­­­ Cathodic phosphate coating containing nano zinc particles on magnesium alloy

NIU Li-yuan(牛丽媛)

Zhejiang Industry and Trade Polytechnic, Advanced Material Research and Development Center,

Wenzhou 325003, China

Received 12 June 2008; accepted 5 September 2008

Abstract: A technology for preparation of a cathodic phosphate coating mainly containing nano metallic zinc particles and phosphate compounds on magnesium alloy was developed. The influence of cathodic current density on the microstructure of the cathodic phosphate coating was investigated. The results show that the crystals of the coating are finer and the microstructures of the outer surface of the coatings are zigzag at the cathodic density of 0.2-0.5 A/dm². The content of nano metallic zinc particles in the coating decreases with the increase of the thickness of the coatings and tends to be zero when the coating thickness is 4.14 mm. The cathodic phosphate coating was applied to be a transition coating for improving the adhesion between the paints and the magnesium alloys. The formation mechanism of the cathodic phosphate coating was investigated as well.

Key words: cathodic phosphate coating; magnesium alloy; formation mechanism

1 Introduction

Studies on the surface treatments to prevent the corrosion of the magnesium alloys have been developed quickly in the recent years[1-3]. Chemical conversion films of the magnesium alloy were widely used to the base film of paint for ensuring good contact between the paint and the magnesium alloy substrate[4]. The chromate conversion treatment was a traditional method to obtain anticorrosion film of metal[5-6]. However, in the interests of pollution control, there is a tendency to replace the chromium passivating baths by the chromium-free treatment baths.

The phosphate coatings were the most promising substitutes to the chromate coating that can create good bonding between the paint film and the metal[7]. Based on this consideration, the zinc phosphating process has been widely used in the automobile industry as a pretreatment method on the metal surface[8-9]. There were some reports on the phosphating processes of magnesium alloys being revealed in these years[10-12]. However, few investigations have been made for the electrochemical phosphating process of the magnesium alloys.

In the present study, a small cathode current was used to accelerate the phosphatization of the magnesium alloy samples. In the previous studies, chlorate, sodium metanitrobenzene sulphonate[13], nitrate[14-15] and nitrite[16] were used to be the accelerating agents in the phosphating baths. However, the poisonous accelerator such as nitrate and nitrite gave out poisonous gases, and the excessive accumulation of dissolution production of accelerators would shorten the life of the phosphating bath.

In this test, the cathodic phosphate coatings were prepared in a simple phosphating bath without accelerators. Micro performance and adhesion characteristic of the coatings were estimated and the formation mechanism of the coating was investigated.

2 Experimental

AZ91D magnesium alloy samples with the size of 50 mm×50 mm×3 mm were used in the experiments. The samples were degreased in a 10.5% KOH and rinsed in de-ionized water to remove all the alkali before the phosphating treatment. The cleaned specimens were then treated in the phosphating bath and dried. The phosphating temperature ranged from 20 to 30 ℃. The detailed process in this study is listed in Table 1.

Two graphite plates (100 mm×50 mm×5 mm) were applied to the anodes at two sides of the magnesium

Table 1 Cathodic phosphating process of AZ91D magnesium alloy

alloy plates. The zinc ions were provided by the supplement of zinc oxide. The operation temperature was 20-30 ℃.

The adhesion tests were conducted according to the ISO2409 standard. Adhesion of the paint was divided to six classifications from 0 classification (best) to 5th classification (worst) in the ISO2409 standard. For comparison, the cathodic phosphate coatings (obtained according to Table 1), the normal phosphate coatings[16] and the chromate conversion coatings[5] were applied to the transition coating between the paints and the magnesium alloy.

3 Results and discussion

3.1 Morphology and composition of cathodic phosphate coating

Fig.1 shows the SEM morphologies of the cathodic  phosphate coating on the AZ91D magnesium alloy obtained from the phosphating bath at 0.5 mA/cm². It can be seen in Fig.1(a) that the diameter of the crystals of the phosphate coating is 1-5 mm. The visible crystals in Fig.1(a) are phosphate compounds of the coatings and they are usually more than 10 mm in a normal phosphating process. The metallic zinc particles cannot be clearly seen because they are too small.

Fig.1 SEM morphologies of cathodic phosphate coating on AZ91D: (a) Surface image; (b) Cross-section image

Fig.1(b) shows the cross-section image of the cathodic phosphate coating. It can be seen that the microstructure of the coating is zigzag, which helps to improve the adhesion between the paint and the magnesium alloy.

The composition of the phosphate coating is determined by the energy spectrum and XRD pattern (Fig.2). According to the energy spectrum, the coating consists of Zn, P, O, Mg and Al. The XRD pattern shows that coating consists of mainly metallic zinc particles and various phosphate compounds including Zn₃(PO₄)₂×4H₂O, AlPO₄ and MgZn₂(PO₄)₂.

The grain sizes of the coatings were measured through the Sherrer equation. The FWHM (full wave at half maximum) of the plane was used for the measurement of the crystals size. In Fig.2(b), the FWHM of the diffraction peak of the zinc plane (2θ=43.34?) is 0.235, hence the average size of the metallic zinc and the coating is about 28.2 nm.

Fig.3 shows the relation between the metallic zinc content and the coating thickness. It can be seen that the increase of the thickness from 0.53 to 4.14 mm causes a decrease of the metallic zinc content from 46.4% to 0. The external layer of the coating (beside magnesium alloy base) has the highest concentration of the metallic zinc particles. The outer layer of the cathodic phosphate coating on the magnesium alloy mainly consists of the phosphate compounds without zinc particles.

3.2 Influence of phosphate coatings on paint adhesion

The results of the adhesion test of the paint coatings  on the phosphate coatings and chromate conversion coatings are listed in Table 2. According to ISO2409, “0” classification means the best adhesion performance.

Fig.2 Composition analysis of phosphate coatings formed from baths at 0.5 mA/cm²: (a) Energy spectrum; (b) XRD pattern

Table 2 shows that the adhesion classifications of the paints are “0” when the cathodic phosphate coatings are the transition coatings of cataphoric paints (about 20 μm in paint thickness) and powder paints (about 60 μm in paint thickness). Therefore, the cathodic phosphate coating leads to an evident improvement of the paint  adhesion on the magnesium alloy. This is because the cathodic phosphate coating has the zigzag structure (Fig.1(b)).

Fig.3 Variation of zinc content with increase of coating thickness

When the normal phosphate coatings and the chromate conversion coatings are the transition coatings of cataphoric paint, the adhesion classifications of the paints are “0” and “1”, respectively.

Also, the adhesion of powder paints based on the normal phosphate coatings of magnesium alloy is worse (“1” classification). The crystals of the normal phosphate coatings of the magnesium alloy[16] were larger than those cathodic phosphate coatings.

The adhesion of the powder paints based on the chromate conversion coatings of the magnesium alloy is the worst (“2” classification) because the surface of chromate conversion coating of the magnesium alloy is not uneven.

3.3 Formation mechanism of cathodic phosphate coating

In the process of the cathodic phosphating, there are many micro-anodes and micro-cathodes on the surface of the magnesium alloy.

Table 2 Results of adhesion tests

On the micro-anode sites of the magnesium alloy surface, Mg and Al are dissolved:

                            (1)

                              (2)

At the same time, metallic zinc particles are generated at the sites of the Mg atoms:

                             (3)

At the micro-cathode sites, hydrogen is given out:  

                              (4)

The reduction of Reaction (4) results in the increase of local pH at the metal and solution interface, which facilitates the precipitation of insoluble phosphate compounds. Therefore, the formation reactions of insoluble phosphates occur as follows:

                      (5)

                        (6)

       (7)

A cathodic current accelerates the reduction reaction on the surface of the magnesium alloy samples (Reactions (3), (4) and (5)). The growth of the crystals of the zinc particles and the phosphate compounds inhibits each other. As a result, the coating crystals become finer.

There is an electrochemical replacement reaction between magnesium and zinc, causing the deposition of the zinc particles (Reactions (1) and (3)). With the increase of the coating thickness, the surfaces of the magnesium alloy substrate are covered gradually. Therefore, the content of the zinc particles decreases with the increase of the coating thickness and tends to be zero.

4 Conclusions

1) Finer cathodic phosphate coatings are formed on the AZ91D magnesium alloy in the phosphating bath at 0.2-0.5 A/dm². The optimal bath compositions are: 15.5-17.8 g/L of 85% phosphoric acid; 2.5-3.9 g/L of zinc oxide; 1.5-1.8 g/L of sodium fluoride; 0.03-0.05 g/L of sodium dodecylsulfate.

2) Zn₃(PO₄)₂?4H₂O, AlPO₄, MgZn₂(PO₄)₂ and zinc particles are the main compositions of the cathodic phosphate coating. The average grain size of the metallic zinc in the film is about 28.2 nm and the content of the metallic Zn particles in the coating decreases with the increase of the coating thickness.

3) The cathodic phosphate coatings were applied to a transition coating of the magnesium alloy for improving the adhesion between the paints and the magnesium alloy. The results show that phosphate coatings obtained at 0.2-0.5 A/dm² exhibit the best adhesion performance, which is suitable to not only the cataphoric paint but also the power paint.

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(Edited by LI Xiang-qun)