influence of anodising current

8/6/2019 Influence of Anodising Current

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Influence of anodising current on the corrosionresistance of anodised AZ91D magnesium alloy

Zhiming Shi, Guangling Song *, Andrej Atrens

CRC for Cast Metals Manufacturing (CAST), Materials Engineering, School of Engineering,

The University of Queensland, St. Lucia, Brisbane, Qld 4072, Australia

Received 18 August 2004; accepted 9 August 2005Available online 28 September 2005

Abstract

The thickness, chemical composition and microstructure of anodised coatings formed on magne-sium alloy AZ91D at various anodising current densities were measured. It was found that all these

parameters could be affected by anodising current density, and hence the coatings formed at differentanodising current densities had different corrosion resistances. This suggests that the corrosion per-formance of an anodised coating could be improved if a properly designed current waveform is usedfor anodising. In addition, based on the experimental results, some physical, chemical and electro-chemical reactions involved in the anodising process were proposed to explain the anodising behav-iour in this paper. 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Magnesium; B. SEM; C. Anodic film

1. Introduction

Corrosion, particularly galvanic corrosion, is a complicated and serious problem inapplications of magnesium alloys [19]. Coating is an effective solution to the problem[1012]. So far, various anodised coatings have been developed, such as HAE, DOW17,Tagnite, ANOMAG, MGZ and Keronite. These coatings not only serve as a corrosionresistant layer, but also provide a good base for paint coatings.

0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2005.08.004

*

Corresponding author. Tel.: +61 7 33 65 41 97; fax: +61 7 33 65 38 88.E-mail address: [email protected] (G. Song).

Corrosion Science 48 (2006) 19391959

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Anodising magnesium alloys normally requires high voltages and high current densitiesapplied, which can usually cause sparking in anodising. Various anodising current densi-ties that can result in different anodising behaviours have been tried. For example, Bartonand John [13] anodised AZ91 magnesium alloy with a slowly increasing anodising cell-

voltage before sparking began, then adjusted the cell-voltage to maintain a constant spark-ing until the cell-voltage reached 90 V. Sometimes, Barton also tried even higher cellvoltages (e.g. 170350 V [14]) for anodising. Other researchers, such as Zozulin and Bartak[15], Sharma et al. [16] and Khaselev et al. [17], anodised magnesium alloys using a con-stant current density. The anodic behaviour of Mg and Mg alloys in NaOH or KOH solu-tions over a wide voltage range has been described by Evangelides [27], Huber [28] andEmley [29]. An anodising process can normally be divided into several stages [29,30].For example, Khaselev and Yahalom [31] divided the anodising of MgAl alloys inKOH-aluminate solution into four different regions based on the polarisation curve: (1)primary passivity, (2) breakdown of primary passivity and metal dissolution, (3) secondarypassivity, and (4) the breakdown of the secondary passivity. Khaselev and Yahalom [32]reported that the current density in the passive state decreased with an increase in the alu-minate content in the solution and/or the aluminium content in the alloy. It is believed [15]that sparking occurs in anodising when the applied voltage is greater than the dielectricbreakdown voltage of the layer produced in the pre-treatment process. In addition, itwas found that the breakdown of film associated with the sparking process was controlledby the interface of film and electrolyte solution [37].

Although the previous investigations have indicated that anodising current density cansignificantly influence the corrosion performance of an anodised magnesium alloy, no sys-

tematic study has been carried out to reveal the mechanism. So far, it is not well under-stood how the anodising current density or cell voltage influences the corrosionperformance of an anodised coating. This paper aims to understand the anodising mech-anism and to reveal the influence of the anodising current density on an anodised coatingin terms of its corrosion performance.

2. Experimental

Specimens were cut from an AZ91D ingot. The AZ91D alloy contained 8.96%aluminium, 0.77% zinc, 0.23% manganese,


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The applied anodising current waveforms were as follows:

(1) G0: the anodising current density was 20 mA/cm2 for 10 min.(2) G1: the anodising current density was 15 mA/cm2 for 20 min.

(3) G2: the anodising current density was 20 mA/cm2 for 7 min, 15 mA/cm2 for 7 minand 10 mA/cm2 for 7 min.

(4) G3: the anodising current density was 25 mA/cm2 for 7 min, 20 mA/cm2 for 4 minand 15 mA/cm2 for 7 min.

(5) G4: the anodising current density was 20 mA/cm2 for 5 min.(6) G5: the anodising current density was 20 mA/cm2 for 10 min, 10 mA/cm2 for 10 min

and 5 mA/cm2 for 10 min.(7) G6: the anodising current density was 20 mA/cm2 for 1 min.

The waveforms of the above anodising current densities are schematically illustrated inFig. 1.

Apart from the waveforms of the anodising current densities, some specimens were alsoanodised by constant current densities 10, 15, 20 and 25 mA/cm2, respectively, in order toinvestigate the influence of anodising current density on the coating performance. In addi-tion, a polarisation curve of AZ91D was measured in the bath solution using (a) the DCpower supply and (b) a potentiostat with a saturated silver/silver chloride electrode as ref-erence. The difference in potential between a saturated silver/silver chloride electrode andthe stainless steel container was 255 mV, which is negligible compared with the anodisingcell voltage. Nevertheless, in the study, the potentials relative to the saturated silver/silver

chloride electrode measured by the potentiostat were converted into the cell voltages. The

0 4 8 12 16 20 24 280

10

20300

10

20300

10

20300

10

20

300

10

20

300

10

20300

1020

30

Time (min)

G0

G1

G2

G3

G4

G5

Currentdensity(mA

/cm

2)

G6

Fig. 1. Waveforms of controlled anodising current densities.

Z. Shi et al. / Corrosion Science 48 (2006) 19391959 1941


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other potentials or voltages presented in this paper if not specified are the cell voltages rel-ative to the 304 stainless steel container.

Oxygen evolution from the specimens during anodising was measured using a burette toevaluate the efficiency of the anodisation. The burette, initially full of the bath solution,

was mounted over a funnel that covered the AZ91D electrode being anodised. This set-up is similar to that one used by Song et al. [2024] in measuring hydrogen evolution fromMg alloys and proven to be convenient and reliable [1,12,25].

After being anodised, the specimens were washed with demineralised water and driedfor the following measurements.

The microstructures of the anodised coatings were analysed using a Philips XL30 scan-ning electron microscope (SEM) and EDAX was used to determine the compositions ofthe anodised coatings in the same time. The elemental compositions were determined byX-ray photon spectroscopy (XPS) after the coating surface was cleaned by argon-ion-etch-ing for 1 min. The corrosion performance of the anodised coatings was evaluated byimmersion in 5% NaCl solution. The appearance of the samples after immersion wasrecorded.

3. Results and discussion

3.1. Anodising behaviours

The variations of the cell voltages of the specimens anodised by various waveforms ofanodising current are presented in Fig. 2. Fig. 3 shows the results of similar experiments

that used the same total charge for the anodising experiments at different applied currentdensities. The increasing cell voltages indicate the growth of the anodised coatings.According to the differences in cell voltage (Fig. 2), sparking behaviour and oxygen evo-

lution, an anodisation process can be divided into three stages. In the initial stage, when

0 2 4 6 8 10 12 14 16 18 20 22 240

100

200

300

400

G4

G0

G2

G1

G3

Cellvoltage(Volts)

Time (min)

G0

G1

G2G3

G4

Fig. 2. The cell voltage response curves of AZ91D under different waveforms of current densities.

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the cell voltage was lower than 10 V, there was no sparking and oxygen evolution was notsignificant, either. In the second stage (cell voltage between 10 V and 190 V), there was nosparking but oxygen evolution was more significant. In the third stage, sparking arcs werevisible. The cell voltage was higher than 190 V and oxygen evolution became vigorous.

It should be noted that in the initial and second stages, the cell voltage increased morequickly at a higher anodising current density. For example, when the current density in-creased from 15 mA/cm2 to 25 mA/cm2 (curves G1, G2 and G3 in Fig. 2), it took a shorter

time for the cell voltage to climb up to the sparking voltage range. The cell voltage duringthe anodisation process was approximately proportional to the coating thickness. There-fore, the increase of cell voltage in the initial stage corresponded to the formation andthickening of an anodised film on the AZ91D surface. For a constant-current, a highercurrent density resulted in faster film formation and hence the cell voltage increased morequickly.

In the third stage, the cell voltage increased slowly with anodising time. Sparking andoxygen evolution were the major phenomena. Under a given anodising current density, thecell voltage gradually approached a stable value. The sparking intensity can be controlledby the anodising current density. In experiments, a higher current density caused a more

intensive sparking process and resulted in a coarser anodised coating.The efficiency of anodisation or anodising current is determined by the formation rates

of the coating and the by-products involved in the anodising process. The evolved oxygenis one of the most important by-products. So, a higher rate of oxygen evolution means alower efficiency of anodisation. Fig. 4(a) presents the volume of oxygen evolved in G0 toG4 anodising processes. To clearly reveal the oxygen evolution during anodising, a typicaloxygen evolution curve and the corresponding cell voltage curve are presented in Fig. 4(b).The volume of oxygen increased more rapidly for G2 at 20 mA/cm2 than for G1 at 15 mA/cm2. This indicates that in the same anodising stage, the efficiency of anodisation wasdependent on current density.

The current efficiency can be defined by

g IF

IT 100% 1

0 50 100 150 200 250 300

0

50

100

150

200

Stage II

Stage I

25 mA/cm2

20 mA/cm2

15 mA/cm2

10 mA/cm2

Cellvoltage(V

olts)

Time (seconds)0

0

100

200

300

400

400 800 1200 1600 2000

Stage III

25 mA/cm2

10 mA/cm2

15 mA/cm2

20 mA/cm2

Cellvoltage(Volts)

Time (seconds)(a) (b)

Fig. 3. The cell voltage response curves of AZ91D anodised by different current densities with the same amount

of electric charge (18 C/cm2). (a) Early stages and (b) whole anodising process.



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where IT is the total applied current and IF is the current resulting in film formation. If it isassumed that the only by-product is oxygen generated by reaction:

4OH 2H2O O2 4e 2

then IF can be estimated by

IF IT IO2 3

where IO2 is the current density used for the electrochemical production (2) of oxygen,which can be estimated from the amount of the measured gas. Thus, the current efficiencycan be calculated (see Appendix 1):

g IT IO2

IT 100% 4

It should be noted that the current efficiency as defined by Eq. (4) could lead to a negativevalue if there is a sufficient amount of gas generated by a non-electrochemical reaction.

In the initial stage, the evolved oxygen volume was very small (see Fig. 4(a) and (b)),which means that almost all of the applied current was used for coating formation. Inthe second stage, the evolved oxygen volume increased gradually (Fig. 4(a) and (b)), whichcan be interpreted as a gradual decrease in current efficiency. In the third stage, the rate of

0 4 8 12 16 20 24

0.0

0.5

1.0

1.5

2.0

2.5

G4

G3

G1

G2G0

VolumeofO

2(ml/c

m2)

Time(min)

G0

G1G2

G3

G4

0 2 4 6 8 10 12 14 16 18 20 22 24

0

50

100

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400

III

Cell voltageVolume of O

2

Time (min)

CellVoltage(V

)

0.0

0.2

0.4

0.6

0.8

II

I

Vo

lumeofO2(ml/cm2)

a b

c d

0

-2 0 2 4 6 8 10 12 14 16 18 20 22

-200

-150

-100

-50

50

100

Efficiencyofcurrent(%)

Time (min)

G0G1

G2G3

G4

0.00

0.05

0.10

0.15

0.20

0.25

0 4 8 12 16 20

Derivativeofoxygenevolution(ml/min.cm

2)

Time (min)

G0

G1G2

G3G4

Fig. 4. (a) The oxygen evolution curves, (b) the cell voltage and oxygen evolution curves (G1), (c) the curves ofcurrent efficiency and (d) the derivative of oxygen evolution of AZ91D ingot during anodising processes.

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oxygen evolution increased dramatically (Fig. 4(a) and (b)). Fig. 4(c) presents the currentefficiency calculated according to Eq. (4). The efficiency varied with time in anodising. Thecurrent efficiency became low and even negative in the third stage (see curves G1, G2 andG3 in Fig. 4(c)). The negative efficiency could be due to the thermal decomposition of

water during anodising, which will be analysed later.Fig. 4(d) shows the derivative of the evolved oxygen volume standing for the oxygen

evolution rate. It also varied in anodising, corresponding well to the variation of currentefficiency (Fig. 4(c)). In the first stage, the oxygen evolution rate was low. In the secondstage, the oxygen evolution rate became higher. The derivative of the oxygen evolutionexhibited complicated tendencies in the third stage. The oxygen evolution rate didnot simply increase or decrease with time. Similar to the strange negative current effi-ciency, the complicated oxygen evolution in this stage could also be associated with anon-electrochemical reaction.

Fig. 5(a) shows that under a constant electric charge, the total amount of the measuredgas (over the whole anodisation process) increased with increasing current density, and thetotal current efficiency decreased with increasing current density. Fig. 5(b) shows how theoxygen evolution increased and the current efficiency decreased with an increasing cellvoltage at different current densities. It implies that the evolved oxygen volume is depen-dent on the cell voltage.

The growth of the anodised coating should be proportional to the electrical charge usedin coating formation, which is proportional to the total electrical charge used during anod-isation minus the charge used for oxygen evolution. The electrical charges and the mea-sured thicknesses of the coatings are listed in Table 1. It is clearly shown that the

measured thicknesses of the anodised coatings were proportional to the applied densitiesof electrical charges, which is further illustrated in Fig. 6(a). Fig. 6(a) also shows that thefinal cell voltage was proportional to the density of electricity charge. To further confirmthis, the data from Fig. 6(a) are re-plotted in Fig. 6(c). It can be seen that there is a linearrelationship between the final cell voltage and the thickness of the coating. This isunderstandable as the final cell voltage is closely associated with the thickness of ananodised coating. Fig. 6(b) shows that the measured total current efficiency decreases with

0 100 200 300 400

0

1

2

3

O2

evolution

Efficiency

Current density (mA/cm2)

10 15 20 25

Oxygenevolution(ml/cm

2)

Potential (volts)

-500

-400

-300

-200

-100

0

100

10 15 20 25

Efficiencyofcurrent(%)

(%)

fficie

8 12 16 20 24 28

0.10

0.12

0.14

0.16

0.18

0.20 O2

evolution

O2evolution(ml/cm

2.C)

Current density (mA/cm2)

70

75

80

85

90

E

ncy

Efficiency

a b

Fig. 5. Under a constant electric charge (18 C/cm2) (a) total amount of oxygen evolved and current efficiency vs.current density and (b) oxygen evolution and current efficiency vs. cell voltage.



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increasing charge density, particularly at a high charge density. This is probably due tomore electrical charge used in the third stage of anodising at a higher charge density.The data in Fig. 6(a) were replotted in Fig. 6(d) to show the relationship between the totalapplied charge and the measured thickness. The apparent linear relationship could be for-tuitous, because only some of the total charge resulted in growth of the anodised film andthe other resulted in oxygen generation.

To examine the charge used for oxygen evolution, the theoretical electrical charges usedfor the coating formation and oxygen evolution were calculated and presented in Fig. 6(d).It should be noted that in the calculation is based on the assumptions that (a) oxygen was

the only gas produced and measured during anodising and (b) all the oxygen was pro-duced by electrochemical reaction (2). It is found that (CTCgas) $ thickness is a curve(which is approximately a straight line when coating thickness is small) with a maximumfollowed by a steep decrease to negative values. The steep decrease and the negative chargevalues indicate that assumption (a) is not valid at high voltage for a thick coating. It ispossible that the intense sparking caused thermal decomposition of water into 2H2 andO2 at a high temperature. Therefore, it is interesting to estimate the amount of thermolysisgas by analysing the difference between CT and (CTCgas), which is denoted as thermol-ysis gas (O2 + H2) in Fig. 6(d). The difference is quite significant. The significant amountof thermolysis gas (O2 + H2) in the third stage confirms the previous postulation about

the occurrence of water thermo-decomposition caused by sparking.The composition of the anodised coating can also be roughly estimated based on the

relationship between the coating thickness and the charge Cq used for generation of thecoating. The curves of Cq that are obtained by assuming that the film is MgO, Mg(OH)2or MgSiO3 with 15% porosity are plotted in Fig. 6(d) (see Appendix 2). It can be seen thatCT is located between Cq-MgO and Cq-MgOH2 or Cq-MgSiO3 . This suggests that the coatingcould be a mixture of MgO and Mg(OH)2, or a mixture of MgO and MgSiO3 or a mixtureof MgO, Mg(OH)2 and MgSiO3.

3.2. Anodised coating

The microstructures of the anodised coatings are shown in Fig. 7. The pores of coatingG4 (20 mA/cm2 for 5 min) are generally smaller than 1 lm in diameter and those of the G0(20 mA/cm2 for 10 min) coating are near 2 lm (see Fig. 7). These results indicate that in

Table 1Thickness of the anodised coatings

G4 G0 G1 G2 G3

Density of charge

mA min/cm2 100 200 300 315 340C/cm2 6 12 18 18.9 20.4

Thickness (lm) (measured) 1011 17 1920 1821 2023Amount of O2 evolved (ml/cm

2) 0.063 0.169 0.758 0.657 2.429Charge for evolved oxygen (C/cm2)a 0.995 2.66 11.96 10.364 38.33Charge used for coatinga (C/cm2) 5.0 9.34 6 8.5 18.33Efficiency (%)a 83 77.8 33.6 45.2 87.9Final cell voltage (V) 252 316 355 371 372

a Calculation based on the reaction 4OH 4e = 2H2O + O2.



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the second stage before the cell voltage reached 190 V, the anodised coating was relatively

compact with only few tiny pores. In anodising, only occasional tiny sparking was ob-served in this stage. The reason for the microstructure of the G0 coating coarser than thatof the G4 coating is that the more vigorous sparking occurred during anodising, especiallyin the last period. The G1 (15 mA/cm2 for 20 min) coating is irregularly porous with tiny

4 6 8 10 12 14 16 18 20 22 24

-80

-40

0

40

80

Currentefficiency

(%)

Density of charge (C/cm2) Density of charge (C/cm2)

10 12 14 16 18 20 22

240

260

280

300

320

340

360

380

Finalcellvoltage(Volts)

Thickness (m)

Thickness (m)

0 6 12 18 24

8

12

16

20

24

Thickness

G3G2G1

G0

G4

G4

G3

G2G1

G0

Thickness(m)

0

100

200

300

400

Final cell voltage

Finalcellvoltage(V

olts)

0 4 8 12 16 20

-20

-15

-10

-5

0

5

10

15

20 Porosity = 15%

Thermolysis gas (O2+H

2)

CT-C

gas

CT

Cq-MgO

Cq-Mg(OH)

2

Cq-MgSiO3

Charge(C/cm

2)

a b

c

d

Fig. 6. Relationships between coating thickness, final cell voltage, and density of electrical charges. (a) Thicknessand final cell voltage vs. density of electrical charge, (b) current efficiency vs. density of charge, (c) final cellvoltage vs. thickness and (d) dependence of theoretical electrical charges of MgO, Mg(OH)2 and MgSiO3. ChargeCOe associated with electrochemical oxygen and total charge CT on coating thickness.



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pores less than 12 lm in diameter. The G2 coating has some large pores near 3 lm indiameter. The G3 coating has a microstructure similar to that of G1 coating, but withmore large pores.

Fig. 8 shows the sealed pores in a local sparking spot on the G5 coating. It seems thatlocalised sparking caused a bump or a ridge on the coating (such a bump can evenbe visualised by the naked eye). The EDAX analysis (see Table 2) shows that the compo-

sition of the coating at localised sparking spots is similar to that of the normal coatingareas, except that the Mg content is slightly lower and the silicon level slightly higher inthe localised sparking spots than in the normal coating areas. The image shown inFig. 8 provides evidence to support that some pores can be sealed during the anodisation

Fig. 7. The SEM images of anodised coatings on AZ91D ingot.



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process. Smaller pores and sealed pores in an anodised coating should be beneficial to thecorrosion resistance of the coating. The sealing could be attributed to the lower currentdensity applied in the final anodising stage.

With the same amount of electric charge (18 C/cm2), the influence of the anodising cur-rent density on the microstructure is insignificant; the irregular porous microstructures ofall these coatings formed at different anodising current densities are almost the same (see

Fig. 9). Comparing the microstructures of the coatings formed by different waveforms ofcurrent densities, it is found that the microstructure of a coating is closely associated withthe final cell voltage. If the final cell voltage is low, the coating is fine and compact. Other-wise, the coating is coarser and porous.

Fig. 8. The cross-section of a localised sparking spot on anodised AZ91D alloy under G5 anodising condition.

Table 2Compositions of the normal coating areas and localised sparking spots analysed by EDAX

At.% Normal coating areas Localised sparking spots

Postion 1 Position 2 Position 3 Spot 1 Spot 2 Spot 3

C 27.91 28.10 29.42 29.06 28.57 29.80O 39.00 39.9 38.03 37.97 39.98 40.30Mg 13.92 14.62 14.03 14.20 10.15 8.00Al 1.33 0.96 1.15 1.42 0.88 0.81Si 16.28 15.64 15.87 15.83 17.59 18.47K 1.56 1.58 1.50 1.52 2.83 2.61



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Fig. 9. SEM images of AZ91D ingot anodised under different current densities with the same amount of totalelectric charge.



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The compositions of the anodised coatings are presented in Table 3. In this study, theXPS detected area was about 3 3 mm2. In this area of an anodised coating, there wereplenty of pores. The coating is thin inside the pores and thick outside the pores. Therefore,the measured XPS should be the average information of the thick film outside the poresand the thin film inside the pores. Hence, the XPS results should represent the averagecomposition of an anodised coating. The anodised coatings contain mainly magnesium,silicon, and oxygen. The content of oxygen is near 55%, the content of silicon is in therange between 23% and 35%, and the magnesium content is from 3% to 16%. The highcontent of silicon in the coating should result from the deposition of the potassium silicatefrom the bath electrolyte. Sodium and boron in the coatings may come from the pre-treat-

ment electrolyte. The alloying elements aluminium and zinc could not be found in thecoating by XPS. This is probably because Al only exists in the coating adjacent to theinterface of substrate and coating. It has been reported [26] that the diffusion of Al inthe surface film on a Mg alloy is much slower than Mg under normal oxidation conditionsand Al was also be found mainly in the zone adjacent to the interface between oxide filmand substrate. Moreover, it should be noted that the anodised coatings were formed in anAl-free bath solution. Even if there was some Al from the substrate, it should exist in thecoating next to the substrate/coating interface, completely covered by Al-free oxides andhydroxides. Therefore, it is not easy to detect the Al-containing compounds. For a lowlevel of Al in the coating, it is unlikely to form some spinels, like Al2MgO4.

Table 3 indicates that the general composition of the coatings can be expressed by(MgO)x (SiO2)y H2O. The coefficients x and y changed with the waveform of the currentdensity. It is to some extent equivalent to a mixture of MgO, Mg(OH)2 and MgSiO3 inchemical composition, which is consistent with the estimated composition based on theelectrical charge analysis (Fig. 6(d)). For G2 and G3, x was low and y was high. Thatmeans that the ratio of MgO and SiO2 in the coating can change with the applied currentdensity. The corrosion performance of the coating could change with the ratio of MgOand SiO2, because in corrosive environments SiO2 is more chemically stable than MgO.

In order to explore the anodisation mechanism of the Mg alloy, the compositions of theanodised coatings formed in the different stages are also analysed (see Table 4). Table 4

shows that the atomic ratio of Si/Mg increases from 0.5 to 6, which means that the outerlayer of the coating is silicate rich and the different anodising reactions could occur in thedifferent stages. In the first stage, the atomic ratio of Si/Mg was 0.5, which can be inter-preted as an indication that the deposition rate of Mg(OH)2 was the same as that of

Table 3Chemical compositions of the anodised coatings by XPS

Conditions Atomic concentration (%)

C O Mg Si Na B Si/Mg

G0 0.69 56.16 7.89 26.51 5.66 3.09 3.36G1 8.725 64.51 5.1 21.61 0.00 0.07 4.24G2 3.01 65.38 2.62 14.85 8.67 5.48 5.66G3 4.44 69.10 2.83 23.51 0.12 0.00 8.3G4 3.98 65.36 10.10 20.10 0.39 0.07 1.99G5 4.42 63.64 12.38 19.51 0.05 0.00 1.57G6 21.05 53.11 8.62 10.57 2.32 4.34 1.22



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MgSiO3. In the second stage, the atomic ratio of Si/Mg was higher, suggesting that moresilicate was deposited into the coating. The explanation for the high ratio of Si/Mg in stage3 is that the amount of silicate deposited into the coating was much greater than Mg(OH) 2.

This could be attributed to the sparking induced thermo-decomposition of the bath solu-tion (containing silicates) in this stage. In addition, a high content of carbon in these coat-ings is also noticed. This can be simply ascribed to the absorption of carbon dioxide fromthe atmosphere after anodising.

In addition, high resolution XPS also exhibits a difference in the binding energies be-tween hydroxide and oxide, which confirmed the existence of hydroxyl in the coating.

3.3. Anodising process

Based on the above results, a three stage anodising process of AZ91D under a constantcurrent density can be proposed.

(a) Initial stage:It is proposed that the reactions on the Mg anode include dissolution of Mg, deposition

of Mg(OH)2 and MgSiO3 and oxygen evolution:

4OH 2H2O O2 4e 2

Mg Mg2 2e 5

Mg2 2OH MgOH2 # 6

Mg2 SiO23 ! MgSiO3 # 7

Reaction (2) is the only side reaction, which is not significant in this stage. Reactions (5)and (6) have been reported by Mizutani et al. [33] who believes that the film on Mg alloysformed at a low cell voltage in alkaline solution consists of Mg(OH)2. The postulation ofreaction (7) in this paper is inspired by Khaselev et al.s work [17] in which MgAl2O4 wasfound to be the composition of the anodised coating on Mg alloys in an aluminate con-taining solution. Most of the anodising current in this stage is consumed by the reaction(5). The formed coating in this stage is thin because the anodising time is short, less than10 s and electrochemical reaction (5) is relatively slow, so chemical reaction (6) and (7)cannot be fast. The XPS results (Mg peak) of the coating formed in this stage suggest that

the coating mainly contains Mg(OH)2 and MgSiO3.(b) Second stage:Evolution of some tiny oxygen bubbles can be observed. There is no sparking. The cell

voltage increases rapidly from 10 V to 190 V (see Figs. 3 and 4(b)). Most of the current is

Table 4Chemical compositions of the anodised coating formed in the anodisation different stages by XPS

Anodising stage Anodising conditions Atomic composition (%)

C O Mg Si Si/Mg

Stage 1 2 V, 2 min 66.13 22.78 7.36 3.73 0.5Stage 1 7 V, 2 min 36.78 41.83 14.67 6.71 0.5Stage 1 12 V, 2 min 36.51 43.44 13.73 6.31 0.5Stage 2 60 V, 2 min 16.96 53.47 15.24 14.33 1Stage 3 370 V, 21 min (G2) 3.01 65.38 2.62 14.85 5.7

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used to form the anodised coating. It is postulated that the reactions include reaction (2),(5)(7) as well as

Mg2 2OH MgOH2 MgO # H2O 8

Mizutani et al. [33] reported that the anodised coating of Mg alloys formed at 60 V in analkaline solution contained a mixture of magnesium hydroxide and magnesium oxidewhich can be confirmed by XRD analysis. Therefore, the involvement of reactions (5),(6) and (8) in this stage is reasonable. The evidence for reaction (7) is that magnesium sil-icate has been detected by XPS analysis in the anodised coating. Moreover, the limitedoxygen evolution indicates that reaction (2) is still slow in the second stage (seeFig. 4(b)). The XPS results have indicated that the coating consisted of Mg(OH)2, MgOand MgSiO3 in this stage. The second stage usually lasts about 25 min depending onthe applied current density (see Fig. 3(a)). The coating formed in this stage is compactand smooth. The thickness of the coating increases with anodising time. However, themaximum thickness of the coating is only several micrometers. With a coating on the sur-face, reaction (5) should be controlled by the mass-transfer processes in the coating. Underthe circumstances, the cell voltage increases with the thickness of the coating in order tomaintain the reaction rate.

(c) Third stage:The obvious phenomena in this stage are sparking and vigorous oxygen evolution on

the sample surface. The cell voltage continues to increase with anodising time but increas-ing rate becomes slow (see Figs. 2 and 3). In addition to reactions (2), (5)(8), the followingreactions will occur at high temperatures in this stage.

MgOH2 MgO # H2O 92Mg O2 2MgO # 10

MgO yMgSiO3 MgOx SiO2y 11

2H2O 2H2 " O2 " 12

The sparking activity can lead to a high temperature at sparking areas. At high tempera-tures, magnesium hydroxide formed in the first two stages could be dehydrated (reaction(9)) and magnesium can react with active oxygen to form magnesium oxide directly (reac-tion (10)). However, reaction (6) still continue in the non-sparking areas because the XPS

results suggest that there is still magnesium hydroxide in the coating. In addition to thechemical changes, at the high temperatures, anodised film can be melted and mixed to-gether around the pores resulting from sparking (reaction (11)). Reaction (11) is confirmedby SEM (see Fig. 8), which shows a localised melted area. It seems that the coating depos-ited in the first two stages is locally melted by sparking to form a thick and coarse coating.

Due to sparking and local high temperature, thermal-decomposition of water (12) nearthe sparking area becomes possible. The thermal-decomposition temperature of water hasbeen reported to range from 2000 C to 5500 C [34,35]. It has been reported that the hightemperature at the sparking location was estimated to be above 1000 C [37]. The temper-ature could be much higher in the centre of a sparking flame, which could result in fusion

of silicate, magnesium hydroxide and magnesium oxide onto the metal surface. The inten-sive gas evolution and the decrease in measured current efficiency or a negative efficiencycan all be easily understood as a result of the thermo-decomposition (reaction (12)) ofwater in this stage.

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The three-stage anodising process can be further verified by a polarisation curve (poten-tial-current density) for AZ91D in the bath solution (see Fig. 10). There are three regionscorresponding to the different anodising stages on the curve. The first stage of anodisingcorresponds to a peak in the polarisation curve and t very limited oxygen evolution. Oxy-

gen evolution rate became slightly higher and anodic current density increased dramati-cally and achieved a maximum when the cell voltage exceeded 4 V. In the second stage,the cell voltage higher than 10 V, the anodising current density became lower than thepeak of the first stage, corresponding to deposition of magnesium hydroxide on the sur-face. With increasing cell voltage, the anodised coating continued growing and thickeningand oxygen evolution also increased. After the cell voltage exceeded the dielectric break-down voltage of the coating (in this study, round 150190 V), sparking occurred and theanodising system entered the third stage. The formation of the anodised coating continuedthrough vigorous sparking induced breakdown of the coating and repairing the coatingafterward at the broken areas. At the same time, intensive gas evolved from the sparkingspots where thermo-decomposition of water occurred.

3.4. Corrosion resistance of anodised coating

The corrosion of the coatings initiated from tiny pits, progressed into pitting corrosionand finally became localised or filiform corrosion. The typical images are shown in Fig. 11.The corrosion specks and pits became visible after 16-h immersion. After 88-h immersion,the specks had become larger and developed into localised corrosion and filiform corro-sion. Based on the morphologies of the corroded specimens, the ratio of corroded area

of the specimens after 88 h exposure testing can be obtained. The ratio of the corroded

0 100 200 300 400

0

1

2

3

4

5

Stage III

Stage II

Stage I

I(mA/cm

2)

Cell voltage (Volts)

-2 0 2 4 6 8 10 12

0.000

0.002

0.004

0.006

0.008

0.010

0.012

I(mA/cm

2)

Cell voltage (volts)

(a)

(b)

Fig. 10. Polarisation curve of AZ91D in the anodising solution measured by high voltage DC power supply (b)measured by potentiostat with a saturated silver/silver chloride electrode as a reference (the potentials relative tothe silver/silver chloride reference electrode has been converted into the cell voltages in this figure).



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specimens was 1620.1% for G0 coating, 24.9% for G1 coating, 3.17.0% for G2 coating,0.410.1% for G3 coating and 3.022.4% for G4 coating. So, the rating of corrosion resis-tance of these coatings decreased in the following order: G1 > G2 > G3 > G0 > G4.

According to the above discussion, the microstructure of the anodised coating shouldbe dependent on the waveform of the current and the final cell voltage. On the other hand,the corrosion resistance of the anodised coating is determined by the microstructure, com-position and thickness of the coating. Therefore, the corrosion performance can be reason-ably related to the anodising current density. This explains the corrosion performance ofthe coating as shown in Fig. 11.

For a short time or low current density anodisation, when the coating is thin (like the

G4 anodised coating in Stage I and Stage II), the thickness of the coating is the main factoraffecting the corrosion resistance of the anodised coating. That is why the G4 coatingshows a low corrosion resistance even if the microstructure of the coating looks verycompact.

Fig. 11. The corrosion images of the coatings after immersion in 5% NaCl solution for (a) 16 h and (b) 88 h.



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In the third stage, the coating is relatively thick. The effect of the thickness becomes lessimportant as the coatings become more porous due to sparking. In this case, other factorssuch as the composition of the coating should be considered. For example, the content ofsilicate can play an important role in the corrosion performance of the anodised coating. A

compact and silicate rich coating can provide good corrosion resistance. It is found thatthe microstructure of G0 is similar to G3, but G3 is thicker and richer in silicate thanG0, so the corrosion resistance of G3 is higher than that of G0.

3.5. Optimisation of anodising current density

The above analyses suggest that the control of anodising current density is important. Ifthe current density is very high after the anodised coating has already very thick, the ano-dised coating may break down in a relatively large area by intensive sparking, and the sub-sequent anodisation of the damaged area becomes difficult. A intensive localised plasmasparking process can cause a local breakdown of the film and result in a bad quality ano-dised coating. Therefore, in the last stage after a thick anodised coating has been formed,anodising current density should be reduced to a suitable value to maintain a non-vigorousand uniform sparking process. However, before an anodised coating is formed, if a toolow anodising current density is applied, then the formation of an anodised coating willbecome too slow or even unachievable. Therefore, selection of a suitable current densityis critical for anodising.

An ideal anodising current density should be like this. At the beginning of anodising, ahigh anodising current density is essential to obtain an initial anodised coating quickly. In

the later stage while sparking becomes too intensive or too localised, a low current densitywill be appropriate, which can lead to a coating less porous. An optimised waveform ofcurrent density can improve the microstructure, surface appearance, and the final qualityof the coating and the efficiency of the anodising reactions. Consequently, the resultingcoating with suitable composition, microstructure and thickness will have a good corro-sion resistance. For example, in anodising G0, the anodised coating formed quickly inthe first and second stages, but vigorous sparking occurred in the third stage. Hence,the anodised coating was coarse. Its corrosion resistance was bad. In anodising G2, thecurrent density was decreased in the later stages. Therefore, the corrosion performanceof the coating is better than the coating G0. For G3, the coating was formed quickly in

the first stage with higher current density. However, the current densities in the later stageswere only slightly reduced, not sufficiently. Thus, its corrosion performance was not asgood as coating G2.

4. Conclusions

1. Various reactions are involved in the anodisation of Mg alloy AZ91D in a hydroxidesilicate solution at a constant applied current density: such as the deposition ofMg(OH)2 and MgSiO3 in the initial stage, the dehydration of Mg(OH)2 in the secondstage and the vigorous gas evolution, sparking, deposition of MgO, thermal-decompo-

sition of bath solution, oxidation of magnesium by active oxygen, melting and sealingof coating in the final stage.

2. The thickness, composition and microstructure are dependent on the anodising currentdensity and they can influence the corrosion performance of the anodised coatings.



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Therefore, the corrosion performance of anodised coating can be adjusted by the ano-dising current density.

3. In order to improve the corrosion performance of an anodised coating, an optimisedanodising current waveform should be applied. In earlier stages, a higher current den-

sity is essential for high production efficiency, but in the later stages, a low current den-sity can minimise or seal the pores and improve the corrosion resistance of an anodisedcoating.

Acknowledgement

The research work was supported by UQIPRS and CAST scholarships. The Coopera-tive Research Centre for Cast Metals Manufacturing (CAST) was established and is sup-ported by the Australian Governments Cooperative Research Centres Program. Theauthors would also like to acknowledge Ms. Ying Yu s assistance in the SEM and EDXanalyses in this study.

Appendix 1. Calculation of the current efficiency

At time ti, if the volume of O2 is Vi and the total area of the specimen is A, then the rate(RO2 ) of O2 evolution per area at time ti can be expressed as:

RO2 Vi1 Vi1=ti1 ti1=A

If only the following electrochemical reaction is responsible for the oxygen evolution:

4OH 2H2O O2 " 4e

then the charge used for oxygen evolution CO2 can be calculated:

CO2 xnF

where x is the molar number of oxygen produced, n (number of electron of the above reac-tion) = 4, and F (Faradays constant) = 96485.3 C/M. In addition,

x PV=RT 1 RO2Dt=1000=8:2057 102 298 13

in which oxygen pressure P= 1, Vis volume of oxygen, R = 8.2057 102 at l K1 mol1,and absolute temperature T= 296 K. Hence:

CO2 4xF 4 96500 x 15:783RO2Dt

and the current density IO2 used for O2 evolution should be

IO2 CO2=Dt 15:783RO2 14

Normally, the current efficiency g can be defined as

g Ifilm=IT 100%

where IT is the total anodising current density, which is equal to the applied anodising cur-rent density I, and Ifilm is the current used to form the coating. Since

I IT Ifilm IO2



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we have

g IT IO2 =IT I 15:783RO2 =IT 100%

Appendix 2. Calculation of the theoretical charge during anodising process

It is assumed that the density (q) and porosity (d) of an anodised coating are constants.Reaction (5) is considered as the main electrochemical reaction during the anodising

process. The amount of the coating in mole (mcoating) can be expressed as

mcoating A h qcoating 1 d=Mcoating 15

in which A is the area (assumed to be 1 cm2), h is the thickness of coating (lm) and Mcoatingis molar weight (g mol1).

If the coating consists of MgO, Mg(OH)2 and MgSiO3 alone, the densities and molarweights of the coatings can be calculated and listed in the following table [36].

MgO Mg(OH)2 MgSiO3

q 3.58 2.36 3.19Mcoating 40.3 58 100

The charge used for the coating formation is

Ccoating 2 mcoating F 2 1 h qcoating 1 dcoating F=Mcoating

Based on the statistical estimation of the porosity of the anodised coating on AZ91D,dcoating was about 15% (estimated from the SEM images). Substitution of it in the aboveequation gives:

CMgO 1:4573h 16

CMgOH2 0:6675h 17

CMgSiO3 0:5236h 18

The relationships between the charge and thickness of the coatings described by Eqs. (16)(18) are plotted in Fig. 6(d).

References

[1] G. Song, Advanced Engineering Materials 7 (2005) 563586.[2] G. Song, Materials Science Forum 488489 (2005) 649652.[3] G. Song, B. Johannesson, S. Hapugoda, D. St John, Corrosion Science 46 (2004) 955977.[4] G. Song, A. Bowles, D. St John, Materials Science and Engineering: A 336 (1) (2004) 7486.[5] G. Song, D. St John, Corrosion Science 46 (2004) 13811399.[6] G. Song, D. St John, Materials and Corrosion 56 (1) (2005) 1523.[7] G. Song, D. St John, T. Abbott, International Journal of Cast Metals Research 18 (3) (2005) 174180.

[8] G. Song, D. St John, International Journal of Cast Metals Research 12 (2000) 327334.[9] G. Song, D. St John, Corrosion inhibition of magnesium alloys in coolants, in: N.R. Neelameggham, H.I.

Kaplan, B.R. Powell (Eds.), Magneisum Techlology 2005, TMS (The Minerals, Metals and MaterialsSociety), 2005, pp. 469474.



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[10] Z. Shi, G. Song, D. St John, A method for evaluating the corrosion resistance of anodised magnesium alloys,Corrosion and Prevention 2001, ACA, Newcastle, Paper #058, 2001.

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[14] T.F. Barton, US Patent 5792335, Magnesium Technology Limited, Auckland, New Zealand, 1998.[15] A.J. Zozulin, D.E. Bartak, Metal Finishing 92 (1994) 39.[16] A.K. Sharma, R.U. Rani, K. Giri, Metal Finishing 95 (1997) 43.[17] O. Khaselev, D. Weiss, J. Yahalom, Corrosion Science 43 (2001) 1295.[18] D.E. Bartak, B.E. Lemieux, E.R. Woolsey, US Patent: 5470664, Technology Applications Group, Grand

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[23] G. Song, D. St John, Journal of Light metals 2 (2002) 1.[24] G. Song, A. Atrens, D. St John, in: J. Hryn (Ed.), Magnesium Technology, TMS, New Orleans, 2001, p. 255.[25] G. Song, The Journal of Corrosion Science and Engineering, 6 C104.[26] S.J. Splinter, W.S. McIntyre, Surface Science 314 (1994) 157171.[27] H.A. Evangelides, Metal Finishing 7 (1951) 56.[28] K.J. Huber, Journal of Electrochemistry Society 100 (1952) 372.[29] E.F. Emley, Principles of Magnesium Technology, Pergamon Press, London, New York, 1966.[30] M. Takaya, Gypsum and Lime 223 (1989) 40.[31] O. Khaselev, J. Yahalom, Corrosion Science 40 (1998) 1149.[32] O. Khaselev, J. Yahalom, Journal of the Electrochemical Society 145 (1998) 190.[33] Y. Mizutani, S.J. Kim, R. Ichino, M. Okido, Surface and Coatings Technology 169170 (2003) 143.[34] L. Brown, G. Besenbruch, K. Schultz, S. Showalter, A. Marshall, P. Pickard, J. Funk, in: AIChE 2002 Spring

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influence of anodising current

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