2014Analysis of characteristics of vanadate conversion coating on the surface of magnesium alloy

Letter

Analysis of characteristics of vanadate conversion coating on the surface of magnesium

alloy

Liyuan Niu a ,c ,Shiuan-Ho Chang a ,?,Xian Tong a ,Guangyu Li b ,Zimu Shi a

Department of Material Engineer,Zhejiang Industry &Trade Vocational College,Wenzhou 325000,China b

College of Materials Science and Engineering,Jilin University,Nanling Campus,Changchun 130025,China c

Wenzhou Laite Laser Institute Ltd.,Wenzhou 325003,China

a r t i c l e i n f o Article history:

Received 24May 2014

Received in revised form 23July 2014Accepted 5August 2014

Available online 14August 2014Keywords:

Magnesium alloy Conversion coating Electrophoresis Vanadate

a b s t r a c t

The vanadate conversion coating (VCC)was formed on the surface of magnesium (Mg)alloy in vanadium phosphate solution,as well as the cathode electrophoresis and bake-curing treatments of the conversion coating proceeded.According to the addition of vanadate in the solution,the conversion coating is re?ned crystalline and possesses low weight loss during electrophoresis and bake-curing treatment processes.Besides,when the content of NaVO 3is 4g/L in vanadate solution,not only the microstructure of conver-sion coating is the most re?ned,but also the adhesion and corrosion resistance of electrophoretic paint coating (EPC)is the best.On the other hand,the ‘‘rare earth phosphating VS low-temperature electropho-resis’’technique is suitable for Mg alloy coatings.As seen in scanning electron microscope (SEM),the VCC reveals three-dimensional net structure,which provides a well underlayer for the adhesion between elec-trophoretic paint and the samples.

1.Introduction

Magnesium alloy which is a modern and one of the lightest structural materials has many well physical and mechanical prop-erties such as high strength and stiffness,ease of cutting machining and casting,good shock absorption,high resistance of shock and vibration loads,good conductivity of heat and electricity,and well electromagnetic shielding effectiveness [1,2].Now,many parts and components made by Mg alloy have been widely used in the auto-motive industry,machinery manufacturing,aerospace,electronics,telecommunication,military,optical equipment,computer manu-facturing,and other ?elds.The weak anticorrosion is the main rea-son for restricting the application of Mg alloy,so that the protection measures must be taken for enhancing corrosion resis-tance to develop high performance of Mg alloy in industry.In order to increase the corrosion resistance,the most effective method is to form a coating on the surface between the metal and its environ-ment.There are many surface treatments,such as chromate con-version [3–5],phosphate [6],and sol–gel [7]coatings widely applied to Mg alloys.Due to the high reactivity of Mg alloys,the microstructure of conversion coatings formed with conventional methods produces microcracks and is macrocrystalline as well as

incomplete [8–10].First,most of the conversion coatings on Mg alloy have rare microcracks appeared during the initial reaction.Then,after ?ve minutes,the surface of coating was rough while the microcracks became deeper and larger.Finally,after ten min-utes,the surface of coating got accidented and the deep cracks appeared [11].In recent years,there were a few researches of van-adate ?lms on Mg alloys but the related dissertations were rarely reported.For instance,Hamdy et al.reported that vanadate conver-sion coating (VCC)formed in 50g/L NaVO 3solution has a self-heal-ing ability and can provide corrosion protection for AZ31HP-O alloy [12,13].Yang et al.indicated that the VCC has potential to replace chromate conversion coatings and improve the corrosion resistance of AZ61alloy [14].In this paper,we characterized the surface morphology and composition of the VCC by scanning elec-tron microscope with energy dispersion spectroscopy (SEM-EDS)and X-ray diffraction (XRD)(D/max-2500PC,Cu K a ),whereas the cathode electrophoresis and bake-curing treatments of the conver-sion coatings also proceeded.

2.Experiment

In this work,a substrate of 10mm ?20mm ?5mm die cast Mg alloy (AZ91D)with alloy composition of 8.77%Al,0.74%Zn,0.18%Mn,90.31%Mg was treated by the following processes.Firstly,the specimen was pre-degreased in sodium hydrox-ide solution and rinsed with distilled water.Then,it was activated by hydro?uoric acid,degreased by sodium hydroxide,and ?ushed with distilled water.Next,the sample was immersed in vanadium phosphate solution at 50°C for ten minutes

https://www.360docs.net/doc/4a8423273.html,/10.1016/j.jallcom.2014.08.044

?Corresponding author.

E-mail address:1802186169@https://www.360docs.net/doc/4a8423273.html, (S.-H.Chang).

to form VCC.The vanadium phosphate solution consists of10g/L phosphoric acid, 3.0–5.5g/L NaVO3,6.5–8.2g/L zinc nitrate,1.0–1.6g/L ammonium hydrogen?uo-ride,1.5–6.5mL/L ammonia solution,and0.5–1.2g/L molybdate additive.Finally, cathode electrophoresis and bake-curing treatments of the conversion coating were carried out.After bake-curing treatments with different temperatures,electropho-retic coating was removed by paint remover so as to measure the weight loss of conversion coating during electrophoresis process.Furthermore,the sample was investigated by salt spray test(SF850cabinet)using5wt%NaCl solution for contin-uous spray at35±2°C,while the adhesion between conversion coating and the sur-face of Mg alloy was examined by mesh partition experiment(ISO2409).

3.Results and discussions

3.1.Weight and microstructure of vanadate conversion coating

In vanadium phosphate solution,the effective components of ?lm formation reacted with Mg alloy to form VCC and liberate hydrogen.When Mg alloy is immersed in solution the reaction begins,whereas it stops when the specimen surface does not liber-ate hydrogen again.Normally,it takes a long time to make conver-sion coatings on Mg alloy and the weight of coatings is low,while not only the coatings are easy to be erased but also the reaction of ?lm forming is dif?cult to occur.In order to overcome the above disadvantages,we added vanadate and molybdate additives in the solution.The acid,vanadium and molybdenum,which reacted with Mg alloy to form VCC,effectively promote the?lm forming and make the coating dense as well as re?ned crystalline.In gen-eral,the conversion coating whose weight is more than20g/m2 on Mg alloy can provide certain protection performance[15].As shown in Fig.1,with other components unchanged in vanadium phosphate solution,when the content of NaVO3is either below 3.0g/L or above4g/L,the weight of conversion coatings corre-spondingly is either less than25g/m2or steady around26.5g/ m2.Hence,the vanadate additive enhanced the stability of the coating weight.Fig.2shows the SEM-EDS photo for AZ91D Mg alloy surface covered by VCC,while its main elements are vana-dium,phosphorus,zinc,molybdenum,and oxygen.Based on the EDS analysis,the presence of oxygen reveals the presence of the oxide or hydroxide layer formed on the alloy[16].The existence of phosphorus,zinc,and molybdenum indicates that VCC was formed on the surface of Mg alloy from vanadium phosphate solu-tion.In Fig.2(a),the VCC made from3.0g/L NaVO3solution is non-uniform and incomplete microstructure as well as exists cracks. When the content of NaVO3is increased to4.0g/L in the solution, the microstructure of conversion coatings is re?ned and crack-free, as seen in Fig.2(b).It is obvious that the cracks of conversion coatings decrease greatly with the increase of NaVO3content in the solution.As revealed in Fig.2(c),when the content of NaVO3 is increased up to5.5g/L in the solution,the cracks distribute across the coating surface again.These phenomena mean that high or low NaVO3concentration has bad effect on the formation of uni-form coatings and the appropriate concentration of NaVO3is4.0g/ L in the vanadium phosphate solution.

For comparison,Fig.3(a)–(d)shows the SEM photos of composite phosphate?lms on the AZ91D Mg alloy formed in the baths with different CeCl3concentrations for2.5min at45°C,by use of phosphoric acid and sodium hydroxide solution whose pH value was adjusted to2.5.As illustrated in Fig.3,when the con-tents of CeCl3in phosphating solution equal to0,1,and1.5g/L,

NaVO3 content (g/L)

Variations of coating weight with various NaVO3contents in vanadium solution.

(b)

(c)

crack

(a)

crack

SEM morphographies of vanadate conversion coating on Mg alloy formed vanadium phosphate solution with different NaVO3concentrations:(a)

(b)4.0g/L NaVO3and(c)5.5g/L NaVO3.

L.Niu et al./Journal of Alloys and Compounds617(2014)214–218215

crystallizations of the ?lms are macrocrystalline,while when CeCl 3amount to 2.0g/L,the ?lm crystallization becomes ?ne-grained.Macroscopically,it is not conducive to form the ?ne-grained ?lms if CeCl 3content in phosphating solution is below 2.0g/L so as to make the crystallization of the coatings gets accidented.

3.2.Variation of vanadate conversion coating on Mg alloy during cathode electrophoresis process

3.2.1.Weight loss of vanadate conversion coating on Mg alloy during cathode electrophoresis process in alkaline solution

During the cathode electrophoresis process by use of Mg alloy with VCC as the cathode,when transient voltage rises to cause hydrogen ion away from the sample surface,the pH value rapidly raises to 13.If the alkali resistance of conversion coating is poor to lead coating thinning even dissolved in the solution,the coating may lost competent function.Furthermore,poor alkali resistance of conversion coating will ?rstly make EPC produce pits,and then the dissolved chemical ions are adsorbed in the pits and the paint coating that condense the ions after drying,as well as ?nally reduce the protection performance.On the other hand,in order to measure the weight loss of VCC before and after electrophoresis treatment,cathode electrophoresis of the sample proceeded for ?ve minutes.Subsequently,the electrophoretic coating was rapidly removed by solvent and then the weight loss of VCC was measured.As shown in Fig.4,the weight loss of VCC decreases with the increased content of NaVO 3in vanadium phosphate solu-tion during the cathode electrophoresis process.On the assump-tion of uniform corrosion in the specimens,the weight loss rate

of corroded metal can be evaluated using Faraday’s equation of general chemistry [17].

3.2.2.Weight loss of VCC on Mg alloy during bake-curing process after electrophoresis treatment

Generally,conversion coatings consist of phosphate containing crystal water,which vanishes when the coatings are heated to a certain temperature.After phosphating,the electrophoresis

(a)(b)

(c)(d)

morphographies of composite phosphate ?lms on the Mg alloy formed in the baths with different CeCl 3concentrations:(a)without CeCl and (d)2.0g/L CeCl 3.

NaVO 3 content (g/L)

W e i g h t l o s s (g /m 2)

Fig. 4.The weight loss of vanadate conversion coatings decreases with the increased content of NaVO 3in vanadium phosphate solution during the cathode electrophoresis process.

216L.Niu et al./Journal of Alloys and Compounds 617(2014)214–218

treatment of conversion coatings proceeds and then bake-curing is carried out.During bake-curing process,dehydration of conversion coating can increase pore size of coating and make electrophoretic paint coating(EPC)enter the pores.Afterwards,when conversion coating meets water,rehydration causes coating volume to increase and result in stress,which makes EPC produce shrinkage or blistering[18].In recent years,low-temperature electrophoretic paint is applied to Mg alloy and solidi?ed for?fty minutes at 120°C,whereas it has less weight loss than the traditional electro-phoretic paint with bake-curing temperature of160–180°C[19], as shown in Fig.5from our experiment.As a result,VCC is dense and almost no dehydration after bake-curing at120°C.Therefore, the‘‘rare earth phosphating VS low-temperature electrophoresis’’technique is suitable for Mg alloy coatings.As illustrated in Fig.5,the weight loss of VCC decreases with the increased content of NaVO3in vanadium phosphate solution,while4–5.5g/L NaVO3 additive make the coatings possess less weight loss so as to protect Mg alloys effectively in strict corrosion environment.

3.2.3.Adhesion between cathodic EPC and VCC on Mg alloy

Table1shows the test result of adhesion between cathodic EPC and VCC on AZ91D Mg alloy.From ISO2409standard,adhesion-grade1is quali?ed while grade0is the best.As revealed in Table 1,VCC developed in vanadium phosphate solution containing4–5.5g/L NaVO3have excellent adhesion-grade0,but they are just quali?ed for vanadate-free conversion coatings whose thicknesses of EPC must be above22l m.

3.2.

4.Corrosion resistance of EPC formed on VCC of Mg alloy

Initially,electrophoresis as well as bake-curing treatments of AZ91D Mg alloy substrate directly proceeded,and then the EPC was inscribed for salt spray test.After salt spray test of this painting substrate,the diffusion corrosion width along the notch is about 2.6mm,which is larger than1.6–2.1mm diffusion corrosion width of EPC formed on VCC of Mg alloy.At the same time,the EPC on Mg alloy substrate delaminated and appeared ridge along the notch. Table2reveals the standard salt spray test(ASTM B117-03)for EPC on VCC formed in the solution with various contents of NaVO3. In Table2,the1.6–2.1mm diffusion corrosion width of EPC on vanadate-free conversion coating along the notch is larger than that of EPC on VCC.Hence,especially for VCC made from the solution containing4g/L NaVO3,the corrosion resistance of EPC on VCC is better than that of EPC on vanadate-free conversion coating of Mg alloy.Additionally,the EPC on VCC did not produce blistering during salt spray test for480h.As seen in Fig.2,the VCC reveals three-dimensional net structure,which provides a well underlayer for the adhesion between electrophoretic paint and the samples.It is obvious for AZ91D Mg alloy that EPC on VCC reduce the reaction activity of Mg alloy surface,and the VCC improves interface condi-tion between EPC and Mg alloy so as to delay the development of EPC corrosion.

4.Conclusion

1.The vanadate conversion coating(VCC)of Mg alloy consists of

vanadium,phosphorus,zinc,molybdenum,and oxygen.More-over,the microstructure of VCC formed in the solution contain-ing4g/L NaVO3is the most re?ned and crack-free.

2.4–5.5g/L NaVO3additive make VCC possess less weight loss so

as to protect Mg alloys effectively in strict corrosion environment.

3.Low-temperature electrophoretic paint on VCC is applied and

solidi?ed for?fty minutes at120°C,while it has less weight loss than the traditional electrophoretic paint with bake-curing temperature of160–180°C.

4.The VCC formed in vanadium phosphate solution containing

4–5.5g/L NaVO3reveals three-dimensional net structure,which provides a well underlayer for the adhesion between electrophoretic paint and the samples.

Acknowledgement

This work has received?nancial supports from the Science and Technology Project in Wenzhou under contracts J20130020. References

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NaVO3 content (g/L)

The weight loss of vanadate conversion coatings decreases with the increased content of NaVO3in vanadium phosphate solution during the bake-process.

Table1

Adhesion between cathodic electrophoretic paint coatings and vanadate conversion coatings formed in the solution with various contents of NaVO3.

Paint coating thickness(l m)NaVO3(g/L)

034 5.5

16–21.52100Adhesion(grade) 22–261000Table2

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thickness(l m)

Test time

(h)

NaVO3(g/L)

04 5.5

20–21.5240 1.60.6 1.3Diffusion corrosion

width(mm)

480 2.10.9 1.8

22–26240 1.60.5 1.1

480 1.90.9 1.9

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