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Influence of surface pretreatment on the anodizing film of Mg alloy and the mechanism of the ultrasound during the pretreatment
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Surface & Coatings Technology 202 (2008) 4210 – 4217 www.elsevier.com/locate/surfcoat
Influence of surface pretreatment on the anodizing film of Mg alloy and the mechanism of the ultrasound during the pretreatment Wang Ximei⁎, Zhu Liqun, Liu Huicong, Li Weiping School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, 37# Xueyuan Road, Haidian District, Beijing 100083, People's Republic of China Received 8 January 2008; accepted in revised form 10 March 2008 Available online 25 March 2008
Abstract The effect of surface pretreatments on the micro-morphology, composition and the corrosion resistance of the anodic films formed on AZ91D magnesium alloys was investigated. The results showed that the aluminium content increased layer can be formed on the surface of the AZ91D magnesium alloys by the immersion pretreatment in aluminium nitrate solution with or without ultrasound. The pretreated surface with ultrasound was more uniform except for some pits. The anodic oxidation films on the pretreated magnesium alloys with ultrasound contained more aluminium and the pretreatment improved the uniformity of the anodic films. The use of ultrasound in the pretreatment process could significantly decrease the quantity and size of the micro-pores on the anodic films. The corrosion resistance of the anodic films formed on the pretreated magnesium alloys was improved. The corrosion resistance with ultrasound was better than that without ultrasound in the pretreatment process. In this paper, the effect and mechanism of the ultrasound during the immersion pretreatment process in aluminium nitrate solution was also discussed. © 2008 Elsevier B.V. All rights reserved. PACS: 81.65.-b Keywords: Magnesium alloys; Anodizing; Pretreatment; Immersion; Ultrasound
1. Introduction Magnesium and its alloys possess a series of inherent superior properties which include the lowest density, high specific strength, excellent damping capacity, stronger vibration load resistance, machinability, etc. As a result, they are increasingly applied to a number of components on automobile, mobile personal computers and cellular phones. However, the application was limited by its poor corrosion resistance [1–3]. One of major techniques for solving the problem is surface treatment including chemical conversion coating, anodization, electroplating, electroless plating, physical vapor deposition and so on. These surface treatments often have certain limitations [4–11]. Anodization treatment is one of the popular methods for improving the corrosion resistance of magnesium and its alloys. It can produce a relatively thick, hard, adherent and abrasion-
⁎ Corresponding author. E-mail address: [email protected] (W. Ximei). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.018
resistance film. Extensive investigations have been performed to study the effects of electrolyte composition, electric parameters (such as voltage, current, waveform), post-treatment on the properties of the anodic film produced [12–15]. Many new and environmentally friendly anodizing electrolytes have been developed in recent years. The anodizing conditions have a deep influence on the composition and microstructure of the anodic film as well. However, the effect of pre-anodization treatment on the anodic films' properties was scarcely investigated. Most investigations [12,16–18] have shown that the pretreatment of magnesium alloys specimens is often as follows: ground with abrasive paper to a certain grit, degreased with acetone, and then rinsed with distilled water before anodizing. In this investigation, therefore, the effect of pretreatment on the characteristics of AZ91D magnesium alloy and anodized AZ91D was studied. It is well known that aluminium has a beneficial effect on the corrosion resistance of magnesium. As a result, many Mg–Al alloys have been developed for improving the corrosion resistance of magnesium alloys. The anodic films formed at the electrolyte with Al(NO3)3 or NaAlO2 also exhibited superior corrosion
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resistance for magnesium alloy, compared with the films without aluminate ion [19,20]. On the other hand, the power ultrasound used to clean the surfaces of manufactured objects. It has also been found that the ultrasonic agitation and cavitation phenomenon of the ultrasound can also help to induce chemical modification on many materials [21,22]. Therefore, in this investigation, the pretreatment was mainly applied to improve the Al content on the surface of AZ91D magnesium alloy before anodizing. At the same time, the pretreatment of magnesium alloy was processed by the combination of ultrasound with impregnation in the aluminium nitrate electrolyte. The effect of the pretreatment in aluminium nitrate electrolyte with and without ultrasound on the micromorphology, composition and the corrosion resistance of the anodic films formed on AZ91D magnesium alloy was studied. The mechanism of ultrasound was discussed as well. 2. Experimental 2.1. The film process Die-cast AZ91D Mg alloy(9.1 wt.% Al, 0.50 wt.% Zn) was employed in this study. The AZ91D samples were cut to size of 20 mm × 20 mm × 5 mm. Electrical connection was made by using screws into tapped holes. The samples were mounted with a PTFE resin with one surface exposed. The exposed surface of the sample was ground with water proof abrasive papers up to a grit of 800, and then rinsed with demineralised water. In this paper, we call this treatment to the surface of AZ91D magnesium alloys as the cleaning pretreatment. Besides, the other two methods were employed to pretreat the specimens before anodized. The first one was to immerse the specimens in aluminium nitrate solution for 30 min at room temperature (about 15 °C), called as “immersion pretreatment”. The second one was as follows: immersing the specimens in aluminium nitrate for 10 min with ultrasonic wave power at ambient temperature → only immersing for 10 min(without ultrasound)→ immersing for 5 min with ultrasonic wave power →only immersing for 5 min. And the second pretreatment method was called as “immersion & ultrasound pretreatment” in the paper. The ultrasonic wave field was applied with a constant frequency of 40 kHz and a constant power of 440 W. The concentration of the aluminium nitrate solution was 0.1 mol/L and the pH value was about 4. The immersion and immersion & ultrasound pretreatments were both processed after the cleaning pretreatment. The magnesium alloy samples were used as an anode in the electrolytic cell, whereas a stainless steel plate was served as a counter-electrode. The constant current with current density of 10 mA/cm2 was employed. Three kinds of the samples used to be anodized are as follows: 1) The AZ91D magnesium alloys with the cleaning pretreatment; 2) The AZ91D magnesium alloys with the immersion pretreatment; 3) The AZ91D magnesium alloys with the immersion & ultrasound pretreatment; The three kinds of samples were anodized in the aqueous electrolyte contained 1 mol/L Na2SiO3 and 10 vol.% silica sol.
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The preparation of silica sol was shown in the previous paper [23]. The solution temperature is 15–20 °C and the anodic time is 10 min. All samples were rinsed in distilled water before anodized. 2.2. Characterization The data acquisition and storage recorder (named ZF-10) was employed to detect the change of the anodic voltage with the anodic time during the anodization. The thickness of anodic films was measured by eddy current thickness meter. All samples were scaled by analytical balance in order to evaluate the weight loss of samples due to the immersion and immersion & ultrasound pretreatment. The surface morphologies of the treated magnesium alloys were characterized by scanning electron microscopy (HITACHI S-530) and the elemental compositions were determined by energy dispersion spectrometry (OXFORD LINK ISIS). The depth of the pits on the surface of AZ91D magnesium alloys treated at aluminium nitrate solution with ultrasound was measured with 3D microscope (Hirox KH-3000VD). The potentiodynamic polarization scan was performed in 5 wt.% NaCl solution to determine the corrosion resistance of the anodic films. The experiment was carried out using a three-electrode configuration: the specimen with 1.0 cm2 exposure area was used as working electrode and saturated calomel electrode (SCE) as a reference electrode, while a platinum (Pt) was used as an auxiliary electrode. Potentiodynamic polarization curves were measured with scan rate of 2 mV/s at 25 °C. 3. Results and discussion 3.1. The effects of different pretreatments on the anodic oxidation films Fig. 1 shows the variation of voltage between the cathode and the anode with time during the anodic oxidation process at room temperature. According to the different rising rates of the voltages and the range of the voltage oscillation, three stages can be classified. As shown in Fig. 1, each stage was marked with arrows and dashed lines. In the first stage of anodizing under constant current density condition (10 mA/cm2), for all the AZ91D samples pretreated with the three methods, the voltage increased linearly until it reached a breakdown voltage as depicted in Fig. 1. It also could be seen that the breakdown voltage of the AZ91D magnesium alloy pretreated in Al(NO3)3 solution decreased. As for the AZ91D magnesium alloy with the cleaning pretreatment, the breakdown voltage was 121 V. However, when the magnesium alloys were immersed at Al (NO3)3 solution with and without ultrasonic wave power before anodized, the breakdown voltages fell to nearly 90 V and 100 V, respectively. Beyond this point, the breakdown voltage, intensive and white small sparking was observed and the voltage started to oscillate slightly; which was named the stage II. Consequently, the voltage, in stage III, increased with a rate slower than that in the initial linear stage and oscillated chaotically with a larger range voltage than in the stage II. The
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Fig. 1. Potential transient of AZ91D Mg alloys during anodic oxidation after different pretreatments.
sparking caused the formation of a high impedance anodic film on the magnesium alloy surface. In the stage III, sparks which were large and orange in color were not intensive and only appeared on some particular sites. As a result, the anodized films formed on the surface of the magnesium alloy samples were not uniform. The uniform and intensive sparking could result in the formation of uniform, compact and adherent anodic film on the surface of magnesium alloys [20]. So, the intensive sparks in the stage II could help to obtain more uniform and compact anodic films than the large and scattered sparks in the stage III. That is to say, the longer the time of stage II was occupied during the whole anodic oxidation process, more uniform the anodic oxidation films were. During the anodic oxidation process, we observed that intensive sparks on the surface of magnesium alloy samples with the immersion & ultrasound pretreatment lasted for a longer time than that with the cleaning pretreatment and the immersion pretreatment. At the same time, in Fig. 1, the time of stage II for the magnesium alloy with the immersion & ultrasound pretreatment was longer than that with the other two pretreatments. Consequently, the more uniform anodic oxidation film should be obtained on the surface of the magnesium alloy pretreated in Al(NO3)3 solution with ultrasonic field. Fig. 2 shows the surface SEM images of the anodic films formed on AZ91D magnesium alloys which were pretreated differently. The A and B areas of Fig. 2(a) were zoomed in and showed in Fig. 2(b) and (c), respectively. In the same way, Fig. 2 (e) and (f) was corresponding to Fig. 2(d) and Fig. 2(h) and (i)
was corresponding to Fig. 2(g). It could be seen that all the anodic films have porous microstructure and the micro-pores often caused by spark discharge. The anodic films on the surface of cleaning and immersion pretreated magnesium alloys are uneven and coarse, and some island-like area can be observed on the surface of the anodic oxidation films which was prepared on the surface of cleaning pretreated magnesium alloys. The surface of the anodic oxidation film formed on the immersion & ultrasound magnesium alloy was more uniform than that on the other two pretreated magnesium alloys. This is consistent with the influence of the anodic oxidizing voltage. The size of the micro-pores on the surface of the anodic oxidation film was obviously influenced by the pretreatments. After the AZ91D magnesium alloy was pretreated at Al(NO3)3 solution with ultrasound field, the diameter of the micro-pores on the surface of the anodic film was the smallest. Nevertheless, we could also see some pits on the surface of the anodic oxidation films. In the pits, the micro-pores caused by the spark discharge were also small. These pits, which existed before anodizing, were not caused by the spark discharge but caused by the pretreatment of immersing in Al(NO3)3 solution with ultrasonic wave power. The size of pits was much larger than that of the micro-pores, whereas the quantity of pits was much less than that of the micro-pores. The diameter of the pits was about several tens of μm, and that of the micro-pores was about several μm. The depth of the pits was 12–60 μm and most of them were 28–40 μm. After anodized, the quantity, size and depth of the pits caused by pretreatment can be reduced. And
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Fig. 2. The SEM micrographs of the anodic films on differently pretreated AZ91D magnesium alloys: (a–c) pretreated by the cleaning pretreatment; (d–f) pretreated by the immersion pretreatment; (g–i) pretreated by the immersion & ultrasound pretreatment.
the depth of the pits was reduced to about 6–28 μm. The ultrasonic wave played a role in the formation of the pits on the surface of the AZ91D magnesium alloys, when they were immersed in Al(NO3)3 solution. This would be discussed at length in the latter. Fig. 3 presents the thickness distribution of the anodic films formed on AZ91D magnesium alloys pretreated differently. The average thickness of the anodic film on the surface of cleaning pretreated magnesium alloy was the largest. When the magnesium alloys were immersed in aluminium nitrate solution with and without ultrasound, the average thickness of the anodic oxidation films decreased. The distance between the maximum thickness and the minimum one could indicate the uniformity of the anodic films. The larger the distance, the more uneven the film was. So, the uniformity of the anodic film was improved
when AZ91D magnesium alloy was immersed in aluminium nitrate electrolyte with and without ultrasonic wave field before anodizing. The ultrasonic wave, especially combined with immersion has a more beneficial effect on the uniformity of the anodic film. Fig. 4 shows the element content of the surface of the anodic films formed on AZ91D magnesium alloys pretreated differently. The anodic films mainly contained O, Mg, Al and Si, and a small quantity of Na was also detected. The adsorption of electrolyte by the porous film should be responsible for the presence of Na in the film. When the magnesium alloys were pretreated by immersion in aluminium nitrate electrolyte with or without ultrasound before anodizing, the amount of Na on the anodic film surface decreased. Especially when ultrasound was applied in the immersion pretreatment, the amount of Na on the
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Fig. 3. Thickness distribution of anodic films formed on AZ91D Mg alloys pretreated differently.
anodic film decreased from 5.94% to 1.72%. This may be owed to the decrease of the size of the micro-pores on the film surface. The more Al could be detected on the surface of the anodic oxidation films of the magnesium alloys pretreated in aluminium nitrate electrolyte with or without ultrasound. Aluminium is known to have a beneficial effect on the protective performance of magnesium alloys and their anodic films. Thereby, the anodic oxidation films formed on the surface of immersion and immersion & ultrasound pretreated magnesium alloys could have a better corrosion resistance because of a higher content of Al. Fig. 5 demonstrates the potentiodynamic polarization curves of AZ91D Mg alloy and the anodic oxidation films formed on different pretreated magnesium alloys in 5 wt.% NaCl solutions. The Potentiodynamic Extension Method is used to measure corrosion current densities. The corrosion potential (Ecorr) and the corrosion current density (icorr) derived from the potentiodynamic polarization curves are shown in Table 1. It is well known that corrosion potential and corrosion current density of coated samples are often used to characterize
Fig. 5. The potentiodynamic polarization curves of the original sample (AZ91D) and the anodic oxidation films on differently pretreated magnesium alloys surface in 5 wt.% NaCl solutions.
the corrosion protective property of the films. The high corrosion potential and low corrosion current density of the film suggest that it exhibits a low corrosion rate and a good corrosion resistance. The results shown in Table 1 demonstrate that the anodic oxidation films could greatly improve the corrosion resistance of AZ91D magnesium alloys. The anodic films formed on the AZ91D Mg alloys pretreated in Al(NO3)3 solution with or without ultrasound has a higher Ecorr and lower icorr than that un-pretreated in Al(NO3)3 solution, indicating that it has a lower corrosion rate. The application of ultrasound during the immersion pretreatment can help to improve the Al content in the anodic oxidation film and its surface uniformity, decrease the diameter of micro-pores on the surface of the anodic oxidation film, and accordingly enhance the corrosion resistance of the anodic oxidation film formed on the AZ91D magnesium alloy. 3.2. The role of ultrasonic power during the pretreatment The surface micrographs of the AZ91D magnesium alloys immersed in aluminium nitrate with and without ultrasonic wave power are shown in Fig. 6. Fig. 6(b) and (d) is the magnification of Fig. 6(a) and (c), respectively. It is shown in Fig. 6(a) and (b) that the surface of specimen is covered with a layer which may be some oxide and hydrate of magnesium and aluminium when the AZ91D magnesium alloy was immersed in
Table 1 The results of the potentiodynamic corrosion tests in 5 wt.% NaCl solution
Fig. 4. Element quantitative analysis of anodic films formed on AZ91D Mg alloys pretreated differently.
Samples
icorr Ecorr (± 0.002 V) (±0.53 mA/cm2)
AZ91D substrate Film on the cleaning pretreated AZ91D surface Film on the immersion pretreated AZ91D surface Film on the immersion & ultrasound pretreated AZ91D surface
− 1.560 − 1.510 − 1.457 − 1.412
58.65 1.54 1.50 0.18
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Fig. 6. The SEM micrographs of the AZ91D magnesium alloy treated by immersing in the Al(NO3)3 solution: (a–b) without ultrasound, and (c–d) with ultrasound.
0.1 mol/L aluminium nitrate solution for 30 min. The oxide and hydrate of magnesium and aluminium formed on the magnesium alloy is deduced from the result of the element composition of the surface shown in Table 2. Because the element hydrogen cannot be detected by the EDS, the layer possibly contains the hydrate of magnesium and aluminium. The surface layer was formed mainly by the chemical reactions and was loose. The adherence between the surface layer and the magnesium alloy substrate may be not good. It also can be seen that the oxide or hydrate layer was block-like and tiny cracks were obviously distributed all over the surface. The surface of the AZ91D magnesium alloy immersed in aluminium nitrate solution with ultrasonic field was bestrewed with many pits as depicted in Fig. 6(c). However, except for these pits, the surface was compact and uniform and network-like cracks also could be observed on the surface. Table 2 Element quantitative analysis of the surface of the AZ91D Mg alloys immersed in solution with and without ultrasound, and the area A and B in Fig. 6 Different treatment
Element content O
Immersion in Al(NO3)3 Immersion & ultrasound Area A Area B
Mg
Al
wt.%
at.%
wt.%
at.%
wt.%
at.%
26.01 28.33 8.21 32.22
35.42 38.18 12.44 42.48
54.49 51.72 51.00 52.58
48.83 45.87 50.88 45.65
19.50 19.95 40.79 15.20
15.74 15.95 36.67 11.87
The element contents of the surface layer formed by immersing in aluminium nitrate solution with and without ultrasonic field are shown in Table 2. It indicates that the both layers are composed of oxygen, magnesium and aluminium and the content of the three elements are similar. The compositions of local areas A and B in Fig. 6(d) are also shown in Table 2. The value of aluminium in local area A is higher than that of local area B, which indicates that where area A located in is the position of β phase of magnesium alloy substrate. The formation process of the surface layers, after the AZ91D magnesium alloys immersed in aluminium nitrate solution, could be explained briefly as follows. The AZ91D magnesium alloy was consisted of α and β phases, and the β grain is nobler than the α grain. This creates a local cell effect which leads to hydrogen being released at the cathode and the dissolving of metallic elements Mg, Al at the anode in aluminium nitrate solution. The reduction of H+ resulted in a decrease of the acidic value of the cathodic site, thus the pH value of these local cathodic sites increased. This resulted in the formation of Mg, Al-hydroxide. Mg(OH)2 and Al(OH)3, which may be oxidized into Mg, Al-oxide, were deposited on the surface of the magnesium alloys, and the surface layer was formed. The cracks on the surface layer may possibly be due to hydrogen being released via the chemical reaction during the immersion treatment and/or the dehydration of the surface layer after treatment. According to the results described above, the form of those pits on the surface should be attributed to the application of
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Fig. 7. The weight loss of AZ91D Mg alloys before and after different pretreatment.
ultrasound. It is the cavitation damage of power ultrasound that induced so many pits on the surface. Cavitation may be defined as the growth and collapse of vapor bubbles due to localized pressure changes in a liquid [24]. The bubbles are formed in the liquid during negative pressure period of sound wave. The collapse process of the bubbles takes place extremely rapidly, producing a strong shock wave that damages the material around. Because a few million bubbles may collapse in a second, damage quickly occurs. In the present work, when the AZ91D magnesium alloys surface are in the vicinity of the cavitation bubbles, the strong shock waves caused by the implosive collapse of bubbles will propagate to it causing microscopic turbulence. Subsequently, the loose, block-like oxide or hydrate of magnesium and aluminium would be removed from the surface of magnesium alloy. For example, after attacked by the ultrasonic cavitation, the blocklike oxide in the rectangle region in Fig. 6(b) may be brushed off and the surface of the rectangle region could turn into that of the area A in Fig. 6(d). In the circle region of Fig. 6(b), the protrusive surface can be observed. When the protrusive surface in the circle region is in close proximity to the bubble, it is unstable to collapse symmetrically and microjets form [25]. These powerful microjets act like microscopic ‘bombs’, and they explode the surface of the circle region of Fig. 6(b). Then, pieces of oxide materials are likely to be removed and pitting and erosion can be formed on the surface, as shown in Fig. 6(c). The circle region surface of Fig. 6(b) which was attacked by bubble implosions could be changed into the pit like the area B in Fig. 6(d). Based on the analysis above, weight loss from the AZ91D magnesium alloy immersed in aluminium nitrate solution with ultrasound may be caused by the cavitation damage of ultrasound. The weight changes of the AZ91D magnesium alloy sample before and after being immersed in aluminium nitrate solution with or without ultrasound are shown in Fig. 7. It can be seen that the weight of the AZ91D magnesium alloys both decreased after immersed in aluminium nitrate solution whether with or without ultrasound. But, with the use of ultrasound, the weight loss was dramatically increased, which proved the
cavitation damage of ultrasound to the surface of the AZ91D magnesium alloy. For fine explaining the cavitation damage of ultrasound to the AZ91D magnesium alloy immersed in aluminium nitrate solution, we measured the weight of the AZ91D magnesium alloys samples in each step during the immersion & ultrasound pretreatment. The weight loss of the samples after each step was shown in Fig. 8. In the first and third steps, both with ultrasound applied, the weight loss was much larger than that of the other two steps. It can be deduced that the weight loss is mainly owed to the period of the use of ultrasound. That is to say, the ultrasonic cavitation induced the large weight loss of the AZ91D magnesium alloy when it was immersed in aluminium nitrate solution. The implosion of the bubbles can also cause enhanced agitation and higher temperature of the liquid. The enhanced agitation effect produced by ultrasound allows the surface of the AZ91D magnesium alloy to be replenished with fresh aluminium nitrate solution more quickly than that only immersed in aluminium nitrate solution without ultrasound. Consequently, the weight of the AZ91D magnesium alloys may be lessened more heavily than that without ultrasound. The increased temperature of the liquid caused by ultrasonic cavitation could also contribute to the weight loss of the AZ91D magnesium alloy which was immersed in aluminium nitrate solution with ultrasound. So, the test which measured the weight reduction caused by the solution temperature was carried out. After the immersion & ultrasound pretreatment finished, the temperature of the aluminium nitrate solution was about 42 °C. The weight loss of the AZ91D magnesium alloy immersed in 0.1 mol/L Al (NO3)3 solution without ultrasound at about 50 °C for 30 min was 0.47–0.54 mg/cm2. As shown in Fig. 7, the weight loss of magnesium alloy immersed in the same solution at room temperature (15 °C) for 30 min was 0.03–0.14 mg/cm2. The effect of solution temperature on the weight loss of magnesium
Fig. 8. The weight loss of the AZ91D Mg alloys in each step during the immersion & ultrasound pretreatment; U-10 min: immersed in aluminium nitrate solution with ultrasound for 10 min; I-10 min: only immersed in the same solution after U-10 min; U-5 min: immersed in the same solution with ultrasound after I-10 min; I-5 min: only immersed in the same solution after U5 min.
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alloys was obvious. The higher the solution temperature, the larger the weight loss is. We also know from Fig. 7 that the weight loss of magnesium alloys immersed in aluminium nitrate solution for 30 min with ultrasound was 1.79–2.91 mg/cm2 which was much larger than that at about 50 °C (0.47–0.54 mg/ cm2). As a result, the higher solution temperature caused by the implosion of the bubbles with ultrasound may be one factor influencing the weight loss of the magnesium alloys immersed in aluminium nitrate solution. Therefore, the enhanced agitation and higher temperature of the solution caused by the implosion of the bubbles partly contribute to the weight loss of the AZ91D magnesium alloy. As mentioned above, the effects of ultrasonic pretreatment in the aluminium nitrate solution on the AZ91D magnesium alloys are generally characterized by cavitation and are accompanied by a local increase in pressure and temperature and a strong mechanical effect such as the enhanced agitation of the liquid. The cavitation, pressure, local high temperature and enhanced agitation made the surface of AZ91D magnesium alloy more compact and uniform except for some pits, which produced a clear and uniform surface for the subsequent anodic oxidation process. Therefore, the anodic film of ultrasonic immersed magnesium alloy surface was more uniform and the micropores which caused by anodizing sparks were small, although there still were some pits which were smaller than that before anodizing. If the anodic time be prolonged, the pits caused by immersion & ultrasound pretreatment may be filled in. After pretreated in aluminium nitrate solution with or without ultrasound, the anodic films of AZ91D magnesium alloys contained more aluminium owing to that the aluminium in the pretreatment layer became incorporated into the anodic film. The process that the aluminium became incorporated into the anodic film could be described as follows. Firstly, the sol particles and the SiO32 − anion in the electrolyte were adsorbed on the pretreatment layer surface at the beginning of the oxidation process. The adsorption could be chemical adsorption or physical adsorption. The composite barrier layers were formed in the surface of magnesium alloys. Secondly, the anodic voltage increased and the sparks appeared on the surface. The composite layers were destroyed by the high energy generated by the sparking during the oxidation process, and the products were formed with the anodic reaction. 4. Conclusions 1) After the AZ91D magnesium alloy was pretreated in 0.1 mol/L aluminium nitrate solution with or without ultrasound, especially with ultrasound, the duration of intensive sparks on the Mg alloy surface was longer during the anodizing process and the uniformity of the anodic oxidation film was improved. The micro-pores on the film
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surface are decreased in quantity and size and the thickness of the anodic film was reduced. But some pits appeared in the surface of the magnesium alloy because of the ultrasound treatment, and these pits, as though being partly filled, still existed after the subsequent anodizing. 2) The pretreatment in aluminium nitrate solution with or without ultrasound resulted in the increase of Al both in the anodic film and in the pretreated surface of AZ91D magnesium alloy. The content of Al increased more with the use of ultrasound in the pretreatment. 3) The anodic film formed on the AZ91D Mg alloy pretreated in aluminium nitrate solution with or without ultrasound improved the corrosion resistance of AZ91D Mg alloy substrate more significantly than that without pretreatment in aluminium nitrate solution. The corrosion resistance with ultrasound was better than that without ultrasound in the pretreatment process. References [1] G.L. Song, A. Atrens, Adv. Eng. Mater. 5 (2003) 837. [2] R. Ambat, N.N. Aung, W. Zhou, Corros. Sci. 42 (2000) 1433. [3] G.L. Song, B. Johannesson, S.H. David StJohn, Corros. Sci. 46 (2004) 955. [4] J.E. Gray, B. Luan, J. Alloys Comp. 336 (2002) 88. [5] C. Blawert, W. Dietzel, E. Ghali, G.L. Song, Adv. Eng. Mater. 8 (2006) 511. [6] Y.H. Liu, Z.X. Guo, Y. Yang, H.Y. Wang, J.D. Hu, Y.X. Li, A.N. Chumakov, N.A. Bosak, Appl. Surf. Sci. 253 (2006) 1722. [7] Z.M. Liu, W. Gao, Surf. Coat. Technol. 200 (2006) 5087. [8] K.H. Yang, M.D. Ger, W.H. Hwu, Y. Sung, Y.C. Liu, Mater. Chem. Phys. 101 (2007) 480. [9] S.X. Yu, Q.Y. Lu, J. Han, Z.W. Zhang, Q.Y. Zhang, J. Rare Earths 24 (2006) 397. [10] A.N. Khramov, V.N. Balbyshev, L.S. Kasten, R.A. Mantz, Thin Solid Films 514 (2006) 174. [11] M. Dabala, K. Brunelli, E. Napolitani, M. Magrini, Surf. Coat. Technol. 172 (2003) 227. [12] H.Y. Hsiao, P. Chung, W.T. Tsai, Corros. Sci. 49 (2007) 781. [13] Z.M. Shi, G.L. Song, A. Atrens, Corros. Sci. 48 (2006) 1939. [14] W.P. Li, L.Q. Zhu, H.C. Liu, Surf. Coat. Technol. 201 (2006) 2505. [15] C. Blawert, V. Heitmann, W. Dietzel, H.M. Nykyforchyn, M.D. Klapkiv, Surf. Coat. Technol. 201 (2007) 8709. [16] H.F. Guo, M.Z. An, S. Xu, H.B. Huo, Thin Solid Films 485 (2005) 53. [17] H.Y. Hsiao, H.C. Tsung, W.T. Tsai, Surf. Coat. Technol. 199 (2005) 127. [18] Y. Mizutani, S.J. Kim, R. Ichino, M. Okido, Surf. Coat. Technol. 169–170 (2003) 143. [19] J. Liang, B.G. Guo, J. Tian, H.W. Liu, J.F. Zhou, W.M. Liu, T. Xu, Surf. Coat. Technol. 199 (2005) 121. [20] H.Y. Hsiao, W.T. Tsai, Surf. Coat. Technol. 190 (2005) 299. [21] G. Cravotto, S. Tagliapietra, B. Robaldo, M. Trotta, Ultrason. Sonochem. 12 (2005) 95. [22] M. Aurousseau, N.T. Pham, P. Ozil, Ultrason. Sonochem. 11 (2004) 23. [23] W.P. Li, L.Q. Zhu, H.C. Liu, Surf. Coat. Technol. 201 (2006) 2573. [24] A. Al-hashem, W. Riad, Mater. Charact. 48 (2002) 37. [25] X.W. Guo, W.J. Ding, C. Lu, C.Q. Zhai, Surf. Coat. Technol. 183 (2004) 359.
Surface & Coatings Technology 202 (2008) 4210 – 4217 www.elsevier.com/locate/surfcoat
Influence of surface pretreatment on the anodizing film of Mg alloy and the mechanism of the ultrasound during the pretreatment Wang Ximei⁎, Zhu Liqun, Liu Huicong, Li Weiping School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, 37# Xueyuan Road, Haidian District, Beijing 100083, People's Republic of China Received 8 January 2008; accepted in revised form 10 March 2008 Available online 25 March 2008
Abstract The effect of surface pretreatments on the micro-morphology, composition and the corrosion resistance of the anodic films formed on AZ91D magnesium alloys was investigated. The results showed that the aluminium content increased layer can be formed on the surface of the AZ91D magnesium alloys by the immersion pretreatment in aluminium nitrate solution with or without ultrasound. The pretreated surface with ultrasound was more uniform except for some pits. The anodic oxidation films on the pretreated magnesium alloys with ultrasound contained more aluminium and the pretreatment improved the uniformity of the anodic films. The use of ultrasound in the pretreatment process could significantly decrease the quantity and size of the micro-pores on the anodic films. The corrosion resistance of the anodic films formed on the pretreated magnesium alloys was improved. The corrosion resistance with ultrasound was better than that without ultrasound in the pretreatment process. In this paper, the effect and mechanism of the ultrasound during the immersion pretreatment process in aluminium nitrate solution was also discussed. © 2008 Elsevier B.V. All rights reserved. PACS: 81.65.-b Keywords: Magnesium alloys; Anodizing; Pretreatment; Immersion; Ultrasound
1. Introduction Magnesium and its alloys possess a series of inherent superior properties which include the lowest density, high specific strength, excellent damping capacity, stronger vibration load resistance, machinability, etc. As a result, they are increasingly applied to a number of components on automobile, mobile personal computers and cellular phones. However, the application was limited by its poor corrosion resistance [1–3]. One of major techniques for solving the problem is surface treatment including chemical conversion coating, anodization, electroplating, electroless plating, physical vapor deposition and so on. These surface treatments often have certain limitations [4–11]. Anodization treatment is one of the popular methods for improving the corrosion resistance of magnesium and its alloys. It can produce a relatively thick, hard, adherent and abrasion-
⁎ Corresponding author. E-mail address: [email protected] (W. Ximei). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.018
resistance film. Extensive investigations have been performed to study the effects of electrolyte composition, electric parameters (such as voltage, current, waveform), post-treatment on the properties of the anodic film produced [12–15]. Many new and environmentally friendly anodizing electrolytes have been developed in recent years. The anodizing conditions have a deep influence on the composition and microstructure of the anodic film as well. However, the effect of pre-anodization treatment on the anodic films' properties was scarcely investigated. Most investigations [12,16–18] have shown that the pretreatment of magnesium alloys specimens is often as follows: ground with abrasive paper to a certain grit, degreased with acetone, and then rinsed with distilled water before anodizing. In this investigation, therefore, the effect of pretreatment on the characteristics of AZ91D magnesium alloy and anodized AZ91D was studied. It is well known that aluminium has a beneficial effect on the corrosion resistance of magnesium. As a result, many Mg–Al alloys have been developed for improving the corrosion resistance of magnesium alloys. The anodic films formed at the electrolyte with Al(NO3)3 or NaAlO2 also exhibited superior corrosion
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resistance for magnesium alloy, compared with the films without aluminate ion [19,20]. On the other hand, the power ultrasound used to clean the surfaces of manufactured objects. It has also been found that the ultrasonic agitation and cavitation phenomenon of the ultrasound can also help to induce chemical modification on many materials [21,22]. Therefore, in this investigation, the pretreatment was mainly applied to improve the Al content on the surface of AZ91D magnesium alloy before anodizing. At the same time, the pretreatment of magnesium alloy was processed by the combination of ultrasound with impregnation in the aluminium nitrate electrolyte. The effect of the pretreatment in aluminium nitrate electrolyte with and without ultrasound on the micromorphology, composition and the corrosion resistance of the anodic films formed on AZ91D magnesium alloy was studied. The mechanism of ultrasound was discussed as well. 2. Experimental 2.1. The film process Die-cast AZ91D Mg alloy(9.1 wt.% Al, 0.50 wt.% Zn) was employed in this study. The AZ91D samples were cut to size of 20 mm × 20 mm × 5 mm. Electrical connection was made by using screws into tapped holes. The samples were mounted with a PTFE resin with one surface exposed. The exposed surface of the sample was ground with water proof abrasive papers up to a grit of 800, and then rinsed with demineralised water. In this paper, we call this treatment to the surface of AZ91D magnesium alloys as the cleaning pretreatment. Besides, the other two methods were employed to pretreat the specimens before anodized. The first one was to immerse the specimens in aluminium nitrate solution for 30 min at room temperature (about 15 °C), called as “immersion pretreatment”. The second one was as follows: immersing the specimens in aluminium nitrate for 10 min with ultrasonic wave power at ambient temperature → only immersing for 10 min(without ultrasound)→ immersing for 5 min with ultrasonic wave power →only immersing for 5 min. And the second pretreatment method was called as “immersion & ultrasound pretreatment” in the paper. The ultrasonic wave field was applied with a constant frequency of 40 kHz and a constant power of 440 W. The concentration of the aluminium nitrate solution was 0.1 mol/L and the pH value was about 4. The immersion and immersion & ultrasound pretreatments were both processed after the cleaning pretreatment. The magnesium alloy samples were used as an anode in the electrolytic cell, whereas a stainless steel plate was served as a counter-electrode. The constant current with current density of 10 mA/cm2 was employed. Three kinds of the samples used to be anodized are as follows: 1) The AZ91D magnesium alloys with the cleaning pretreatment; 2) The AZ91D magnesium alloys with the immersion pretreatment; 3) The AZ91D magnesium alloys with the immersion & ultrasound pretreatment; The three kinds of samples were anodized in the aqueous electrolyte contained 1 mol/L Na2SiO3 and 10 vol.% silica sol.
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The preparation of silica sol was shown in the previous paper [23]. The solution temperature is 15–20 °C and the anodic time is 10 min. All samples were rinsed in distilled water before anodized. 2.2. Characterization The data acquisition and storage recorder (named ZF-10) was employed to detect the change of the anodic voltage with the anodic time during the anodization. The thickness of anodic films was measured by eddy current thickness meter. All samples were scaled by analytical balance in order to evaluate the weight loss of samples due to the immersion and immersion & ultrasound pretreatment. The surface morphologies of the treated magnesium alloys were characterized by scanning electron microscopy (HITACHI S-530) and the elemental compositions were determined by energy dispersion spectrometry (OXFORD LINK ISIS). The depth of the pits on the surface of AZ91D magnesium alloys treated at aluminium nitrate solution with ultrasound was measured with 3D microscope (Hirox KH-3000VD). The potentiodynamic polarization scan was performed in 5 wt.% NaCl solution to determine the corrosion resistance of the anodic films. The experiment was carried out using a three-electrode configuration: the specimen with 1.0 cm2 exposure area was used as working electrode and saturated calomel electrode (SCE) as a reference electrode, while a platinum (Pt) was used as an auxiliary electrode. Potentiodynamic polarization curves were measured with scan rate of 2 mV/s at 25 °C. 3. Results and discussion 3.1. The effects of different pretreatments on the anodic oxidation films Fig. 1 shows the variation of voltage between the cathode and the anode with time during the anodic oxidation process at room temperature. According to the different rising rates of the voltages and the range of the voltage oscillation, three stages can be classified. As shown in Fig. 1, each stage was marked with arrows and dashed lines. In the first stage of anodizing under constant current density condition (10 mA/cm2), for all the AZ91D samples pretreated with the three methods, the voltage increased linearly until it reached a breakdown voltage as depicted in Fig. 1. It also could be seen that the breakdown voltage of the AZ91D magnesium alloy pretreated in Al(NO3)3 solution decreased. As for the AZ91D magnesium alloy with the cleaning pretreatment, the breakdown voltage was 121 V. However, when the magnesium alloys were immersed at Al (NO3)3 solution with and without ultrasonic wave power before anodized, the breakdown voltages fell to nearly 90 V and 100 V, respectively. Beyond this point, the breakdown voltage, intensive and white small sparking was observed and the voltage started to oscillate slightly; which was named the stage II. Consequently, the voltage, in stage III, increased with a rate slower than that in the initial linear stage and oscillated chaotically with a larger range voltage than in the stage II. The
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Fig. 1. Potential transient of AZ91D Mg alloys during anodic oxidation after different pretreatments.
sparking caused the formation of a high impedance anodic film on the magnesium alloy surface. In the stage III, sparks which were large and orange in color were not intensive and only appeared on some particular sites. As a result, the anodized films formed on the surface of the magnesium alloy samples were not uniform. The uniform and intensive sparking could result in the formation of uniform, compact and adherent anodic film on the surface of magnesium alloys [20]. So, the intensive sparks in the stage II could help to obtain more uniform and compact anodic films than the large and scattered sparks in the stage III. That is to say, the longer the time of stage II was occupied during the whole anodic oxidation process, more uniform the anodic oxidation films were. During the anodic oxidation process, we observed that intensive sparks on the surface of magnesium alloy samples with the immersion & ultrasound pretreatment lasted for a longer time than that with the cleaning pretreatment and the immersion pretreatment. At the same time, in Fig. 1, the time of stage II for the magnesium alloy with the immersion & ultrasound pretreatment was longer than that with the other two pretreatments. Consequently, the more uniform anodic oxidation film should be obtained on the surface of the magnesium alloy pretreated in Al(NO3)3 solution with ultrasonic field. Fig. 2 shows the surface SEM images of the anodic films formed on AZ91D magnesium alloys which were pretreated differently. The A and B areas of Fig. 2(a) were zoomed in and showed in Fig. 2(b) and (c), respectively. In the same way, Fig. 2 (e) and (f) was corresponding to Fig. 2(d) and Fig. 2(h) and (i)
was corresponding to Fig. 2(g). It could be seen that all the anodic films have porous microstructure and the micro-pores often caused by spark discharge. The anodic films on the surface of cleaning and immersion pretreated magnesium alloys are uneven and coarse, and some island-like area can be observed on the surface of the anodic oxidation films which was prepared on the surface of cleaning pretreated magnesium alloys. The surface of the anodic oxidation film formed on the immersion & ultrasound magnesium alloy was more uniform than that on the other two pretreated magnesium alloys. This is consistent with the influence of the anodic oxidizing voltage. The size of the micro-pores on the surface of the anodic oxidation film was obviously influenced by the pretreatments. After the AZ91D magnesium alloy was pretreated at Al(NO3)3 solution with ultrasound field, the diameter of the micro-pores on the surface of the anodic film was the smallest. Nevertheless, we could also see some pits on the surface of the anodic oxidation films. In the pits, the micro-pores caused by the spark discharge were also small. These pits, which existed before anodizing, were not caused by the spark discharge but caused by the pretreatment of immersing in Al(NO3)3 solution with ultrasonic wave power. The size of pits was much larger than that of the micro-pores, whereas the quantity of pits was much less than that of the micro-pores. The diameter of the pits was about several tens of μm, and that of the micro-pores was about several μm. The depth of the pits was 12–60 μm and most of them were 28–40 μm. After anodized, the quantity, size and depth of the pits caused by pretreatment can be reduced. And
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Fig. 2. The SEM micrographs of the anodic films on differently pretreated AZ91D magnesium alloys: (a–c) pretreated by the cleaning pretreatment; (d–f) pretreated by the immersion pretreatment; (g–i) pretreated by the immersion & ultrasound pretreatment.
the depth of the pits was reduced to about 6–28 μm. The ultrasonic wave played a role in the formation of the pits on the surface of the AZ91D magnesium alloys, when they were immersed in Al(NO3)3 solution. This would be discussed at length in the latter. Fig. 3 presents the thickness distribution of the anodic films formed on AZ91D magnesium alloys pretreated differently. The average thickness of the anodic film on the surface of cleaning pretreated magnesium alloy was the largest. When the magnesium alloys were immersed in aluminium nitrate solution with and without ultrasound, the average thickness of the anodic oxidation films decreased. The distance between the maximum thickness and the minimum one could indicate the uniformity of the anodic films. The larger the distance, the more uneven the film was. So, the uniformity of the anodic film was improved
when AZ91D magnesium alloy was immersed in aluminium nitrate electrolyte with and without ultrasonic wave field before anodizing. The ultrasonic wave, especially combined with immersion has a more beneficial effect on the uniformity of the anodic film. Fig. 4 shows the element content of the surface of the anodic films formed on AZ91D magnesium alloys pretreated differently. The anodic films mainly contained O, Mg, Al and Si, and a small quantity of Na was also detected. The adsorption of electrolyte by the porous film should be responsible for the presence of Na in the film. When the magnesium alloys were pretreated by immersion in aluminium nitrate electrolyte with or without ultrasound before anodizing, the amount of Na on the anodic film surface decreased. Especially when ultrasound was applied in the immersion pretreatment, the amount of Na on the
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Fig. 3. Thickness distribution of anodic films formed on AZ91D Mg alloys pretreated differently.
anodic film decreased from 5.94% to 1.72%. This may be owed to the decrease of the size of the micro-pores on the film surface. The more Al could be detected on the surface of the anodic oxidation films of the magnesium alloys pretreated in aluminium nitrate electrolyte with or without ultrasound. Aluminium is known to have a beneficial effect on the protective performance of magnesium alloys and their anodic films. Thereby, the anodic oxidation films formed on the surface of immersion and immersion & ultrasound pretreated magnesium alloys could have a better corrosion resistance because of a higher content of Al. Fig. 5 demonstrates the potentiodynamic polarization curves of AZ91D Mg alloy and the anodic oxidation films formed on different pretreated magnesium alloys in 5 wt.% NaCl solutions. The Potentiodynamic Extension Method is used to measure corrosion current densities. The corrosion potential (Ecorr) and the corrosion current density (icorr) derived from the potentiodynamic polarization curves are shown in Table 1. It is well known that corrosion potential and corrosion current density of coated samples are often used to characterize
Fig. 5. The potentiodynamic polarization curves of the original sample (AZ91D) and the anodic oxidation films on differently pretreated magnesium alloys surface in 5 wt.% NaCl solutions.
the corrosion protective property of the films. The high corrosion potential and low corrosion current density of the film suggest that it exhibits a low corrosion rate and a good corrosion resistance. The results shown in Table 1 demonstrate that the anodic oxidation films could greatly improve the corrosion resistance of AZ91D magnesium alloys. The anodic films formed on the AZ91D Mg alloys pretreated in Al(NO3)3 solution with or without ultrasound has a higher Ecorr and lower icorr than that un-pretreated in Al(NO3)3 solution, indicating that it has a lower corrosion rate. The application of ultrasound during the immersion pretreatment can help to improve the Al content in the anodic oxidation film and its surface uniformity, decrease the diameter of micro-pores on the surface of the anodic oxidation film, and accordingly enhance the corrosion resistance of the anodic oxidation film formed on the AZ91D magnesium alloy. 3.2. The role of ultrasonic power during the pretreatment The surface micrographs of the AZ91D magnesium alloys immersed in aluminium nitrate with and without ultrasonic wave power are shown in Fig. 6. Fig. 6(b) and (d) is the magnification of Fig. 6(a) and (c), respectively. It is shown in Fig. 6(a) and (b) that the surface of specimen is covered with a layer which may be some oxide and hydrate of magnesium and aluminium when the AZ91D magnesium alloy was immersed in
Table 1 The results of the potentiodynamic corrosion tests in 5 wt.% NaCl solution
Fig. 4. Element quantitative analysis of anodic films formed on AZ91D Mg alloys pretreated differently.
Samples
icorr Ecorr (± 0.002 V) (±0.53 mA/cm2)
AZ91D substrate Film on the cleaning pretreated AZ91D surface Film on the immersion pretreated AZ91D surface Film on the immersion & ultrasound pretreated AZ91D surface
− 1.560 − 1.510 − 1.457 − 1.412
58.65 1.54 1.50 0.18
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Fig. 6. The SEM micrographs of the AZ91D magnesium alloy treated by immersing in the Al(NO3)3 solution: (a–b) without ultrasound, and (c–d) with ultrasound.
0.1 mol/L aluminium nitrate solution for 30 min. The oxide and hydrate of magnesium and aluminium formed on the magnesium alloy is deduced from the result of the element composition of the surface shown in Table 2. Because the element hydrogen cannot be detected by the EDS, the layer possibly contains the hydrate of magnesium and aluminium. The surface layer was formed mainly by the chemical reactions and was loose. The adherence between the surface layer and the magnesium alloy substrate may be not good. It also can be seen that the oxide or hydrate layer was block-like and tiny cracks were obviously distributed all over the surface. The surface of the AZ91D magnesium alloy immersed in aluminium nitrate solution with ultrasonic field was bestrewed with many pits as depicted in Fig. 6(c). However, except for these pits, the surface was compact and uniform and network-like cracks also could be observed on the surface. Table 2 Element quantitative analysis of the surface of the AZ91D Mg alloys immersed in solution with and without ultrasound, and the area A and B in Fig. 6 Different treatment
Element content O
Immersion in Al(NO3)3 Immersion & ultrasound Area A Area B
Mg
Al
wt.%
at.%
wt.%
at.%
wt.%
at.%
26.01 28.33 8.21 32.22
35.42 38.18 12.44 42.48
54.49 51.72 51.00 52.58
48.83 45.87 50.88 45.65
19.50 19.95 40.79 15.20
15.74 15.95 36.67 11.87
The element contents of the surface layer formed by immersing in aluminium nitrate solution with and without ultrasonic field are shown in Table 2. It indicates that the both layers are composed of oxygen, magnesium and aluminium and the content of the three elements are similar. The compositions of local areas A and B in Fig. 6(d) are also shown in Table 2. The value of aluminium in local area A is higher than that of local area B, which indicates that where area A located in is the position of β phase of magnesium alloy substrate. The formation process of the surface layers, after the AZ91D magnesium alloys immersed in aluminium nitrate solution, could be explained briefly as follows. The AZ91D magnesium alloy was consisted of α and β phases, and the β grain is nobler than the α grain. This creates a local cell effect which leads to hydrogen being released at the cathode and the dissolving of metallic elements Mg, Al at the anode in aluminium nitrate solution. The reduction of H+ resulted in a decrease of the acidic value of the cathodic site, thus the pH value of these local cathodic sites increased. This resulted in the formation of Mg, Al-hydroxide. Mg(OH)2 and Al(OH)3, which may be oxidized into Mg, Al-oxide, were deposited on the surface of the magnesium alloys, and the surface layer was formed. The cracks on the surface layer may possibly be due to hydrogen being released via the chemical reaction during the immersion treatment and/or the dehydration of the surface layer after treatment. According to the results described above, the form of those pits on the surface should be attributed to the application of
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Fig. 7. The weight loss of AZ91D Mg alloys before and after different pretreatment.
ultrasound. It is the cavitation damage of power ultrasound that induced so many pits on the surface. Cavitation may be defined as the growth and collapse of vapor bubbles due to localized pressure changes in a liquid [24]. The bubbles are formed in the liquid during negative pressure period of sound wave. The collapse process of the bubbles takes place extremely rapidly, producing a strong shock wave that damages the material around. Because a few million bubbles may collapse in a second, damage quickly occurs. In the present work, when the AZ91D magnesium alloys surface are in the vicinity of the cavitation bubbles, the strong shock waves caused by the implosive collapse of bubbles will propagate to it causing microscopic turbulence. Subsequently, the loose, block-like oxide or hydrate of magnesium and aluminium would be removed from the surface of magnesium alloy. For example, after attacked by the ultrasonic cavitation, the blocklike oxide in the rectangle region in Fig. 6(b) may be brushed off and the surface of the rectangle region could turn into that of the area A in Fig. 6(d). In the circle region of Fig. 6(b), the protrusive surface can be observed. When the protrusive surface in the circle region is in close proximity to the bubble, it is unstable to collapse symmetrically and microjets form [25]. These powerful microjets act like microscopic ‘bombs’, and they explode the surface of the circle region of Fig. 6(b). Then, pieces of oxide materials are likely to be removed and pitting and erosion can be formed on the surface, as shown in Fig. 6(c). The circle region surface of Fig. 6(b) which was attacked by bubble implosions could be changed into the pit like the area B in Fig. 6(d). Based on the analysis above, weight loss from the AZ91D magnesium alloy immersed in aluminium nitrate solution with ultrasound may be caused by the cavitation damage of ultrasound. The weight changes of the AZ91D magnesium alloy sample before and after being immersed in aluminium nitrate solution with or without ultrasound are shown in Fig. 7. It can be seen that the weight of the AZ91D magnesium alloys both decreased after immersed in aluminium nitrate solution whether with or without ultrasound. But, with the use of ultrasound, the weight loss was dramatically increased, which proved the
cavitation damage of ultrasound to the surface of the AZ91D magnesium alloy. For fine explaining the cavitation damage of ultrasound to the AZ91D magnesium alloy immersed in aluminium nitrate solution, we measured the weight of the AZ91D magnesium alloys samples in each step during the immersion & ultrasound pretreatment. The weight loss of the samples after each step was shown in Fig. 8. In the first and third steps, both with ultrasound applied, the weight loss was much larger than that of the other two steps. It can be deduced that the weight loss is mainly owed to the period of the use of ultrasound. That is to say, the ultrasonic cavitation induced the large weight loss of the AZ91D magnesium alloy when it was immersed in aluminium nitrate solution. The implosion of the bubbles can also cause enhanced agitation and higher temperature of the liquid. The enhanced agitation effect produced by ultrasound allows the surface of the AZ91D magnesium alloy to be replenished with fresh aluminium nitrate solution more quickly than that only immersed in aluminium nitrate solution without ultrasound. Consequently, the weight of the AZ91D magnesium alloys may be lessened more heavily than that without ultrasound. The increased temperature of the liquid caused by ultrasonic cavitation could also contribute to the weight loss of the AZ91D magnesium alloy which was immersed in aluminium nitrate solution with ultrasound. So, the test which measured the weight reduction caused by the solution temperature was carried out. After the immersion & ultrasound pretreatment finished, the temperature of the aluminium nitrate solution was about 42 °C. The weight loss of the AZ91D magnesium alloy immersed in 0.1 mol/L Al (NO3)3 solution without ultrasound at about 50 °C for 30 min was 0.47–0.54 mg/cm2. As shown in Fig. 7, the weight loss of magnesium alloy immersed in the same solution at room temperature (15 °C) for 30 min was 0.03–0.14 mg/cm2. The effect of solution temperature on the weight loss of magnesium
Fig. 8. The weight loss of the AZ91D Mg alloys in each step during the immersion & ultrasound pretreatment; U-10 min: immersed in aluminium nitrate solution with ultrasound for 10 min; I-10 min: only immersed in the same solution after U-10 min; U-5 min: immersed in the same solution with ultrasound after I-10 min; I-5 min: only immersed in the same solution after U5 min.
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alloys was obvious. The higher the solution temperature, the larger the weight loss is. We also know from Fig. 7 that the weight loss of magnesium alloys immersed in aluminium nitrate solution for 30 min with ultrasound was 1.79–2.91 mg/cm2 which was much larger than that at about 50 °C (0.47–0.54 mg/ cm2). As a result, the higher solution temperature caused by the implosion of the bubbles with ultrasound may be one factor influencing the weight loss of the magnesium alloys immersed in aluminium nitrate solution. Therefore, the enhanced agitation and higher temperature of the solution caused by the implosion of the bubbles partly contribute to the weight loss of the AZ91D magnesium alloy. As mentioned above, the effects of ultrasonic pretreatment in the aluminium nitrate solution on the AZ91D magnesium alloys are generally characterized by cavitation and are accompanied by a local increase in pressure and temperature and a strong mechanical effect such as the enhanced agitation of the liquid. The cavitation, pressure, local high temperature and enhanced agitation made the surface of AZ91D magnesium alloy more compact and uniform except for some pits, which produced a clear and uniform surface for the subsequent anodic oxidation process. Therefore, the anodic film of ultrasonic immersed magnesium alloy surface was more uniform and the micropores which caused by anodizing sparks were small, although there still were some pits which were smaller than that before anodizing. If the anodic time be prolonged, the pits caused by immersion & ultrasound pretreatment may be filled in. After pretreated in aluminium nitrate solution with or without ultrasound, the anodic films of AZ91D magnesium alloys contained more aluminium owing to that the aluminium in the pretreatment layer became incorporated into the anodic film. The process that the aluminium became incorporated into the anodic film could be described as follows. Firstly, the sol particles and the SiO32 − anion in the electrolyte were adsorbed on the pretreatment layer surface at the beginning of the oxidation process. The adsorption could be chemical adsorption or physical adsorption. The composite barrier layers were formed in the surface of magnesium alloys. Secondly, the anodic voltage increased and the sparks appeared on the surface. The composite layers were destroyed by the high energy generated by the sparking during the oxidation process, and the products were formed with the anodic reaction. 4. Conclusions 1) After the AZ91D magnesium alloy was pretreated in 0.1 mol/L aluminium nitrate solution with or without ultrasound, especially with ultrasound, the duration of intensive sparks on the Mg alloy surface was longer during the anodizing process and the uniformity of the anodic oxidation film was improved. The micro-pores on the film
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surface are decreased in quantity and size and the thickness of the anodic film was reduced. But some pits appeared in the surface of the magnesium alloy because of the ultrasound treatment, and these pits, as though being partly filled, still existed after the subsequent anodizing. 2) The pretreatment in aluminium nitrate solution with or without ultrasound resulted in the increase of Al both in the anodic film and in the pretreated surface of AZ91D magnesium alloy. The content of Al increased more with the use of ultrasound in the pretreatment. 3) The anodic film formed on the AZ91D Mg alloy pretreated in aluminium nitrate solution with or without ultrasound improved the corrosion resistance of AZ91D Mg alloy substrate more significantly than that without pretreatment in aluminium nitrate solution. The corrosion resistance with ultrasound was better than that without ultrasound in the pretreatment process. References [1] G.L. Song, A. Atrens, Adv. Eng. Mater. 5 (2003) 837. [2] R. Ambat, N.N. Aung, W. Zhou, Corros. Sci. 42 (2000) 1433. [3] G.L. Song, B. Johannesson, S.H. David StJohn, Corros. Sci. 46 (2004) 955. [4] J.E. Gray, B. Luan, J. Alloys Comp. 336 (2002) 88. [5] C. Blawert, W. Dietzel, E. Ghali, G.L. Song, Adv. Eng. Mater. 8 (2006) 511. [6] Y.H. Liu, Z.X. Guo, Y. Yang, H.Y. Wang, J.D. Hu, Y.X. Li, A.N. Chumakov, N.A. Bosak, Appl. Surf. Sci. 253 (2006) 1722. [7] Z.M. Liu, W. Gao, Surf. Coat. Technol. 200 (2006) 5087. [8] K.H. Yang, M.D. Ger, W.H. Hwu, Y. Sung, Y.C. Liu, Mater. Chem. Phys. 101 (2007) 480. [9] S.X. Yu, Q.Y. Lu, J. Han, Z.W. Zhang, Q.Y. Zhang, J. Rare Earths 24 (2006) 397. [10] A.N. Khramov, V.N. Balbyshev, L.S. Kasten, R.A. Mantz, Thin Solid Films 514 (2006) 174. [11] M. Dabala, K. Brunelli, E. Napolitani, M. Magrini, Surf. Coat. Technol. 172 (2003) 227. [12] H.Y. Hsiao, P. Chung, W.T. Tsai, Corros. Sci. 49 (2007) 781. [13] Z.M. Shi, G.L. Song, A. Atrens, Corros. Sci. 48 (2006) 1939. [14] W.P. Li, L.Q. Zhu, H.C. Liu, Surf. Coat. Technol. 201 (2006) 2505. [15] C. Blawert, V. Heitmann, W. Dietzel, H.M. Nykyforchyn, M.D. Klapkiv, Surf. Coat. Technol. 201 (2007) 8709. [16] H.F. Guo, M.Z. An, S. Xu, H.B. Huo, Thin Solid Films 485 (2005) 53. [17] H.Y. Hsiao, H.C. Tsung, W.T. Tsai, Surf. Coat. Technol. 199 (2005) 127. [18] Y. Mizutani, S.J. Kim, R. Ichino, M. Okido, Surf. Coat. Technol. 169–170 (2003) 143. [19] J. Liang, B.G. Guo, J. Tian, H.W. Liu, J.F. Zhou, W.M. Liu, T. Xu, Surf. Coat. Technol. 199 (2005) 121. [20] H.Y. Hsiao, W.T. Tsai, Surf. Coat. Technol. 190 (2005) 299. [21] G. Cravotto, S. Tagliapietra, B. Robaldo, M. Trotta, Ultrason. Sonochem. 12 (2005) 95. [22] M. Aurousseau, N.T. Pham, P. Ozil, Ultrason. Sonochem. 11 (2004) 23. [23] W.P. Li, L.Q. Zhu, H.C. Liu, Surf. Coat. Technol. 201 (2006) 2573. [24] A. Al-hashem, W. Riad, Mater. Charact. 48 (2002) 37. [25] X.W. Guo, W.J. Ding, C. Lu, C.Q. Zhai, Surf. Coat. Technol. 183 (2004) 359.