Improvement of anticorrosion and adhesion to magnesium alloy by phosphate coating formed at room temperature

NIU Li-yuan(牛丽媛), LIN Ji-xing(林继兴), LI Yong(李 勇), SHI Zi-mu(史子木), XU Lin-chao(徐临超)

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

Wenzhou 325003, China

Received 23 September 2009; accepted 30 January 2010

Abstract: A new surface protection process was developed to magnesium alloy against corrosion in aggressive environments. Firstly, a phosphate coating was formed on rinsed magnesium alloy. Then, powder painting was carried out on the phosphated magnesium alloy. Surface morphologies and phase compositions of the phosphate coating were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that the phosphate coatings formed in bath containing earth additives at room temperature have dense and fine microstructure. The phosphate coating provides excellent paint adhesion to the magnesium alloy. Salt spray tests indicate that the corrosion resistance of the phosphate coating plus paint could meet the demand of magnesium alloy automobile components in aggressive environments.

Key words: anticorrosion; adhesion; magnesium alloy; paint; phosphate coating

1 Introduction

Magnesium alloy components have higher specific strength and lower environmental risks. The demand for light-weight products has grown recently due to the rapid development of automobile, aircraft, and 3C (computer, communication and consumer electronic) industry[1-6]. Under normal conditions, however, magnesium alloy components could not generally meet the demand of automobile components in aggressive environments. Researches on anticorrosion performance of magnesium alloys have been a hot topic nowadays[7-9].

Adhesion between paint and magnesium alloy is poor because of high electrochemical activity of magnesium. To improve the adhesion of magnesium alloy components, they were covered conversion coatings including earth conversion coatings[10-12], stannate coatings[13-14], phosphate-permanganate conversion coatings[15-17], manganese phosphate coating[18] and zinc phosphate coatings[19-24]. However, paints on magnesium alloys tend to produce local corrosion in aggressive environments. Investigations on how to avoid paint local corrosion of magnesium alloys are less.

In this study, the poor adhesion and local corrosion resistance of magnesium alloy were improved by phosphate coating. The influences of phosphate bath compositions on the surface morphologies, anticorrosion and adhesion of paint were studied as well.

2 Experimental

Cast AZ91D magnesium alloy samples (50 mm?50 mm?3 mm) were used. The compositions of the alloy are given in Table l.

The process of surface treatments of magnesium alloys is listed in Table 2. Magnesium alloys were degreased in the alkaline bath and rinsed with distilled water. Afterward, they were treated in the phosphating bath. The composition of the phospating bath is listed in  Table 3. Then, samples were cleaned with overflowing distilled water. At last, FL-11 powder painting was carried out on the phosphate coating and dried.

Table 1 Compositions of AZ91D magnesium alloy (mass fraction, %)

Table 2 Process of surface treatments to AZ91D magnesium alloys

Table 3 Chemical compositions of phosphating baths (g/L)

Surface morphology observations of phosphate coatings were conducted with scanning electron microscope (SEM). The phases in the phosphate coatings were analyzed by X-ray diffraction (XRD).

The potential—time curves were measured using platinum sheet as counter electrode (500 mm²). The working electrode was controlled by a saturated calomel electrode (SCE).

Adhesion tests were conducted according to ISO 2409. This method assessed the adhesion of paints on the substrates when a right-angle lattice pattern was cut onto the coating and penetrated to the substrate. The test results were classified to six grades. The classification of “0” indicates the best adhesion of the coating and that of “5” depicts the worst adhesion of the coating.

The first adhesion of the paint occurred when the coating was painted on the magnesium samples. The second adhesion of the paint came from the reaction when the painted magnesium samples were soaked in water at (80±2) ?C for 100 h.

The salt spray tests were conducted in SF850 salt spray cabinet (Atlas Electric Devices Company). The paints on AZ91D samples were in the size of 100 mm?100 mm?3 mm.

3 Results and discussion

3.1 Effects of phosphate bath compositions on coating formation time

The compositions of basic bath are: 11.3 g/L phosphoric acid, 2.2 g/L zinc oxide, 2.3 g/L sodium fluoride, 1.2 g/L organic amine, 12.5 g/L zinc nitrate. In Bath 1, Bath 2 and Bath 3, alkyl phosphate salt, hydrogen peroxide, sodium cerium and sodium chlorate were used to accelerate the coating formation reactions (Table 3).

Fig.1 shows the open circuit potentials of AZ91D samples in Basic bath, Bath 1, Bath 2 and Bath 3 at room temperature with time.

Fig.1 Potential—time curves of AZ91D magnesium alloys in Basic bath, Bath 1, Bath 2 and Bath 3

In Basic bath, the open circuit potential increased steadily. The potential was stabilized after 54 min of immersion. This meant that the coating formation finished after 54 min in Basic bath. The potential—time curve of magnesium alloys in Bath 1 was similar to that in Basic bath; the open circuit potential was stabilized after 57 min. Hence, alkyl phosphate salt could not effectively accelerate the coating reactions.

In Bath 2, the open circuit potential increased more rapidly and was stabilized after 14 min of immersion. This indicated that hydrogen peroxide effectively accelerated the reactions of the coating formation.

In Bath 3, the phosphating reaction proceeded at the highest speed. The open circuit potential increased significantly during the initial 5 min of immersion. Hence, complex accelerator (sodium cerium and sodium chlorate) in the phosphating bath could effectively accelerate the phosphating reaction.

3.2 Effects of phosphate bath compositions on microstructure of phosphate coatings

The phosphate coatings were prepared in Bath 2 and Bath 3 at (20±2) ?C and (45±2) ?C, respectively, and the surface morphologies of phosphate coatings are shown in Fig.2.

It can be seen that the crystals of the phosphate coating obtained in Bath 3 are finer and more even than that in Bath 2 (Fig.2). In Bath 2, the coating formed at (20±2) ?C was porous and had cracked-mud microstructure (Fig.2(a)). However in Fig.2(b), the coating crystals formed at (45±2) ?C were not even.

In Bath 3, the complex accelerator contained sodium cerium and sodium chlorate accelerated effectively phosphating reaction (Figs.2(c) and (d)). The addition of complex accelerator made the zinc phosphate films dense and improved the integrality of zinc phosphate films. So, the phosphate coating formed in Bath 3 containing earth additive at room temperature had dense and fine microstructure.

Fig.3 shows the XRD pattern of the phosphate coating obtained from Bath 3 in 5 min at (20±2) ?C. It could be seen that the coating was composed of Zn₃(PO₄)₂×4H₂O, AlPO₄ and Zn.

When magnesium alloy was soaked in the bath, α-Mg phases dissolved and hydrogen was evolved. The formation of hydrogen resulted in the increase of local pH value at the interface of magnesium alloy and solution, which facilitated the precipitation of insoluble phosphate (Zn₃(PO₄) ₂×4H₂O, AlPO₄). During phosphating process, Zn²⁺ ions were also reduced to Zn and deposited on the magnesium alloy surface.

3.3 Effects of phosphate bath compositions on adhesion of paint to magnesium alloy

The first adhesion and second adhesion of the paint on the phosphatized magnesium alloy were measured by ISO 2409. The low grade “0” denoted the best adhesion. The adhesion demand of automobile components was grade “0” and grade “1”.

Painting Samples 1-4 were produced on the phosphate coating on the magnesium alloy obtained from Bath 2 at (20±2) ?C, Bath 2 at (45±2) ?C, Bath 3 at (20±2) ?C and Bath 3 at (45±2) ?C, respectively. The coating mass per area of the phosphate film for Samples 1-4 were 0.84, 8.57, 2.23 and 3.19 g/m². Fig.4 shows the adhesion grade of paint for the four samples. It could be seen that Samples 3 and 4 could meet the adhesion demand of automobile components. And Sample 3 showed the best adhesion in the tests of first adhesion and second adhesion.

Fig.2 Surface morphologies of phosphate coating on AZ91D magnesium alloy: (a) Bath 2, (20±2) ?C; (b) Bath 2, (45±2) ?C; (c) Bath 3, (20±2) ?C; (d) Bath 3, (45±2) ?C

Fig.3 XRD pattern of phosphate coating obtained from Bath 3 in 5 min

Fig.4 Adhesion of painted magnesium alloy with phosphate coating from Bath 2 at (20±2) ?C (Sample 1), Bath 2 at (45±2) ?C (Sample 2), Bath 3 at (20±2) ?C (Sample 3) and Bath 3 at (45±2) ?C (Sample 4)

3.4 Effects of phosphate bath compositions on corrosion resistance of phosphate coating for magnesium alloy

Salt spray test results of phosphate film and paint on magnesium alloy are depicted in Fig.5.

The paints on Samples 1 and 2 showed the local corrosion. The rust spots on the Samples 1 and 2 appeared in 220-340 h in salt spray test, and the rust pits grew as pitting corrosion.

Rust on Samples 3 and 4 emerged approximately in 650 h and exhibited uniform corrosion on the whole surface. Rust time of paints on the phosphate coating containing earth additives was about 3 times of that of paints the phosphate coating without earth additives in salt spray test. The paints on Samples 3 and 4 exhibited uniform corrosion, and the local corrosion was avoided by their uniform phosphate coating.

Fig.5　Rust areas of painted magnesium alloy with phosphate coating from Bath 2 at (20±2) ?C (Sample 1); Bath 2 at (45±2) ?C (Sample 2); Bath 3 at (20±2) ?C (Sample 3) and Bath 3 at (45±2) ?C (Sample 4)

4 Conclusions

1) An even phosphate coating were prepared in 5 min of immersion time at room temperature. The addition of complex accelerator made cracks of zinc phosphate films vanish and improved the integrality of zinc phosphate films.

2) The optimal composition of phosphate bath for obtaining more dense crystals at room temperature was: 11.3 g/L phosphoric acid, 2.2 g/L zinc oxide, 2.3 g/L sodium fluoride, 1.2 g/L organic amine, 12.5 g/L zinc nitrate, 0.1 g/L alkyl phosphate salt and 1.6 g/L complex accelerator (with ratio of cerium nitrate to sodium chlorate 1?25).

3) The first adhesion and second adhesion of the phosphate coating on magnesium alloy substrates were improved by the complex accelerator.

4) Salt spray test showed that paints on the phosphate coating containing earth additives exhibited uniform corrosion. Rust time of paints on the phosphate coating containing earth additives was about 3 times of that of paints on the phosphate coating without earth additives.

References

[1] SONG G L, JOHANNESSON B, HAPUGODA S, STJOHN D. Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc [J]. Corros Sci, 2004, 46(4): 955-977.

[2] LIN Ji-xing, NIU Li-yuan, GAO Guang-liang. Study on the technology of the environment-friendly conversioncoating containing phytic acid for magnesium surface [J]. Materials Science Forum, 2009, 610/613(2): 905-910

[3] SONG G L, BOWLES A L, STJOHN D H. Corrosion resistance of aged die cast magnesium alloy AZ91 [J]. Mater Sci Eng A, 2004, 366(1): 74-86.

[4] DELAUNOIS F, PETITJEAN J P, LIENARD P, JACOB-DULIERE M. Autocatalytic electroless nickel-boron plating on light alloys [J]. Surf Coat Technol, 2000, 124(2/3): 201-209.

[5] NIU Li-yuan. Cathodic phosphate coating containing nano zinc particles on magnesium alloy [J]. Trans Nonferrous Met Soc China, 2008, 18(z1): s365-368.

[6] YEROKHIN A L, SHATROV A, SAMSONOV V, SHASHKOV P, LEYLAND A, MATTHEWS A. Fatigue properties of keronite coatings on a magnesium alloy [J]. Surf Coat Technol, 2004, 182(1): 78-84.

[7] SHI Z, SONG G, ATRENS A. Influence of the β phase on the corrosion performance of anodized coatings on magnesium aluminium alloys [J]. Corros Sci, 2005, 47(11): 2760-2777.

[8] SONG G L, ATRENS A. Corrosion mechanisms of magnesium alloys [J]. Adv Eng Mater, 1999, 1(1): 11-33.

[9] SONG G, ATRENS A, DARGUSCH M. Influence of microstructure on the corrosion of diecast AZ91D [J]. Corros Sci, 1998, 41(2): 249-273.

[10] DABAL? M, BRUNELLI K, NAPOLITANI E, MAGRINI M. Cerium-based chemical conversion coating on AZ63 magnesium alloy [J]. Surf Coat Technol, 2003, 172 (2/3): 227-232.

[11] BRUNELLI K, DABALA M, CALLIARI I, MAGRINI M. Effect of HCl pre-treatment on corrosion resistance of cerium-based conversion coatings on magnesium and magnesium alloys [J]. Corros Sci, 2005, 47(4): 989-1000.

[12] RUDDA L, BRESLINA C B, MANSFELD F. The corrosion protection afforded by rare earth conversion coatings applied to magnesium [J]. Corros Sci, 2000, 42(2): 275-288.

[13] LIN C S, LIN H C, LIN K M, LAI W C. Formation and properties of stannate conversion coatings on AZ61 magnesium alloys [J]. Corros Sci, 2006, 48(1): 93-109.

[14] HUO H W, LI Y, WANG F H. Corrosion of AZ91D magnesium alloy with a chemical conversion coating and electroless nickel layer [J]. Corros Sci, 2004, 46(6): 1467-1477.

[15] HAWKE D, ALBRIGHT D L. A phosphate-permanganate conversion coating for magnesium [J]. Met Finish, 1995, 93(10): 34-37.

[16] ZHAO M, WU S, LUO J R, FUKUDA Y, NAKAE H. A chromium-free conversion coating of magnesium alloy by a phosphate–permanganate solution [J]. Surf Coat Technol,2006, 200(18/19): 5407-5412.

[17] KWO Z C, TENG S S. Conversion-coating treatment for magnesium alloys by a permanganate-phosphate solution [J]. Mater Chem Phys, 2003, 80(1): 191-200.

[18] KOUISNI L, AZZI M, ZERTOUBI M, DALARD F, MAXIMOVITCH S. Phosphate coatings on magnesium alloy AM60 (Part 1): Study of the formation and the growth of zinc phosphate coatings [J]. Surf Coat Technol, 2004, 185(1): 58-67.

[19] KOUISNI L, AZZI M, DALARD F, MAXIMOVITCH S. Phosphate coatings on magnesium alloy AM60 (Part 2): Electrochemical behaviour in borate buffer solution [J]. Surf Coat Technol, 2005, 192(2/3): 239-246.

[20] NIU L Y, JIANG Z H, LI G Y, GU C D, LIAN J S. A study and application of zinc phosphate coating on AZ91D magnesium alloy [J]. Surf Coat Technol, 2006, 200(9): 3021-3026.

[21] LI G Y, LIAN J S, NIU L Y, JIANG Z H, JIANG Q. Growth of zinc phosphate coatings on AZ91D magnesium alloy [J]. Surf Coat Technol, 2006, 201(3/4): 1814-1820.

[22] LI G Y, LIAN J S, NIU L Y, JIANG Z H. Influence of pH of phosphating bath on the zinc phosphate coating on AZ91D magnesium alloy [J]. Adv Eng Mater, 2006, 8(1/2): 123-127.

[23] NIU Li-yuan, LI Guang-yu, JIANG Zhong-hao, SUN Li-ping, HAN Dong, LIAN Jian-she. Influence of sodium metanitrobenzene sulphonate on structures and surface morphologies of phosphate coating on AZ91D [J]. Trans Nonferrous Met Soc China, 2006, 16(3): 567-571.

[24] LIAN J S, LI G Y, NIU L Y, GU C D, JIANG Z H, JIANG Q. Electroless Ni-P deposition plus zinc phosphate coating on AZ91D magnesium alloy [J]. Surf Coat Technol, 2006, 200(20/21): 5956-5962.

(Edited by LI Xiang-qun)

Foundation item: Projects(Y20090108, G20080115) supported by Education Department of Zhejiang and Technology Department of Wenzhou, China

Corresponding author: NIU Li-yuan; Fax: +86-577-88313017; E-mail: ccnly@sina.com

DOI: 10.1016/S1003-6326(09)60304-6