*Ming Liu

aNanjing Tech University,No.30 South Puzhu Road,Nanjing 211816,China

bNanjing Haorang Environment Science&Technology CO.,Ltd,No.30 South Puzhu Road,Nanjing 211816,China

cGeneral Motors China Science Lab,No.56 Jinwan Road,Shanghai 201206,China

dXi'an University of Technology,No.5 South Jinhua Road,Xi'an 710048,China

Preparation and characterization of the micro-arc oxidation composite coatings on magnesium alloys

Yanfeng Gea,b,*,Bailing Jianga,b,Ming Liuc,Congjie Wangd,Wenning Shend

aNanjing Tech University,No.30 South Puzhu Road,Nanjing 211816,China

bNanjing Haorang Environment Science&Technology CO.,Ltd,No.30 South Puzhu Road,Nanjing 211816,China

cGeneral Motors China Science Lab,No.56 Jinwan Road,Shanghai 201206,China

dXi'an University of Technology,No.5 South Jinhua Road,Xi'an 710048,China

The magnesium alloys attract the light-weight manufacture due to its high strength to weight ratio,however the poor corrosion resistance limits the application in automobile industry.The Micro-arc Composite Ceramic(MCC)coatings on AZ91D magnesium alloys were prepared by Micro-arc Oxidation(MAO)and electrophoresis technologies.The microstructure,corrosion resistance,abrasion resistance,stone impact resistance and adhesion of MCC coatings were studied respectively.The cross section morphologies showed that the outer organic coating was f i lled into the hole on surface of MAO coating,and it acted as a shelter against corrosive products.The copper-accelerated acetic acid salt spray Test,abrasion resistance test,stone impact resistance test,thermal shock resistance test and adhesion test were used to evaluate the protective characterization by the third testing organization which approved by GM.The test results showed the composite coatings meet all the requirements.The MCC coating on Mg presents excellent properties,and it is a promising surface treatment technology on magnesium alloys for production vehicles.

AZ91D;Micro-arc oxidation;Electrophoresis;Composite coatings;Properties

1.Introduction

Recently,limited fossil fuel and environmental problems have promoted the use of lightweight material,such as magnesium,for automotive use in order to improve fuel economy consumption and decreasing exhaust emissions [1,2]. Magnesium-based alloys are of increasing attraction for many industrial applications on account of their low density,high specif i c strength,good cast-ability,good weld-ability and mechanical properties,especially in automotive production [3,4].Porsche engineers striving to progressively reduce engine weight and ref i ne part production selected magnesium for the V6 and V8 engine front covers,using precisely the same lightweight cover part for both engine types.The front cover weighs just 2.14 kg,compared to 3.89 kg using an aluminum cover a 45 percent weight savings[5].

Unfortunately,magnesium has a number of undesirable properties including poor corrosion and wear resistance,poor creep resistance and high chemical reactivity.Magnesium and its alloys were extremely susceptible to galvanic corrosion, which can cause severe pitting in the metal resulting in decreased mechanical stability and an unattractive appearance [6].Surface coating technology is one of the most effective methods to prevent magnesium from degradation[7,8].Coatings can protect a substrate by providing a barrier between the metal and its environment[7].There are many possible coating technologies available for magnesium and its alloys,such as electrochemical plating,conversion coatings, hydride coating,anodizing,gas-phase deposition process,laser surface alloying/cladding,and organic/polymercoatings [7-9].However each has some disadvantages.Organic coatings can be prepared by many approaches,including painting [9],powder coating[10,11],E-coating[12],sol-gel coating [13],and plasma polymerization[14].E-coating(also known as Electrocoating or electrophoresis coating)is a process of using an anodic or cathodic current to apply paint on metallic part surfaces.Cathodic E-coating is a popular process in the automotive industry due to its excellent corrosion resistance, great covering ability on complex metallic components,short formation time and simple apparatus[15,16].But it usually has poor adhesion with substrates due to the loose MgO and/or Mg(OH)2f i lm formed on bare Mg alloys[17].A pretreatment is always recommended before the E-coat is deposited onto Mg alloys to achieve good corrosion resistance and paint adhesion performance,such as chromate conversion coatings [7],silane treatment[18]and phosphating which is currently the most widely used pretreatment for E-coating in industry [19],but the phosphating processes are complicated and harmful to the environment[20].

Among these methods,Micro-arc oxidation(MAO)is a novel and attractive surface engineering process for Mg.In an electrolytic bath with high electric energy,the surface of Mg alloy can be converted into ceramic oxide coating which can be used in diverse applications as a corrosion control technique[6].Due to simplif i ed pretreatment,well adhere performance and environmentally coating process,the MAO technology has emerged as an important alternative to anodizing techniques in certain areas.The exposed surface of the MAO ceramic layer is porous,which is not conducive to improving the corrosion resistance under harsh conditions and to prevent galvanic corrosion in contact with other metals,but allows mechanical locking between the MAO coat and subsequent top layers.

In this paper,the MAO and E-coat method were combined to provide a double-layer coating which presents the advantages of MAO and E-coat technology for AZ91D magnesium alloy,in which MAO was used to deal with bare Mg and then an E-coat covered on the top.The microstructure of MAO and Micro-arc Composite Ceramic(MCC)coated AZ91D Mg was investigated.According to General Motors Corp.(GM)worldwide engineering standards,corrosion resistance,abrasion resistance, stone impact resistance,thermal shock resistance and adhesion of MCC coating on Mg were studied respectively.

2.Experimental

2.1.Materials and equipment

Die-cast AZ91D Mg alloy test coupons (150 mm × 100 mm × 10 mm)were used in the study,and chemical compositions of the test materials are shown inTable 1.The specimens were manually polished with sand papers up to 1500 grade.The electrolyte consisted of 10 g/L sodium silicate(Na2SiO3),8 g/L potassium hydroxide(KOH) and 5 g/L potassium f l uoride(KF).The pH and conductivity of the electrolyte were 12-13 and 3.5 × 104μs/cm,and temperature of electrolyte was maintained at approximately 30°C during MAO process.

Table 1Chemical composition(wt%)of AZ91D test coupons.

The MAO coating facility,manufactured by the research team at Xi'an University of Technology(XAUT),uses pulse current anodizing,the peak current,frequency and pulse width can be adjusted independently.Electrocoating was supplied by Xi'an Emerging Motor manufacturing Co.,Ltd.

2.2.MCC coating preparation

The MAO coating was formed on the surface of AZ91D coupons under 50 A/dm2of peak current,and the thickness was controlled about 10 μm.The samples were then cleaned to ensure that there were no remnant solutions on the coating before drying.The MAO coated AZ91D coupons were immersed in a cathodic PPG(ED7000P)E-coat bath solution (70-78 wt.%deionized water,17-25 wt.%epoxy resin, 3.2 wt.%titanium dioxide and 1.8 wt.%aluminum silicate)at voltage 225 V for 2 min as cathode while the anode is a stainless steel plate.After that,the coupons were rinsed with deionized water.A post deposition process is required in common standards,which can further densify the deposits and has been shown to create uniform coatings on all exposed surfaces,including around corners and edges.This process was taken in the oven at 170°C for 30 min.

2.3.MCC coating characterization

2.3.1.Microstructure

The MAO and MCC coatings on Mg samples were examined for their microstructures using scanning electron microscopy(SEM).Coated samples were cut to expose their cross section.The cross section of samples was polished down to 0.1 μm before SEM examination.A thin layer of Pd was sputtered over the surface and polished cross section specimens to ensure adequate electrical conductivity during SEM observation.

2.3.2.Copper-accelerated acetic acid salt spray(CASS) test

The copper-accelerated acetic acid salt spray(CASS)test was used to evaluate the corrosion resistance of MCC coating, and the standard for testing was strictly performed according to GMW14458(evaluation standard of GM).The solutions were sodium chloride concentration of 50 g/L ± 5 g/L and copper(II)chloride dehydrate(CuCl2?2H2O)of 0.26 g/L ± 0.02 g/L.The pH and operating temperature were kept between 3.1 and 3.3 and 55 ± 2°C,respectively.The average collection rate for a horizontal collecting area of 80 cm2was adjusted about 1.5 ml/h ± 0.5 ml/h.In addition,a scribe line (cut into the metal substrate)of 8 cm length was made on the surface of MCC coated specimens before the CASS test.

2.3.3.Abrasion resistance test

The abrasion resistance of MCC coatings was analyzed according to GMW15487(evaluation standard of GM). Abrasion resistance as determined by this test method was def i ned as a measure of effect of sand,falling from a def i ned height onto the inclined surface of the coating.A def i ned volume of sand falls through a tube onto the surface.This process shall be repeated until an area of 35 mm2of the coating has been removed.The quantity of the needed sand has to be measured.This value divided by the thickness was the abrasion resistance of a coating.The test temperature was 23 ± 5°C.A sketch of the device was shown in Fig.1.

Fig.1.A sketch of device for abrasion resistance test.

Fig.2.The mathematic def i nition diameter after stone impact test.

2.3.4.Stone impact test

The stone impact resistance was evaluated by GMW14700 Method B and C(evaluation standard of GM).The coatings received gravel shock from a gravelometer.The condition of Method B was that the test samples were cooled to a temperature of-18 ± 2°C,and Method C were at temperature 22 ± 5°C.After testing,tapes were used to remove remnants dropped during the test process.Then the stone impact resistance was determined by the stone impact average diameter. The mathematic def i nition of D1and D2were shown in Fig.2. And the rating standards on maximum stone impact diameter listed in Table 2.

2.3.5.Thermal shock resistance test

The thermal shock resistance test followed GMW15919 (evaluation standard of GM)was used to determine the resistance to coating adhesion loss of MCC coating when subjected to a wet steam blast similar to that produced by vehicle wash equipment.Firstly,the samples were immersed in a water bath at 38 ± 2°C for 3 h,and then immediately put into a freezer at-29 ± 3°C.Then,an “X”through the coating into the substrate,with angle of 60 ± 15°,was scribed into the samples by a straight-shank tungsten carbide tip.Next,within 60 s from freezer removal,the steam blast which was produced by a steam generator impacted at the scribe lines for a rain of 30 s.

2.3.6.Tape adhesion test

The adhesion of MCC coating was determined in accordance with GMW14829(evaluation standard of GM). Firstly,the samples were scribed with a cross hatch cut made by sharp blades as shown in Fig.3.All cuts were about20 mm long,and the numbers of lines upon the MCC coating were 12.Tape was placed over the center of grid,and pressed the tape down f i rmly onto the surface with suff i cient pressure to remove air bubbles and insure good contact between the tape and the paint surface.After 5-10 s,the tail end of the tape was grasped between thumb and foref i nger and pulled upward with a rapid motion perpendicular to the paint f i lm.

Table 2Rating standards on maximum stone impact diameter.

Fig.3.The schematic diagram of tape adhesion test.

3.Results and discussion

3.1.Microstructure

Fig.4.Surface micrographs of(a)MAO and(b)MCC coated Mg specimens.

Fig.5.Cross-sectional micrographs of(a)MAO and(b)MCC coated Mg specimens.

Fig.6.The photograph of MCC coated specimens after 168 h CASS test.

The surface morphology of MAO and MCC coating on Mg were showed in Fig.4.Fig.4a illustrates the surface morphology of MAO coated Mg,which was a typical structure consists of agglomeration of uniformly distributed oxide particles and inter oxide particles gaps.The pores were formed by the molten oxide and gas bubbles thrown out of micro-arc discharge channels;while the micro cracks were resulted from the thermal stress due to the rapid solidif i cation of the molten oxide in the relatively cooling electrolyte.It can be seen that there were relatively similar size pores randomly distributed on surface of MAO coat,which would permit the penetration of corrosive ions to the Mg substrate and allow corrosion to proceed.Comparing to Fig.4a,the surface morphology of MCC coating(as shown in Fig.4b)was smooth and without micro pores,there were organic compounds mainly epoxy resin present with an amorphous state. The E-coat can cover the whole MAO f i lm and has a more smooth and compact surface without defects.

Fig.5 shows the cross-sectional micrographs of MAO and MCC coated specimens.The MAO coating(Fig.5a)exhibits two distinct layer structures:a thin,dense layer at thesubstrate/coating interface,and a much thicker porous outer layer.The porous nature of the outer layer provides a good base for the topcoats(E-coat and/or powder coat).The interface between the substrate and MAO coat was not smooth,this could be the combined result of a rough starting surface and the aggressive etching action of the plasma discharges.From cross-sectionalmicrographsofMCC coated specimens (Fig.5b),it can be seen that the E-coats adheres tightly to the MAO coating,and the open pores on the surface of the MAO coating are f i lled in nicely with the E-coat material.The thickness of the uniform organic layer was about 20 μm.The interface between MAO and E-coating was very rough,and their bond was in the form of mechanical interlock.This structure greatly increases the contact area between the inorganic and organic layers which enhanced the binding energy.

Table 3The results of CASS test for MCC coated Mg.

3.2.Copper-accelerated acetic acid salt spray(CASS) test

Fig.6 shows the photographs of MCC coated specimens after CASS test.There was no evidence of corrosion or coating degradation,and the creep back from edges and scribes was no more than 1 mm after the 168 h CASS test.The test results were summarized in Table 3.Firstly,it indicates that the organic coating which acts as a shelter to avoid direct contact with the corrosive solutions prevents the corrosive liquid penetrating into the AZ91D substrate through the pores on MAO coating,and greatly reduces the possibility of corrosive solutions penetrating into the MCC coating.Secondly,there is little corrosive liquid penetrating into the MCC coating from the scribes which reach the AZ91D substrate.It proves that the adhesion supplied by mechanical interlock between the E-coating and MAO coating was excellent.The CASS results illustrate the corrosion resistance of the MCC coating on AZ91D has met the requirement of GMW14458.

Table 4The results of abrasion resistance test.

3.3.Abrasion resistance test

The photographs of the MCC coated specimens after the abrasion test were shown in Fig.7,and the results were illustrated in Table 3.As can be seen from Table 4,the standard requirement is 1.4 L/μm at least,and the tested results were both 4.8 L/um,so the abrasion resistance of the MCC coating far surpasses the requirement.The component of the outer E-coating is bicomponent epoxy resin,and it possesses good compactness and cohesion after curing at 170°C for 30 min.The compact composite structure is characterized by an organic outer layer and inorganic inner coating,and it furnishes the MCC coating with excellent abrasion resistance when the composite coating suffered the impact from the fallen sands.The abrasion resistance of MCC coating on AZ91D alloy absolutely satisf i ed the requirement of GMW15487.

3.4.Stone impact test

Fig.8 shows the photographs of the MCC coated specimens testedbystoneimpactresistancetest(MethodBandC).Ascanbe seen from Fig.8,there were many pits on the surface of MCC coatingduetotheimpactofstones.Thestoneimpactresistanceis determinedbythe average diameterofthe maximum pit,and the average diameter was calculated by formula(1):

Fig.7.The photographs of MCC coated specimens after abrasion resistance test.

Fig.8.The photographs of MCC coated specimens after stone impact tests using(a)Method B and(b)Method C.

According to the results shown in Table 5,the stone impact resistance of the MCC coated specimens tested by Method B and C had a rating of 8,so the evaluation results met the requirement.Additionally,Fig.9 was the enlarged surface photograph from one of the stone impact pits.It can be seen that the pits on the MCC coating were concave along the AZ91D substrate,and the coating hardly dropped off.The MCC coating was enough to resist the stone impact.It also can be shown that the outer organic coating could absorb the shock from the outside impact,and the MCC coating met the requirements of GMW14700.

3.5.Thermal shock resistance

The photographs of the MCC coated specimens after thermal shock test were shown in Fig.10.It can be seen that the coating at the vicinity of the cross without pull off underthe shock of 37.5 Kpa steam.It shows that the bond between the MCC coating and the substrate was strong.The excellent strength between the substrate and the MAO coating was derived from a metallurgical bond.The mechanical interlock between the MAO coating and E-coating also display excellent bond strength.The results of thermal shock test showed that the MCC coating on AZ91D satisf i ed the requirement of GMW15919,and possessed excellent thermal shock resistance and environmental stability.

Table 5The results of stone impact test for MCC coated Mg.

Fig.9.Enlarged surface photograph of one stone impact pits after stone impact test.

Fig.10.The photographs of MCC coated specimens after thermal shock test.

Fig.11.The photographs of MCC coated specimens after the tape adhesion test.

3.6.Tape adhesion test

The cross hatch tape test was used to assess the adhesion between the MAO and E-coat.The photographs of the MCC coated specimens after the tape adhesion test were shown in Fig.11.It can be seen that E-coat adheres well with the MAO coating without peeling off.According to the rating standards for the tape adhesion test shown in Table 6,the remnant coating on the Mg specimens after the tape adhesion tests accounted for at least 99%by contrasting with the rating standards,and the rating was level 0.This excellent adhesion between the E-coat and the MAO coating comes from inf i ltration of the E-coat into theporoussurfaceoftheMAOlayer.Aftercuring,E-coatforms a strong bond with the MAO coating.

4.Conclusions

In order to promote the application of magnesium alloys in automobile production,the Micro-arc Composite Ceramic(MCC)coatings were prepared on AZ91D magnesium alloys in combination with Micro-arc Oxidation and electrophoresis technologies.The cross-section morphologies of the MCC coating showed that the outer E-coating f i lled the holes on the surface of the MAO coating.The corrosion resistance,abrasion resistance,stone impact resistance,thermal shock resistance and adhesion of MCC coating completely met the automotive evaluation standard of GM.The MCC coating on magnesium alloys have demonstrated very attractive properties,and it is a promising surface treatment technology on magnesium alloys for production vehicles.

Table 6The results of tape adhesion test.

Acknowledgment

Support from National Natural Science Foundation of China(Grant NO.51271144),as well as from the National Key Technologies Research and Development Program of China(Grant No.2011BAE22B05)is gratefully acknowledged.And thanks to General Motors research and development center for provided test equipment.

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Received 23 September 2014;revised 14 November 2014;accepted 18 November 2014 Available online 9 December 2014

*Corresponding author.Nanjing Tech University,No.30 South Puzhu Road, Nanjing 211816,China.

E-mail addresses:geyanf@163.com,geyanfeng@aliyun.com(Y.Ge).

Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China,Chongqing University.

http://dx.doi.org/10.1016/j.jma.2014.11.006.

2213-9567/Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.

Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.