidation of the magnesium alloy AM60 releases Al

ions. These ions slow down the phosphatation process. Fluoride ions act by a scour- ing action on the aluminium surface (see Section 3.1.3).

Fig. 4a shows that fluoride ions influence the coating formation by a reduction in the size of the first cathodic nuclei and consequently the increase in their number.

SEM photographs for phosphated surfaces after vari- ous immersion times in bath 3 are shown in Fig. 5 for several times: a (30 s), b (60 s), c (120 s), d (300 s) and e (600 s).

From the SEM photographs shown in Fig. 5a (30 s) it can be seen that two kinds of coating can be found on the surface of the AM60 samples (the clear and the dark ones).

EDX analysis showed that the clear particles contain more zinc than the dark ones (Fig. 4b).

From Fig. 5b (60 s), it can be noted that both the clear and the dark particles can be seen, but only the dark ones are developed. However, for larger immersion times (120 s) (Fig. 5c), the potential increases, and only one type of zinc phosphate particle is formed (dark). It can be noted that zinc phosphate starts to crystallise with a covering rate of approximately 90%. These are small crystals. They develop starting from the clear zones that could represent cathodic sites. Fig. 5c highlights the beginning of crystallisation and the presence of homo- geneous zinc phosphate layers. A short immersion time of approximately 30 s is sufficient to start the germina-

Fig. 2. Cathodic and anodic polarisation plots of magnesium alloy AM60 in: bath 1; (b) bath 2 (c) bath 3 bath 1=Na

HPO

(20 g l

)+H

PO

(7.4 ml)+NaNO

(3 g l

)+NaNO

(1.84 g l

), bath 2=Bath 1+Zn(NO

)

(5 g l

), bath 3=Bath 2+NaF (1 g l

), temperature=45 j C.

tion of the phosphate. Moreover, at long immersion period (300 s), (Fig. 5d) the magnesium surface is completely covered with the dark phosphate coating, these immersion steps are characterised by a small potential increase and a maximum covering of the surface by phosphate. Hence we have to take into account the impact of the longer immersion time on the zinc phosphate growth. After 600 s of immersion in phosphating bath (Fig. 5e), the morphology of the coating shows dispersion of micro-cavities. Photographs

‘d’ and ‘e’ show that it is needless to increase the immersion time treatment since the micro-cavities cannot be eliminated. A 300 s immersion in the phosphating bath seems to be sufficient for the zinc phosphate layer

formation on magnesium AM60. These SEM observa- tions confirm the result obtained by the OCP and the polarisation curves.

On the basis of the above discussion, it is suggested that the magnesium phosphatation occurs in at least three steps: (i) A short incubation step of approximately 30 s with activation of the cathodic sites on magnesium surface; (ii) a growth step where the zinc phosphate crystals precipitate on the cathodic sites, along with the rapid growth of individual crystals; and (iii) a final step in which the surface coverage increases, leading to the formation of a more protective zinc phosphate film. The formation of magnesium phosphate continues also on the anodic area.

Fig. 3. SEM photographs and EDX analysis for magnesium alloy samples after immersion in: (a) bath 1, (b) bath 2, (c) bath 3.

The fact that there are distinct steps in the zinc phosphate crystal growth makes this process similar to that observed in the case of aluminium phosphatation [5], but is different from that described in the case of zinc and iron. Gaarenstoom and Ottaviani [11] showed that there are no distinct stages in the phosphatation of these metals.

Typical X-ray spectra for the nuclei, crystals and sub- strate are shown in Fig. 6. The exposed substrate exhibited X-ray peaks for magnesium. There is not a large difference between the substrate and nuclei compositions. The crystals exhibited other peaks that correspond to tetra-hydrated zinc phosphate (hopeite) [27]. This result confirms that the conversion coating of magnesium alloy is principally com- posed of hopeite crystals.

Raman spectroscopy was applied to clarify the chemical structure of the hopeite crystal films. Orthophosphate [PO

] forms a regular tetrahedron as shown in Fig. 7 [28], m

, m

, m

and m

are, respectively, the symmetrical stretching vibration A, the double degeneracy vibration E with deformation, the triple degeneracy vibration F

of stretching type, and the triple degeneracy vibration F

with deformation. At 800 – 1300 cm

, only m

and m

are observed [28,29].

Previous authors have considered the relationship between the metallic components in hopeite films and Raman spectra [30,31]. It was found that four peaks of the Raman spectrum for [PO

] were observed and that the Raman bands shifted according to the metallic component composition in the hopeite crystals. However,

Fig. 4. (a) Photographs SEM of magnesium alloy AM60 immersed in: (i) bath 2 without fluoride ions and (ii) bath 3 after addition of fluoride ions. (b) EDX

analysis of the particles formed on the magnesium alloy: (i) clear particle and (ii) dark particle.

in the case of a solid crystal such as hopeite, the symmetry will become distorted by interaction with the surrounding crystalline structure. As a result, it is thought that the degeneracy of the vibration mode shows in Fig. 7 will be split [31].

In this study, conventional hopeite films were analysed ex-situ in solid state and the attribution of Raman bands for [PO

] was studied.

Raman spectrum for hopeite Zn

(PO

)

4H

O, in the solid state is shown in Fig. 8, because the degeneracy of m

is three-fold, four peaks were observed in the region 800 – 1300 cm

with the main peak for m

and three other peaks for m

. This is consistent with the result given by Minami and Sato [30] and Sommer and Leidheiser [32].

This spectrum corresponds to [PO

] in hopeite films and is attributable to stretching vibration.

Magnesium phosphate is obtained in the bath without zinc ions. In the bath with zinc ions and after the activation of the cathodic sites on magnesium surface, the hopeite film precipitates on the cathodic sites. Addition of fluoride ions

in the phosphating bath (bath 3) allows an increase in the number of cathodic nuclei and consequently eases the formation of a thin and more protective hopeite film.

4. Conclusion

It is possible to form a well-crystallised zinc phosphate layer on magnesium alloy AM60 in solution (bath 3) con- taining phosphoric acid, phosphates, nitrates, nitrites, zinc and fluoride ions. The microanalysis of the coating by EDX confirms the presence of the zinc, phosphorus and oxygen that must be found in the hopeite films. X-Ray diffraction and Raman spectroscopy confirm that the conversion coating of magnesium alloy is principally composed of hopeite crystals.

The zinc phosphate layer (hopeite) formation is strongly influenced by the presence of zinc and fluoride ions in the phosphating bath. The addition of zinc ions to the phosphating solution encourages the formation of the crystals of an insoluble zinc phosphate film. The

Fig. 5. Photographs SEM of AM60 samples immersed in bath 3 for (a) 30 s, (b) 60 s, (c) 120 s, (d) 300 s and (e) 600 s, bath 3=Na

HPO

(20 g l

)+H

PO

(7.4

ml)+NaNO

(3 g l

)+Zn(NO

)

(5 g l

)+NaF (1 g l

), temperature=45 j C, pH=3.

fluoride ions increase the number of nuclei (microcatho- dic sites) formed on AM60. They decrease the size of the crystals and allow the formation of a more compact layer, and then complex the Al

ions that release area for the cathodic sites.

The germination of the hopeite crystals is achieved in three significant steps:

An incubation time of approximately 30 s after immersion, necessary for the activation of the micro- cathodic sites on the magnesium surface;

A precipitation of the zinc phosphate crystals on the microcathodic sites;

A determining step, occurring 300 s after immersion, in which the individual zinc phosphate crystals continue to grow and form a coating covering the whole metal surface.

Within the framework of this study, we have shown the possibility of forming a well-crystallised layer on a magne- sium surface and have discussed the growth of the phos- phate layer.

Acknowledgements

Thanks are due to Mr L. Maniguet and Mr A. Crisci of CMTC, INPGrenoble for assistance with scanning elec- tron microscopy observations and Raman Spectroscopy analysis.

Fig. 6. X-Ray diffraction spectra of coating formed on magnesium in the phosphating bath 3.

Fig. 7. Structural model of basic vibration for [PO

] [28].

Fig. 8. Raman spectrum of the film formed on magnesium surface at

different phosphatation duration: (a) 30 s, (b) 300 s and (c) 600 s.

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