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Formation mechanism of oxide layer on AZ31 Mg alloy subjected to micro-arc oxidation considering surface roughness
Applied Surface Science 497 (2019) 143772
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Formation mechanism of oxide layer on AZ31 Mg alloy subjected to microarc oxidation considering surface roughness N. Nashraha, M.P. Kamila, D.K. Yoona, Y.G. Kimb, Y.G. Koa, a b
T
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Materials Electrochemistry Group, School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea Extreme Fabrication Technology Group, Korea Institute of Industrial Technology, Daegu 42994, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Mg alloy Micro-arc oxidation Surface roughness Nucleation Growth
This study investigated the formation mechanism of the oxide layer generated by micro-arc oxidation (MAO) considering surface roughness of AZ31 Mg alloy. For this purpose, a series of MAO treatments under alternating current in a silicate-based electrolyte were performed on two present samples with grooved and flat surfaces. The time-voltage response presented that the grooved sample reached the breakdown voltage earlier than the flat sample. This indicated the appearance of micro-discharges on the grooved sample occurred earlier than that of the flat sample since the oxide fragments nucleated preferentially at the ridge areas where electrons were concentrated severely under high electrical field. The oxide layer of the grooved sample was relatively less dense than that of the flat sample. This was attributed to the fact that the oxide layer of the grooved sample was likely to grow with ease on pre-existing coating layer whereas the oxide layer of the flat sample developed conformably throughout the whole areas. Thus, the growth rate of the grooved sample was higher than that of the flat sample, which agreed well with micro-discharge activities. Such formation mechanism was proposed in relation to nucleation and growth of the oxide layer formed on different surface structures.
1. Introduction In recent years, magnesium (Mg) and its alloys have been investigated extensively as one of the most promising metallic candidates suitable for a variety of commercial applications within the scopes of electronic and automobile components due to high electromagnetic shielding properties and good specific strength at room temperature. Despite their attractive properties, these applications were unfortunately obstructed by inherent poor corrosion resistance of Mg alloy against corrosive environment [1–3]. Thus, a variety of surface modification methods, such as metal-based plating, chemical-conversion coating, and micro-arc oxidation (MAO, also known as plasma electrolytic oxidation), have been applied [4–6]. MAO, where the electrochemical reactions accompanying a myriad of micro-arcs over dielectric ignition triggered the nucleation of the oxide layer in an eco-friendly electrolyte, was considered to be desirable to fabricate the protective oxide layer with excellent adhesion to the metal substrate. This technique differed from the conventional hard-coatings [7]. Earlier studies reported the effects of several parameters on the improvement in the electrochemical properties as well as on the formation of the oxide layer by modifying electrolytes with the additions of organic and/or inorganic compounds [8,9], by changing electrical
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parameters of rectifier [10,11], and by controlling substrate features, such as grain size and surface topography [12,13]. Among processing variables, the surface morphologies of the substrate would be known to affect the formation of the oxide layer during MAO coating. Moreover, the influences of surface roughness on corrosion, wear, and fatigue properties were significant [14–16]. For instance, Yoo et al. [17] postulated from direct current PEO coating on AZ91 Mg alloy samples whose surface roughness values were measured to be 0.5, 1, and 2.5 μm that, with an increment in surface roughness, corrosion resistance in 3.5 wt% NaCl solution decreased to some extent whereas the transfer of current across the oxide layer became easier due to the fact that the flat surface might cause crystalline phase in the oxide layer during growth [13,17]. Another study reported by Wang et al. [18] suggested that the regular micro-grooves with a trapezoidal shape formed on Al alloy would lead to the high fractions of α-Al2O3 and 3Al2O3·2SiO2 working as hard constituents in the oxide layer due to high temperature and pressure in the sharp interface. This would give rise to superior wear resistance to the flat counterpart [18]. According to the fatigue results of 2024 Al alloy samples processed by MAO, the fatigue life was seen to decrease with increasing initial surface roughness in which hill and valley were identified [16]. While it has been well accepted that surface roughness was a crucial
Corresponding author. E-mail address: [email protected] (Y.G. Ko).
https://doi.org/10.1016/j.apsusc.2019.143772 Received 28 May 2019; Received in revised form 19 August 2019; Accepted 22 August 2019 Available online 23 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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factor affecting the inherent performances of the oxide layer, the fundamental mechanism that would describe nucleation and growth of the oxide layer during plasma electrolysis remained less understood by considering a role of surface roughness. Therefore, the present study attempts to scrutinize the nucleation and growth influenced by surface roughness of the initial sample. The nucleation phenomena of the oxide fragments formed at the ridge and valley of the initial substrate are compared at the onset of breakdown where the plasma discharges start to appear. Subsequently, both growth pattern and rate of the oxide layer are studied to propose formation mechanism in relation to microstructural interpretation. Two different corrosion assessments are used to describe the difference in electrochemical performances related to surface roughness. In addition, the stress state of the oxide layer is discussed based on high-energy plasma discharges formed on the surface during MAO. 2. Experimental procedures A commercial-graded AZ31 Mg alloy plate was cut into a dimension of 15 (L) × 25 (W) × 4 (T) mm3. Prior to MAO coating, two samples were polished mechanically with SiC paper up to #2400 grit followed by diamond suspension to obtain a nearly flat surface (FS). Then, one of the samples was ground again using SiC paper with #100 grit in order to produce a grooved surface (GS). Both samples were rinsed with distilled water, cleaned ultrasonically with pure ethanol, and dried in a stream of warm air. Surface roughness of the present samples was measured by atomic force microscopy (AFM, Park systems NX-10). To quantify surface roughness, the value of Ra that expressed an average deviation of the oscillations along a regression line drawn through particular roughness profile between the ridge and valley areas in the samples was used. Such values for both FS and GS samples were measured to be 0.025 and 1 μm as shown in Fig. 1. A series of MAO treatments were performed in an aqueous electrolyte with 0.1 M KOH and 0.05 M Na2SiO3 (pH: 11.83, conductivity: 12.39 mS·cm−1) at current density and frequency of 50 mA·cm−2 and 60 Hz, respectively. The present samples worked as the anode while the stainless steel net was set as the cathode. A glass vessel was used as the electrolyte container, equipped with magnetic stirrer and water cooling system to maintain the electrolyte temperature around 288 K for the stabilization of electrochemical reactions throughout the process. Details of the experimental setup were found elsewhere [19]. During MAO, the real-time images of micro-discharges formed on the surface were taken with respect to processing time. The morphologies of the surface and cross section in the oxide layer were observed by fieldemission scanning electron microscope (FE-SEM, HITACHI S-4800) coupled with energy-dispersive spectrometer. The electrochemical response of the oxide layer was evaluated through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution near to sea water. The polarization tests were scanned from −0.25 to +0.4 V with respect to open circuit potential at a scan rate of 1 mV·s−1. EIS tests were performed at frequency ranging 106 to 10 Hz with 10 mV rms perturbation. For each sample, electrochemical tests were repeated at least three times to ensure that the data obtained would be feasible.
Fig. 1. Representative AFM images showing the surface morphologies of the present samples with (a) flat and (b) grooved surfaces prior to MAO coating. These structures are uniform throughout the surfaces after mechanical preparation.
sample via MAO for 20 s is not provided due to less noticeable microdischarges on the anode surface. From the increasing tendency of responding voltage shown in Fig. 2a, two stages were divided clearly by their own slopes. In stage I, the responding voltage would tend to increase linearly, obeying Ohm's law due to an increase in electrical resistance of the thin passive film that formed on the substrate by chemical reactions [9,12,19]. Unless the thin passive film would withstand the intense electric field any longer, breakdown phenomenon began to appear together with a number of micro-discharges as the lighting dots. Then, stage II was started. The dotted red-circle in both curves indicated the breakdown voltage whose values were measured to be ~220 V for both cases. Interestingly, times to reach breakdown were different from each other in this study. FS and GS samples reached such ignition in 30 and 20 s respectively, suggesting that surface roughness would influence the initial stability and concurrent nucleation of microdischarges on the sample. It is inferred that the higher surface roughness, the earlier breakdown took place. After breakdown, the voltage in stage II increased with slope value lower than that in stage I as the electrical resistance increased by the growth of the oxide layer covering the anode surface. After MAO for 300 s, the final voltages of FS and GS samples reached ~290 and ~320 V, respectively. Such a difference in the final voltage would be associated with the thickness and morphologies of the oxide layer, which would be discussed microstructurally in the following section. The inset images in Fig. 2a evidenced that micro-discharges on FS and GS samples were evolved in different manners that allowed their size and fraction to vary moderately with increasing coating time. These results were analyzed in Fig. 2b. After MAO for 20 s, a number of fine micro-discharges with a dim color started to appear on GS sample
3. Results and discussion 3.1. Voltage-time response with micro-discharge Fig. 2 presents the voltage-time curves and histograms of FS and GS samples coated by MAO at 50 mA·cm−2 as a function of coating time ranging from 20 to 300 s. The changes in the responding voltages together with couples of inset images showing the characteristics of micro-discharges for 20, 30, 120, and 300 s are displayed in Fig. 2a, and the histograms of both size distribution and fraction of micro-discharges with respect to coating time are provided in Fig. 2b. A histogram of the 2
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Fig. 2. (a) Responding voltage vs. coating time curves of FS and GS samples during MAO at 50 mA·cm−2 for 300 s. The insets show the evolution of micro-discharges formed on surfaces with respect to coating time, and (b) the size distribution and fraction of micro-discharges taken from the insets.
Fig. 3. Representative SEM images showing the surface and cross section structures after MAO for 30 and 20 s when breakdown took place for the first time: (a), (b) FS and (c), (d) GS samples. Points A and B are used for compositional analyses in EDS.
MAO at breakdown point corresponding to FS and GS samples for 30 and 20 s, respectively. As presented in Figs. 3a and b, a thin oxide layer with a fairly uniform thickness of 1.2 μm was observed clearly on the whole surface of FS sample. Several micro-pores with an average size of 0.5 μm were also detected due to micro-discharges with high energy. Those structures were typical characteristic of metal-oxide layer formed by MAO [20,21]. In case of GS sample, the formation of the oxide layer seemed to be premature. From Fig. 3c, both ridge and valley were still visible even after breakdown. To confirm the elemental composition, EDS point measurements were carried out at least five times to show the element difference on the ridge and valley areas as indicated by the black (A) and white (B) dots in Fig. 3c. The amount of O and Si in wt% on the ridge was relatively higher than that on the valley as listed in Table 1, confirming the un-uniform formation of oxides on the ridge at the initial stage of MAO coating. From the observations on the cross section, it is important to note that some oxide fragments were also found to nucleate locally around the ridge to a greater extent than that near to the valley as shown in Fig. 3d. This fact implied that breakdown phenomenon was initiated around the ridge earlier than around the adjacent valley. It is found that both chemical decay of metal and formation of passive film were processed in the same way that allowed the movement of the anion species in the wet-process and their interaction with
Table 1 EDS results of GS sample after MAO for 30 s when breakdown took place for the first time. The measurements are taken from the representative points at the ridge and valley areas, which are denoted as points A and B in Fig. 3c. Area Ridge Valley
Point A B
Mg (wt%)
O (wt%)
Si (wt%)
48.8 ± 0.8 63.6 ± 2.7
41.0 ± 1.3 29.1 ± 0.5
10.2 ± 0.6 7.30 ± 0.7
while no micro-discharge was detected on FS sample. This fact was confirmed again by the different times to reach breakdown in the voltage-time data. In particular after 300 s, the size distribution of microdischarges in FS sample was more homogenous than that in GS sample which showed a bimodal distribution where coarse and fine sparks coexisted. Thus, the fraction of micro-discharges in GS sample was lower than that in FS counterpart, which was attributed mainly to the cluster of micro-discharges in local areas of GS sample. This finding suggested that growth behavior of the oxide layers was expected to be different in the samples with different surface topographies. 3.2. Nucleation behavior upon breakdown Fig. 3 shows an initial occurrence of the oxide layers produced by 4
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sample, intense plasma arcs would result in the formation of discharge channel present in the oxide layer. On the other hand, the size and distribution of the micro-pores were relatively uniform in FS sample after MAO for 300 s. This phenomenon was explained by continuous monitoring on short-duration PEO treatment that the serial formation of micro-discharge was expected on the local region with low electrical resistivity as discussed by Clyne research group [24]. The oxide layer with either deep micro-pores or thin thickness would be the plausible candidate responsible for electron transfer across the insulation barrier during the wet-based oxidation coating. It should be noted that this tendency was understood on the same system in a ‘relative’ manner. In this study, the valley area with lower volume of the oxide fragments would be susceptible to next breakdown accompanying intense cascades because nucleation of the oxide layer appeared first at the ridge area as evidenced in Fig. 3d. Thus, micro-discharges concentrated at the valley area would play an important role in the formation of type-B discharge channels (defined by Hussein et al. [25]) as well as relatively fast growth of the oxide layer in the local area. Both XRD data of two samples coated by MAO for 300 s were composed of peaks associated with Mg, MgO, and MgSiO3 as shown in the Fig. 4g. A significant difference was detected rarely due to the same substrate and electrolyte used in this study. The thickness values of the final oxide layers in FS and GS samples were measured to be ~11 and ~14 μm, respectively after MAO for 300 s as shown in Fig. 4h. Hence, the growth rate of GS sample was higher than that of FS sample. This finding would be in line with final responding voltage of GS sample higher than that of FS counterpart. All results were also reinforced indirectly by AFM observations as shown in Fig. 5. For FS sample with initial substrate roughness was measured to be ~0.025 μm, the values of Ra would tend to increase with respect to coating time, reaching ~0.05, however, the initial Ra value of GS sample measured to be ~1 μm would tend to decrease, reaching ~0.1 μm. Thus, it is inferred that the growth rate of the oxide layer at the valley area was higher than that at the ridge area in the grooved condition. In general, the stress would be considered during MAO coating. As discussed by Clyne and Troughton [26] in the area of plasma electrolysis, the generation of stress was known to be related to discharge characteristics of high-energy plasma arcs that were affected by several variables, such as electrical conditions (current density, frequency, etc.) and substrate states (crystallinity, surface roughness, etc.). Thus, the formation of stress might depend on surface roughness. More importantly, however, stress would be relaxed by thermal contribution of plasma discharges. During MAO where plasma-assisted electrochemical reactions governed, the local temperature of transient plasma arcs was reported to be as high as ~2000 K, exceeding the melting temperature of the newly-formed oxide layer. For example, Hussein et al. [25] estimated the plasma electron temperature ranging from 4500 to 10,000 K linked with collective discharge behavior by means of optical emission spectroscopy. This result was obtained from 1100 Al alloy. Indeed, the present group reported the temperature of plasma arcs on AZ91 Mg alloy in the range of 2116–2643 K [27]. Thank to high-temperature, the spontaneous phenomena including solidification and (re-) melting of Mg oxide would take place. Although the evident expansion of plasma arcs after critical voltage affected the local surface area, thermal relaxation resulting from the concurrence of high-energy plasma arcs was dominant. As a result, stress generated would tend to be relieved. Thus, the present process differed from the stress development during conventional depositions [28]. In this study, the morphologies of the oxide layer as well as defect structure were related closely to plasma characteristics.
metal anode to be attained during natural chemical reactions. This process was influenced significantly by how the surface features after mechanical finish would be formed, such as artificial grid in the whole surface and the related surface roughness in local area per se. According to the recent investigation in the correlation between surface groove and chemical reactions reported by Zuo et al. [22] using 316 L stainless steel in 0.01 M NaCl solution, the aspect ratio of the surface groove defined as w/d (openness parameter), in which w and d were the width and depth of one groove at the opening, would control the rate of local chemical reactions. They suggested that, depending upon surface finish method, the value of w/d higher than threshold would lead to the nucleation of localized decay by the accumulation of anions. In this study, the present value of openness parameter in any ridge-valley region was measured to be ~2.5 (w: ~10 and d: ~4). It is certain that the formation of the thin passive film during anodization prior to ignition accompanying plasma arcs was prone to be initiated primarily in the vicinity of valley area with a high surface area. Thus, the passive film at the valley was relatively thicker than that at the ridge. This fact would affect the nucleation of the oxide fragments on the onset of breakdown. The thin passive film with low electrical resistivity would possess a number of electrons to move across the interfaces easier than what the thick passive film acted unless the thickness of the passive film was uniform entirely [23]. As a result, myriads of electrons at the ridge area in PE field would trigger the initial appearance of micro-discharge with ease. This was consistent with Fig. 2a that the time to undergo breakdown in GS sample was shorter than that in FS counterpart. It is concluded that the preferential nucleation of the oxide fragments was facilitated first at the ridge area whereas a relatively uniform the oxide layer with a thin thickness of 1.2 μm was formed on FS sample, probably owing to the occurrence of homogeneous nucleation. As the nucleation of the oxide fragments related closely to surface roughness might have an influence on subsequent process to develop the oxide layer, it is necessary to look into the growth behavior of the oxide layer in the next section. 3.3. Growth behavior of oxide layer Fig. 4 displays the growth behavior of the present oxide layer through SEM micrographs of the cross-sectional morphologies in FS and GS samples subjected to MAO for 120 and 300 s together with XRD data of both samples after MAO for 300 s. A graph of the thickness of the oxide layer with respect to coating time up to 300 s is provided to figure out the different growth rate of the oxide layer. As seen from Figs. 4a and b, the oxide layers with not only fairly uniform thickness but also relatively dense structure were detected clearly on FS sample. If the nucleation of the oxide layer occurred uniformly on the substrate during ignition, the occurrence of micro-discharges would take place evenly throughout the surface. This finding was confirmed by surface morphologies of the oxide layer that showed micro-pores which are fairly uniform in size (Fig. 4c). In case of GS sample, the contour of the interface between the oxide layer and substrate that defined the ridge and valley areas was still visible as shown in Figs. 4d and e. In addition, the oxide layer in GS sample exhibited more porous structure than that in FS sample due to the occurrence of discharge channels that would provide a short path for harmful ions from corrosive environment. The present work confirmed the effect of surface roughness on controlling the overall characteristics of the micro-defects present in the oxide layer. Surface roughness affected directly discharge characteristics during growth in which GS sample exhibited plasma arcs with various size and distribution as shown in the inset of Fig. 2a. These plasma activities would modify the defect-structure of the oxide layer in terms of density and size on the basis of the morphological comparison between Figs. 4c and f. The pores density on the surface in GS and FS sample treated by MAO for 300 s showed some differences in different locations as shown in Figs. 4c and f. Considering the bimodal size distribution of plasma arcs shown in the inset of Fig. 2a in case of GS
3.4. Electrochemical response of oxide layer Electrochemical responses of the oxide layers on GS and FS samples evaluated by potentiodynamic polarization and electrochemical 5
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Fig. 4. Representative SEM images showing the cross section structures after MAO of (a) FS and (d) GS samples for 120 s, and of (b) FS and (e) GS samples for 300 s, the surface structures of (c) FS and (f) GS samples for 300 s. (g) XRD spectra of FS and GS samples. The scan ranges are from 10 to 90° with Cu Kα radiation source and (h) the thickness variation of the oxide layers formed on FS and GS samples with respect to coating time. The growth rate is calculated based on a linear approximation at the point of 120 s.
FS sample showed higher impedance value than that on GS sample. Based on equivalent circuit model, it is of importance to note that a significant difference in Ro value was found between FS and GS samples whereas the value of Ri remained unchanged each other. On the contrary, the value of Ro in FS sample was approximately one-order higher than that in GS sample. This was attributed to high porosity in the oxide layer formed on GS sample, which was consistent with microstructural observations shown in Fig. 4. In contrast to FS sample, the existence of appreciable discharge channels in GS sample would provide the short paths for Cl− ions that penetrated toward the substrate (Table 3). Although the micro-defects exhibited negative effect on corrosion resistance aforementioned, the enlarged surface area by the micropores would be beneficial for interface anchoring and heterogeneous compositing. For instance, the ‘porous’ Mg-phosphate layer after MAO provided preferential sites for secondary substances via chemical interlocking, such as TiO2 and chitosan on OH-terminated Mg-phosphate
impedance tests are presented in Fig. 6. The values of corrosion parameters, such as corrosion current density (Icorr), corrosion potential (Ecorr), and Tafel slopes of anodic and cathodic branches (βa and βc) were tabulated in Table 2. Polarization resistance (Rp) was calculated using representative equation proposed by Stern-Geary [29]. The corrosion analysis based on Tafel iteration revealed that the Rp values of GS sample was approximately two-order lower than that of FS sample due to the porous structure of the oxide layer formed on GS sample. EIS analysis was used to describe the protective capability by considering microstructural characteristics of the oxide layer of FS and GS samples in a quantitative manner. As reported earlier [30], the present oxide layer formed by MAO would comprise two different layers where the inner layer were thin and compact while the outer layer were thick and porous in nature. From Nyquist plot shown in Fig. 6b, the impedance features of the samples were proportional directly to the whole diameters of their semicircles, suggesting that the oxide layer formed on 6
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Fig. 5. Representative AFM images showing the surface roughness after MAO of (a) FS and (c) GS samples for 120 s, and of (b) FS and (d) GS samples for 300 s.
Fig. 6. (a) Potentiodynamic polarization curves of FS and GS samples treated by MAO for 300 s, ranging from −0.25 to 0.4 V vs. open circuit potential at a scan rate of 1 mV·s−1 and (b) Nyquist plots taken from 106 to 10 Hz with 10 mV rms perturbation. The inset in Fig. 6b shows the equivalent circuit model that describes electrochemical response. Both testing are performed in 3.5 wt% NaCl solution. Table 2 Potentiodynamic polarization results of FS and GS samples treated by MAO for 300 s, which are measured from −0.25 to 0.4 V vs. open circuit potential at a scan rate of 1 mV·s−1 in 3.5 wt% NaCl solution. Sample FS GS
Ecorr (V)
Icorr (A·cm−2)
βa (V·decade−1)
βc (V·decade−1)
Rp (Ω·cm2)
−0.64 −0.94
7.38 × 10−9 2.25 × 10−7
0.78 0.93
0.31 0.24
3.09 × 107 6.37 × 105
7
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Table 3 Electrochemical impedance results of FS and GS samples treated by MAO for 300 s, which are measured from 106 to 10 Hz with 10 mV rms perturbation in 3.5 wt% NaCl solution. All values are obtained based on equivalent circuit model in inset of Fig. 6b. In this model, Rs, Ro, and Ri refer to the resistance of the electrolyte, outer layer, and inner layer, respectively. CPEo and CPEi are the constant phase elements related to the outer and inner layers, respectively. Sample FS GS
Rs (Ω·cm2)
Ro (Ω·cm2)
CPEo (S·sn·cm−2)
Ri (Ω·cm2)
CPEi (S·sn·cm−2)
24.3 23.6
4.04 × 105 1.83 × 104
1.15 × 10−13 2.15 × 10−12
5.38 × 106 1.91 × 106
1.74 × 10−11 9.69 × 10−10
Fig. 7. A schematic illustration on formation mechanism of the oxide layer formed on grooved surface of AZ31 Mg alloy: (a) initial condition with movement of anions, (b) the onset of breakdown, (c) nucleation of the oxide fragments formed on ridge area, and (d) growth of the oxide layer as a final stage.
was in contrast to FS sample where any flaws were removed on surface. In GS sample with relatively large ratio of w/d, the valleys with higher surface area than the ridges would provide a preferable site for the accumulation of anion species, such as OH– and SiO32− in the present electrolyte as shown in Fig. 7a. This promoted the chemical reactions at the valleys to a greater extent than that at the ridges [22,35]. Hence, the thickness of the passive film formed on the valley was presumably thicker than that on the ridges, resulting in non-uniform resistivity on surface. The formation of the passive film exhibited a linear increase in responding voltage due to the steady growth of the passive film as shown in Fig. 2a. Once the passive film would no longer withstand the high electric field by MAO, the number of electrons from the electrolyte would be injected into the passive film, generating the emission of plasma sparks on the surface as depicted in Fig. 7b. Since the dielectric breakdown would take place first at the ridge area due to lower electrical resistance as compared to that at the valley area, it was clear that the oxide fragments were nucleated preferentially on the ridge area.
surface [31–33]. Such anchoring, in turn, sealed the micro-pores effectively to improve corrosion resistance of Mg alloy, enhancing the potential of MAO. 3.5. Formation mechanism of oxide layer on grooved surface All microstructural evidences aforementioned supported that the different surface roughness of the substrate played a critical role in the nucleation and growth behavior of the oxide layer formed on AZ31 Mg alloy via MAO coating. The schematic illustration shown in Fig. 7 describes the formation mechanism of the oxide layer in a chronological order by considering meaningful variation in surface topographies. Prior to MAO, the interaction of ions in the present electrolyte with the metal substrate would result in the formation of the fragile passive film through chemical reactions similar to those by anodization [34]. These chemical reactions to form the passive film were affected locally by the ridge and valley in GS sample with a uniform period of ~10 μm, which 8
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After the nucleation occurred at the ridge area, the concurrent transition to develop next breakdown from the ridge to the valley would be favored by the reason that the presence of premature surface that has not fully covered by the oxide fragments would create a pathway with possessing relatively low electrical resistance. This was described in Fig. 7c. Thus, consecutive breakdown phenomena of GS sample would differ from FS sample, which was confirmed by both population and size distribution of micro-discharges shown in Fig. 2b. Such distribution of micro-discharges was observed to be bimodal in GS sample after MAO coating for 300 s. These micro-discharges concentrated at the valley region would facilitate plasma-enhanced electrochemical reactions to thicken the oxide layer with an aid of increasing surrounding temperature which would induce local overheating [36]. This fact allowed the oxide layer at the valley area to grow at a relatively higher rate than that at the ridge area. On the other hand, the group of intense micro-discharges at the valley led to the formation of appreciable discharge channels as short-circuit path as shown in Fig. 7d. The final morphologies of the oxide layer in GS sample were found to be more porous than those in FS sample. Although the porous structure would suffer from the facile penetration of various corrosive ions onto the metal substrate, the present study would offer a new insight into the significant role of surface roughness on the modification of micro-discharge characteristics. The plasma activities determined by surface roughness of the substrate would modify the defect-structure of the oxide layer in terms of density and size. As a result, the surface roughness of the substrate that would facilitate the distinctive discharge characteristics during MAO would affect the structure of the oxide layer in a direction toward the enlarged surface area that would be anticipated to be beneficial for inducing emerging properties like bio-implanting.
[3]
[4]
[5]
[6] [7]
[8]
[9] [10]
[11] [12] [13]
[14] [15]
[16]
[17]
4. Summary [18]
The formation mechanism of the oxide layer in AZ31 Mg alloy processed by MAO coating was investigated by taking the effect of surface roughness into account. Irrespective of surface roughness used in this study, the time to reach breakdown in GS sample was earlier than that in FS sample due to the fact that the nucleation of the oxide fragments was prone to occur preferentially at the ridge area. This result suggested that the nucleation kinetics would be triggered with increasing surface roughness. Once the micro-discharge became intensified subsequent to breakdown, the oxide layer of GS sample grew locally on the pre-existing oxide fragments whereas the oxide layer of FS sample grew uniformly throughout the entire surface which implied that the growth rate of GS sample was higher than that of FS counterpart. Although GS sample possessed the thicker oxide layer of ~14 μm than FS sample after MAO for 300 s, however, the oxide layer contained somewhat noticeable discharge channels with an aid of intense microdischarges that were favored at the valley area during growth of the oxide layer. Controlled by grooved surface, such a porous structure of the oxide layer that might sacrifice long-term electrochemical stability would be desirable to achieve the enlarged surface area responsible for functional properties, such as medical implant, photo catalysis, dyesensitized solar cell, etc.
[19]
[20]
[21] [22] [23]
[24]
[25]
[26]
[27]
[28]
Acknowledgment [29]
This work was supported by the Fundamental Research Project funded by the National Research Foundation of Korea, Republic of Korea (NRF-2017R1D1A1A09000921).
[30]
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Formation mechanism of oxide layer on AZ31 Mg alloy subjected to microarc oxidation considering surface roughness N. Nashraha, M.P. Kamila, D.K. Yoona, Y.G. Kimb, Y.G. Koa, a b
T
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Materials Electrochemistry Group, School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea Extreme Fabrication Technology Group, Korea Institute of Industrial Technology, Daegu 42994, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Mg alloy Micro-arc oxidation Surface roughness Nucleation Growth
This study investigated the formation mechanism of the oxide layer generated by micro-arc oxidation (MAO) considering surface roughness of AZ31 Mg alloy. For this purpose, a series of MAO treatments under alternating current in a silicate-based electrolyte were performed on two present samples with grooved and flat surfaces. The time-voltage response presented that the grooved sample reached the breakdown voltage earlier than the flat sample. This indicated the appearance of micro-discharges on the grooved sample occurred earlier than that of the flat sample since the oxide fragments nucleated preferentially at the ridge areas where electrons were concentrated severely under high electrical field. The oxide layer of the grooved sample was relatively less dense than that of the flat sample. This was attributed to the fact that the oxide layer of the grooved sample was likely to grow with ease on pre-existing coating layer whereas the oxide layer of the flat sample developed conformably throughout the whole areas. Thus, the growth rate of the grooved sample was higher than that of the flat sample, which agreed well with micro-discharge activities. Such formation mechanism was proposed in relation to nucleation and growth of the oxide layer formed on different surface structures.
1. Introduction In recent years, magnesium (Mg) and its alloys have been investigated extensively as one of the most promising metallic candidates suitable for a variety of commercial applications within the scopes of electronic and automobile components due to high electromagnetic shielding properties and good specific strength at room temperature. Despite their attractive properties, these applications were unfortunately obstructed by inherent poor corrosion resistance of Mg alloy against corrosive environment [1–3]. Thus, a variety of surface modification methods, such as metal-based plating, chemical-conversion coating, and micro-arc oxidation (MAO, also known as plasma electrolytic oxidation), have been applied [4–6]. MAO, where the electrochemical reactions accompanying a myriad of micro-arcs over dielectric ignition triggered the nucleation of the oxide layer in an eco-friendly electrolyte, was considered to be desirable to fabricate the protective oxide layer with excellent adhesion to the metal substrate. This technique differed from the conventional hard-coatings [7]. Earlier studies reported the effects of several parameters on the improvement in the electrochemical properties as well as on the formation of the oxide layer by modifying electrolytes with the additions of organic and/or inorganic compounds [8,9], by changing electrical
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parameters of rectifier [10,11], and by controlling substrate features, such as grain size and surface topography [12,13]. Among processing variables, the surface morphologies of the substrate would be known to affect the formation of the oxide layer during MAO coating. Moreover, the influences of surface roughness on corrosion, wear, and fatigue properties were significant [14–16]. For instance, Yoo et al. [17] postulated from direct current PEO coating on AZ91 Mg alloy samples whose surface roughness values were measured to be 0.5, 1, and 2.5 μm that, with an increment in surface roughness, corrosion resistance in 3.5 wt% NaCl solution decreased to some extent whereas the transfer of current across the oxide layer became easier due to the fact that the flat surface might cause crystalline phase in the oxide layer during growth [13,17]. Another study reported by Wang et al. [18] suggested that the regular micro-grooves with a trapezoidal shape formed on Al alloy would lead to the high fractions of α-Al2O3 and 3Al2O3·2SiO2 working as hard constituents in the oxide layer due to high temperature and pressure in the sharp interface. This would give rise to superior wear resistance to the flat counterpart [18]. According to the fatigue results of 2024 Al alloy samples processed by MAO, the fatigue life was seen to decrease with increasing initial surface roughness in which hill and valley were identified [16]. While it has been well accepted that surface roughness was a crucial
Corresponding author. E-mail address: [email protected] (Y.G. Ko).
https://doi.org/10.1016/j.apsusc.2019.143772 Received 28 May 2019; Received in revised form 19 August 2019; Accepted 22 August 2019 Available online 23 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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factor affecting the inherent performances of the oxide layer, the fundamental mechanism that would describe nucleation and growth of the oxide layer during plasma electrolysis remained less understood by considering a role of surface roughness. Therefore, the present study attempts to scrutinize the nucleation and growth influenced by surface roughness of the initial sample. The nucleation phenomena of the oxide fragments formed at the ridge and valley of the initial substrate are compared at the onset of breakdown where the plasma discharges start to appear. Subsequently, both growth pattern and rate of the oxide layer are studied to propose formation mechanism in relation to microstructural interpretation. Two different corrosion assessments are used to describe the difference in electrochemical performances related to surface roughness. In addition, the stress state of the oxide layer is discussed based on high-energy plasma discharges formed on the surface during MAO. 2. Experimental procedures A commercial-graded AZ31 Mg alloy plate was cut into a dimension of 15 (L) × 25 (W) × 4 (T) mm3. Prior to MAO coating, two samples were polished mechanically with SiC paper up to #2400 grit followed by diamond suspension to obtain a nearly flat surface (FS). Then, one of the samples was ground again using SiC paper with #100 grit in order to produce a grooved surface (GS). Both samples were rinsed with distilled water, cleaned ultrasonically with pure ethanol, and dried in a stream of warm air. Surface roughness of the present samples was measured by atomic force microscopy (AFM, Park systems NX-10). To quantify surface roughness, the value of Ra that expressed an average deviation of the oscillations along a regression line drawn through particular roughness profile between the ridge and valley areas in the samples was used. Such values for both FS and GS samples were measured to be 0.025 and 1 μm as shown in Fig. 1. A series of MAO treatments were performed in an aqueous electrolyte with 0.1 M KOH and 0.05 M Na2SiO3 (pH: 11.83, conductivity: 12.39 mS·cm−1) at current density and frequency of 50 mA·cm−2 and 60 Hz, respectively. The present samples worked as the anode while the stainless steel net was set as the cathode. A glass vessel was used as the electrolyte container, equipped with magnetic stirrer and water cooling system to maintain the electrolyte temperature around 288 K for the stabilization of electrochemical reactions throughout the process. Details of the experimental setup were found elsewhere [19]. During MAO, the real-time images of micro-discharges formed on the surface were taken with respect to processing time. The morphologies of the surface and cross section in the oxide layer were observed by fieldemission scanning electron microscope (FE-SEM, HITACHI S-4800) coupled with energy-dispersive spectrometer. The electrochemical response of the oxide layer was evaluated through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution near to sea water. The polarization tests were scanned from −0.25 to +0.4 V with respect to open circuit potential at a scan rate of 1 mV·s−1. EIS tests were performed at frequency ranging 106 to 10 Hz with 10 mV rms perturbation. For each sample, electrochemical tests were repeated at least three times to ensure that the data obtained would be feasible.
Fig. 1. Representative AFM images showing the surface morphologies of the present samples with (a) flat and (b) grooved surfaces prior to MAO coating. These structures are uniform throughout the surfaces after mechanical preparation.
sample via MAO for 20 s is not provided due to less noticeable microdischarges on the anode surface. From the increasing tendency of responding voltage shown in Fig. 2a, two stages were divided clearly by their own slopes. In stage I, the responding voltage would tend to increase linearly, obeying Ohm's law due to an increase in electrical resistance of the thin passive film that formed on the substrate by chemical reactions [9,12,19]. Unless the thin passive film would withstand the intense electric field any longer, breakdown phenomenon began to appear together with a number of micro-discharges as the lighting dots. Then, stage II was started. The dotted red-circle in both curves indicated the breakdown voltage whose values were measured to be ~220 V for both cases. Interestingly, times to reach breakdown were different from each other in this study. FS and GS samples reached such ignition in 30 and 20 s respectively, suggesting that surface roughness would influence the initial stability and concurrent nucleation of microdischarges on the sample. It is inferred that the higher surface roughness, the earlier breakdown took place. After breakdown, the voltage in stage II increased with slope value lower than that in stage I as the electrical resistance increased by the growth of the oxide layer covering the anode surface. After MAO for 300 s, the final voltages of FS and GS samples reached ~290 and ~320 V, respectively. Such a difference in the final voltage would be associated with the thickness and morphologies of the oxide layer, which would be discussed microstructurally in the following section. The inset images in Fig. 2a evidenced that micro-discharges on FS and GS samples were evolved in different manners that allowed their size and fraction to vary moderately with increasing coating time. These results were analyzed in Fig. 2b. After MAO for 20 s, a number of fine micro-discharges with a dim color started to appear on GS sample
3. Results and discussion 3.1. Voltage-time response with micro-discharge Fig. 2 presents the voltage-time curves and histograms of FS and GS samples coated by MAO at 50 mA·cm−2 as a function of coating time ranging from 20 to 300 s. The changes in the responding voltages together with couples of inset images showing the characteristics of micro-discharges for 20, 30, 120, and 300 s are displayed in Fig. 2a, and the histograms of both size distribution and fraction of micro-discharges with respect to coating time are provided in Fig. 2b. A histogram of the 2
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Fig. 2. (a) Responding voltage vs. coating time curves of FS and GS samples during MAO at 50 mA·cm−2 for 300 s. The insets show the evolution of micro-discharges formed on surfaces with respect to coating time, and (b) the size distribution and fraction of micro-discharges taken from the insets.
Fig. 3. Representative SEM images showing the surface and cross section structures after MAO for 30 and 20 s when breakdown took place for the first time: (a), (b) FS and (c), (d) GS samples. Points A and B are used for compositional analyses in EDS.
MAO at breakdown point corresponding to FS and GS samples for 30 and 20 s, respectively. As presented in Figs. 3a and b, a thin oxide layer with a fairly uniform thickness of 1.2 μm was observed clearly on the whole surface of FS sample. Several micro-pores with an average size of 0.5 μm were also detected due to micro-discharges with high energy. Those structures were typical characteristic of metal-oxide layer formed by MAO [20,21]. In case of GS sample, the formation of the oxide layer seemed to be premature. From Fig. 3c, both ridge and valley were still visible even after breakdown. To confirm the elemental composition, EDS point measurements were carried out at least five times to show the element difference on the ridge and valley areas as indicated by the black (A) and white (B) dots in Fig. 3c. The amount of O and Si in wt% on the ridge was relatively higher than that on the valley as listed in Table 1, confirming the un-uniform formation of oxides on the ridge at the initial stage of MAO coating. From the observations on the cross section, it is important to note that some oxide fragments were also found to nucleate locally around the ridge to a greater extent than that near to the valley as shown in Fig. 3d. This fact implied that breakdown phenomenon was initiated around the ridge earlier than around the adjacent valley. It is found that both chemical decay of metal and formation of passive film were processed in the same way that allowed the movement of the anion species in the wet-process and their interaction with
Table 1 EDS results of GS sample after MAO for 30 s when breakdown took place for the first time. The measurements are taken from the representative points at the ridge and valley areas, which are denoted as points A and B in Fig. 3c. Area Ridge Valley
Point A B
Mg (wt%)
O (wt%)
Si (wt%)
48.8 ± 0.8 63.6 ± 2.7
41.0 ± 1.3 29.1 ± 0.5
10.2 ± 0.6 7.30 ± 0.7
while no micro-discharge was detected on FS sample. This fact was confirmed again by the different times to reach breakdown in the voltage-time data. In particular after 300 s, the size distribution of microdischarges in FS sample was more homogenous than that in GS sample which showed a bimodal distribution where coarse and fine sparks coexisted. Thus, the fraction of micro-discharges in GS sample was lower than that in FS counterpart, which was attributed mainly to the cluster of micro-discharges in local areas of GS sample. This finding suggested that growth behavior of the oxide layers was expected to be different in the samples with different surface topographies. 3.2. Nucleation behavior upon breakdown Fig. 3 shows an initial occurrence of the oxide layers produced by 4
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sample, intense plasma arcs would result in the formation of discharge channel present in the oxide layer. On the other hand, the size and distribution of the micro-pores were relatively uniform in FS sample after MAO for 300 s. This phenomenon was explained by continuous monitoring on short-duration PEO treatment that the serial formation of micro-discharge was expected on the local region with low electrical resistivity as discussed by Clyne research group [24]. The oxide layer with either deep micro-pores or thin thickness would be the plausible candidate responsible for electron transfer across the insulation barrier during the wet-based oxidation coating. It should be noted that this tendency was understood on the same system in a ‘relative’ manner. In this study, the valley area with lower volume of the oxide fragments would be susceptible to next breakdown accompanying intense cascades because nucleation of the oxide layer appeared first at the ridge area as evidenced in Fig. 3d. Thus, micro-discharges concentrated at the valley area would play an important role in the formation of type-B discharge channels (defined by Hussein et al. [25]) as well as relatively fast growth of the oxide layer in the local area. Both XRD data of two samples coated by MAO for 300 s were composed of peaks associated with Mg, MgO, and MgSiO3 as shown in the Fig. 4g. A significant difference was detected rarely due to the same substrate and electrolyte used in this study. The thickness values of the final oxide layers in FS and GS samples were measured to be ~11 and ~14 μm, respectively after MAO for 300 s as shown in Fig. 4h. Hence, the growth rate of GS sample was higher than that of FS sample. This finding would be in line with final responding voltage of GS sample higher than that of FS counterpart. All results were also reinforced indirectly by AFM observations as shown in Fig. 5. For FS sample with initial substrate roughness was measured to be ~0.025 μm, the values of Ra would tend to increase with respect to coating time, reaching ~0.05, however, the initial Ra value of GS sample measured to be ~1 μm would tend to decrease, reaching ~0.1 μm. Thus, it is inferred that the growth rate of the oxide layer at the valley area was higher than that at the ridge area in the grooved condition. In general, the stress would be considered during MAO coating. As discussed by Clyne and Troughton [26] in the area of plasma electrolysis, the generation of stress was known to be related to discharge characteristics of high-energy plasma arcs that were affected by several variables, such as electrical conditions (current density, frequency, etc.) and substrate states (crystallinity, surface roughness, etc.). Thus, the formation of stress might depend on surface roughness. More importantly, however, stress would be relaxed by thermal contribution of plasma discharges. During MAO where plasma-assisted electrochemical reactions governed, the local temperature of transient plasma arcs was reported to be as high as ~2000 K, exceeding the melting temperature of the newly-formed oxide layer. For example, Hussein et al. [25] estimated the plasma electron temperature ranging from 4500 to 10,000 K linked with collective discharge behavior by means of optical emission spectroscopy. This result was obtained from 1100 Al alloy. Indeed, the present group reported the temperature of plasma arcs on AZ91 Mg alloy in the range of 2116–2643 K [27]. Thank to high-temperature, the spontaneous phenomena including solidification and (re-) melting of Mg oxide would take place. Although the evident expansion of plasma arcs after critical voltage affected the local surface area, thermal relaxation resulting from the concurrence of high-energy plasma arcs was dominant. As a result, stress generated would tend to be relieved. Thus, the present process differed from the stress development during conventional depositions [28]. In this study, the morphologies of the oxide layer as well as defect structure were related closely to plasma characteristics.
metal anode to be attained during natural chemical reactions. This process was influenced significantly by how the surface features after mechanical finish would be formed, such as artificial grid in the whole surface and the related surface roughness in local area per se. According to the recent investigation in the correlation between surface groove and chemical reactions reported by Zuo et al. [22] using 316 L stainless steel in 0.01 M NaCl solution, the aspect ratio of the surface groove defined as w/d (openness parameter), in which w and d were the width and depth of one groove at the opening, would control the rate of local chemical reactions. They suggested that, depending upon surface finish method, the value of w/d higher than threshold would lead to the nucleation of localized decay by the accumulation of anions. In this study, the present value of openness parameter in any ridge-valley region was measured to be ~2.5 (w: ~10 and d: ~4). It is certain that the formation of the thin passive film during anodization prior to ignition accompanying plasma arcs was prone to be initiated primarily in the vicinity of valley area with a high surface area. Thus, the passive film at the valley was relatively thicker than that at the ridge. This fact would affect the nucleation of the oxide fragments on the onset of breakdown. The thin passive film with low electrical resistivity would possess a number of electrons to move across the interfaces easier than what the thick passive film acted unless the thickness of the passive film was uniform entirely [23]. As a result, myriads of electrons at the ridge area in PE field would trigger the initial appearance of micro-discharge with ease. This was consistent with Fig. 2a that the time to undergo breakdown in GS sample was shorter than that in FS counterpart. It is concluded that the preferential nucleation of the oxide fragments was facilitated first at the ridge area whereas a relatively uniform the oxide layer with a thin thickness of 1.2 μm was formed on FS sample, probably owing to the occurrence of homogeneous nucleation. As the nucleation of the oxide fragments related closely to surface roughness might have an influence on subsequent process to develop the oxide layer, it is necessary to look into the growth behavior of the oxide layer in the next section. 3.3. Growth behavior of oxide layer Fig. 4 displays the growth behavior of the present oxide layer through SEM micrographs of the cross-sectional morphologies in FS and GS samples subjected to MAO for 120 and 300 s together with XRD data of both samples after MAO for 300 s. A graph of the thickness of the oxide layer with respect to coating time up to 300 s is provided to figure out the different growth rate of the oxide layer. As seen from Figs. 4a and b, the oxide layers with not only fairly uniform thickness but also relatively dense structure were detected clearly on FS sample. If the nucleation of the oxide layer occurred uniformly on the substrate during ignition, the occurrence of micro-discharges would take place evenly throughout the surface. This finding was confirmed by surface morphologies of the oxide layer that showed micro-pores which are fairly uniform in size (Fig. 4c). In case of GS sample, the contour of the interface between the oxide layer and substrate that defined the ridge and valley areas was still visible as shown in Figs. 4d and e. In addition, the oxide layer in GS sample exhibited more porous structure than that in FS sample due to the occurrence of discharge channels that would provide a short path for harmful ions from corrosive environment. The present work confirmed the effect of surface roughness on controlling the overall characteristics of the micro-defects present in the oxide layer. Surface roughness affected directly discharge characteristics during growth in which GS sample exhibited plasma arcs with various size and distribution as shown in the inset of Fig. 2a. These plasma activities would modify the defect-structure of the oxide layer in terms of density and size on the basis of the morphological comparison between Figs. 4c and f. The pores density on the surface in GS and FS sample treated by MAO for 300 s showed some differences in different locations as shown in Figs. 4c and f. Considering the bimodal size distribution of plasma arcs shown in the inset of Fig. 2a in case of GS
3.4. Electrochemical response of oxide layer Electrochemical responses of the oxide layers on GS and FS samples evaluated by potentiodynamic polarization and electrochemical 5
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Fig. 4. Representative SEM images showing the cross section structures after MAO of (a) FS and (d) GS samples for 120 s, and of (b) FS and (e) GS samples for 300 s, the surface structures of (c) FS and (f) GS samples for 300 s. (g) XRD spectra of FS and GS samples. The scan ranges are from 10 to 90° with Cu Kα radiation source and (h) the thickness variation of the oxide layers formed on FS and GS samples with respect to coating time. The growth rate is calculated based on a linear approximation at the point of 120 s.
FS sample showed higher impedance value than that on GS sample. Based on equivalent circuit model, it is of importance to note that a significant difference in Ro value was found between FS and GS samples whereas the value of Ri remained unchanged each other. On the contrary, the value of Ro in FS sample was approximately one-order higher than that in GS sample. This was attributed to high porosity in the oxide layer formed on GS sample, which was consistent with microstructural observations shown in Fig. 4. In contrast to FS sample, the existence of appreciable discharge channels in GS sample would provide the short paths for Cl− ions that penetrated toward the substrate (Table 3). Although the micro-defects exhibited negative effect on corrosion resistance aforementioned, the enlarged surface area by the micropores would be beneficial for interface anchoring and heterogeneous compositing. For instance, the ‘porous’ Mg-phosphate layer after MAO provided preferential sites for secondary substances via chemical interlocking, such as TiO2 and chitosan on OH-terminated Mg-phosphate
impedance tests are presented in Fig. 6. The values of corrosion parameters, such as corrosion current density (Icorr), corrosion potential (Ecorr), and Tafel slopes of anodic and cathodic branches (βa and βc) were tabulated in Table 2. Polarization resistance (Rp) was calculated using representative equation proposed by Stern-Geary [29]. The corrosion analysis based on Tafel iteration revealed that the Rp values of GS sample was approximately two-order lower than that of FS sample due to the porous structure of the oxide layer formed on GS sample. EIS analysis was used to describe the protective capability by considering microstructural characteristics of the oxide layer of FS and GS samples in a quantitative manner. As reported earlier [30], the present oxide layer formed by MAO would comprise two different layers where the inner layer were thin and compact while the outer layer were thick and porous in nature. From Nyquist plot shown in Fig. 6b, the impedance features of the samples were proportional directly to the whole diameters of their semicircles, suggesting that the oxide layer formed on 6
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Fig. 5. Representative AFM images showing the surface roughness after MAO of (a) FS and (c) GS samples for 120 s, and of (b) FS and (d) GS samples for 300 s.
Fig. 6. (a) Potentiodynamic polarization curves of FS and GS samples treated by MAO for 300 s, ranging from −0.25 to 0.4 V vs. open circuit potential at a scan rate of 1 mV·s−1 and (b) Nyquist plots taken from 106 to 10 Hz with 10 mV rms perturbation. The inset in Fig. 6b shows the equivalent circuit model that describes electrochemical response. Both testing are performed in 3.5 wt% NaCl solution. Table 2 Potentiodynamic polarization results of FS and GS samples treated by MAO for 300 s, which are measured from −0.25 to 0.4 V vs. open circuit potential at a scan rate of 1 mV·s−1 in 3.5 wt% NaCl solution. Sample FS GS
Ecorr (V)
Icorr (A·cm−2)
βa (V·decade−1)
βc (V·decade−1)
Rp (Ω·cm2)
−0.64 −0.94
7.38 × 10−9 2.25 × 10−7
0.78 0.93
0.31 0.24
3.09 × 107 6.37 × 105
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Table 3 Electrochemical impedance results of FS and GS samples treated by MAO for 300 s, which are measured from 106 to 10 Hz with 10 mV rms perturbation in 3.5 wt% NaCl solution. All values are obtained based on equivalent circuit model in inset of Fig. 6b. In this model, Rs, Ro, and Ri refer to the resistance of the electrolyte, outer layer, and inner layer, respectively. CPEo and CPEi are the constant phase elements related to the outer and inner layers, respectively. Sample FS GS
Rs (Ω·cm2)
Ro (Ω·cm2)
CPEo (S·sn·cm−2)
Ri (Ω·cm2)
CPEi (S·sn·cm−2)
24.3 23.6
4.04 × 105 1.83 × 104
1.15 × 10−13 2.15 × 10−12
5.38 × 106 1.91 × 106
1.74 × 10−11 9.69 × 10−10
Fig. 7. A schematic illustration on formation mechanism of the oxide layer formed on grooved surface of AZ31 Mg alloy: (a) initial condition with movement of anions, (b) the onset of breakdown, (c) nucleation of the oxide fragments formed on ridge area, and (d) growth of the oxide layer as a final stage.
was in contrast to FS sample where any flaws were removed on surface. In GS sample with relatively large ratio of w/d, the valleys with higher surface area than the ridges would provide a preferable site for the accumulation of anion species, such as OH– and SiO32− in the present electrolyte as shown in Fig. 7a. This promoted the chemical reactions at the valleys to a greater extent than that at the ridges [22,35]. Hence, the thickness of the passive film formed on the valley was presumably thicker than that on the ridges, resulting in non-uniform resistivity on surface. The formation of the passive film exhibited a linear increase in responding voltage due to the steady growth of the passive film as shown in Fig. 2a. Once the passive film would no longer withstand the high electric field by MAO, the number of electrons from the electrolyte would be injected into the passive film, generating the emission of plasma sparks on the surface as depicted in Fig. 7b. Since the dielectric breakdown would take place first at the ridge area due to lower electrical resistance as compared to that at the valley area, it was clear that the oxide fragments were nucleated preferentially on the ridge area.
surface [31–33]. Such anchoring, in turn, sealed the micro-pores effectively to improve corrosion resistance of Mg alloy, enhancing the potential of MAO. 3.5. Formation mechanism of oxide layer on grooved surface All microstructural evidences aforementioned supported that the different surface roughness of the substrate played a critical role in the nucleation and growth behavior of the oxide layer formed on AZ31 Mg alloy via MAO coating. The schematic illustration shown in Fig. 7 describes the formation mechanism of the oxide layer in a chronological order by considering meaningful variation in surface topographies. Prior to MAO, the interaction of ions in the present electrolyte with the metal substrate would result in the formation of the fragile passive film through chemical reactions similar to those by anodization [34]. These chemical reactions to form the passive film were affected locally by the ridge and valley in GS sample with a uniform period of ~10 μm, which 8
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After the nucleation occurred at the ridge area, the concurrent transition to develop next breakdown from the ridge to the valley would be favored by the reason that the presence of premature surface that has not fully covered by the oxide fragments would create a pathway with possessing relatively low electrical resistance. This was described in Fig. 7c. Thus, consecutive breakdown phenomena of GS sample would differ from FS sample, which was confirmed by both population and size distribution of micro-discharges shown in Fig. 2b. Such distribution of micro-discharges was observed to be bimodal in GS sample after MAO coating for 300 s. These micro-discharges concentrated at the valley region would facilitate plasma-enhanced electrochemical reactions to thicken the oxide layer with an aid of increasing surrounding temperature which would induce local overheating [36]. This fact allowed the oxide layer at the valley area to grow at a relatively higher rate than that at the ridge area. On the other hand, the group of intense micro-discharges at the valley led to the formation of appreciable discharge channels as short-circuit path as shown in Fig. 7d. The final morphologies of the oxide layer in GS sample were found to be more porous than those in FS sample. Although the porous structure would suffer from the facile penetration of various corrosive ions onto the metal substrate, the present study would offer a new insight into the significant role of surface roughness on the modification of micro-discharge characteristics. The plasma activities determined by surface roughness of the substrate would modify the defect-structure of the oxide layer in terms of density and size. As a result, the surface roughness of the substrate that would facilitate the distinctive discharge characteristics during MAO would affect the structure of the oxide layer in a direction toward the enlarged surface area that would be anticipated to be beneficial for inducing emerging properties like bio-implanting.
[3]
[4]
[5]
[6] [7]
[8]
[9] [10]
[11] [12] [13]
[14] [15]
[16]
[17]
4. Summary [18]
The formation mechanism of the oxide layer in AZ31 Mg alloy processed by MAO coating was investigated by taking the effect of surface roughness into account. Irrespective of surface roughness used in this study, the time to reach breakdown in GS sample was earlier than that in FS sample due to the fact that the nucleation of the oxide fragments was prone to occur preferentially at the ridge area. This result suggested that the nucleation kinetics would be triggered with increasing surface roughness. Once the micro-discharge became intensified subsequent to breakdown, the oxide layer of GS sample grew locally on the pre-existing oxide fragments whereas the oxide layer of FS sample grew uniformly throughout the entire surface which implied that the growth rate of GS sample was higher than that of FS counterpart. Although GS sample possessed the thicker oxide layer of ~14 μm than FS sample after MAO for 300 s, however, the oxide layer contained somewhat noticeable discharge channels with an aid of intense microdischarges that were favored at the valley area during growth of the oxide layer. Controlled by grooved surface, such a porous structure of the oxide layer that might sacrifice long-term electrochemical stability would be desirable to achieve the enlarged surface area responsible for functional properties, such as medical implant, photo catalysis, dyesensitized solar cell, etc.
[19]
[20]
[21] [22] [23]
[24]
[25]
[26]
[27]
[28]
Acknowledgment [29]
This work was supported by the Fundamental Research Project funded by the National Research Foundation of Korea, Republic of Korea (NRF-2017R1D1A1A09000921).
[30]
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