Phosphate coatings on magnesium alloy AM60 Part 2: Electrochemical behaviour in borate buffer solution

Phosphate coatings on magnesium alloy AM60

Part 2: Electrochemical behaviour in borate buffer solution

L. Kouisni

, M. Azzi

, F. Dalard

*, S. Maximovitch

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

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

Received 2 November 2003; accepted in revised form 25 May 2004 Available online 14 August 2004

Abstract

A zinc phosphate coating was formed on the surface of magnesium alloy AM60 by immersing the specimen in a phosphating bath.

Corrosion potential measurements, anodic polarisation curves and electrochemical impedance spectroscopy (EIS) were used to assess the corrosion protection of the coating. The electrochemical measurements were performed in borate solution at pH = 9.2. This slightly alkaline solution was chosen because it can be used to differentiate the electrochemical behaviour of phosphated and non-phosphated magnesium alloys. The phosphate coating considerably slowed down the metal dissolution process. Measurements performed on phosphated and non- phosphated specimens in borate solution showed that the phosphate coating afforded considerable protection against corrosion. From the results obtained, it was deduced that the optimum phosphating time was 10 min. The results were fitted to an equivalent electrical circuit.

D

2004 Published by Elsevier B.V.

Keywords:Magnesium alloy; Zinc phosphate; Phosphate coating; Impedance spectroscopy; Borate buffer solution

1. Introduction

Magnesium alloy exhibits an attractive properties for automotive industry but its low corrosion resistance has limited its use [1,2]. Among pre-treatment, phosphating has become popular because of its ability to improve adhesion of the organic topcoat and prevention of underfilm corrosion [3 – 5].

Phosphate coatings were studied in different solutions:

dilute CuSO

solution [5], 0.5 M sodium phosphate buffer at pH 7 [6], 5% NaCl [7], 0.01 M NaOH, 0.5 M Na

SO

[8].

Flis et al. [9] used Na

HPO

solutions of concentration from 5 10

to 0.1 mol/l on steel and electrodeposited Zn and Zn – 12% Ni. They observed that 0.005 M Na

HPO

was most appropriate as a test solution owing to its low aggressiveness and its ability to strongly differentiate the electrochemical behaviour of treated and non-treated mate- rials. They showed that ratios of charge transfer resistance

and interfacial capacitance for phosphated and non-phosph- ated materials could be used for quality evaluation.

Coatings based on zinc phosphating rapidly dissolve in acidic and alkaline solutions and in the presence of com- plexants [5]. The Na

HPO

solution of approximately pH 9 does not dissolve the coating. Weng et al. [10] studied the corrosion and protection characteristics of zinc and manga- nese phosphate coatings formed on steel. They found that chemical dissolution, induced by electrochemical corrosion of the substrate, was the primary form of failure of phos- phate coatings. The corrosion of phosphated steel did not exhibit the characteristics of diffusion in acidic solution, a finite-length diffusion in neutral medium, and a semi-infi- nite diffusion in alkaline solution [10].

Some authors [5,7,8] have observed that phosphate anions are less aggressive than chloride or sulphate anions, and therefore, surface distortion would be smaller than in Na

SO

and NaCl solutions, which are used for coating evaluation. However, like sulphate, phosphate and chloride anions, borate anions form soluble complexes with cations [11], and they can dissolve phosphate coating. Ellipsometric studies [12,13] in de-aerated borate solutions revealed a decrease in the optical parameters of passivating films after

0257-8972/$ - see front matterD2004 Published by Elsevier B.V.

doi:10.1016/j.surfcoat.2004.05.028

* Corresponding author. Tel.: +33-4-76-82-65-91; fax: +33-4-76-82- 67-77.

E-mail address:francis.dalard@lepmi.inpg.fr (F. Dalard).

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addition of oxyanions of phosphorus, suggesting an incor- poration of these anions into the films.

The pH of solutions has a considerable effect on the corrosion rate of magnesium [14]. However, it is difficult to keep pH constant, especially in a neutral solution, because the corrosion product of magnesium, magnesium hydroxide, is readily dissolved into the solution and causes a substantial pH increase [14]. In a test solution containing a buffer agent, such as borate, the pH can be kept constant, and corrosion – resistance measurements could therefore be expected to be accurate. Recently, buffer solutions have frequently been used as test solutions to examine the corrosion behaviour of magnesium and its alloys [15 – 18]. Inoue et al. [19] showed that the corrosion rate of magnesium in the pH 6.5 buffer was higher than that in the pH = 9 by a factor of about 8.

However, there have been no investigations on the effect of buffer capacity on the corrosion behaviour of magnesium.

In our previous report [20], we studied the phosphating of magnesium alloy AM60. The formation of a well-crystal- lised zinc phosphate layer was pointed out in three phos- phatation solutions containing phosphoric acid, phosphate ions, nitrates and nitrites added with zinc and fluorides.

The aim of the present study was to study the electro- chemical behaviour of phosphated and non-phosphated magnesium alloy AM60. A borate buffer solution of about pH = 9.2 was chosen because it was not expected to cause noticeable degradation of the coating. The magnesium substrate was phosphate-treated according to the method described in our previous paper [20]. The electrochemical behaviour of the magnesium alloy was studied with the use of electrochemical impedance spectroscopy (EIS), a method that has also be used to investigate the phosphate coating formation in situ [21].

2. Experimental methods

2.1. Sample preparation

Samples of 8 mm in diameter were prepared from magnesium alloy AM60 which contains 6% Al and 0.28%

Mn. The magnesium electrodes were polished successively to a 1200- and 2400-grit finish. The specimens were ultrasonically degreased in an acetone bath for 3 min between each polishing step. They were cleaned in ethanol and dried under a warm air stream then immersed in a phosphating bath of pH 3.0 containing: 20 g/l Na

HPO

, 7.4 ml H

PO

, 3 g/l NaNO

, 5 g/l ZnNO

and 1 g/l NaF. The phosphate treatment was carried out at about 47 j C for several immersion times lasting between 30 s and 20 min.

2.2. Test procedures

Phosphated and non-phosphated samples were mounted in a cylindrical specimen holder. They were immersed in borate buffer solution of pH about 9.2 (0.93 g/l H

BO

and

9.86 g/l Na

B

O

). Potentiodynamic electrochemical tests were carried out using a computer-monitored Solartron/

Schlumberger Model 1287 potentiostat and the electrochem- ical software CWare for Windows. The results were ana- lysed with CView graphing and analysis software for Windows. In the potentiodynamic polarisation tests, the working electrode was immersed for 5 min in solution and then polarised from the corrosion potential at a scan rate of 0.2 mV s

in the anodic direction. A Saturated Calomel Electrode (SCE) was used as a reference electrode.

Impedance measurements were carried out at corrosion potential with a computer-monitored Solartron/Schlumberger 1255 HF Frequency Response Analyser in conjunction with the Solartron/Schlumberger Model 1287 potentiostat. The frequency range was between 20 kHz and 10 MHz. The instruments were operated through the electrochemical soft- ware ZPlot for Windows and the results were analysed with ZView graphing and analysis software for Windows.

3. Experimental results

3.1. Phosphate coating of surface

In Part 1 of the present study [20], the formation and growth of phosphate coatings on the surface of magnesium alloy AM60 were studied in a phosphating bath containing phosphoric acid, phosphate ions, nitrates, nitrites, zinc and fluorides at pH = 3. Deposition occurred simultaneously with corrosion potential increase, with a stabilisation after about 6 min.

Fig. 1 shows that the corrosion potential increased from 1250 mV to approximately 1100 mV, reaching a

Fig. 1. Open circuit potential of magnesium alloy AM60 as a function of immersion time in phosphating bath atT= 310 K.

maximum after 3-min immersion, then decreased following possible attack of the metal due to the formation of free phosphoric acid. The formation of the zinc phosphate coating on the alloy surface was dependent on the immer- sion time in the phosphating bath. Scanning Electron Microscopy (SEM) was used to show the formation of the coating after immersion times of 30 s, 1, 2, 5 and 10 min in the phosphating bath [20].

Phosphate deposition and oxidation of the magnesium alloy occurred simultaneously in the acidic phosphating bath. After 30-s immersion (Fig. 2a), two types of particles were observed (light and dark) containing phosphorus, oxygen, zinc, magnesium and aluminium. The light par- ticles were richer in zinc and might be the starting point for phosphate germination. After 1 min of phosphating (Fig.

2b), both types of particles were present, with the dark ones developing in preference to the light ones. After 2-min immersion (Fig. 2c), only dark particles were visible and the phosphate coating started to crystallise. After 5 min of phosphating (Fig. 2d), the coating was compact and well crystallised, covering about 90% of the surface. Crystalli- sation started on the dark particles that developed in preference to the clear ones which represented the phosphate germination sites. The image in Fig. 2e shows the growth of the phosphate coating. On the basis of these results, three

stages can be identified in the formation of the phosphate coating: an incubation period of approximately 30 s neces- sary for activation of the metal surface and probably for the formation of the first zinc phosphate nuclei; a growth stage, during which the zinc nuclei on the AM60 surface acted as cathodic sites and promoted the precipitation of zinc phos- phate crystals; a final stage in which the phosphate coating was formed [20]. The coating was analysed using X-ray diffraction and Raman spectroscopy. These analyses con- firmed the presence of a zinc phosphate coating called hopeite [20]. By monitoring the behaviour of phosphate coatings in the borate solution, it should be possible to determine the optimum phosphating time.

3.2. Behaviour of phosphated specimens after immersion in borate buffer solution

3.2.1. Changes in corrosion potential (E

) of phosphated and non-phosphated magnesium alloy specimens

The changes in corrosion potential of phosphated and non-phosphated AM60 after different immersion times in borate solution at pH = 9.2 are represented in Fig. 3.

In the case of non-phosphated AM60, the corrosion potential (E

) increased from an initial value of 1950 mV to a stable value of E = 1850 mV. For the phosphated

Fig. 2. Formation of zinc phosphate coating on the surface of magnesium alloy AM60 as a function of immersion time in phosphating bath (a) = 30 s, (b) = 1 min, (c) = 2 min, (d) = 5 min, (e) = 10 min.

specimens, the initial corrosion potential was more anodic (E = 1150 mV) but decreased with time. Its evolution depended on the duration of the phosphate treatment. They reached about the same limit of 1600 mV at 7000 s for 5-, 10- and 20-min phosphating periods. For 30-s immersion time, the corrosion potential reached that of non-treated specimen.

3.2.2. Electrochemical polarisation curves

The properties of the phosphate coatings were studied by plotting the polarisation curves in borate buffer solution.

Generally, crystalline phosphate coatings are porous. Poros- ity is related to the polarisation resistance.

Fig. 4 presents the anodic polarisation curves for the non- phosphated AM60 specimen and those phosphated for different times (30 s, 5, 10 and 20 min).

The most negative corrosion potential (E

= 1850 mV) was obtained for the non-phosphated AM60, which also exhibited higher oxidation currents.

In the case of the phosphated specimens, E

exhibited more positive values. The anodic current densities of the phosphated specimens were considerably lower than that of the non-treated ones. The most noble corrosion potential was obtained for the specimen phosphated for 10 min.

These results are in agreement with the SEM photo- graphs of the coatings formed on the surface of the magne- sium alloy. The porosity of the coating decreased as the phosphating time increased up to 10 min. The results shown in Fig. 4 were used to determine the polarisation resistance values, which are presented in Table 1.

Polarisation resistance increased with phosphating time and reached a maximum at t = 10 min. In this case, the phosphate coating was compact and limited electrolyte

penetration into the pores of the protective coating. For phosphating times longer than 10 min, R

decreased and the protective properties of the coating deteriorated. This can be explained by the fact that the phosphate coating becomes thicker with immersion time, but with longer times, cracks can appear in the coating.

The polarisation curves were thus found to be correlated with the zinc phosphate crystals growth. After crystallisation of the phosphate coating, the system was more resistant to corrosion.

3.3. Stability of zinc phosphate coating

Electrochemical impedance spectroscopy was used to study the stability of the coatings as a function of phosphat- ing time and immersion time in the borate buffer solution.

3.3.1. Impedance diagrams as a function of phosphating time

Fig. 5 shows the Nyquist diagrams for the AM60 speci- mens after 2-h immersion in the borate buffer solution: non- phosphated specimen and specimens phosphated for differ- ent times. Measurements were performed at corrosion potential.

The shape of the diagrams for the non-phosphated specimen and the AM60 treated in phosphating bath for

Fig. 4. Anodic polarisation curves for non-phosphated AM60 specimen (+) and specimens phosphated for 30 s (o), 5 min (D), 10 min (w) and 20 min (

.

Table 1 Phosphatation time (min)

0 0,5 5 10 20

Rp(Vcm²) 110F5 200F10 690F10 850F10 690F10 Fig. 3. Corrosion potential (Ecorr) of non-phosphated AM60 (E) and AM60

phosphated for 30 s (o), 5 min (

.

q

in borate buffer solution.

30 s is identical after 2-h immersion in the borate solution.

These diagrams are composed of three parts: two capacitive loops (one high frequency HF and one low frequency LF) and one inductive loop (low frequency LF). In the case of the non-phosphated AM60 specimen, the first capacitive loop (HF) can be attributed to the formation of the MgO passive film on the surface of the alloy, and the second capacitive loop (LF) to the diffusion of the electrolyte in the pores. The inductive loop (LF) can be attributed to the relaxation reaction of the oxidation products (adsorbed electroactive species, Mg

or Mg(OH)

). Baril et al. [22]

studied the corrosion mechanism of alloys AM50, AZ91 and AZ91Si in sodium sulphate 0.1 M. They ascribed the high frequency (HF) loop to the charge transfer process. The resistance of this loop could be assimilated to the resistance of the layer of corrosion products. The medium frequency (MF) loop was attributed to a solid phase diffusion process.

The inductive loop (LF) was attributed to relaxation reaction of an adsorbed species.

Fig. 5 shows that the resistance of the specimen phosph- ated for 30 s was higher than that of the non-phosphated specimen. This difference in resistance value might be due to the presence of the first zinc phosphate nuclei formed after 30 s phosphating and which would reduce the active surface of the treated metal compared with the untreated specimen.

In the case of specimens phosphated for 5, 10 and 20 min, the diagrams have two distinct parts according to the literature [10,17]:

– the HF part representative of the zinc phosphate coatings, – the LF capacitive loop display a linear part at 45 j which can be ascribed to diffusion of the electrolyte in the pores.

The resistance of the phosphate coating (R

) corres- ponding to the HF part increased with phosphating time.

The maximum resistance value observed corresponds to the AM60 phosphated for 10 min. For phosphating times longer than 10 min, no improvement in resistance R

was observed. This is in agreement with the results of the polarisation curves (Table 1).

As shown in Fig. 5, the biggest difference was observed between the non-phosphated AM60 and the AM60 phosph- ated for 10 min. After 2-h immersion in the borate solution, the HF resistance value was approximately 103 V for the non-phosphated AM60 and 740 V for the specimen phosph- ated for 10 min. The resistance of the phosphate coatings was up to seven times greater than for non-phosphated AM60. Longer phosphating times did not improve the protective properties of the coating: the resistance of the protective coating formed on the magnesium electrode after 20-min immersion in the phosphating bath was lower than or the same as that of the AM60 phosphated for 10 min.

These impedance diagrams show that the inductive loop observed for the non-phosphated specimen and attributed to magnesium corrosion disappeared in the case of the phosph- ated AM60.

3.3.2. Impedance diagrams as a function of immersion time in borate buffer solution and for different phosphating times The electrochemical impedance diagrams were obtained for AM60 as a function of immersion time in the borate solution and for different phosphating times (Fig. 6). Meas- urements were performed at the corrosion potential of the electrode.

The impedance diagrams obtained for the non-phosph- ated AM60 and the AM60 phosphated for 30 s present two capacitive loops (HF and LF) and an inductive loop (LF) that disappears with immersion time in the borate solution.

In contrast, the impedance diagrams obtained for the speci- mens phosphated for 5, 10 and 20 min present two HF and LF capacitive parts for all the immersion times.

These diagrams can be described by the equivalent electric circuit shown in Fig. 7. R

represents the solution resistance, R

represents the resistance of the phosphate coating, CPE is the constant-phase element representing the phosphate coating (pc). Z

represents the diffusion imped- ance. This term is indicative of electrolyte diffusion through the pores of the coating.

A good correlation was found between the simulated curves and the experimental results (Fig. 8).

Fig. 9a shows the changes in HF resistance as a function of immersion time for non-phosphated AM60 and the specimens phosphated for different periods. The corrosion resistance of the non-phosphated specimen, which was initially very low, continued to increase, even after several hours of immersion (Fig. 9a). The passive layer has been found to get thicker after formation of MgO or Mg(OH)

[22,23]. The same observation was made for the AM60 phosphated for 30 s. Similarly, in the case of the specimen

Fig. 5. Electrochemical impedance diagrams after 2-h immersion in borate buffer solution for alloy AM60, non phosphated (z) and phosphated for different times 30 s (o); 1 min (n); 10 min (

.

Fig. 6. Impedance diagram for non-phosphated AM60 (a) and AM60 phosphated for 30 s (b), 5 min (c), 10 min (d), 20 min (e) immersed in borate buffer solution for 2 h (5), 15 h (

.

phosphated for 5 min, the HF and LF resistance values increased with immersion time in the borate solution. This may be indicative of the formation of MgO or Mg(OH)

on the surface of the metal via the corrosion reaction.

For AM60 phosphated for 10 min, the impedance dia- grams did not change as a function of immersion time in the buffer solution. The HF and LF resistance values remained virtually constant. This result indicates that the phosphate coating formed after 10-min immersion in the phosphating bath was more stable versus corrosion, given that the impedance changed very little with immersion time.

For AM60 phosphated for 20 min, a variation in the protective properties of the coating was observed. The HF resistance increased with immersion time in the borate buffer solution, with the formation of MgO or Mg(OH)

. By comparing the HF resistance values for the AM60 phosph- ated for 10 min and with those for AM60 phosphated for 20 min, it can be seen that the protective properties of the coatings were not improved by prolonging phosphating time.

The resistance of the coating formed on the electrode im- mersed in the phosphating bath for 20 min and then immersed for 2 h in the buffer solution was lower than that of the specimen phosphated for 10 min (Fig. 9a). There is therefore no purpose in prolonging phosphating time beyond 10 min since no improvement in protective properties is obtained.

R

is related to diffusion resistance in the pores. Fig.

9(b) shows that R

increased with immersion time in the borate solution with an optimum value for the specimen phosphated for 10 min. Throughout the immersion period in the borate solution, R

for phosphated AM60 was two to three times higher than that of the non-phosphated alloy.

The results given in Fig. 9 showed that at the experi- mental pH of 9.2, the phosphate coating deteriorated for phosphating times longer than 10 min. The cracks in the phosphate coating increased and the electrolyte penetrated through the pores of the coating, reacting with the substrate and leading to the formation of MgO. Amy [17] showed that the protective properties of a cerium-based coating on magnesium decreased with immersion time in a borate buffer solution at pH = 8.5.

The low polarisation resistance (a few hundred ohms) obtained for phosphated AM60 immersed in the borate buffer solution show that the zinc phosphate coating alone was not sufficient to provide AM60 with good protection against corrosion in a slightly alkaline solution. An alterna- tive way of obtaining better protective properties would be to modify the composition of the phosphating bath, using a trication phosphating process.

These results show that for long time, resistance is relatively stable. Then, borate buffer solution at pH = 9.2 is not very aggressive and can be a good medium for measuring the corrosion resistance of phosphate coatings and compare the electrochemical behaviour of phosphated and non-phosphated magnesium alloy.

Fig. 9. (a) Resistance of phosphate coating (RHF) and (b) diffusion resistance (RLF) of AM60 specimens phosphated for different times: non phosphated (w); 30 s (E); 5 min (o); 10 min (); 20 min (*); as a function of immersion time in borate solution.

Fig. 7. Equivalent circuit used to fit data for phosphated AM60 specimens.

Fig. 8. Impedance spectra for non-phosphated Mg (

z

.

4. Conclusion

The protective properties of the zinc phosphate coating deposited on the surface of the alloy were dependent on the duration of treatment in the phosphating bath.

Borate buffer solution at pH = 9.2 proved to be a good medium not too aggressive for measuring the corrosion resistance of phosphate coatings on magnesium alloy AM60 because of the relative stability for long time measurements.

On the basis of corrosion potentials, polarisation curves and EIS results, it was deduced that the optimum phosphat- ing time was 10 min and that the phosphate coating improved corrosion resistance even after 15 h immersion in the buffer solution. Nevertheless, this resistance value is not sufficiently high in the field of corrosion protection.

In a further study, research efforts will focus on improve- ments in the corrosion resistance of phosphate coatings formed on magnesium alloy surfaces through immersion in a trication phosphating bath (Zn, Ni, Mn).

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