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Growth characterization of anodic film on AZ91D magnesium alloy in an electrolyte of Na2SiO3 and KF
Journal of University of Science and Technology Beijing Volume 13, Number 5, October 2006, Page 450
Materials
Growth characterization of anodic film on AZ91D magnesium alloy in an electrolyte of Na,SiO, and KF Weiping Li’), Liqun Zhu”, Ehong Li’), and Bo Zhao2) I ) Materials Science and Engineering School, Beihang University, Beijing 100083, China 2) Beijing Special Vehicles Research Institute, Beijing 100072, China (Received 2005- 10-26)
Abstract: Anodization of AZ91D magnesium alloy in the electrolyte solution of 0.5 mom of sodium silicate and 1.0 mom of potassium fluoride was investigated. The anodic films were characterized using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The corrosion resistance of the various anodized alloys was evaluated by a fast corrosion test using the solution of hydrochloric acid and potassium dichromate. The results showed that the addition of KF resulted in the presence of NaF in the anodic film. The thickness of the anodic film formed under a constant current density of 20 mA/cm2 for 16 min at 60°C exceeded 100 pm. The growth of the anodic film could be divided into three stages based on the anodizing time; the growth rate was much faster during stage I1 than in stages I and III. The anodic film exhibited the highest corrosion resistance for the AZ91 alloy, which is attributed to the fact that the anodization was maintained until the end of stage IT. Key words: magnesium alloy; anodic film; growth characterization; chromate-free anodizing
[The work was financially supported by the National Natural Science Foundation of China (Nos0541003)and the Aeronautic Science Foundation of China (No.04H51002).]
1. Introduction Anodization is one of the surface treatments for Mg and its alloys to improve their corrosion resistance and to enhance the adhesiveness for organic coatings [l-61. The development of the bow17 processes promoted the commercial applications of anodizing treatment for magnesium alloys. However, in view of low environmental impact, it is significant to develop a chromatefree anodizing process. Anodic behaviors of Mg and Mg alloys depend on the process parameters employed, the chemical composition of the materials anodized, and the electrolytes used. The effects of electrolyte composition on the anodizing process have been widely investigated [7121. For example, H.Y. Hsiao et al. [9-101 have investigated the effects of Al(NO,), and its concentration on anodizing die-cast AZ91D Mg alloy in 3 molL KOH+0.21 molL Na,PO,+O.6 m o m KF electrolyte. They found that the addition of Al(N0J3 into the base electrolyte resulted in the formation of A1,0, and Al(OH), in the anodic film. 0. Khaselev et al. [l 11 have studied the effects of aluminate addition on anodizing Mg-A1 alloys in KOH electrolyte. They reported Corresponding authnr: Weiping Li, E-mail: [email protected],edu.cn
that anodic films were formed on magnesium and MgA1 alloys under continuous sparking conditions in alkaline solutions containing aluminate. H. Fukuda et al. [ 121 have studied the effects of the addition of Na,SiO, in 3 m o m KOH solution on the anodization of the Mg-Al-Zn alloy. They found that a little silicon was present as Mg2Si04in the anodic film. In this study, the anodizing process in a simple chromate-free electrolyte of 0.5 molL Na,SiO, with and without KF was investigated. The surface and cross-sectional morphologies and the structure and composition of the anodic films on the AZ91D Mg alloy were examined. Especially, the growth characteristics of the anodic films are discussed in detail.
2. Experimental 2.1. Materials Die-cast AZ91D Mg alloy (9.1 wt% Al, 0.5 wt% Zn) was employed for this study. Samples of AZ91D were cut to a size of 20 mmx20 mmx5 mm. All the samples were mounted using neoprene with one exposed surface. Before anodization, the exposed surface was polished with alumina paper and was activated in 35-40
W.P. Li et al., Growth characterization of anodic film on AZ91D magnesium alloy.. .
g/L NaF at 5 3 5 ° C for 5-10 min. The electrical connection was provided using screws into tapped holes.
2.2. Anodizing process The base anodizing electrolyte was 0.5 m o m Na,SiO, with and without 1 .0 m o m KF, respectively. A stainless steel plate was used as the cathode. A constant current density of 20 mA/cm2 was provided by a DH1716A-13 power supply. The treatment time was varied in the range from 1 to 16 min, and the electrolyte temperature was maintained at 60°C.
451
were observed moving across the sample surface, which exhibited the potential behavior as oscillating with rather high amplitudes (after point B in Fig. l(b)). However, in the electrolyte without KF, the position of point B was not obvious, and the potential oscillated with lower amplitudes for the whole process.
2.3. Evaluation of the anodic film The surface morphology and thickness of the anodic film were observed using scanning electron microscopy (SEM) and optical microscopy (OM). The elemental composition of the anodic film was studied using energy dispersion spectrometry (EDS). The phase composition of the anodized sample was determined by X-ray diffraction (XRD). The corrosion resistance of the anodic film was evaluated using a fast corrosion test. A drop of solution composed of 250 mL/L HC1 and 3 g/L K2Cr20, was dripped on the surface of the sample. The time at which the color of the solution transformed from orange to green was recorded for evaluating the corrosion resistance of the anodic film.
3. Results and discussion Fig. 1 shows the variation of potential with time during the anodizing treatment under the condition of 20 mA/cm2 and 60°C in the electrolyte of 0.5 mol/L Na,SiO, with and without 1.O m o m KF respectively. In the electrolyte without m, the potential increased linearly until nearly 85 V, namely the breakdown potential, beyond which tiny sparks were observed and the potential starts oscillating. However, with addition of 1.0 m o m of KF to the electrolyte, the breakdown potential decreased to about 55 V. Moreover, in the electrolyte with KF, the potential behaviors during anodization could be described as follows. First, the potential increased linearly during the first several seconds until it reached the breakdown potential (point A in Fig. l(b)). The potential then started to oscillate with low amplitudes (between points A and B in Fig. l(b)). Finally, the potential oscillated with rather high amplitudes (after point B in Fig. l(b)). It is noticeable that the potential oscillated slightly for a period of time (between points A and B in Fig. l(b)). In this stage, many tiny sparks were observed over the whole surface of the sample, and this process continued for about 2 min. Later, a few large sparks
5051
(b) 0 5 molLNar%O,+l OmoliLKI
J.
Time I s Fig. 1. Potential transient during anodization under the conditions of 20 mA/cm2 and 6OoCin different electrolytes.
Fig. 2 shows the surface micrographs of the anodic film on AZ91D formed under the condition of 20 mA/cm2 and 60°C for 6 min in the electrolyte of 0.5 moVL Na,SiO, with and without 1.O moVL KF, respectively. The presence of KF led to a significant change in the macro- and microsurface morphologies of the anodic film. The film formed in KF-free electrolyte was gray with an uneven macrosurface, but in the micrograph, the surface was uniform with many small holes (Fig. 2(a)). However, with the addition of 1.O m o m KF to the electrolyte, the anodic film became considerably more nonuniform in the micrograph (Fig. 2(b)) but showed an even macrosurface with silver white color. Table 1 shows the results of the quantitative analysis of the anodic film surface formed in the electrolyte of 0.5 mol/L Na2Si0, with and without 1.0 m o m KF, respectively. It could be seen that the addition of KF resulted in the decrease of the elements 0,Mg, and Si and in the presence of F and K in the film. The XRD patterns of as-cast and as-anodized AZ91D Mg alloy are shown in Fig. 3. The sample anodized in the KF-free electrolyte consisted of a solid phase a and an intermetallic compound phase y (MgI7A1,,) (Fig. 3(a)), which was similar to the as-cast substrate. The XRD patterns showed that a-Mg and NaF were the main constituents of the sample anodized in 0.5 m o m Na,SiO,+l.O moVL KF solution (Fig. 3(b)). The results indicated that the addition of KF into the electrolyte of Na2Si0, resulted in the presence of NaF. However, no compound of Si was detected using XRD although there was more than 20wt% Si in the anodic film formed in the electrolyte of 0.5 mol/L Na2Si03either with or without KF. Comparing the XRD patterns of the anodized AZ91D with that of the
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as-cast AZ91D, it was obvious that the background of the XRD patterns of the as-anodized sample had considerably more intensity than that of the as-cast Mg al-
loy. Many experiments are worthy to be performed to testify the existing state of silicon in the anodic film.
Fig. 2. Surface micrographs of the anodic films on AZ91D formed under the condition of 20 mA/cm2 and 6OoC for 6 min in different electrolytes: (a) 0.5 mom Na,SiO,; (b) 0.5 m o m Na,SiO,+l.O m o m KF. Table 1. Quantitative analysis of the anodic film formed in different electrolytes
at%
32.1
at% 44.0
wt%
0.5 moILNqSi0,
-
-
wt% 10.7
at% 10.0
Mg wt% at% 22.3 19.8
0.5 moVL NqSiO,+ 1.0 mom KF
25.4
35.4
11.1
13.1
14.5
14.1
12.3
0
Electrolyte
wt%
Na
F
K
Si wt% 34.2
at%
wt%
at%
26.2
-
-
11.3
20.9
16.8
15.8
9.1
each stage, the growth of the film included two steps with different growth rates, which were described as the rapid increase step (AB, CD and EF in Fig. 4) and the smooth spread step (BC, DE and FG in Fig.4).
10
20
30
40
50
60
70
80
20/(7 Fig. 3. XRD of anodic films on AZ91D formed under the conditions of 20 mA/cmZ and 6OoC for 6 min in different electrolytes: (a) 0.5 m o m Na,SiO,; (b) 0.5 m o m Na,SiO,+l.O m o m KF.
Generally, the constant current can ensure that anodic films grow at an almost uniform rate [l]. However, in this investigation, the anodic films grew with varied rates in different periods. Fig. 4 shows the average thickness of the anodic films formed as a function of the anodizing time in an electrolyte of 0.5 m o m Na2Si0,+ 1.O m o m KF at 60°C under 20 mA/cm2. According to the potential and sparking behavior during anodization, the growth of the anodic film was divided into three stages described as (I), (II), (111) in Fig. 4. In
Time / min Fig. 4. Average thickness of anodic films formed in the electrolyte of 0.5 m o m Na,SiO,+l.O m o m KF under the condition of 20 mA/cm2and 6OoCfor varied anodizing time from 1 to 16 min.
Fig. 5 shows the surface and cross-sectional micrographs of the AZ9lD Mg alloy after anodization in an electrolyte of 0.5 m o m Na,SiO,+l.O mom KF for 4,8 and 12 min, which could exhibit the morphology of the anodic film formed in stages I, 11, and 111, respectively. The surface micrographs showed that the morphology became increasingly nonuniform with the increase in treatment time. By comparing the film shown
W.P. Li et al., Growth characterizationof anodic film on AZ91D magnesium alloy...
in Fig. 5(b) with that shown in Fig. 5(d), which were formed in stages I and 11, respectively, it can be seen that the anodic film thickness increased rapidly from
453
about 20 to nearly 80 pm. The growth characteristics of the anodic film could be described by combining it with the morphology of each stage as follows.
Fig. 5. Surface ((a), (c), and (e)) and cross-section micrographs ((b), (d), and (0)of AZ91D Mg alloy after anodization in the electrolyte of 0.5 moVL Na,SiO,+l.O m o m KF for different times: (a) and (h) 4 min; (c) and (d) 8 min; (e) and (0 12 min.
First, the anodic film was grown in two steps in each stage, and the growth rate of the first step was higher than that of the second step. It can be considered that a new anodic film was formed in the first step, which became increasingly compact in the second step. For example, in stage I, after the potential reached the breakdown potential, many tiny sparks occurred over the whole surface at the same time, which resulted in the potential oscillation with low amplitudes, as shown in Fig. l(b) between points A and B; this process continued for about 2 min. Then, a very thin film was formed, which can be regarded as the first step of this stage. The film formed in this step was thin and incompact, and thus it could not cover the substrate compactly. Even though the film was not compact, it was difficult to generate many sparks at the same time, and
hence, the sparks became fewer and the potential 0scillated with higher amplitudes (after point B in Fig. 1 (b)), which can be regarded as the second step of stage I until the whole surface was covered by a thin compact film (shown in Figs. 5(a) and 5(b)). Second, the growth rate was much higher during stage I1 than in stages I and 111. During anodization, the sparking can establish a condition similar to that of sintering and promote the formation of the anodic film. As discussed above, a thin compact film was formed during stage I. The anodic film in stage I1 grew by sparlung, and it should generated by breaking the film formed in stage I. Although the film formed in stage I was relatively thin compared with the film formed in stage 11, it was about 20 pm and was not easy to break. It was difficult to generate many sparks at the same
J. Univ. Sci. Technol. Beijing, Vo1.13,NOS, Oct 2006
454
time. Thus, the sparks in stage I1 could be described as a large spark accompanied with a few tiny sparks moving across the sample surface. Once a large spark had occurred, and the film was broken at one place, a considerably lower potential was needed to keep the film growing in such a position. On the potential transient curve, the potential exhibited much higher amplitudes. The impact of the large spark was so intense that the impact could be compared to that of a bomb blast that formed many craters as shown in Fig. 5(c), which were in contrast to the numerous holes formed in stage I as shown in Fig. 5(a). This kind of surface morphology resulted in the rapid increase in average thickness of the film. Thus, the growth rate in stage I1 was relatively higher than that in stage I. In fact, after stage 11, the thickness of the film nearly reached 80 pm (shown in Fig. 5(d) and point E in Fig. 4). In this manner, the thickness should increase even faster in stage I11 than in stage 11; however, the film growth rate decreased in stage I11 (as shown in Fig. 4). In fact, the growth of the anodic film became increasingly difficult after stage I1 because the film was too thick to break. In addition, the anodic film formed after stage I1 became increasingly uneven with many macroholes on the surface that could be observed even visually. Hence, it could be considered that the film formed in stage 111 was devastated. Fig. 6 shows the corrosion resistance of the anodic films formed for different anodizing time periods. The film formed in less than 5 min, namely in stage I, was too thin to provide adequate protection for the substrate. The anodic film exhibited the highest corrosion resistance for the AZ91 alloy by maintaining the anodizing process until the end of stage 11, the anodizing time of stage I1 being about 10 min (point E in Fig. 4); the corrosion resistance of the anodic film decreased obviously when the anodizing time exceeded 10 min (in 1700
.A m
2
4
6
8
10
12-14
16
Time I min
Fig. 6. Average thickness (a) and corrosion resistance in fast corrosion test (b) of the anodic films formed in the electrolyte of 0.5 mom Na,SiO,+l.O m o m KF under the condition of 20 mA/cm2and 6OoC for varied anodizing time.
stage 111), although the film thickness formed in this stage was beyond 100 pm. Such results were consistent with the growth characterization of the anodic film discussed above.
4. Conclusions (1) During anodization of the AZ91D Mg alloy in the electrolyte solution of 0.5 m o m Na,SiO,, the addition of 1.0 m o m KF resulted in the decrease of the breakdown potential and the presence of element F in the film. (2) a-Mg and NaF were the main phase constituents of the AZ91D Mg alloy anodized in 0.5 molL Na,SiO,+l .O m o m KF solution. However, the sample anodized in the electrolyte of 0.5 m o m Na,SiO, as well as the as-cast AZ91D consisted of the a-Mg and y phase (Mg17A1,z).
( 3 ) The anodic film formed in 0.5 m o m Na,SiO,+ 1.0 m o m KF solution showed a varied growth rate in different periods during anodization, which could be divided into three stages. The growth rate was much faster during stage I1 than in stages I and 111. The anodic film exhibited the highest corrosion resistance for the AZ91 alloy by maintaining anodization until the end of stage 11.
References J.E. Gray and B. Luan, Protective coatings on magnesium and its alloys-a critical review, J. Alloys Compd., 336(2002), p.88. Y.J. Zhang, C.W. Yan, F.H. Wang, et al., Study on the environmentally friendly anodizing of AZ91D magnesium alloy, Surf Coat. Technol., 161(2002), p.36. Y.J. Zhang, C.W. Yan, F.H. Wang, et al., Electrochemical behavior of anodized Mg alloy AZ91D in chloride containing aqueous solution, Corros. Sci., 47(2005), N0.11, p.2816. L.H. Chiu, C.C. Chen, and C.F. Yang, Improvement of corrosion properties in an aluminum-sprayed AZ3 1 magnesium alloy by a post-hot pressing and anodizing treatment, Surf Coat. Technol., 1 9 1(2OO5),p. 181. X.W. Guo, W.J. Ding, C. Lu, et al., Influence of ultrasonic power on the structure and composition of anodizing coatings formed on Mg alloys, Surf Coat. Technol., 183(2004), p.359. M. Abulsain, A. Berkani, F.A. Bonilla, et al., Anodic oxidation of Mg-Cu and Mg-Zn alloys, Electrochim. Acta, 49(2004), p.899. S . Ono and N. Masuko, Anodic films grown on magnesium and magnesium alloys in fluoride solution, Muter: Sci. Forum, 419-422(2003), p.897. Y. Mizutani, S.J. Kim, R. Ichino, et al., Anodizing of Mg alloys in alkaline solutions, Surf Coat. Technol., 169170(2003), p.143. H.Y. Hsiao, H.CH. Tsung, and W.T. Tsai, Anodization of
W.P. Li et al., Growth characterizationof anodic film on AZ91D magnesium alloy.. . AZ91D magnesium alloy in silicate-containing electrolytes, Sui$ Coat. Technol., 199(2005), No.(2-3), p.127. [101 H.Y. Hsiao and W.T. Tsai, Characterization of anodic films formed on AZ91D magnesium alloy, Sui$ Coat. Technol., 190(2005), p.299. [11] 0. Khaselev, D. Weiss, and J. Yahalom, Structure and com-
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position of anodic films formed on binary Mg-A1 alloys in KOH-aluminate solutions under continuous sparking, Corros. Sci., 43(2001), No.7, p.1295. [ 121 H. Fukuda and Y. Matsumoto, Effects of Na,SiO, on anodization of Mg-Al-Zn alloy in 3 M KOH solution, Corros. Sci., 46(2004), No.9, p.2135.
Materials
Growth characterization of anodic film on AZ91D magnesium alloy in an electrolyte of Na,SiO, and KF Weiping Li’), Liqun Zhu”, Ehong Li’), and Bo Zhao2) I ) Materials Science and Engineering School, Beihang University, Beijing 100083, China 2) Beijing Special Vehicles Research Institute, Beijing 100072, China (Received 2005- 10-26)
Abstract: Anodization of AZ91D magnesium alloy in the electrolyte solution of 0.5 mom of sodium silicate and 1.0 mom of potassium fluoride was investigated. The anodic films were characterized using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The corrosion resistance of the various anodized alloys was evaluated by a fast corrosion test using the solution of hydrochloric acid and potassium dichromate. The results showed that the addition of KF resulted in the presence of NaF in the anodic film. The thickness of the anodic film formed under a constant current density of 20 mA/cm2 for 16 min at 60°C exceeded 100 pm. The growth of the anodic film could be divided into three stages based on the anodizing time; the growth rate was much faster during stage I1 than in stages I and III. The anodic film exhibited the highest corrosion resistance for the AZ91 alloy, which is attributed to the fact that the anodization was maintained until the end of stage IT. Key words: magnesium alloy; anodic film; growth characterization; chromate-free anodizing
[The work was financially supported by the National Natural Science Foundation of China (Nos0541003)and the Aeronautic Science Foundation of China (No.04H51002).]
1. Introduction Anodization is one of the surface treatments for Mg and its alloys to improve their corrosion resistance and to enhance the adhesiveness for organic coatings [l-61. The development of the bow17 processes promoted the commercial applications of anodizing treatment for magnesium alloys. However, in view of low environmental impact, it is significant to develop a chromatefree anodizing process. Anodic behaviors of Mg and Mg alloys depend on the process parameters employed, the chemical composition of the materials anodized, and the electrolytes used. The effects of electrolyte composition on the anodizing process have been widely investigated [7121. For example, H.Y. Hsiao et al. [9-101 have investigated the effects of Al(NO,), and its concentration on anodizing die-cast AZ91D Mg alloy in 3 molL KOH+0.21 molL Na,PO,+O.6 m o m KF electrolyte. They found that the addition of Al(N0J3 into the base electrolyte resulted in the formation of A1,0, and Al(OH), in the anodic film. 0. Khaselev et al. [l 11 have studied the effects of aluminate addition on anodizing Mg-A1 alloys in KOH electrolyte. They reported Corresponding authnr: Weiping Li, E-mail: [email protected],edu.cn
that anodic films were formed on magnesium and MgA1 alloys under continuous sparking conditions in alkaline solutions containing aluminate. H. Fukuda et al. [ 121 have studied the effects of the addition of Na,SiO, in 3 m o m KOH solution on the anodization of the Mg-Al-Zn alloy. They found that a little silicon was present as Mg2Si04in the anodic film. In this study, the anodizing process in a simple chromate-free electrolyte of 0.5 molL Na,SiO, with and without KF was investigated. The surface and cross-sectional morphologies and the structure and composition of the anodic films on the AZ91D Mg alloy were examined. Especially, the growth characteristics of the anodic films are discussed in detail.
2. Experimental 2.1. Materials Die-cast AZ91D Mg alloy (9.1 wt% Al, 0.5 wt% Zn) was employed for this study. Samples of AZ91D were cut to a size of 20 mmx20 mmx5 mm. All the samples were mounted using neoprene with one exposed surface. Before anodization, the exposed surface was polished with alumina paper and was activated in 35-40
W.P. Li et al., Growth characterization of anodic film on AZ91D magnesium alloy.. .
g/L NaF at 5 3 5 ° C for 5-10 min. The electrical connection was provided using screws into tapped holes.
2.2. Anodizing process The base anodizing electrolyte was 0.5 m o m Na,SiO, with and without 1 .0 m o m KF, respectively. A stainless steel plate was used as the cathode. A constant current density of 20 mA/cm2 was provided by a DH1716A-13 power supply. The treatment time was varied in the range from 1 to 16 min, and the electrolyte temperature was maintained at 60°C.
451
were observed moving across the sample surface, which exhibited the potential behavior as oscillating with rather high amplitudes (after point B in Fig. l(b)). However, in the electrolyte without KF, the position of point B was not obvious, and the potential oscillated with lower amplitudes for the whole process.
2.3. Evaluation of the anodic film The surface morphology and thickness of the anodic film were observed using scanning electron microscopy (SEM) and optical microscopy (OM). The elemental composition of the anodic film was studied using energy dispersion spectrometry (EDS). The phase composition of the anodized sample was determined by X-ray diffraction (XRD). The corrosion resistance of the anodic film was evaluated using a fast corrosion test. A drop of solution composed of 250 mL/L HC1 and 3 g/L K2Cr20, was dripped on the surface of the sample. The time at which the color of the solution transformed from orange to green was recorded for evaluating the corrosion resistance of the anodic film.
3. Results and discussion Fig. 1 shows the variation of potential with time during the anodizing treatment under the condition of 20 mA/cm2 and 60°C in the electrolyte of 0.5 mol/L Na,SiO, with and without 1.O m o m KF respectively. In the electrolyte without m, the potential increased linearly until nearly 85 V, namely the breakdown potential, beyond which tiny sparks were observed and the potential starts oscillating. However, with addition of 1.0 m o m of KF to the electrolyte, the breakdown potential decreased to about 55 V. Moreover, in the electrolyte with KF, the potential behaviors during anodization could be described as follows. First, the potential increased linearly during the first several seconds until it reached the breakdown potential (point A in Fig. l(b)). The potential then started to oscillate with low amplitudes (between points A and B in Fig. l(b)). Finally, the potential oscillated with rather high amplitudes (after point B in Fig. l(b)). It is noticeable that the potential oscillated slightly for a period of time (between points A and B in Fig. l(b)). In this stage, many tiny sparks were observed over the whole surface of the sample, and this process continued for about 2 min. Later, a few large sparks
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(b) 0 5 molLNar%O,+l OmoliLKI
J.
Time I s Fig. 1. Potential transient during anodization under the conditions of 20 mA/cm2 and 6OoCin different electrolytes.
Fig. 2 shows the surface micrographs of the anodic film on AZ91D formed under the condition of 20 mA/cm2 and 60°C for 6 min in the electrolyte of 0.5 moVL Na,SiO, with and without 1.O moVL KF, respectively. The presence of KF led to a significant change in the macro- and microsurface morphologies of the anodic film. The film formed in KF-free electrolyte was gray with an uneven macrosurface, but in the micrograph, the surface was uniform with many small holes (Fig. 2(a)). However, with the addition of 1.O m o m KF to the electrolyte, the anodic film became considerably more nonuniform in the micrograph (Fig. 2(b)) but showed an even macrosurface with silver white color. Table 1 shows the results of the quantitative analysis of the anodic film surface formed in the electrolyte of 0.5 mol/L Na2Si0, with and without 1.0 m o m KF, respectively. It could be seen that the addition of KF resulted in the decrease of the elements 0,Mg, and Si and in the presence of F and K in the film. The XRD patterns of as-cast and as-anodized AZ91D Mg alloy are shown in Fig. 3. The sample anodized in the KF-free electrolyte consisted of a solid phase a and an intermetallic compound phase y (MgI7A1,,) (Fig. 3(a)), which was similar to the as-cast substrate. The XRD patterns showed that a-Mg and NaF were the main constituents of the sample anodized in 0.5 m o m Na,SiO,+l.O moVL KF solution (Fig. 3(b)). The results indicated that the addition of KF into the electrolyte of Na2Si0, resulted in the presence of NaF. However, no compound of Si was detected using XRD although there was more than 20wt% Si in the anodic film formed in the electrolyte of 0.5 mol/L Na2Si03either with or without KF. Comparing the XRD patterns of the anodized AZ91D with that of the
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J. Univ. Sci. Technol. Beijing, V01.13, No.& Oct 2006
as-cast AZ91D, it was obvious that the background of the XRD patterns of the as-anodized sample had considerably more intensity than that of the as-cast Mg al-
loy. Many experiments are worthy to be performed to testify the existing state of silicon in the anodic film.
Fig. 2. Surface micrographs of the anodic films on AZ91D formed under the condition of 20 mA/cm2 and 6OoC for 6 min in different electrolytes: (a) 0.5 mom Na,SiO,; (b) 0.5 m o m Na,SiO,+l.O m o m KF. Table 1. Quantitative analysis of the anodic film formed in different electrolytes
at%
32.1
at% 44.0
wt%
0.5 moILNqSi0,
-
-
wt% 10.7
at% 10.0
Mg wt% at% 22.3 19.8
0.5 moVL NqSiO,+ 1.0 mom KF
25.4
35.4
11.1
13.1
14.5
14.1
12.3
0
Electrolyte
wt%
Na
F
K
Si wt% 34.2
at%
wt%
at%
26.2
-
-
11.3
20.9
16.8
15.8
9.1
each stage, the growth of the film included two steps with different growth rates, which were described as the rapid increase step (AB, CD and EF in Fig. 4) and the smooth spread step (BC, DE and FG in Fig.4).
10
20
30
40
50
60
70
80
20/(7 Fig. 3. XRD of anodic films on AZ91D formed under the conditions of 20 mA/cmZ and 6OoC for 6 min in different electrolytes: (a) 0.5 m o m Na,SiO,; (b) 0.5 m o m Na,SiO,+l.O m o m KF.
Generally, the constant current can ensure that anodic films grow at an almost uniform rate [l]. However, in this investigation, the anodic films grew with varied rates in different periods. Fig. 4 shows the average thickness of the anodic films formed as a function of the anodizing time in an electrolyte of 0.5 m o m Na2Si0,+ 1.O m o m KF at 60°C under 20 mA/cm2. According to the potential and sparking behavior during anodization, the growth of the anodic film was divided into three stages described as (I), (II), (111) in Fig. 4. In
Time / min Fig. 4. Average thickness of anodic films formed in the electrolyte of 0.5 m o m Na,SiO,+l.O m o m KF under the condition of 20 mA/cm2and 6OoCfor varied anodizing time from 1 to 16 min.
Fig. 5 shows the surface and cross-sectional micrographs of the AZ9lD Mg alloy after anodization in an electrolyte of 0.5 m o m Na,SiO,+l.O mom KF for 4,8 and 12 min, which could exhibit the morphology of the anodic film formed in stages I, 11, and 111, respectively. The surface micrographs showed that the morphology became increasingly nonuniform with the increase in treatment time. By comparing the film shown
W.P. Li et al., Growth characterizationof anodic film on AZ91D magnesium alloy...
in Fig. 5(b) with that shown in Fig. 5(d), which were formed in stages I and 11, respectively, it can be seen that the anodic film thickness increased rapidly from
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about 20 to nearly 80 pm. The growth characteristics of the anodic film could be described by combining it with the morphology of each stage as follows.
Fig. 5. Surface ((a), (c), and (e)) and cross-section micrographs ((b), (d), and (0)of AZ91D Mg alloy after anodization in the electrolyte of 0.5 moVL Na,SiO,+l.O m o m KF for different times: (a) and (h) 4 min; (c) and (d) 8 min; (e) and (0 12 min.
First, the anodic film was grown in two steps in each stage, and the growth rate of the first step was higher than that of the second step. It can be considered that a new anodic film was formed in the first step, which became increasingly compact in the second step. For example, in stage I, after the potential reached the breakdown potential, many tiny sparks occurred over the whole surface at the same time, which resulted in the potential oscillation with low amplitudes, as shown in Fig. l(b) between points A and B; this process continued for about 2 min. Then, a very thin film was formed, which can be regarded as the first step of this stage. The film formed in this step was thin and incompact, and thus it could not cover the substrate compactly. Even though the film was not compact, it was difficult to generate many sparks at the same time, and
hence, the sparks became fewer and the potential 0scillated with higher amplitudes (after point B in Fig. 1 (b)), which can be regarded as the second step of stage I until the whole surface was covered by a thin compact film (shown in Figs. 5(a) and 5(b)). Second, the growth rate was much higher during stage I1 than in stages I and 111. During anodization, the sparking can establish a condition similar to that of sintering and promote the formation of the anodic film. As discussed above, a thin compact film was formed during stage I. The anodic film in stage I1 grew by sparlung, and it should generated by breaking the film formed in stage I. Although the film formed in stage I was relatively thin compared with the film formed in stage 11, it was about 20 pm and was not easy to break. It was difficult to generate many sparks at the same
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time. Thus, the sparks in stage I1 could be described as a large spark accompanied with a few tiny sparks moving across the sample surface. Once a large spark had occurred, and the film was broken at one place, a considerably lower potential was needed to keep the film growing in such a position. On the potential transient curve, the potential exhibited much higher amplitudes. The impact of the large spark was so intense that the impact could be compared to that of a bomb blast that formed many craters as shown in Fig. 5(c), which were in contrast to the numerous holes formed in stage I as shown in Fig. 5(a). This kind of surface morphology resulted in the rapid increase in average thickness of the film. Thus, the growth rate in stage I1 was relatively higher than that in stage I. In fact, after stage 11, the thickness of the film nearly reached 80 pm (shown in Fig. 5(d) and point E in Fig. 4). In this manner, the thickness should increase even faster in stage I11 than in stage 11; however, the film growth rate decreased in stage I11 (as shown in Fig. 4). In fact, the growth of the anodic film became increasingly difficult after stage I1 because the film was too thick to break. In addition, the anodic film formed after stage I1 became increasingly uneven with many macroholes on the surface that could be observed even visually. Hence, it could be considered that the film formed in stage 111 was devastated. Fig. 6 shows the corrosion resistance of the anodic films formed for different anodizing time periods. The film formed in less than 5 min, namely in stage I, was too thin to provide adequate protection for the substrate. The anodic film exhibited the highest corrosion resistance for the AZ91 alloy by maintaining the anodizing process until the end of stage 11, the anodizing time of stage I1 being about 10 min (point E in Fig. 4); the corrosion resistance of the anodic film decreased obviously when the anodizing time exceeded 10 min (in 1700
.A m
2
4
6
8
10
12-14
16
Time I min
Fig. 6. Average thickness (a) and corrosion resistance in fast corrosion test (b) of the anodic films formed in the electrolyte of 0.5 mom Na,SiO,+l.O m o m KF under the condition of 20 mA/cm2and 6OoC for varied anodizing time.
stage 111), although the film thickness formed in this stage was beyond 100 pm. Such results were consistent with the growth characterization of the anodic film discussed above.
4. Conclusions (1) During anodization of the AZ91D Mg alloy in the electrolyte solution of 0.5 m o m Na,SiO,, the addition of 1.0 m o m KF resulted in the decrease of the breakdown potential and the presence of element F in the film. (2) a-Mg and NaF were the main phase constituents of the AZ91D Mg alloy anodized in 0.5 molL Na,SiO,+l .O m o m KF solution. However, the sample anodized in the electrolyte of 0.5 m o m Na,SiO, as well as the as-cast AZ91D consisted of the a-Mg and y phase (Mg17A1,z).
( 3 ) The anodic film formed in 0.5 m o m Na,SiO,+ 1.0 m o m KF solution showed a varied growth rate in different periods during anodization, which could be divided into three stages. The growth rate was much faster during stage I1 than in stages I and 111. The anodic film exhibited the highest corrosion resistance for the AZ91 alloy by maintaining anodization until the end of stage 11.
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