effects of current density on the microstructure and...

Indian Journal of Engineering & Materials Sciences Vol. 24, August 2017, pp. 313-320

Effects of current density on the microstructure and corrosion resistance of MAO coating on extruded AZ80 alloy

Degang Zhao*, Quan Yu, Jiai Ning & Na Liu School of Materials Science and Engineering, University of Jinan, Jinan, 250022, China

Received 24 April 2015; accepted 30 March 2017

In this study, microarc oxidation coatings are prepared on the extruded AZ80 magnesium alloy in the silicate electrolyte solution under different current densities. The phase composition and microstructure of the MAO coating are characterized by XRD and SEM, respectively. The MAO coatings are mainly composed of MgO, MgSiO3 and MgAl2O4. Typical porous microstructure can be observed in the coatings. The average diameter of micropores is less than 1 μm. With the current density increasing, the diameter of micropores increased. When the current density is higher than 5 A/dm2, the hydrate with flocculent structure is found on the edge of micropores indicating the higher current density could induce hydration to seal the micropores of coating at relatively low temperature in the silicate electrolyte solution. The thickness of MAO coating increased with the current density increasing. The electrochemical corrosion results showed that the MAO coating prepared at 6 A/dm2 has the best corrosion resistance.

Keywords: Alloys, Coating, Microstructure, Corrosion

Magnesium and its alloys, an important class of structural materials with the properties of low density and high strength-to-weight ratio, are widely utilized in the automotive, electronic and aeronautical industries1-3. However, the application of magnesium alloys has been restricted due to the poor corrosion resistance. Therefore, many surface modification technologies, such as anodizing treatment, electrochemical plating, micro-arc oxidation (MAO), gas-phase deposition process and laser surface alloying have been applied to improve the surface properties4-6. In recent years, MAO technology based on the anodizing treatment has attracted great interest because MAO can form a remarkably thick, uniform, and hard film, which can enhance the corrosion resistance of magnesium alloys. In addition, the oxide film formed by MAO technology can strongly adhere to the substrate with complex geometrix. At present, MAO process has successfully been used for producing oxide coating on magnesium alloys to provide sufficient corrosion protection7, 8.

The quality of MAO coating is determined by the alloy composition, the electrolyte constituent, and the MAO parameters including applied voltage, current density and current frequency etc. Current density, which could greatly influence the microstructure and properties of MAO coatings, is one of the most

important process parameters. Many MAO treatments on magnesium alloys have been conducted under constant current mode9-11. Yang et al.12 studied the effects of current density on the MAO coatings of AZ91 magnesium alloy and found that the MAO coatings formed at the current density of 5 A/cm2 showed the best corrosion resistance. Liang et al.13 compared the effects of three different current modes on the corrosion properties of MAO coatings on the AM60 alloy and demonstrated that the surface morphology and corrosion resistance could be improved by adjusting the current density. However, the substrates of magnesium alloy in the MAO process are mostly as-cast alloys in the present research related with MAO technology14-16. Compared with conventional as-cast magnesium alloy, the extruded magnesium alloy has greater application prospect because of its better strength and plasticity, especially in the automobile industry17-19. With the development of lightweight of automobile, the extruded AZ80 magnesium alloy has usually been selected as the structure framework of electric vehicle bus for its high strength and plastic formability. An understanding of MAO coating on the extruded AZ80 magnesium alloy is therefore required in order to improve the corrosion resistance of the extruded AZ80 magnesium alloy. In this study, the effects of current density on the microstructure and properties of MAO coating on the extruded AZ80 magnesium alloy

————— *Corresponding author (E-mail: [email protected])

INDIAN J. ENG. MATER. SCI., AUGUST 2017

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were discussed. The results are expected to be beneficial to the application of the extruded AZ80 magnesium alloy in the electric vehicle bus. Experimental Procedure

Materials The substrate used in the experiment was the

extruded AZ80 magnesium alloy (10×20×3 mm) with nominal composition shown in Table 1. The specimens were ground with SiC abrasive paper, ultrasonically washed with acetone, and finally dried for MAO treatment. Coating preparation

Electrolyte was prepared from the solution of Na2SiO3 (15 g/L), NaOH (3 g/L), NaF (3 g/L) and pH value adjusting additives in distilled water. A 20 kW MAO equipment consisting of a high power bi-polar pulsed electrical source, a stainless steel container with a sample-holder as the electrolyte cell, and a stirring and cooling system was employed to generate voltage waveform. The current frequency was 800 Hz, and the current density was fixed at 4.0 A/dm2, 5.0 A/dm2, 6.0 A/dm2 and 7.0 A/dm2 in the MAO process, respectively. The current density was maintained by controlling the voltage amplitude in the deposition process. The electrolyte temperature was retained in the range of 22-45oC during MAO process. The MAO treatment time was 15 min for all the samples. Characterization of MAO coating

The constituent phases of MAO coatings were determined by X-ray diffractometry (Cu-Kα, Rigaku, Rint2000). The surface morphologies of MAO coatings were observed by JXA-8100v scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) analysis facilities. Before observation, the MAO coated samples were mounted in the epoxy resin, polished with SiC paper along the cross-section of the coating, and then ultrasonically cleaned with distilled water. The thickness of coating was determined by a MINITEST 1100 microprocessor coating thickness gauge (Elektro-physik Koln), which utilizes eddy currents induced in the substrate to measure coating thickness with an accuracy of about ±0.5 μm. Micro-hardness of coatings was measured using a Bluehler Micromet hardness tester with a load of 25 g for 10 s. The hardness value was obtained by averaging the results of 5 measurements.

Electrochemical tests The potentiodynamic polarization measurement for

the samples with and without MAO coating were performed using an electrochemical workstation at room temperature. A 3.5 wt% NaCl solution was used as the corrosive medium. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate was used as the counter electrode. Prior to the beginning of the potentiodynamic polarization test, the samples were kept in the solution for 30 min to allow the open circuit potential to become stable. The polarization scan continued in the anodic direction with a potential scan rate of 0.5 mV/s.

Results and Discussion

XRD analyses of MAO coating Figure 1 shows the X-ray diffraction patterns of

micro-arc oxidation coatings prepared at different current densities on extruded AZ80 magnesium alloy. It can be seen that the MAO coatings were all mainly composed of cubic MgO, MgAl2O4 and MgSiO3. With the current density increasing, the intensity of peaks of magnesium decreased which should be related with the thickness of MAO coating. The presence of MgSiO3 phase indicated that Si ions in the electrolyte had directly engaged in the chemical reactions near the micro-arc zone during the MAO process. The content of MgAl2O4 in the coating was low owing to the low content of Al in the substrate. In addition, no compound of sodium in the coatings was detected, which should be due to the very low content of sodium

Table 1 — Chemical composition of extruded AZ80 magnesium alloy

Elements wt%

Mg Bal.

Al 8.0

Zn 0.7

Mn 0.2

Cu ≤0.05

Fe ≤0.01

Ni ≤0.005

Fig. 1 — X-ray diffraction patterns of MAO coatings prepared at different current densities on the extruded AZ80 magnesium alloy

ZHAO et al.: MICROSTRUCTURE AND CORROSION RESISTANCE OF MAO COATING

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in the coatings. Cai et al.20 and Wang et al.21 ever studied the MAO coating on as-cast AZ91 magnesium alloy and reported the coating was composed of MgO and Mg2SiO4, which was different from the present results. The difference in the composition of coating is due to the different microstructure of magnesium alloy substrates. Under the condition of constant current density of MAO process, the formation of coating resulted from the plasma chemical oxidation reaction of the substrate and nearby the electrolyte in the discharging channels produced by sparks. During the sparking process, the dissolving of Mg2+ from the substrate outward migrated owing to the effect of electrical field; meanwhile SiO3

2- and OH- inward migrated into the channels. The chemical reactions could occur in the discharging channels. Therefore, MgO and MgSiO3 were formed by the following reaction:

22 )(2 OHMgOHMg … (1)

OHMgOOHMg 22)( … (2)

32

32 MgSiOSiOMg … (3)

Figure 2 displays the microstructure of as-cast and extruded AZ80 alloys. The microstructure of the AZ80 alloy mainly consists of α-Mg phase and β-Mg17Al12 phase. The MAO coating formation during the initial stages preferentially happens on the α phase field due to the difference of chemical potential between α-Mg and β-Mg17Al12, then the coating forms on the β phase at later stages. Although the MAO coating growth does not initiate on α-Mg and β phase simultaneously, the β phase neither

destroys the integrity and continuity of MAO coating nor induces any structural defects. Therefore, the corrosion-resistant coatings can be deposited on Mg alloys having heterogeneous microstructures through the MAO technique. Compared with as-cast AZ80 alloy, the extruded AZ80 magnesium alloy with the fine-grained microstructure has higher dislocation density, high-energy defects and modified distribution of β-Mg17Al12 phase, which contribute to increase the growth rate of MAO coating. The smaller β-phase in extruded AZ80 magnesium alloy homogeneously distributed in the α-Mg phase matrix with the band structure, as shown in Fig. 2(b), which could promote more Al3+ to involve in the following chemical reactions in the MAO process:

OHOMgAlOHAlMg 24232 48 … (4)

Therefore, the XRD result of MAO coating on the

extruded AZ80 magnesium alloy show the different content compared with that of as-cast magnesium alloy. Surface morphology and microstructure of MAO coating

The control of applied current density is crucial to the microstructure in the microarc oxidation process. The variation of current density can regulate the spark discharge characteristics as well as the microstructure of formed oxide coatings. Figure 3 shows the SEM of MAO coatings on the extruded AZ80 magnesium alloy prepared at different current densities. Typical porous microstructure can be observed in the coatings. The average diameter of micropores in the MAO coating surface was less than 1 μm. With the current density increasing, the diameter of micropores

Fig. 2 — Microstructure of the as-cast and extruded AZ80 magnesium alloys, (a) as-cast and (b) extruded


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increased. It can be understandable that higher energy density in the electrolyte induced larger discharge sparks and larger “melt discharging channel”, therefore larger micropores would remain on the MAO coating surface after solidification of melting products in the channels. As is known there are three main steps underlying the formation of MAO coating: firstly, a number of discharge channels are formed for the breakdown of the low conductive oxide layer and the alloy elements are melted out of the substrate to the channels; then, the alloy elements react with the electrolytic elements and get oxidized; at last, the oxidized material is ejected to the coating surface and cooled by the electrolyte. The above processes are repeated all through the MAO process, leading to the increase of the coating thickness. Figure 4 shows the enlarged surface morphology of MAO coating on extruded AZ80 magnesium alloys formed at different current densities. It can be clearly observed from Fig. 4(a) and (b) that the micropores of MAO coating do not penetrate through the whole oxide coating.

However, a flocculent structure was found on the edge of micropores of MAO coating when the current density was higher than 5 A/dm2, just as shown in Fig. 4(c) and (d). Moreover, the flocculent phase has a tendency to seal the micropores with increasing the current density. This phenomenon is speculated to be related with the hydration of MAO coating. The hydration reaction can be expressed as

22 )(OHMgOHMgO … (5)

Generally speaking, the reaction (5) occurred during sealing treatment for MAO coating in boiling water (85oC~100oC). In this study, higher current density leads to increased sparking discharge intensity caused by the high pulse energy. The spark discharges could lead to localized high temperature (~2×104 K) and high pressure (~102 MPa)22. According to the results of Khaselev et al.23, the centre of a spark can reach a temperature as high as 2250oC. Such a high temperature would not only trigger plasma chemical reactions to form the porous microstructure of the

Fig. 3 — Surface morphologies of MAO coatings prepared at different current densities on the extruded AZ80 alloy, (a) 4 A/dm2, (b) 5 A/dm2, (c) 6 A/dm2 and (d) 7 A/dm2


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coatings, but also raise the localized temperature of electrolyte. Therefore, the hydration reaction could occur. The formation of the flocculent structure could seal the micropores of MAO coating, which maybe enhance the corrosion resistance of MAO coating.

Figure 5 presents the cross-sectional morphology of MAO coatings formed on the extruded AZ80 magnesium alloy. It can be seen that the interface of coating/substrate was smooth and compact with few defects, which indicated the coating could tightly

Fig. 4 — Enlarged surface morphologies of MAO coatings prepared at different current densities on the extruded AZ80 alloy, (a) 4 A/dm2, (b) 5 A/dm2, (c) 6 A/dm2 and (d) 7 A/dm2

Fig. 5 — Cross-sectional images of MAO coatings on the extruded AZ80 magnesium alloy prepared at different current densities, (a) 4A/dm2 and (b) 7 A/dm2


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adhere to the substrate. The cross-sectional morphology also indicated that the micropores on the surface of MAO coating do not penetrate into the substrate, which is beneficial to enhance the corrosion resistance of extruded AZ80 magnesium alloy. The thickness of MAO coating was about 2.5 μm when the current density was 4 A/dm2, as shown in Fig. 5(a). When the current density increased to 7 A/dm2, however, a multi-layer microstructure composed of the external porous layer and internal dense layer appeared in the MAO coating, just as shown in Fig. 5(b). The formation of two-layer structure of coating should be related with the higher energy discharging. Similar microstructure was also reported in the other studies24-26. With the current density increasing, the thickness of MAO coating increased. The thickness of the coating increases from 2.5 to 5 μm when the current density increases from 4 to 7 A/dm2. It can be concluded that the current density plays a major role on the thickness of coating. The higher current density leads to higher discharge spark intensities and provides larger discharge channels. Therefore, the more ions from the electrolyte will react with the metal cations, producing more oxides and increasing the thickness. Hardness of MAO coating

Figure 6 shows the microhardness and thickness of MAO coating on the extruded AZ80 magnesium alloy as function of current density. The microhardness of the extruded AZ80 magnesium alloy increased visibly after MAO process, which was consistent with the previous studied27. As the thickness of coating increased, the microhardness of coating increased with the current density increasing. However, the microhardness of MAO coating was almost unchanged when the current density increased to 7 A/dm2, which should be related with the two-layer structure of MAO coating. It is well known that the MAO coating is usually composed of a porous outer layer with several large-sized and a compact inner layer with less porosity. The inner layer of compactness have a predominant effect on the microhardness of MAO coatings rather than its thickness. Consequently the microhardness of MAO coating prepared at 7 A/dm2 was nearly same with that of MAO coating prepared at 6 A/dm2. Electrochemical corrosion behavior of MAO coating

Figure 7 displays typical potentiodynamic polarization curves of extruded AZ80 samples with

MAO coating as well as the extruded AZ80 substrate. The corrosion potentials (Ecorr), corrosion current densities (Icorr), anodic/cathodic tafel slopes (ba and bc) and pitting potential (Ept) were derived directly from the polarization curves. The polarization resistance (Rp) was calculated on the basis of approximately linear polarization behavior near open circuit potential by Stern-Geary equation28:

)(3.2 cacorr

cap bbI

bbR

… (6)

The pitting potential Ept is equal to the inflexion potential on polarization curves. When the polarization potential exceeds Ept, the corresponding current density has an abrupt drop in the cathodic

Fig. 6 — Microhardness and thickness of MAO coatings on the extruded AZ80 magnesium alloy as a function of current density

Fig. 7 — Typical potentiodynamic polarization curves of the extruded AZ80 samples with MAO coating as well as theextruded AZ80 substrate


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branch or a sudden increase in the anodic branch, which indicates the breakdown of the surface coating. The electrochemical parameters of potentiodynamic polarization are summarized in Table 2. Generally, the corrosion potentials correspond to the chemical thermodynamic tendency to corrosion of coated samples, which is primarily related to the chemical composition and microstructure of the MAO coating. From the results of polarization tests, it can be seen that the corrosion potentials of all the coated samples were shifted to a positive potential compared with the substrate, which indicated that the corrosion resistance of extruded AZ80 magnesium alloy was significantly enhanced by the MAO coating. This was also in accord with the data in Table 2. With the current density increasing, the Ecorr of coated samples showed a positive shift. When the applied current density was 7 A/dm2, however, the corrosion protective property of MAO coating decreased compared with the MAO coating prepared at 6 A/dm2, which may be attributed to the morphology and microstructure of MAO coating. The major corrosion form of magnesium alloy in NaCl solution is pitting corrosion. Thus, the coating with excellent corrosion resistance should have compact structure. Although the MAO coating prepared at 7 A/dm2 was thickest, the compact inner layer of multi-layer structure was a more important factor for corrosion protection. On the one hand, The formation of hydrate Mg(OH)2 is the consumption or dissolution process MgO. When the current density is 6 A/dm2, the sealing effect of hydrate Mg(OH)2 exceeds the dissolution process of MAO coating. Hence, the corrosion resistance of MAO coating continued to increase. When the process reaches equilibrium, the MAO coating should exhibit the best corrosion resistance. However, when the current density increase to 7 A/dm2, the dissolution process of MAO coating exceeds the sealing effect of hydrate Mg(OH)2. Consequently, the corrosion resistance of MAO coating decreased. On the other hand, some microcracks initiated by the thermal stress during the rapid solidification of molten

oxide product in the large spark discharge channels can be found on the MAO coating prepared at 7 A/dm2, just as shown in Fig. 8. This also decreased the property of corrosion protection of MAO coating. The presence of microcracks or the larger micropores on the surface of the MAO coatings increases the effective surface area and thus the tendency of the corrosive medium to adsorb and concentrate into these sites. This would facilitate quicker infiltration of the corrosive medium into the inner regions of the coating and subsequently down to the substrate, thus deteriorating the corrosion resistance of the coating. Therefore, the microcracks should be avoided and the larger micropores should be sealed. It can be concluded that the corrosion resistance of MAO coating is related to thickness, phase composition, and surface morphology.

Conclusions Microarc oxidation coatings were prepared on the

extruded AZ80 magnesium alloys in the silicate electrolyte solution under different current density. The coatings were composed of MgO, MgSiO3 and MgAl2O4 phases. Moreover, the MAO coating had typical porous microstructure and the average diameter of micropores was less than 1 μm. The hydrate with flocculent structure was found on the edge of micropores when the current density was

Table 2 — Electrochemical parameters of potentiodynamic polarization of extruded AZ80 samples with MAO coating as well as the extruded AZ80 substrate

Samples Ecorr(VSCE) Icorr(A/cm2) ba(VSCE/decade) bc(VSCE/decade) Rp(kΩcm2)

Substrate -1.56 2.61×10-4 1.35 -0.47 1.20 Coating2(4A/dm2) -1.51 2.51×10-5 0.244 -0.400 2.62 Coating3(5A/dm2) -1.41 5.62×10-6 0.264 -0.292 10.73 Coating4(6A/dm2) -1.21 3.16×10-6 0.226 -0.167 13.21 Coating5(7A/dm2) -1.32 5.37×10-6 0.216 -0.227 8.96

Fig. 8 — Microstructure of MAO coating prepared at 7 A/dm2


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higher than 5 A/dm2. Meanwhile the higher current density could induce hydration to seal the micropores of MAO coating in the silicate electrolyte solution.With the current density increasing, the thickness of MAO coating increased. The oxide coating prepared at the current density of 6 A/dm2 had the best corrosion resistance. Acknowledgments

Financial supports from the National Natural Science Foundations of China (51471076 and 51202088) and Shandong Provincial Doctoral Foundation (BS2013CL004) are acknowledged. The authors also acknowledge the help received from the Institute of New Materials, Shandong Academy of Sciences for the measurement of samples. References

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