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Growth process of plasma electrolytic oxidation films formed on magnesium alloy AZ91D in silicate solution
Electrochimica Acta 52 (2007) 5002–5009
Growth process of plasma electrolytic oxidation films formed on magnesium alloy AZ91D in silicate solution Hongping Duan, Chuanwei Yan ∗ , Fuhui Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China Received 17 November 2006; received in revised form 5 February 2007; accepted 6 February 2007 Available online 16 February 2007
Abstract In order to get a clear picture for describing the growth process of the oxide film formed on magnesium alloy AZ91D under plasma electrolytic oxidation (PEO) in alkaline silicate solution, the characteristics of PEO films formed at different reaction stages were systemically investigated. The results of morphologies, compositions and electronic properties indicated that the PEO films had a different growth behavior as the PEO treatment proceeding. At the initial stage (before the occurrence of sparking), the growth rate of PEO films was low, the elements (O, Mg, Al and Si) contents were varied obviously and the donor concentration in the film was kept at a high level. After sparking occurred, the PEO films showed a higher growth rate due to the high transfer rate of ions and electrons and the existence of plasma reactions; simultaneously, the films formed on ␣- and -phase exhibited different growth rate. With treated time increased, the thickness of PEO films and transfer resistance to ions and electrons were also increased; thereby, the growth rate of the PEO films was decreased gently. © 2007 Elsevier Ltd. All rights reserved. Keywords: Magnesium alloy AZ91D; PEO film; Oxidation process; Capacitance measurement; Semiconducting properties
1. Introduction Plasma electrolytic oxidation (PEO) [1–4] developed from conventional anodic oxidation, has been brought for surface modification of magnesium and its alloys due to some significant properties of the PEO films, such as good adhesion, high hardness and high corrosion resistance [4–7]. PEO treatment on magnesium alloys has been developed for several decades and a number of studies focused on the dependence of films performance on PEO conditions (such as electrolyte composition [5], voltage model [8], treated time [9], heating treatment [10] and so on). Recently, some researches also outlined the film growth process on magnesium alloy [11,12]. However, systemic investigations on the variation of structure, composition and electronic properties of the films formed on magnesium alloys during PEO process were seldom reported. In view of barrier effect of PEO films composed of outer porous layer and inner barrier layer [13,14], transportation of
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electrons and ions between electrolyte and magnesium electrode with PEO films would be some certain influenced. This makes the magnesium electrode exhibit different electrochemical behavior in comparison with bare metal electrode, but similar to that of semiconducting or insulating electrode. Thus, some methods used to study semiconducting electrodes can be adopted to investigate the properties of magnesium electrode with PEO films. Capacitance measurement based on Mott–Schottky (MS) theory [15–17] is a conventional technique to probe electronic properties of a semiconducting electrode in aqueous electrolyte. The slope changing of MS curves often indicates the intrinsic changing of semiconducting electrode due to concentrations variation of donor or acceptor. Thus, electronic properties of semiconducting magnesium electrodes can be probed by capacitance measurement. The aim of this work is to reveal the growth process of PEO films and to develop a proper PEO treatment mode for magnesium alloys. The PEO films formed at different reaction stages (treated at different cell potential and for different time) in alkaline silicate electrolyte were prepared. SEM and EDAX were, respectively, used to examine the morphologies and composition of those PEO films. Meanwhile, capacitance measurement was
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carried out in PEO electrolyte to evaluate the donor or accepter levels of the PEO films. Then, PEO films growth process was detailedly analyzed based on the above-mentioned results. 2. Experimental 2.1. Samples and solutions Rectangular samples (with dimensions 10 mm × 10 mm × 25 mm) made of die-cast magnesium alloy AZ91D (Al 8.5–9.5%, Zn 0.50–0.90%, Mn 0.17–0.27%, Mg balance) were used as working electrodes. The working electrodes were sealed with epoxy resin leaving only the polished square surface with an area of 1 cm2 . For clearly observation of the surface morphologies of samples after PEO treatment, the working surface of magnesium electrodes were successively ground with a series of SiC papers and finally polished with diamond paste, then degreased in ethanol and distilled water before PEO treatment. Graphite electrodes were used as counter electrodes. The PEO films used for characteristic measurements were prepared under pulse voltage model and the cell potential were increased step by step till a certain value, then the magnesium electrode was oxidized under this value for some time. The universal PEO electrolyte containing silicate (15–20 g/L Na2 SiO3 and 3–8 g/L KOH) was used. The same electrolyte was used for capacitance measurement. All solutions were made of analytical grade reagents and distilled water.
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still visible clearly (Fig. 1b). As cell potential was increased to 150 V and stabilized at this potential for 30 s, a large amount of small gas bubbles occurred on the sample surface during PEO process. The surface including both ␣- and -phase of magnesium electrode were covered by a thin and uniform oxide film that was composed of many fine granular particles as shown in Fig. 1c. Because this potential (150 V) has not reach the spark discharge potential of magnesium alloy AZ91D in the alkaline silicate solution, the anodizing process was similar to that of the conventional anodic oxidation one. That is to say that the film formation was obeyed the conventional anodic oxidation reactions. On the magnesium electrode, Mg2+ was formed by dissolution of magnesium from the substrate under the effect of electric filed (reaction (i)). Most of the gas produced on the anodic electrode was oxygen generated by decomposition of OH− [18] (as the reaction (ii)): Mg → Mg2+ + 2e− (anodic dissolution)
(i)
4OH− → O2 + 2H2 O + 4e− (side reaction)
(ii)
The formation of uniform granular products on magnesium alloy AZ91D can be attributed to the outward migration of Mg2+ and inward migration of SiO3 2− and OH− under the effect of high potential. When the concentrations of Mg2+ , SiO3 2− and OH− reached a critical value in the electrode/electrolyte interface, film formation reactions would take place. Such as Mg(OH)2 and Mg2 SiO4 would be formed by the following reactions (iii) and (iv), respectively:
2.2. Test methods
Mg2+ + 2OH− → Mg(OH)2
(iii)
The surface morphologies and chemical composition of PEO films were examined by XL-30FEG scanning electron microscopy (SEM) with energy dispersive analysis of X-rays (EDAX). Elemental mapping in the cross-section of PEO films was measured by S-3400N scanning electron microscopy. The thickness of PEO films was measured by HCC-25 eddy-current equipment. Capacitance measurements were conducted using a conventional three electrodes electrochemical cell with the Mg samples as working electrode, a platinum plate as auxiliary electrode and a saturated calomel electrode (SCE) as reference. Capacitance measurements with variable anodic over potential from 0.5 to −0.5 V (SCE) was performed at fixed frequency of 1000 Hz by PAR 2273 potentiostat in cathodic direction with a potential scanning rate of 20 mV/s. All experiments were carried out at room temperature.
2Mg2+ + SiO3 2− + 2OH− → Mg2 SiO4 + H2 O
(iv)
3. Results and discussion 3.1. Morphology characteristics of PEO films The surface morphologies of AZ91D alloy samples formed at different stages of PEO process were shown in Fig. 1. It can be seen that the magnesium alloy AZ91D substrate (Fig. 1a) is consisted of ␣- and -phase (Mg17 Al12 ). When magnesium electrode was anodized at 100 V for 30 s, a few granular products were formed on its surface while the original phase structure
The products resulted from the above reactions would deposit on and cover the surface of magnesium electrode. After the electrode was taken out of electrolyte and desiccated, magnesium oxide (MgO) would be produced by dehydration reaction of Mg(OH)2 as reaction (v): Mg(OH)2 → MgO + H2 O
(v)
When the cell potential was not too high (150 V), these reactions would uniformly take place on the magnesium electrode in alkaline silicate solutions. Thus a thin oxide film formed under 150 V would uniform and cover the whole electrode surface (both ␣- and -phase). The thickness measurement indicated that the thin oxide film was about 2 m (in Fig. 2). This thin oxide film would act as a barrier layer, which not only effectively separates magnesium substrate from electrolyte but also inhibits the transfer of electron and ions between electrolyte and metal substrate. Thus, a high electric field would be formed in the thin oxide barrier layer during the following process when the applied potential was increased further. The high electric field intensity accumulated in the thin oxide layer was the indispensable conditions for spark discharge in the following PEO process. When the cell potential reached about 200 V, which was over the breakdown potential of the thin oxide film formed on the magnesium electrode, there were numerous bright fine sparks
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Fig. 1. Surface morphologies of magnesium alloy AZ91D samples obtained at different PEO reaction stage: (a) the substrate, (b–f) was treated at the labeled potential for 30 s, and (g–k) was treated at 350 V for the labeled time. The value labeled in (e) was Mg/Al ratio (at.%) at corresponding cross-points.
can be seen on the electrode surface. At the same time, a mass of bubbles (oxygen and vapor) companying with shrill sounds escaped from magnesium and graphite electrodes. After treated at 200 V for 30 s, the sample surface represented many evident micro-pores, which connected each other and formed netlike structures (in Fig. 1d). Because the thickness of PEO films formed in the initial spark discharge stage was too low (about 2 m), so that it had low breakdown potential and weak breakdown energy. As a result, the residuals melting pools remained on the magnesium electrode surface for sparking were small. Moreover, the SEM results (Fig. 1d) showed that both ␣- and -phase were all covered by porous film, while the surface morphologies of the PEO films above ␣- and -phase was a little different, i.e. the PEO films grown on ␣-phase had better continuity than those grown on -phase. This can be attributed to
the different reactivity of ␣- and -phase for their different aluminium content. Compared with -phase that had higher content of Al, ␣-phase had higher reactivity and tended to release more Mg2+ to participate in film formation reactions under effect of high electric field and high Joule heat [19]. Because both electrolyte composition and substrate constituents can influence PEO films growth and composition, higher concentration of Mg2+ around ␣-phase would make the PEO films have much more active sites for plasma sparking. Meanwhile, the -phase with higher stability would need stronger energy for sparking than those of ␣-phase. Thus, more large size micro-pores would be formed on the PEO films grown on -phase than that grown on ␣-phase. As cell potential reached 250 V, plasma sparks discharge became much bright with stronger intensity. The PEO films
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Fig. 2. Thickness variation of the oxide films formed on magnesium alloy AZ91D with cell potential and oxidation time during PEO process.
(Fig. 1e) formed under this potential had fewer pores with larger size than those formed at 200 V (Fig. 1d). Moreover, evident concave regions appeared on the PEO films as shown in Fig. 1e. EDAX detection indicated that the area near concave region had lower Mg/Al ratio (≈3.2) than where far away concave region (≈18.2). This indicated that the concave region was above phase of magnesium alloy AZ91D substrate. The above results illustrated the PEO films grown on ␣- and -phase exhibited evident different growth rate for their different reactivity, i.e. the PEO films grown on ␣-phase had higher growth rate than those on -phase. As cell potential reached around 300 V, the PEO films were thickened to about 4 m (as shown in Fig. 2), intensity of spark discharge became even stronger, and the size of micro-pores became larger. At the same time, these sintered products were joined together and piled layer up layer (Fig. 1f). The concave region around -phase was still exposed nevertheless concave area was reduced. This indicated that the growth of PEO films formed above -phase was mainly in virtue of lateral growth of PEO films formed above ␣-phase [15] but was not vertical outward growth from -phase itself during the initial stage of PEO process. When the potential was increased to 350 V for 1 min, thickness of PEO films was increased faster (near 7 m) as shown around the high slope range in Fig. 2. Micro-pores in the PEO films were decreased and aperture of micro-pores was increased further (Fig. 1g). The concave region above -phase in Fig. 1g became deeper than that in Fig. 1f. This indicated that the growth rate of PEO films on ␣-phase was still higher than that on phase. At the same time, the concave area above -phase was narrowed further due to continuous lateral growth of PEO films above ␣-phase. As oxidation time was prolonged to 2 min at the same potential (350 V), numerous small pores were reduced while some bigger micro-pores appeared. The concave region above -phase were diminished and blocked partly because of continuous lateral growth of the PEO films above ␣-phase (Fig. 1h). When treated time was prolonged to 5 min, the morphology of PEO films shown in Fig. 1i was similar to that shown
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in Fig. 1h except the concave region disappeared. By this time, the PEO films formed on magnesium alloy AZ91D showed much uniform surface morphology (above both ␣- and -phase). When magnesium alloy AZ91D electrodes were oxidized at 350 V for 15 and 30 min, respectively, the PEO films as shown in Fig. 1j and k had similar morphologies with uniform structure and larger pores size than those in Fig. 1h. Companying with thickening of PEO films up to around 20 m, the size of micro-pores increased and the oxidized granules were sintered layer by layer, therewith some micro-cracks were induced in the PEO films. This kind of morphological characteristic of PEO films can be attributed to the increasing of energy density during spark discharge as thickening of the PEO films. Higher energy density induced larger discharge sparks and larger “melt pools”. Thereafter the solidification for the larger “melt pool” would remain larger sintered granules. On the other hand, the thickening of the PEO films might bring significant difference of cooling rate between the outer part and inner part of PEO films, i.e. the outer porous layer would take precedence of the inner barrier layer got into solidification, which made the spark discharge channels blocked. After that, when the inner gas in discharge channels escaped outwards, some inner melted products would follow to be spout out and solidified by electrolyte. Thus, large size micro-pores and sintered granules were remained in the PEO films. Some relevant studies [20] have given the relationship between micro-pores of spark discharge with treatment time and showed that porosity of PEO films was exponentially decreased and aperture of micro-pores was linearly increased as the oxidation time prolonged. From the thickness curve (Fig. 2) and the above discussion it can be seen that the growth rate of PEO films was varied with the treatment conditions during PEO process. During the conventional anodic oxidation stage, the film thickness increased slightly and a thin barrier oxide layer was formed. When the cell potential reached the plasma spark discharge voltage (around 200 V), the thickness of PEO films increased sharply but the PEO films were not uniform. As oxidation time prolonged, the PEO films became uniform and their growth rate was gradually decreased. 3.2. Composition characteristics of PEO films The electrode with different thickness oxide films obtained at different PEO stage, would offer dissimilar contents of reactive ions dissolved from the substrate. At the same time, the different applied potential would endow the PEO reactions with different energy and Joule heat. Thus, the composition of PEO films would be varied during PEO process. The EDAX analysis results (shown in Fig. 3) of magnesium alloy AZ91D treated at different PEO stage showed that the PEO films obtained in alkaline silicate solutions were mainly composed of Mg, O, Al, and Si. As some researches [4,21] indicated the constituents of PEO films were influenced not only by the electrolyte composition but also by the ions species of substrate. Thus, it can be concluded that Mg and Al were come from the magnesium substrate; Si was rooted in the PEO electrolyte containing silicate. But the derivation of O was complicated: it
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Fig. 4. Elements distribution on the cross-section surface of the PEO films formed on magnesium alloy AZ91D in alkaline silicate electrolyte. Fig. 3. Qualitative analysis of elements contents in the oxide films formed at different PEO reaction stage on magnesium alloy AZ91D.
might be come from the anions (such as OH− , SiO3 2− ) and the oxygen dissolved in PEO electrolyte. The EDAX analysis also showed that the contents of these elements had evident variation along with the PEO process went on. At the initial stage (before and at the beginning of spark discharge), the content of Mg was decreased sharply and the content of Al was decreased slightly, while the contents of O and Si were increased apparently. This can be attributed to the high reactivity of magnesium electrode due to the thin oxide layer (less than 5 m) and strong electric field intensity during the conventional oxidation stage and initial plasma spark discharge stage. There were high concentrations of Mg2+ , Al3+ , and plasma of Mg and Al, which dissolved from Mg substrate and transferred into the electrode/electrolyte interface. Thus, the PEO films formed at the initial stage had a high content of Mg and Al. As the carrying on of PEO process, the thickness of PEO films was increased. Accordingly, the blocking effect of the PEO films on the outward transfer of some ions (such as Mg2+ , Al3+ ) dissolved from magnesium substrate became stronger and the concentration of these dissolved ions was decreased. Therewith, the contents of Al and Mg in PEO films were decreased gradually. Correspondingly, the contents of O and Si in the PEO films were increased due to the relative higher concentrations of ions (OH− and SiO3 2− et al.) from electrolyte and taking part in film formation reactions at electrode/electrolyte interface. After the potential reached 350 V for 1 min, the contents of those elements in PEO films was tended to stable (Fig. 3) along with the morphology of PEO films uniformization (Fig. 1g). This indicated that the transfer rate and concentrations of ions from both electrolyte and substrate reached a stable level when the thickness of the PEO films reached certain extent. In the later stage, the elemental contents would not be varied apparently with thickening of PEO films. In order to further investigate the constituent characteristics, the elements distribution in cross-section surface of PEO films formed on magnesium alloy AZ91D in silicate electrolyte was detected by line scanning. As shown in Fig. 4, the distributions of O, Mg, Al and Si had similar tendency from inner layer to outer layer as the EDAX results showed (Fig. 3), i.e. oxygen was
rapidly increased and magnesium was rapidly decreased near the interface of substrate/inner oxide film; silicate was increased gradually with the highest value appeared in the outer part of PEO films; while the content of aluminium was slightly change but there existed an aluminium enriched zone at the interface of substrate/PEO films. The appearance of the aluminium enriched zone at the substrate/film interface could be attributed to the different reactivity of Al3+ at the initial and later stage of PEO process [22]. Al3+ was wholly come from the magnesium alloy AZ91D substrate in the aluminate free electrolyte. At the initial stage of PEO process, the aluminium had higher dissolvability than that of magnesium, and Al3+ had higher transfer rate than Mg2+ in the film/substrate interface [23]. The Al3+ dissolved from substrate would react with some anions (such as OH− or SiO3 2− ) preferentially and some sintered products were formed and deposited on the substrate surface. As the PEO process prolonged and the thickness of PEO films increased, the outwards transportation resistance to Al3+ was increased. Then, the concentration of Al3+ would decrease slightly in the later reactions in comparison with the initial stage. The enriched zone of aluminium in inner layer would endow the PEO films with high corrosion resistance for forming the stable products with high aluminium content. 3.3. Electronic properties of PEO film PEO technique is a complicated plasma assisting electrochemical process combined with thermo chemical and physicochemical reactions. The electronic behavior of electrode would have important effects on the PEO process and properties of PEO films. In order to understand the PEO films formation process, the semiconducting properties of magnesium alloy AZ91D were studied by capacitance measurement during PEO process. The properties of a semiconductor/electrolyte interface can be described by Mott–Schottky theory [15–17], which reflects the effect of the applied potential E on capacitance values, based on equation (1): 1 2 kT E − E = − (1) fb 2 εε0 qNq e CSC
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Fig. 5. Mott–Schottky plots of magnesium alloy AZ91D substrate in PEO electrolyte, the solid line is fitting result based on the linear equation.
(8.85 × 10−14 F cm−1 ),
where ε0 is the vacuum permittivity ε the relative permittivity (dielectric constant) of the oxide, q (1.602 × 10−19 C) is the elementary charge (−e for electrons and +e for holes), Nq the donor or acceptor concentration, Efb the flat band potential, E the applied potential, k the Boltzman constant (1.38 × 10−23 J K−1 ), and T is the absolute temperature. kT/e may be neglected, as it is only about 25 mV at room −2 temperature. This equation predicts a linear CSC versus E plot where the slope related with the donor or acceptor concentration of electrode [16,17]. The pH value of the alkaline electrolyte used for PEO treatment was 12–14, which is in the passive region of magnesium alloy AZ91D. Thus, a natural passive film would be formed on magnesium electrode when the electrode was immersed in PEO electrolyte. Capacitance measurement (Fig. 5) showed that the capacitance values of magnesium alloy substrate in PEO electrolyte was decreased with the applied potential, leading to the development of a straight line with positive slope in the −2 CSC versus E plots (MS curve). This indicated that the magnesium substrate represented n-type semiconducting behavior in PEO electrolyte. For an n-type semiconductor, the parameter Nq refers to donor concentration at the electrode/electrolyte interface and the donors were usually some positive defects, such as cation interstitials or anion vacancies [24,25]. During formation of the nature passive film, the atoms of magnesium were divorced from the metal lattice and entered into the oxide lattice of passive film. Then, oxygen vacancies as doping were formed at electrode/passive film interface as the following reaction [26]: Mg → MgM + VO 2+ + 2e−
(vi)
Here, the Mg represents the magnesium atom in metal lattice, MgM the magnesium atom in oxide film lattice and VO 2+ is the oxygen vacancy. Appearance of oxygen vacancy induced the magnesium electrode represented n-type semiconducting character during immersed in PEO electrolyte. Because the natural passive film was ultrathin, the main type of charge transfer would be direct exchange of electrons between redox electronic pairs in electrolyte and the metal substrate by “tunnel effect” [15]. In the initial stage of PEO process (before sparking occurred), the growth of oxide film was similar to these of the conven-
Fig. 6. Variation of donor levels in the oxide films formed on magnesium alloy AZ91D with cell potential and oxidation time during PEO process.
tional anodic oxidation process, i.e. the film nucleation reaction took place at some advantaged points on electrode surface when the potential and ions concentration reached their critical values. As potential increasing and time prolonged, the oxidation reactions took place on the whole electrode and continuous thin oxide film was formed on the magnesium electrode surface (as shown in Fig. 1c). Just for the existence of continuous thin oxide film, the resistance to electrons and ions transfer at the electrode/electrolyte interface was increased. During later PEO process, the growth of PEO films would mainly by virtue of movement of vacancies or interstitials in the oxide film under high electric field assistance [27]. Capacitance measurements revealed that the magnesium alloy AZ91D electrodes treated at different potential for different time were all exhibited n-type semiconducting behavior. But the donor concentrations were varied at the different PEO stage. As shown in Fig. 6, the donor concentrations of the magnesium electrodes was sharply decreased with potential increasing in the initial stage of PEO process. This indicated that the reactivity of magnesium electrode was decreased and the resistance to electrons and ions transfer was increased sharply due to the formation of barrier thin oxide layer on the magnesium electrode (as shown in Fig. 1c). The existence of dielectric thin oxide film was indispensable for formation of strong electric filed and plasma spark discharge during the following PEO reactions. As potential increasing (near the breakdown potential of PEO films in the silicate electrolyte), high electric field would be formed because of storage of electric energy in the thin barrier oxide film. When the electric field with high energy was strong enough to excite doping (such as vacancies or interstitials), some atoms in oxide film lattice would be impacted then electron-cavity pairs were left [27]. Under the continual effect of high electric field, the foregoing reactions were continued and numerous doping with high energy were excited. Eventually high current was formed and avalanche of electrons or ions can run through the oxide films where the films were weak [28]. High energy stored in doping would be released as kinetic energy of ions or electrons, Joule heat and sparks discharge [15].
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During experiment, countless twinkling sparks would be seen glimmering on the whole surface of magnesium electrode. After spark discharge occurred, the film growth process still has some similar characteristics with the conventional anodic oxidation process, but there is evident difference between them. The similar characteristic is the growth of oxide film all mainly depends on the movement of defects or ions among substrate, oxide film and electrolyte under effect of electric field. The representative characteristic of the PEO process that differs from conventional oxidation process is the existence of sparks and the effect of sintered and solidification. Under effect of high electric field and Joule heat, vapor or plasma of substrate and electrolyte constituents would be formed [27]. Consequently, the reactions of PEO films formation would be accelerated because plasmas have higher reactivity and better conductivity than those of ions. Furthermore, the sintered effect of sparking made the PEO reactions thorough and more oxide products formed. At the same time, the molten oxide product would be solidified rapidly because the temperature of electrolyte is much lower than that of the molten oxide formed under sparking. Thus, the synergetic effect of strong electric field and solidification of PEO electrolyte would endow the PEO films with high growth rate. The above inference that sparking promoted film growth was coincided with the thickness measurement results shown in Fig. 2, i.e. the PEO films had much higher growth rate during the potential just reached breakdown potential of the thin oxide film than that of other PEO stages. Just for formation of sintered oxide films and continuous thickening of PEO films, the donor concentration of electrode was decreased gently (as shown in Fig. 6), which indicated that the active positions giving plasma spark discharge on PEO films were reduced gradually. Hence, the number of micro-pores in PEO films would be decreased gradually with PEO reactions carried on (shown in Fig. 1). This result exactly reflected the superiority of PEO technique that the defected and unsubstantial surface in the oxide films always gave birth sparking and the PEO films will get more and more compact [29]. Combining Figs. 1 and 2, it can be seen that the growth rate of PEO films formed on magnesium alloy AZ91D in silicate solution was decreased slightly when the uniform PEO films with certain thickness were formed. This can be explained by the theory of Cabrera–Mott [30]: under high electric filed, ions transfer step was the dominant step of oxide film growth when thickness of oxide film reached certain value and the transfer of ions tightly followed the electric field-assistant mechanism. As films thickness increasing along with PEO process, the transfer resistance to ions in PEO films was increased. Then, the transfer rate of ions, which took part in PEO reactions, was decreased. As a result, the film growth rate was decreased. Therefore, in order to maintain the film formation reactions continual, the cell potential should be increased further to keep necessary electric field density for PEO films growth. On the other hand, the cell potential should be controlled under certain value to avoid inducing local breakdown of the PEO films. This was the reason that cell potential were increased step by step and were stabilized at certain level to keep the PEO films growth uniformly with considerable growth rate during our experiments.
4. Conclusions In this paper, the growth process of the protective oxide films formed on magnesium alloy AZ91D under plasma electrolytic oxidation (PEO) was investigated in alkaline silicate solution. The SEM, EDAX and capacitance measurement results showed that the PEO films formed in this case exhibited different growth behavior at the different PEO stage. The initial stage (before plasma spark discharge) of PEO treatment for magnesium alloy AZ91D was similar to a conventional anodic oxidation process and the oxide film was formed at a low growth rate. While at the beginning of plasma spark discharge, the PEO films had a non-uniform structure and a higher growth rate than any other stage. By then, the PEO films formed on ␣-phase had higher growth rate than those on -phase and the growth of PEO films formed on -phase was mainly in virtue of lateral growth of oxide films formed on ␣-phase. As sparking discharge reactions continued, the PEO films got uniform and thickened accompany with the formation of large size micro-pores and sintered granules. The PEO films formed on magnesium alloy AZ91D in silicate solution were composed of O, Mg, Al and Si. These elements contents were varied evidently during the initial stage of PEO process. There was an aluminium-enriched region in the inner layer of PEO films for formation of stable aluminium oxide. When the structure of the PEO films got uniform, the elements contents were reached a relative stable level. During the PEO process, the magnesium electrodes were all represented n-type semiconducting behavior. The donor concentration of the magnesium electrode was decreased gradually during the PEO process because the transfer resistance to ions or electrons in the PEO films was increased as the films thickness increasing and structure compactification. The growth of PEO films mainly followed electric field-assistant mechanism. Thus, the PEO films growth rate was decreased gently when the thickness of PEO films increased to certain value as increasing of the cell potential and treated time. Acknowledgement This work was supported by 973 Project of China. References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. dowey, Surf. Coat. Technol. 122 (1999) 73. [2] A.L. Yerokhin, A. Shatrov, et al., Surf. Coat. Technol. 182 (2004) 78. [3] X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 125 (2000) 407. [4] H.F. Guo, M.Z. An, Appl. Surf. Sci. 246 (2005) 229. [5] H.Y. Hsial, W.T. Tsai, Surf. Coat. Technol. 190 (2005) 299. [6] H.P. Duan, K.Q. Du, C.W. Yan, F.H. Wang, Electrochim. Acta 51 (2006) 2898. [7] Q.Z. Cai, L.S. Wang, B.K. Wei, Q.X. Liu, Surf. Coat. Technol. 200 (2006) 3727. [8] A.V. Timoshenko, Yu.V. Magurova, Surf. Coat. Technol. 199 (2005) 135. [9] C. Blawert, V. Heitmann, W. Oietzel, Surf. Coat. Technol. 200 (2005) 68. [10] H.Y. Hsiao, W.T. Tsai, J. Mater. Res. 20 (2005) 2763. [11] Y.J. Zhang, C.W. Yan, Surf. Coat. Technol. 201 (2006) 2381.
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Growth process of plasma electrolytic oxidation films formed on magnesium alloy AZ91D in silicate solution Hongping Duan, Chuanwei Yan ∗ , Fuhui Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China Received 17 November 2006; received in revised form 5 February 2007; accepted 6 February 2007 Available online 16 February 2007
Abstract In order to get a clear picture for describing the growth process of the oxide film formed on magnesium alloy AZ91D under plasma electrolytic oxidation (PEO) in alkaline silicate solution, the characteristics of PEO films formed at different reaction stages were systemically investigated. The results of morphologies, compositions and electronic properties indicated that the PEO films had a different growth behavior as the PEO treatment proceeding. At the initial stage (before the occurrence of sparking), the growth rate of PEO films was low, the elements (O, Mg, Al and Si) contents were varied obviously and the donor concentration in the film was kept at a high level. After sparking occurred, the PEO films showed a higher growth rate due to the high transfer rate of ions and electrons and the existence of plasma reactions; simultaneously, the films formed on ␣- and -phase exhibited different growth rate. With treated time increased, the thickness of PEO films and transfer resistance to ions and electrons were also increased; thereby, the growth rate of the PEO films was decreased gently. © 2007 Elsevier Ltd. All rights reserved. Keywords: Magnesium alloy AZ91D; PEO film; Oxidation process; Capacitance measurement; Semiconducting properties
1. Introduction Plasma electrolytic oxidation (PEO) [1–4] developed from conventional anodic oxidation, has been brought for surface modification of magnesium and its alloys due to some significant properties of the PEO films, such as good adhesion, high hardness and high corrosion resistance [4–7]. PEO treatment on magnesium alloys has been developed for several decades and a number of studies focused on the dependence of films performance on PEO conditions (such as electrolyte composition [5], voltage model [8], treated time [9], heating treatment [10] and so on). Recently, some researches also outlined the film growth process on magnesium alloy [11,12]. However, systemic investigations on the variation of structure, composition and electronic properties of the films formed on magnesium alloys during PEO process were seldom reported. In view of barrier effect of PEO films composed of outer porous layer and inner barrier layer [13,14], transportation of
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electrons and ions between electrolyte and magnesium electrode with PEO films would be some certain influenced. This makes the magnesium electrode exhibit different electrochemical behavior in comparison with bare metal electrode, but similar to that of semiconducting or insulating electrode. Thus, some methods used to study semiconducting electrodes can be adopted to investigate the properties of magnesium electrode with PEO films. Capacitance measurement based on Mott–Schottky (MS) theory [15–17] is a conventional technique to probe electronic properties of a semiconducting electrode in aqueous electrolyte. The slope changing of MS curves often indicates the intrinsic changing of semiconducting electrode due to concentrations variation of donor or acceptor. Thus, electronic properties of semiconducting magnesium electrodes can be probed by capacitance measurement. The aim of this work is to reveal the growth process of PEO films and to develop a proper PEO treatment mode for magnesium alloys. The PEO films formed at different reaction stages (treated at different cell potential and for different time) in alkaline silicate electrolyte were prepared. SEM and EDAX were, respectively, used to examine the morphologies and composition of those PEO films. Meanwhile, capacitance measurement was
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carried out in PEO electrolyte to evaluate the donor or accepter levels of the PEO films. Then, PEO films growth process was detailedly analyzed based on the above-mentioned results. 2. Experimental 2.1. Samples and solutions Rectangular samples (with dimensions 10 mm × 10 mm × 25 mm) made of die-cast magnesium alloy AZ91D (Al 8.5–9.5%, Zn 0.50–0.90%, Mn 0.17–0.27%, Mg balance) were used as working electrodes. The working electrodes were sealed with epoxy resin leaving only the polished square surface with an area of 1 cm2 . For clearly observation of the surface morphologies of samples after PEO treatment, the working surface of magnesium electrodes were successively ground with a series of SiC papers and finally polished with diamond paste, then degreased in ethanol and distilled water before PEO treatment. Graphite electrodes were used as counter electrodes. The PEO films used for characteristic measurements were prepared under pulse voltage model and the cell potential were increased step by step till a certain value, then the magnesium electrode was oxidized under this value for some time. The universal PEO electrolyte containing silicate (15–20 g/L Na2 SiO3 and 3–8 g/L KOH) was used. The same electrolyte was used for capacitance measurement. All solutions were made of analytical grade reagents and distilled water.
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still visible clearly (Fig. 1b). As cell potential was increased to 150 V and stabilized at this potential for 30 s, a large amount of small gas bubbles occurred on the sample surface during PEO process. The surface including both ␣- and -phase of magnesium electrode were covered by a thin and uniform oxide film that was composed of many fine granular particles as shown in Fig. 1c. Because this potential (150 V) has not reach the spark discharge potential of magnesium alloy AZ91D in the alkaline silicate solution, the anodizing process was similar to that of the conventional anodic oxidation one. That is to say that the film formation was obeyed the conventional anodic oxidation reactions. On the magnesium electrode, Mg2+ was formed by dissolution of magnesium from the substrate under the effect of electric filed (reaction (i)). Most of the gas produced on the anodic electrode was oxygen generated by decomposition of OH− [18] (as the reaction (ii)): Mg → Mg2+ + 2e− (anodic dissolution)
(i)
4OH− → O2 + 2H2 O + 4e− (side reaction)
(ii)
The formation of uniform granular products on magnesium alloy AZ91D can be attributed to the outward migration of Mg2+ and inward migration of SiO3 2− and OH− under the effect of high potential. When the concentrations of Mg2+ , SiO3 2− and OH− reached a critical value in the electrode/electrolyte interface, film formation reactions would take place. Such as Mg(OH)2 and Mg2 SiO4 would be formed by the following reactions (iii) and (iv), respectively:
2.2. Test methods
Mg2+ + 2OH− → Mg(OH)2
(iii)
The surface morphologies and chemical composition of PEO films were examined by XL-30FEG scanning electron microscopy (SEM) with energy dispersive analysis of X-rays (EDAX). Elemental mapping in the cross-section of PEO films was measured by S-3400N scanning electron microscopy. The thickness of PEO films was measured by HCC-25 eddy-current equipment. Capacitance measurements were conducted using a conventional three electrodes electrochemical cell with the Mg samples as working electrode, a platinum plate as auxiliary electrode and a saturated calomel electrode (SCE) as reference. Capacitance measurements with variable anodic over potential from 0.5 to −0.5 V (SCE) was performed at fixed frequency of 1000 Hz by PAR 2273 potentiostat in cathodic direction with a potential scanning rate of 20 mV/s. All experiments were carried out at room temperature.
2Mg2+ + SiO3 2− + 2OH− → Mg2 SiO4 + H2 O
(iv)
3. Results and discussion 3.1. Morphology characteristics of PEO films The surface morphologies of AZ91D alloy samples formed at different stages of PEO process were shown in Fig. 1. It can be seen that the magnesium alloy AZ91D substrate (Fig. 1a) is consisted of ␣- and -phase (Mg17 Al12 ). When magnesium electrode was anodized at 100 V for 30 s, a few granular products were formed on its surface while the original phase structure
The products resulted from the above reactions would deposit on and cover the surface of magnesium electrode. After the electrode was taken out of electrolyte and desiccated, magnesium oxide (MgO) would be produced by dehydration reaction of Mg(OH)2 as reaction (v): Mg(OH)2 → MgO + H2 O
(v)
When the cell potential was not too high (150 V), these reactions would uniformly take place on the magnesium electrode in alkaline silicate solutions. Thus a thin oxide film formed under 150 V would uniform and cover the whole electrode surface (both ␣- and -phase). The thickness measurement indicated that the thin oxide film was about 2 m (in Fig. 2). This thin oxide film would act as a barrier layer, which not only effectively separates magnesium substrate from electrolyte but also inhibits the transfer of electron and ions between electrolyte and metal substrate. Thus, a high electric field would be formed in the thin oxide barrier layer during the following process when the applied potential was increased further. The high electric field intensity accumulated in the thin oxide layer was the indispensable conditions for spark discharge in the following PEO process. When the cell potential reached about 200 V, which was over the breakdown potential of the thin oxide film formed on the magnesium electrode, there were numerous bright fine sparks
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Fig. 1. Surface morphologies of magnesium alloy AZ91D samples obtained at different PEO reaction stage: (a) the substrate, (b–f) was treated at the labeled potential for 30 s, and (g–k) was treated at 350 V for the labeled time. The value labeled in (e) was Mg/Al ratio (at.%) at corresponding cross-points.
can be seen on the electrode surface. At the same time, a mass of bubbles (oxygen and vapor) companying with shrill sounds escaped from magnesium and graphite electrodes. After treated at 200 V for 30 s, the sample surface represented many evident micro-pores, which connected each other and formed netlike structures (in Fig. 1d). Because the thickness of PEO films formed in the initial spark discharge stage was too low (about 2 m), so that it had low breakdown potential and weak breakdown energy. As a result, the residuals melting pools remained on the magnesium electrode surface for sparking were small. Moreover, the SEM results (Fig. 1d) showed that both ␣- and -phase were all covered by porous film, while the surface morphologies of the PEO films above ␣- and -phase was a little different, i.e. the PEO films grown on ␣-phase had better continuity than those grown on -phase. This can be attributed to
the different reactivity of ␣- and -phase for their different aluminium content. Compared with -phase that had higher content of Al, ␣-phase had higher reactivity and tended to release more Mg2+ to participate in film formation reactions under effect of high electric field and high Joule heat [19]. Because both electrolyte composition and substrate constituents can influence PEO films growth and composition, higher concentration of Mg2+ around ␣-phase would make the PEO films have much more active sites for plasma sparking. Meanwhile, the -phase with higher stability would need stronger energy for sparking than those of ␣-phase. Thus, more large size micro-pores would be formed on the PEO films grown on -phase than that grown on ␣-phase. As cell potential reached 250 V, plasma sparks discharge became much bright with stronger intensity. The PEO films
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Fig. 2. Thickness variation of the oxide films formed on magnesium alloy AZ91D with cell potential and oxidation time during PEO process.
(Fig. 1e) formed under this potential had fewer pores with larger size than those formed at 200 V (Fig. 1d). Moreover, evident concave regions appeared on the PEO films as shown in Fig. 1e. EDAX detection indicated that the area near concave region had lower Mg/Al ratio (≈3.2) than where far away concave region (≈18.2). This indicated that the concave region was above phase of magnesium alloy AZ91D substrate. The above results illustrated the PEO films grown on ␣- and -phase exhibited evident different growth rate for their different reactivity, i.e. the PEO films grown on ␣-phase had higher growth rate than those on -phase. As cell potential reached around 300 V, the PEO films were thickened to about 4 m (as shown in Fig. 2), intensity of spark discharge became even stronger, and the size of micro-pores became larger. At the same time, these sintered products were joined together and piled layer up layer (Fig. 1f). The concave region around -phase was still exposed nevertheless concave area was reduced. This indicated that the growth of PEO films formed above -phase was mainly in virtue of lateral growth of PEO films formed above ␣-phase [15] but was not vertical outward growth from -phase itself during the initial stage of PEO process. When the potential was increased to 350 V for 1 min, thickness of PEO films was increased faster (near 7 m) as shown around the high slope range in Fig. 2. Micro-pores in the PEO films were decreased and aperture of micro-pores was increased further (Fig. 1g). The concave region above -phase in Fig. 1g became deeper than that in Fig. 1f. This indicated that the growth rate of PEO films on ␣-phase was still higher than that on phase. At the same time, the concave area above -phase was narrowed further due to continuous lateral growth of PEO films above ␣-phase. As oxidation time was prolonged to 2 min at the same potential (350 V), numerous small pores were reduced while some bigger micro-pores appeared. The concave region above -phase were diminished and blocked partly because of continuous lateral growth of the PEO films above ␣-phase (Fig. 1h). When treated time was prolonged to 5 min, the morphology of PEO films shown in Fig. 1i was similar to that shown
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in Fig. 1h except the concave region disappeared. By this time, the PEO films formed on magnesium alloy AZ91D showed much uniform surface morphology (above both ␣- and -phase). When magnesium alloy AZ91D electrodes were oxidized at 350 V for 15 and 30 min, respectively, the PEO films as shown in Fig. 1j and k had similar morphologies with uniform structure and larger pores size than those in Fig. 1h. Companying with thickening of PEO films up to around 20 m, the size of micro-pores increased and the oxidized granules were sintered layer by layer, therewith some micro-cracks were induced in the PEO films. This kind of morphological characteristic of PEO films can be attributed to the increasing of energy density during spark discharge as thickening of the PEO films. Higher energy density induced larger discharge sparks and larger “melt pools”. Thereafter the solidification for the larger “melt pool” would remain larger sintered granules. On the other hand, the thickening of the PEO films might bring significant difference of cooling rate between the outer part and inner part of PEO films, i.e. the outer porous layer would take precedence of the inner barrier layer got into solidification, which made the spark discharge channels blocked. After that, when the inner gas in discharge channels escaped outwards, some inner melted products would follow to be spout out and solidified by electrolyte. Thus, large size micro-pores and sintered granules were remained in the PEO films. Some relevant studies [20] have given the relationship between micro-pores of spark discharge with treatment time and showed that porosity of PEO films was exponentially decreased and aperture of micro-pores was linearly increased as the oxidation time prolonged. From the thickness curve (Fig. 2) and the above discussion it can be seen that the growth rate of PEO films was varied with the treatment conditions during PEO process. During the conventional anodic oxidation stage, the film thickness increased slightly and a thin barrier oxide layer was formed. When the cell potential reached the plasma spark discharge voltage (around 200 V), the thickness of PEO films increased sharply but the PEO films were not uniform. As oxidation time prolonged, the PEO films became uniform and their growth rate was gradually decreased. 3.2. Composition characteristics of PEO films The electrode with different thickness oxide films obtained at different PEO stage, would offer dissimilar contents of reactive ions dissolved from the substrate. At the same time, the different applied potential would endow the PEO reactions with different energy and Joule heat. Thus, the composition of PEO films would be varied during PEO process. The EDAX analysis results (shown in Fig. 3) of magnesium alloy AZ91D treated at different PEO stage showed that the PEO films obtained in alkaline silicate solutions were mainly composed of Mg, O, Al, and Si. As some researches [4,21] indicated the constituents of PEO films were influenced not only by the electrolyte composition but also by the ions species of substrate. Thus, it can be concluded that Mg and Al were come from the magnesium substrate; Si was rooted in the PEO electrolyte containing silicate. But the derivation of O was complicated: it
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Fig. 4. Elements distribution on the cross-section surface of the PEO films formed on magnesium alloy AZ91D in alkaline silicate electrolyte. Fig. 3. Qualitative analysis of elements contents in the oxide films formed at different PEO reaction stage on magnesium alloy AZ91D.
might be come from the anions (such as OH− , SiO3 2− ) and the oxygen dissolved in PEO electrolyte. The EDAX analysis also showed that the contents of these elements had evident variation along with the PEO process went on. At the initial stage (before and at the beginning of spark discharge), the content of Mg was decreased sharply and the content of Al was decreased slightly, while the contents of O and Si were increased apparently. This can be attributed to the high reactivity of magnesium electrode due to the thin oxide layer (less than 5 m) and strong electric field intensity during the conventional oxidation stage and initial plasma spark discharge stage. There were high concentrations of Mg2+ , Al3+ , and plasma of Mg and Al, which dissolved from Mg substrate and transferred into the electrode/electrolyte interface. Thus, the PEO films formed at the initial stage had a high content of Mg and Al. As the carrying on of PEO process, the thickness of PEO films was increased. Accordingly, the blocking effect of the PEO films on the outward transfer of some ions (such as Mg2+ , Al3+ ) dissolved from magnesium substrate became stronger and the concentration of these dissolved ions was decreased. Therewith, the contents of Al and Mg in PEO films were decreased gradually. Correspondingly, the contents of O and Si in the PEO films were increased due to the relative higher concentrations of ions (OH− and SiO3 2− et al.) from electrolyte and taking part in film formation reactions at electrode/electrolyte interface. After the potential reached 350 V for 1 min, the contents of those elements in PEO films was tended to stable (Fig. 3) along with the morphology of PEO films uniformization (Fig. 1g). This indicated that the transfer rate and concentrations of ions from both electrolyte and substrate reached a stable level when the thickness of the PEO films reached certain extent. In the later stage, the elemental contents would not be varied apparently with thickening of PEO films. In order to further investigate the constituent characteristics, the elements distribution in cross-section surface of PEO films formed on magnesium alloy AZ91D in silicate electrolyte was detected by line scanning. As shown in Fig. 4, the distributions of O, Mg, Al and Si had similar tendency from inner layer to outer layer as the EDAX results showed (Fig. 3), i.e. oxygen was
rapidly increased and magnesium was rapidly decreased near the interface of substrate/inner oxide film; silicate was increased gradually with the highest value appeared in the outer part of PEO films; while the content of aluminium was slightly change but there existed an aluminium enriched zone at the interface of substrate/PEO films. The appearance of the aluminium enriched zone at the substrate/film interface could be attributed to the different reactivity of Al3+ at the initial and later stage of PEO process [22]. Al3+ was wholly come from the magnesium alloy AZ91D substrate in the aluminate free electrolyte. At the initial stage of PEO process, the aluminium had higher dissolvability than that of magnesium, and Al3+ had higher transfer rate than Mg2+ in the film/substrate interface [23]. The Al3+ dissolved from substrate would react with some anions (such as OH− or SiO3 2− ) preferentially and some sintered products were formed and deposited on the substrate surface. As the PEO process prolonged and the thickness of PEO films increased, the outwards transportation resistance to Al3+ was increased. Then, the concentration of Al3+ would decrease slightly in the later reactions in comparison with the initial stage. The enriched zone of aluminium in inner layer would endow the PEO films with high corrosion resistance for forming the stable products with high aluminium content. 3.3. Electronic properties of PEO film PEO technique is a complicated plasma assisting electrochemical process combined with thermo chemical and physicochemical reactions. The electronic behavior of electrode would have important effects on the PEO process and properties of PEO films. In order to understand the PEO films formation process, the semiconducting properties of magnesium alloy AZ91D were studied by capacitance measurement during PEO process. The properties of a semiconductor/electrolyte interface can be described by Mott–Schottky theory [15–17], which reflects the effect of the applied potential E on capacitance values, based on equation (1): 1 2 kT E − E = − (1) fb 2 εε0 qNq e CSC
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Fig. 5. Mott–Schottky plots of magnesium alloy AZ91D substrate in PEO electrolyte, the solid line is fitting result based on the linear equation.
(8.85 × 10−14 F cm−1 ),
where ε0 is the vacuum permittivity ε the relative permittivity (dielectric constant) of the oxide, q (1.602 × 10−19 C) is the elementary charge (−e for electrons and +e for holes), Nq the donor or acceptor concentration, Efb the flat band potential, E the applied potential, k the Boltzman constant (1.38 × 10−23 J K−1 ), and T is the absolute temperature. kT/e may be neglected, as it is only about 25 mV at room −2 temperature. This equation predicts a linear CSC versus E plot where the slope related with the donor or acceptor concentration of electrode [16,17]. The pH value of the alkaline electrolyte used for PEO treatment was 12–14, which is in the passive region of magnesium alloy AZ91D. Thus, a natural passive film would be formed on magnesium electrode when the electrode was immersed in PEO electrolyte. Capacitance measurement (Fig. 5) showed that the capacitance values of magnesium alloy substrate in PEO electrolyte was decreased with the applied potential, leading to the development of a straight line with positive slope in the −2 CSC versus E plots (MS curve). This indicated that the magnesium substrate represented n-type semiconducting behavior in PEO electrolyte. For an n-type semiconductor, the parameter Nq refers to donor concentration at the electrode/electrolyte interface and the donors were usually some positive defects, such as cation interstitials or anion vacancies [24,25]. During formation of the nature passive film, the atoms of magnesium were divorced from the metal lattice and entered into the oxide lattice of passive film. Then, oxygen vacancies as doping were formed at electrode/passive film interface as the following reaction [26]: Mg → MgM + VO 2+ + 2e−
(vi)
Here, the Mg represents the magnesium atom in metal lattice, MgM the magnesium atom in oxide film lattice and VO 2+ is the oxygen vacancy. Appearance of oxygen vacancy induced the magnesium electrode represented n-type semiconducting character during immersed in PEO electrolyte. Because the natural passive film was ultrathin, the main type of charge transfer would be direct exchange of electrons between redox electronic pairs in electrolyte and the metal substrate by “tunnel effect” [15]. In the initial stage of PEO process (before sparking occurred), the growth of oxide film was similar to these of the conven-
Fig. 6. Variation of donor levels in the oxide films formed on magnesium alloy AZ91D with cell potential and oxidation time during PEO process.
tional anodic oxidation process, i.e. the film nucleation reaction took place at some advantaged points on electrode surface when the potential and ions concentration reached their critical values. As potential increasing and time prolonged, the oxidation reactions took place on the whole electrode and continuous thin oxide film was formed on the magnesium electrode surface (as shown in Fig. 1c). Just for the existence of continuous thin oxide film, the resistance to electrons and ions transfer at the electrode/electrolyte interface was increased. During later PEO process, the growth of PEO films would mainly by virtue of movement of vacancies or interstitials in the oxide film under high electric field assistance [27]. Capacitance measurements revealed that the magnesium alloy AZ91D electrodes treated at different potential for different time were all exhibited n-type semiconducting behavior. But the donor concentrations were varied at the different PEO stage. As shown in Fig. 6, the donor concentrations of the magnesium electrodes was sharply decreased with potential increasing in the initial stage of PEO process. This indicated that the reactivity of magnesium electrode was decreased and the resistance to electrons and ions transfer was increased sharply due to the formation of barrier thin oxide layer on the magnesium electrode (as shown in Fig. 1c). The existence of dielectric thin oxide film was indispensable for formation of strong electric filed and plasma spark discharge during the following PEO reactions. As potential increasing (near the breakdown potential of PEO films in the silicate electrolyte), high electric field would be formed because of storage of electric energy in the thin barrier oxide film. When the electric field with high energy was strong enough to excite doping (such as vacancies or interstitials), some atoms in oxide film lattice would be impacted then electron-cavity pairs were left [27]. Under the continual effect of high electric field, the foregoing reactions were continued and numerous doping with high energy were excited. Eventually high current was formed and avalanche of electrons or ions can run through the oxide films where the films were weak [28]. High energy stored in doping would be released as kinetic energy of ions or electrons, Joule heat and sparks discharge [15].
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During experiment, countless twinkling sparks would be seen glimmering on the whole surface of magnesium electrode. After spark discharge occurred, the film growth process still has some similar characteristics with the conventional anodic oxidation process, but there is evident difference between them. The similar characteristic is the growth of oxide film all mainly depends on the movement of defects or ions among substrate, oxide film and electrolyte under effect of electric field. The representative characteristic of the PEO process that differs from conventional oxidation process is the existence of sparks and the effect of sintered and solidification. Under effect of high electric field and Joule heat, vapor or plasma of substrate and electrolyte constituents would be formed [27]. Consequently, the reactions of PEO films formation would be accelerated because plasmas have higher reactivity and better conductivity than those of ions. Furthermore, the sintered effect of sparking made the PEO reactions thorough and more oxide products formed. At the same time, the molten oxide product would be solidified rapidly because the temperature of electrolyte is much lower than that of the molten oxide formed under sparking. Thus, the synergetic effect of strong electric field and solidification of PEO electrolyte would endow the PEO films with high growth rate. The above inference that sparking promoted film growth was coincided with the thickness measurement results shown in Fig. 2, i.e. the PEO films had much higher growth rate during the potential just reached breakdown potential of the thin oxide film than that of other PEO stages. Just for formation of sintered oxide films and continuous thickening of PEO films, the donor concentration of electrode was decreased gently (as shown in Fig. 6), which indicated that the active positions giving plasma spark discharge on PEO films were reduced gradually. Hence, the number of micro-pores in PEO films would be decreased gradually with PEO reactions carried on (shown in Fig. 1). This result exactly reflected the superiority of PEO technique that the defected and unsubstantial surface in the oxide films always gave birth sparking and the PEO films will get more and more compact [29]. Combining Figs. 1 and 2, it can be seen that the growth rate of PEO films formed on magnesium alloy AZ91D in silicate solution was decreased slightly when the uniform PEO films with certain thickness were formed. This can be explained by the theory of Cabrera–Mott [30]: under high electric filed, ions transfer step was the dominant step of oxide film growth when thickness of oxide film reached certain value and the transfer of ions tightly followed the electric field-assistant mechanism. As films thickness increasing along with PEO process, the transfer resistance to ions in PEO films was increased. Then, the transfer rate of ions, which took part in PEO reactions, was decreased. As a result, the film growth rate was decreased. Therefore, in order to maintain the film formation reactions continual, the cell potential should be increased further to keep necessary electric field density for PEO films growth. On the other hand, the cell potential should be controlled under certain value to avoid inducing local breakdown of the PEO films. This was the reason that cell potential were increased step by step and were stabilized at certain level to keep the PEO films growth uniformly with considerable growth rate during our experiments.
4. Conclusions In this paper, the growth process of the protective oxide films formed on magnesium alloy AZ91D under plasma electrolytic oxidation (PEO) was investigated in alkaline silicate solution. The SEM, EDAX and capacitance measurement results showed that the PEO films formed in this case exhibited different growth behavior at the different PEO stage. The initial stage (before plasma spark discharge) of PEO treatment for magnesium alloy AZ91D was similar to a conventional anodic oxidation process and the oxide film was formed at a low growth rate. While at the beginning of plasma spark discharge, the PEO films had a non-uniform structure and a higher growth rate than any other stage. By then, the PEO films formed on ␣-phase had higher growth rate than those on -phase and the growth of PEO films formed on -phase was mainly in virtue of lateral growth of oxide films formed on ␣-phase. As sparking discharge reactions continued, the PEO films got uniform and thickened accompany with the formation of large size micro-pores and sintered granules. The PEO films formed on magnesium alloy AZ91D in silicate solution were composed of O, Mg, Al and Si. These elements contents were varied evidently during the initial stage of PEO process. There was an aluminium-enriched region in the inner layer of PEO films for formation of stable aluminium oxide. When the structure of the PEO films got uniform, the elements contents were reached a relative stable level. During the PEO process, the magnesium electrodes were all represented n-type semiconducting behavior. The donor concentration of the magnesium electrode was decreased gradually during the PEO process because the transfer resistance to ions or electrons in the PEO films was increased as the films thickness increasing and structure compactification. The growth of PEO films mainly followed electric field-assistant mechanism. Thus, the PEO films growth rate was decreased gently when the thickness of PEO films increased to certain value as increasing of the cell potential and treated time. Acknowledgement This work was supported by 973 Project of China. References [1] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. dowey, Surf. Coat. Technol. 122 (1999) 73. [2] A.L. Yerokhin, A. Shatrov, et al., Surf. Coat. Technol. 182 (2004) 78. [3] X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 125 (2000) 407. [4] H.F. Guo, M.Z. An, Appl. Surf. Sci. 246 (2005) 229. [5] H.Y. Hsial, W.T. Tsai, Surf. Coat. Technol. 190 (2005) 299. [6] H.P. Duan, K.Q. Du, C.W. Yan, F.H. Wang, Electrochim. Acta 51 (2006) 2898. [7] Q.Z. Cai, L.S. Wang, B.K. Wei, Q.X. Liu, Surf. Coat. Technol. 200 (2006) 3727. [8] A.V. Timoshenko, Yu.V. Magurova, Surf. Coat. Technol. 199 (2005) 135. [9] C. Blawert, V. Heitmann, W. Oietzel, Surf. Coat. Technol. 200 (2005) 68. [10] H.Y. Hsiao, W.T. Tsai, J. Mater. Res. 20 (2005) 2763. [11] Y.J. Zhang, C.W. Yan, Surf. Coat. Technol. 201 (2006) 2381.
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