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The anodization of ZK60 magnesium alloy in alkaline solution containing silicate and the corrosion properties of the anodized films
Applied Surface Science 253 (2007) 9387–9394 www.elsevier.com/locate/apsusc
The anodization of ZK60 magnesium alloy in alkaline solution containing silicate and the corrosion properties of the anodized films Wu Hai-lana, Cheng Ying-lianga,*, Li Ling-linga, Chen Zhen-huaa, Wang Hui-mina, Zhang Zhaob a
College of Materials Science and Engineering, Hunan University, Changsha 410082, China Department of Chemistry, Yuquan Campus, Zhejiang University, Hangzhou 310027, China
b
Received 5 April 2007; received in revised form 30 May 2007; accepted 31 May 2007 Available online 19 June 2007
Abstract The anodization of ZK60 magnesium alloy in an alkaline electrolyte of 100 g/l NaOH + 20 g/l Na2B4O710H2O + 50 g/l C6H5Na3O72H2O + 60g/l Na2SiO39H2O was studied in this paper. The corrosion resistance of the anodic films was studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques and the microstructure and composition of films were examined by SEM and XRD. The influence of anodizing time was studied and the results show that the anodizing time of 60 min is suitable for acquiring films with good corrosion resistance. The influence of current density on the corrosion resistance of anodizing films was also studied and the results show that the film anodized at 20 mA/cm2 has the optimum corrosion resistance. The film formed by anodizing in the alkaline solution with optimized parameters show superior corrosion resistance than that formed by the traditional HAE process. The XRD pattern shows that the components of the anodized film consist of MgO and Mg2SiO4. # 2007 Elsevier B.V. All rights reserved. Keywords: ZK60; Anodized film; Corrosion; Alkaline solution; EIS
1. Introduction Magnesium is the lightest structural material, with excellent physical and mechanical properties, such as low density and high specific strength. These properties make magnesium alloys valuable in a number of applications including the automotive industry, computer parts, the aerospace industry and cellular phones where weight reduction is concerned [1–4]. However, magnesium and its alloys are characterized by low corrosion resistance, which has limited their use and the natural oxide layer on magnesium surfaces is very loose and cannot offer an effective resistance to corrosion [5]. Therefore, it is very important to improve the anti-corrosion performances of magnesium alloys in industrial applications. Anodization is one of the effective surface protective treatments for magnesium and its alloys. In contrast to other surface treatments, anodization can produce a relatively thick,
* Corresponding author. Tel.: +86 731 8821648. E-mail address: [email protected] (Y.-l. Cheng). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.05.085
hard, adherent and abrasion-resistance film [6,7]. But the most successful anodization, such as HAE [8] and DOW17 [9], usually contain chromate, fluoride or phosphate which are unfriendly to environment. Recently, a few new environmentally friendly electrolytes have been developed for the anodization of magnesium alloys. Most of solutions are alkaline electrolytes, which contain some additives such as aluminate, tungstate, borate and silicate, etc. [10–12]. All additives, especially aluminate and silicate, play important roles in anodization and affecting the properties of the anodic films. Khaselev [12] found that the addition of Al(OH)3 in KOH electrolyte affected the sparking behavior and the film breakdown potential increased with increasing Al(OH)3 concentration. Hsiao [7] found that the addition of Na2SiO3 into 3 M KOH + 0.21 M Na3PO4 + 0.6 M KF electrolyte resulted in the formation of anodic film on AZ91D magnesium alloy with enhanced polarization resistance. In this investigation, the anodization of ZK60 magnesium alloy in an environmental friendly electrolyte which contains 100 g/l sodium hydroxide + 20 g/l borate + 50 g/l citrate + 60g/l sodium silicate was investigated and the corresponding corrosion
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resistances of the films were evaluated and compared with the traditional anodizing process of HAE. The structure and property of the anodizing film are studied by SEM, XRD, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. 2. Experimental ZK60 Mg alloy with a nominal composition of 5.5 mass% Zn, 0.45 mass% Zr and Mg the balance is chosen in this study. The 5 mm-thick rolled plate was cut into square samples with a dimension of 10 mm 10 mm. All samples were connected with copper wire in one side and then enveloped by epoxy resin with the other surface exposed as the working surface. Before anodizing in the electrolyte, the working surface was polished successively to 1500 grit finish and cleaned in acetone. In the electrolyte cell for anodization, a stainless steel plate was used as the cathode and the sample was the anode. The anodization process was carried out at constant current density on a DYY-6C (600 V, 400 mA) power supplier. The composition of the electrolyte bath is 100 g/l NaOH + 20 g/l Na2B4O710H2O + 50 g/l C6H5Na3O72H2O + 60g/l Na2SiO39H2O, prepared with analytical grade reagent and distilled water. The effect of anodizing time and current density has been studied in this article. Films have been acquired by processing with different parameters and the properties of the films are measured by electrochemical impedance spectroscopy (EIS) tests and potentiodynamic polarization curves. Electrochemical tests are carried out in 3.5% NaCl solution (mass fraction) using a computer-monitored potentiostat CHI660B (CH Instruments, Inc. 3700 Tennison Hill Drive Austin, TX 78738-5012, USA). The set-up is composed of a three-electrode system: a saturated calomel electrode (SCE) is used as a reference electrode; a large platinum sheet is the auxiliary electrode and the sample is the working electrode. The EIS measurements are carried out at open circuit potential (OCP), the amplitude of the perturbative signal is 5 mVand the frequency range is between 0.01 Hz and 100 KHz. After the EIS measurements, the potentiodynamic electrochemical tests are carried out with a scan rate of 0.001 V/s. All of the tests are carried out at room temperature (about 25 8C). Besides the electrochemical tests, other properties of the anodized coating were also measured. The film thicknesses were measured by a coating thickness gauge (type TT260, TIME Group Inc., Beijing, China). The surface morphology of the anodized samples was examined by a SEMCJSM-5610LV scanning electron microscope (SEM) and X-ray diffractometer with Cu Ka radiation (SIMENS D5000).
Fig. 1. Potential transient of the anodizing process at 20 mA/cm2.
the magnified section from 0 min to 2 min) shows that at the initial stage the potential rises at a very high rate, the potential at 5 s reaches 42 V and the potential is 61 V at 10 s; after 10 s, the rising rate of the potential begins to slow down. When the cell potential reaches about 60 V, obvious sparking occurs. The potential corresponding to the occurrence of sparking is designated as breakdown voltage [10]. Once sparking occurs, the rising rate of potential decreases. At this stage, the sparks are numerous, small sized, short-lived, randomly and relatively even distributed on the entire anode surface. About 2 min later, the sparking behaviour changes, the number of the sparks decreases and both the size and lifetime of individual spark increase. The sparks appear only at localized location on anode surface and the potential starts oscillating. About 10 min later, the potential reaches 110 V, after that, it rises much slowly than before, and the fluctuation of potential is more frequent. Due to the fluctuation of the potential, each data point at the late stage of anodization in Fig. 1 is the average of the potential values in a minute recorded by the DYY-6C power supplier. To study the effect of anodizing time on the properties of anodization films, samples with different anodizing time were first prepared with the applied current density of 20 mA/cm2. The film thickness with different anodizing time is measured and the result is shown in Fig. 2. It is shown that the film
3. Results and discussions 3.1. Effect of anodizing time The anodization processes are carried out under the constant current density mode. The transient of the potential during anodization of ZK60 at a constant current density of 20 mA/ cm2 was recorded in Fig. 1. Once circuit is connected correctly and current is supplied, cell potential begins to rise. Fig. 1 (see
Fig. 2. Relationship between film thickness and processing time at 20 mA/cm2.
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thickness goes linearly with anodizing time. This implies that the previous formed film cannot impede the continuous growth of the film. After the thickness measurement, the corrosion resistant properties of these samples were measured by the EIS and potentiodynamic polarization tests. The EIS was measured after about 10 min immersion and the potentiodynamic polarization test was carried out immediately after the EIS measurement. The results of samples with different anodizing time are shown in Figs. 3 and 5, respectively. EIS is a useful technique in the study of corrosion. The corrosion mechanisms can sometimes be estimated by analyzing the measured electrochemical impedance spectrum. The diameter of the capacitive semicircle of a measured Nyquist spectrum is closely related to the corrosion rate [13]. The EIS results in Fig. 3 show that these diagrams are mainly composed of one depressive loops at the high and medium frequency range and some scattered points at low frequency region (below 10 mHz). Although the high and medium region shows one depressive arc, it actually contains two time constants. This can be seen in the Bode phase angle plots in Fig. 3 because the peak in the high and medium frequency range is very broad. And the equivalent circuit in Fig. 4 can be put forward for the EIS of the high and medium frequency part of Fig. 3, where Rs represents the solution resistance, Rf represents the resistance of the anodized film which is related to the micro-pores of the anodized films, the capacitance of the
Fig. 3. EIS of the anodized films with different treatment time at 20 mA/cm2 (a) Nyquist plots, (b) Bode-phase angle plots.
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Fig. 4. Equivalent circuit used to fit the high frequency part of the EIS measurement.
anodized film is replaced by the so called constant phase angle element [CPE, ZCPE = 1/Y0 (jv) n, 0 < n < 1, where ZCPE is the impedance of the constant phase element (V cm2), v is the angular frequency of ac-voltage (rad s 1), Y0 and n are the frequency independent parameters. When n = 1, CPE deserves a pure capacitance. When n = 0, it deserves a pure resistance and an inductance when n = 1]. The presence of CPE has been explained by dispersion effect that can be caused by microscopic roughness of a surface [14,15]. CPE1 in Fig. 4 represents the capacitance of the anodized film; CPE2 is the double layer capacitance between the alloy substrate and the electrolyte solution in the pores. Rct is the charge transfer resistance of the dissolution of magnesium alloy substrate. This equivalent circuit has also been used by Wu et al. [16] in explaining the measured EIS of anodized films. The scattered point in Fig. 2 may be caused by the non-stability of the tested system at very low frequency range; the resistance of the anodized film is very high and the perturbative signal is only 5 mV, thus the response of the system cannot be stable. Fig. 4 is put forward to explain the EIS results and the parameters in Fig. 4 have not been fitted because the corrosion resistance of the anodized films can be directly justified from the radius of the EIS plots. It can be seen from the EIS results in Fig. 3 that when the treatment time is less than 80 min, the radii of the semicircles of the Nyquist plots of the anodized films increase with treatment time. This implies that, when the treatment time is less than 80 min, treatment time is good for the corrosion resistance of the anodized film. When the treatment time reaches 80 min, the radius of the Nyquist plot is less than that of 60 min, indicating that samples with very long treatment time do not have good corrosion resistance too.
Fig. 5. The potentiodynamic polarization tests of the samples with different treatment time.
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Table 1 The fitted result of Icorr in Fig. 5 Treatment time (min)
Icorr (mA/cm2)
20 30 40 50 60 70 80
3.081 10 2.984 10 2.398 10 1.722 10 1.379 10 1.559 10 1.176 10
4 4 4 4 4 4 3
Fig. 5 shows the potentiodyamic polarization tests after the EIS tests and the free corrosion current densities are calculated in Table 1. It is shown in Table 1 that the free corrosion density can be ranked as Icorr (80 min) > Icorr (20 min) > Icorr (30 min) > Icorr (40 min) > Icorr (50 min) > Icorr (70 min) > Icorr (60 min). The free corrosion current density is directly related to the corrosion resistance of the anodized film, the less the free corrosion density, the higher the corrosion resistance of the film is. Then it can be seen that the results of the potentiodynamic tests show nearly the same trends as the EIS results except the samples of 70 min and 80 min, which indicates that the two techniques have slight difference in predicting the corrosion resistance of samples. The EIS is a technique with very small perturbtive signal, the surface condition of the samples will be nearly the same after EIS tests, but the potentiodynamic technique needs large over-potential to polarize the samples, which will result in the changing of the surface condition of the samples. Although the EIS shows the sample with 70 min anodization is more corrosion-resistant, the Icorr (60 min) acquired from polarization tests is smaller than Icorr (70 min). It can be seen in Fig. 5 that both the anodic and the cathodic current density of the sample with 70 min is higher than that of the sample of 60 min, this may imply that the film of 70 min is easier to pitting or cracking under over-potential. The sample of 80 min is even more abnormal; it is shown in Fig. 3 that the radius of the EIS of 80 min is close to that of 60 min and the potentiodynamic test shows that it is the least in corrosion resistance. From the above experimental results, we can know that, although the film thickness increases linearly with treatment time, thicker film does not definitely mean better corrosion resistance. When the processing time is not too long and the film thickness is not too high, the corrosion resistance of film formed on magnesium alloy increase with treatment time. But when the treatment time is too long, not only the energy consumption will increase but also the internal-stress of the film will increase, which will make cracks appear on the film, and the corrosion resistance of the film will become lower than before. It can be concluded that the treatment time of 60min is appropriate for acquiring high corrosion resistance film for ZK60 anodized in this solution at 20 mA/cm2.
Fig. 6. The relationship between potential and time at different current density.
constant current density of 5, 10, 20, 30, 40, 50 mA/cm2, respectively, and the treatment time is chosen as 40 min. The relationship between potential and time during anodizing at different current density is shown in Fig. 6. The behaviour of potential can provide valuable information on the anode. It can reflect directly the course of film growth and it was found that higher potential indicates thicker film while anodizing under the same condition [10]. It is shown in Fig. 6 that breakdown voltage and the terminal voltage of the sample anodized at 5 mA/cm2 and 10 mA/cm2 are lower than that of the samples anodized at other current densities. To learn the growth rate of the film with current density, the film thicknesses at different current densities are also measured and results are shown in Fig. 7. Like the results in Fig. 2, the relationship of the film thickness and current density is linear too. The EIS and potentiodynamic polarization tests of the samples with different anodizing current density are also measured; the results are shown in Figs. 8 and 9, respectively. The EIS features are similar to the EIS behaviour in Fig. 3, the high and medium frequency region in Fig. 8 can be explained by the equivalent circuit of Fig. 4. The Icorr values can be
3.2. Effect of current density To study the effect of current density on the properties of the anodized films, the anodizing processes were carried out at
Fig. 7. The relationship between film thickness and current density.
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Fig. 9. The potentiodynamic polarization tests with different current density.
current density would make the growth of the film too quick, which would result in coarser oxide grain and the internal stress of the anodized film will increase, resulting in micro-cracks on the films. As a result, the corrosion resistance of the anodized films will decrease. The above results indicated that optimum anodizing current density for ZK60 is 20 mA/cm2. 3.3. Morphology and XRD pattern of the anodized film
Fig. 8. EIS of the anodized film with different current density in 3.5 wt.% NaCl solution.
acquired from the potentiodyanmic polarization curves in Fig. 9 and the data are listed in Table 2. It is clearly shown in the EIS results (Fig. 8) and the potentiodynamic polarization tests (Table 2) that the optimum anodizing current density for the anodized film with best corrosion resistance is 20 mA/cm2. When current density is less than 20 mA/cm2, the corrosion resistance of the films increased with current density. This can be explained by the fact that the film growth rate is higher at large current density, the film formed at higher current density with the same anodizing time will be thicker and thus is more protective. However, when the current density exceeds 20 mA/cm2, the corrosion resistance of the anodized films begins to decrease with current density. It can be concluded that although high current density can increase the growth rate of the film and shorten the time to an optimum film thickness, there exists a contrary effect. High Table 2 The fitted result of Icorr in Fig. 9 Current density (mA/cm2)
Icorr (mA/cm2)
5 10 20 30 40 50
1.829 10 1.259 10 2.398 10 2.828 10 4.137 10 5.257 10
2 3 4 3 3 3
Surface and cross-sectional morphology of the anodized specimens were examined. Fig. 10 gives the SEM micrographs for the surface morphologies of ZK60 Mg alloy anodized for different time when 20 mA/cm2 constant current density was applied and Fig. 11 shows the cross-sectional morphologies of the anodized film with different processing time. In Fig. 11b–d, the substrates were etched by a mixture of 5 g picric acid + 5 ml glacial acetic acid + 10 ml distilled water + 100 ml ethanol to reveal the grain boundaries of the substrate alloy. In Fig. 10a, the SEM micrograph was taken when the sample was anodized for 30 s, in this situation, the potential just reached the breakdown potential, and many tiny sparks were just observed on the surface. As a result, the film was only formed on some localized locations on the surface; most of the surface of the alloy is still exposed. Fig. 10b is the morphology of the sample with anodizing time of 1 min, at this time, the potential rose to about 73 V, and the tiny sparks changed into big sparks whose life-span were longer than before, and the big sparks wandered everywhere all the time. It is shown in Fig. 10b that the surface is nearly covered by a porous film except for some small part, but the film was so thin that the grinding scratches formed during sample preparation were still seen. After 2 min anodizing, the surface was completely covered by the porous film (Fig. 10c). Fig. 10d shows that some cracks appear on the film at this time, which might be the result of the internal stress as the film thickness increases. The surface morphology of the samples for 20 min and 40 min anodization has no apparent difference, as can be seen from Fig. 10 (e and f). These photos show that the anodizing films thicken with time and the formation of the films is non-uniform. The crosssectional can afford more details about the film growth process.
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Fig. 10. SEM of the surface morphology of the samples anodized with 20 mA/cm2 at different time (a) 30 s, (b) 1 min, (c) 2 min, (d) 5 min, (e) 20 min, (f) 40 min.
It is shown in Fig. 11a that the thickness of the film with 10 min is still very thin, but the film with 20 min anodization is relatively thick and the thickness of the film becomes thicker when the treatment time is enhanced. It is clearly shown in Fig. 11b and c that the grain boundaries have no apparent effects on the microstructure of the anodization films, there are no cracks leading from the sites of the grain boundaries. It proves that the cracks on the surface of the anodized films are caused by the surface internal stress, not by the substrates. It can also be seen in Fig. 11 that the microstructures of the anodized films are porous, but the film of 60 min is the thickest and the pores are less than the others. This may explain why the sample with 60 min is the best corrosion resistant. The sample with 60 min is also examined by EDS (Energy dispersive spectroscopy) and XRD. The EDS results show that the composition of the anodized film is as follows: O 46.15; Na 1.14; Mg 44.72; Si 6.78 (wt%). It can be seen that the main component of the film are O, Mg and Si. To further learn the phase composition of the film, XRD pattern of the film
anodized for 60 min at 20 mA/cm2 was measured. Before the XRD examination, the anodized film was scratched down by a diamond knife to separate the interference from the substrate. The XRD result of the film with 60 min is shown in Fig. 12. It can be seen that there are two phases in the anodized film. The MgO, not hydroxide, is the main oxide existing in the film. The presence of Mg2SiO4 is also clearly identified in Fig. 12 which shows that silicon exists in SiO42 but not SiO2. 3.4. The comparison with HAE From the above experimental results, the appropriate processing parameters for ZK60 magnesium alloy anodizing in this alkaline solution should be 20 mA/cm2, 60 min. In order to evaluate the corrosion protection provided by anodic films in this new process, the corrosion resistance of the anodized film by this new process is compared with the film formed by HAE process. The HAE is a traditional magnesium anodizing process. The parameters of HAE in this experiment are as
H.L. Wu et al. / Applied Surface Science 253 (2007) 9387–9394
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Fig. 11. SEM of the cross-sectional morphology of the samples anodized with 20 mA/cm2 at different time (a) 10 min, (b) 20 min, (c) 40 min, (d) 60 min.
follows: 120 g l 1 KOH, 34 g l 1 KF, 30 g l 1 Al(OH)3, 34 g l 1 Na3PO4, 19 g l 1 KMnO4, 20 mA/cm2(current density), 40 min (anodizing time). The EIS and polarization curves for ZK60 blank sample and samples with different anodic films are shown in Figs. 13 and 14. It is shown in Fig. 13 that the modulus of the anodized film of new process is higher than that of the anodized film by HAE. The polarization curves experiments in Fig. 14 show the same result. The fitted results of Icorr in Fig. 14 for the blank sample, HAE and the new process are 3.578 10 5 A/cm2, 5.345 10 6 A/cm2 and 1.379 10 7 A/cm2, respectively. This shows that the protective effect of the new process is more excellent than that of the
Fig. 12. XRD of the film anodized for 60 min at 20 mA/cm2.
Fig. 13. EIS plots for ZK60 covered or not covered by anodic films produced by different processes. (a) Bode plots of modulus vs. frequency, (b) Bode plots of phase angle vs. frequency.
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4. Conclusions (1) Both treatment time and current density have obvious influence on the corrosion resistance of anodized films. For ZK60 anodized in alkaline solution containing silicate, the optimum treatment time is about 60 min and the optimum current density is 20 mA/cm2. (2) Compared with HAE, the film anodized in 100 g/l NaOH + 20 g/l Na2B4O710H2O + 50 g/l C6H5Na3O72H2O + 60 g/l Na2SiO39H2O has much higher corrosion resistance. (3) The anodized film consists of MgO and Mg2SiO4, which can make up the volume shrinkage by the formation of MgO, resulting in a uniform and corrosion resistive film.
Acknowledgement Fig. 14. Potentiodynamic polarization curves for ZK60 covered or not covered by anodic films produced by different processes.
traditional HAE process. The excellent protection of the new process can be attributed to the effects of Mg2SiO4 in the anodized film. While the film covered on the surface of Mg or Mg alloy only consists of MgO, the ratio of the volume of MgO to the volume of the consuming Mg will be less than 1. Therefore, the film is loose and cannot offer an effective resistance to corrosion, such as the natural oxide layer on magnesium surface [5]. The protective effect of anodized film by the formation of Mg2SiO4 can be calculated as follows: The densities of Mg, MgO and Mg2SiO4 are 1.74, 3.58, 3.21 (g/cm3), respectively [17]. Then the molar volumes of Mg, MgO and Mg2SiO4 can be calculated and the results are 13.972, 11.260 and 43.835 (cm3/ mol), respectively. If 1 M of Mg is transformed into MgO, the volume ratio of new formed MgO to the consumed Mg is 0.806, there will be shrinkage in the film. However, 1 M of Mg can transformed into 0.5 M Mg2SiO4, the volume ratio of the newly formed Mg2SiO4 to the consumed Mg is 1.569; this clearly shows that the formation of Mg2SiO4 in the anodization film will make up the shrinkage. The result shows that the participation of Mg2SiO4 in the anodization film can make the film more uniform and anti-corrosive.
The authors thank the support of the National Natural Science Foundation of China (no. 50471043). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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The anodization of ZK60 magnesium alloy in alkaline solution containing silicate and the corrosion properties of the anodized films Wu Hai-lana, Cheng Ying-lianga,*, Li Ling-linga, Chen Zhen-huaa, Wang Hui-mina, Zhang Zhaob a
College of Materials Science and Engineering, Hunan University, Changsha 410082, China Department of Chemistry, Yuquan Campus, Zhejiang University, Hangzhou 310027, China
b
Received 5 April 2007; received in revised form 30 May 2007; accepted 31 May 2007 Available online 19 June 2007
Abstract The anodization of ZK60 magnesium alloy in an alkaline electrolyte of 100 g/l NaOH + 20 g/l Na2B4O710H2O + 50 g/l C6H5Na3O72H2O + 60g/l Na2SiO39H2O was studied in this paper. The corrosion resistance of the anodic films was studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques and the microstructure and composition of films were examined by SEM and XRD. The influence of anodizing time was studied and the results show that the anodizing time of 60 min is suitable for acquiring films with good corrosion resistance. The influence of current density on the corrosion resistance of anodizing films was also studied and the results show that the film anodized at 20 mA/cm2 has the optimum corrosion resistance. The film formed by anodizing in the alkaline solution with optimized parameters show superior corrosion resistance than that formed by the traditional HAE process. The XRD pattern shows that the components of the anodized film consist of MgO and Mg2SiO4. # 2007 Elsevier B.V. All rights reserved. Keywords: ZK60; Anodized film; Corrosion; Alkaline solution; EIS
1. Introduction Magnesium is the lightest structural material, with excellent physical and mechanical properties, such as low density and high specific strength. These properties make magnesium alloys valuable in a number of applications including the automotive industry, computer parts, the aerospace industry and cellular phones where weight reduction is concerned [1–4]. However, magnesium and its alloys are characterized by low corrosion resistance, which has limited their use and the natural oxide layer on magnesium surfaces is very loose and cannot offer an effective resistance to corrosion [5]. Therefore, it is very important to improve the anti-corrosion performances of magnesium alloys in industrial applications. Anodization is one of the effective surface protective treatments for magnesium and its alloys. In contrast to other surface treatments, anodization can produce a relatively thick,
* Corresponding author. Tel.: +86 731 8821648. E-mail address: [email protected] (Y.-l. Cheng). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.05.085
hard, adherent and abrasion-resistance film [6,7]. But the most successful anodization, such as HAE [8] and DOW17 [9], usually contain chromate, fluoride or phosphate which are unfriendly to environment. Recently, a few new environmentally friendly electrolytes have been developed for the anodization of magnesium alloys. Most of solutions are alkaline electrolytes, which contain some additives such as aluminate, tungstate, borate and silicate, etc. [10–12]. All additives, especially aluminate and silicate, play important roles in anodization and affecting the properties of the anodic films. Khaselev [12] found that the addition of Al(OH)3 in KOH electrolyte affected the sparking behavior and the film breakdown potential increased with increasing Al(OH)3 concentration. Hsiao [7] found that the addition of Na2SiO3 into 3 M KOH + 0.21 M Na3PO4 + 0.6 M KF electrolyte resulted in the formation of anodic film on AZ91D magnesium alloy with enhanced polarization resistance. In this investigation, the anodization of ZK60 magnesium alloy in an environmental friendly electrolyte which contains 100 g/l sodium hydroxide + 20 g/l borate + 50 g/l citrate + 60g/l sodium silicate was investigated and the corresponding corrosion
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resistances of the films were evaluated and compared with the traditional anodizing process of HAE. The structure and property of the anodizing film are studied by SEM, XRD, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. 2. Experimental ZK60 Mg alloy with a nominal composition of 5.5 mass% Zn, 0.45 mass% Zr and Mg the balance is chosen in this study. The 5 mm-thick rolled plate was cut into square samples with a dimension of 10 mm 10 mm. All samples were connected with copper wire in one side and then enveloped by epoxy resin with the other surface exposed as the working surface. Before anodizing in the electrolyte, the working surface was polished successively to 1500 grit finish and cleaned in acetone. In the electrolyte cell for anodization, a stainless steel plate was used as the cathode and the sample was the anode. The anodization process was carried out at constant current density on a DYY-6C (600 V, 400 mA) power supplier. The composition of the electrolyte bath is 100 g/l NaOH + 20 g/l Na2B4O710H2O + 50 g/l C6H5Na3O72H2O + 60g/l Na2SiO39H2O, prepared with analytical grade reagent and distilled water. The effect of anodizing time and current density has been studied in this article. Films have been acquired by processing with different parameters and the properties of the films are measured by electrochemical impedance spectroscopy (EIS) tests and potentiodynamic polarization curves. Electrochemical tests are carried out in 3.5% NaCl solution (mass fraction) using a computer-monitored potentiostat CHI660B (CH Instruments, Inc. 3700 Tennison Hill Drive Austin, TX 78738-5012, USA). The set-up is composed of a three-electrode system: a saturated calomel electrode (SCE) is used as a reference electrode; a large platinum sheet is the auxiliary electrode and the sample is the working electrode. The EIS measurements are carried out at open circuit potential (OCP), the amplitude of the perturbative signal is 5 mVand the frequency range is between 0.01 Hz and 100 KHz. After the EIS measurements, the potentiodynamic electrochemical tests are carried out with a scan rate of 0.001 V/s. All of the tests are carried out at room temperature (about 25 8C). Besides the electrochemical tests, other properties of the anodized coating were also measured. The film thicknesses were measured by a coating thickness gauge (type TT260, TIME Group Inc., Beijing, China). The surface morphology of the anodized samples was examined by a SEMCJSM-5610LV scanning electron microscope (SEM) and X-ray diffractometer with Cu Ka radiation (SIMENS D5000).
Fig. 1. Potential transient of the anodizing process at 20 mA/cm2.
the magnified section from 0 min to 2 min) shows that at the initial stage the potential rises at a very high rate, the potential at 5 s reaches 42 V and the potential is 61 V at 10 s; after 10 s, the rising rate of the potential begins to slow down. When the cell potential reaches about 60 V, obvious sparking occurs. The potential corresponding to the occurrence of sparking is designated as breakdown voltage [10]. Once sparking occurs, the rising rate of potential decreases. At this stage, the sparks are numerous, small sized, short-lived, randomly and relatively even distributed on the entire anode surface. About 2 min later, the sparking behaviour changes, the number of the sparks decreases and both the size and lifetime of individual spark increase. The sparks appear only at localized location on anode surface and the potential starts oscillating. About 10 min later, the potential reaches 110 V, after that, it rises much slowly than before, and the fluctuation of potential is more frequent. Due to the fluctuation of the potential, each data point at the late stage of anodization in Fig. 1 is the average of the potential values in a minute recorded by the DYY-6C power supplier. To study the effect of anodizing time on the properties of anodization films, samples with different anodizing time were first prepared with the applied current density of 20 mA/cm2. The film thickness with different anodizing time is measured and the result is shown in Fig. 2. It is shown that the film
3. Results and discussions 3.1. Effect of anodizing time The anodization processes are carried out under the constant current density mode. The transient of the potential during anodization of ZK60 at a constant current density of 20 mA/ cm2 was recorded in Fig. 1. Once circuit is connected correctly and current is supplied, cell potential begins to rise. Fig. 1 (see
Fig. 2. Relationship between film thickness and processing time at 20 mA/cm2.
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thickness goes linearly with anodizing time. This implies that the previous formed film cannot impede the continuous growth of the film. After the thickness measurement, the corrosion resistant properties of these samples were measured by the EIS and potentiodynamic polarization tests. The EIS was measured after about 10 min immersion and the potentiodynamic polarization test was carried out immediately after the EIS measurement. The results of samples with different anodizing time are shown in Figs. 3 and 5, respectively. EIS is a useful technique in the study of corrosion. The corrosion mechanisms can sometimes be estimated by analyzing the measured electrochemical impedance spectrum. The diameter of the capacitive semicircle of a measured Nyquist spectrum is closely related to the corrosion rate [13]. The EIS results in Fig. 3 show that these diagrams are mainly composed of one depressive loops at the high and medium frequency range and some scattered points at low frequency region (below 10 mHz). Although the high and medium region shows one depressive arc, it actually contains two time constants. This can be seen in the Bode phase angle plots in Fig. 3 because the peak in the high and medium frequency range is very broad. And the equivalent circuit in Fig. 4 can be put forward for the EIS of the high and medium frequency part of Fig. 3, where Rs represents the solution resistance, Rf represents the resistance of the anodized film which is related to the micro-pores of the anodized films, the capacitance of the
Fig. 3. EIS of the anodized films with different treatment time at 20 mA/cm2 (a) Nyquist plots, (b) Bode-phase angle plots.
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Fig. 4. Equivalent circuit used to fit the high frequency part of the EIS measurement.
anodized film is replaced by the so called constant phase angle element [CPE, ZCPE = 1/Y0 (jv) n, 0 < n < 1, where ZCPE is the impedance of the constant phase element (V cm2), v is the angular frequency of ac-voltage (rad s 1), Y0 and n are the frequency independent parameters. When n = 1, CPE deserves a pure capacitance. When n = 0, it deserves a pure resistance and an inductance when n = 1]. The presence of CPE has been explained by dispersion effect that can be caused by microscopic roughness of a surface [14,15]. CPE1 in Fig. 4 represents the capacitance of the anodized film; CPE2 is the double layer capacitance between the alloy substrate and the electrolyte solution in the pores. Rct is the charge transfer resistance of the dissolution of magnesium alloy substrate. This equivalent circuit has also been used by Wu et al. [16] in explaining the measured EIS of anodized films. The scattered point in Fig. 2 may be caused by the non-stability of the tested system at very low frequency range; the resistance of the anodized film is very high and the perturbative signal is only 5 mV, thus the response of the system cannot be stable. Fig. 4 is put forward to explain the EIS results and the parameters in Fig. 4 have not been fitted because the corrosion resistance of the anodized films can be directly justified from the radius of the EIS plots. It can be seen from the EIS results in Fig. 3 that when the treatment time is less than 80 min, the radii of the semicircles of the Nyquist plots of the anodized films increase with treatment time. This implies that, when the treatment time is less than 80 min, treatment time is good for the corrosion resistance of the anodized film. When the treatment time reaches 80 min, the radius of the Nyquist plot is less than that of 60 min, indicating that samples with very long treatment time do not have good corrosion resistance too.
Fig. 5. The potentiodynamic polarization tests of the samples with different treatment time.
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Table 1 The fitted result of Icorr in Fig. 5 Treatment time (min)
Icorr (mA/cm2)
20 30 40 50 60 70 80
3.081 10 2.984 10 2.398 10 1.722 10 1.379 10 1.559 10 1.176 10
4 4 4 4 4 4 3
Fig. 5 shows the potentiodyamic polarization tests after the EIS tests and the free corrosion current densities are calculated in Table 1. It is shown in Table 1 that the free corrosion density can be ranked as Icorr (80 min) > Icorr (20 min) > Icorr (30 min) > Icorr (40 min) > Icorr (50 min) > Icorr (70 min) > Icorr (60 min). The free corrosion current density is directly related to the corrosion resistance of the anodized film, the less the free corrosion density, the higher the corrosion resistance of the film is. Then it can be seen that the results of the potentiodynamic tests show nearly the same trends as the EIS results except the samples of 70 min and 80 min, which indicates that the two techniques have slight difference in predicting the corrosion resistance of samples. The EIS is a technique with very small perturbtive signal, the surface condition of the samples will be nearly the same after EIS tests, but the potentiodynamic technique needs large over-potential to polarize the samples, which will result in the changing of the surface condition of the samples. Although the EIS shows the sample with 70 min anodization is more corrosion-resistant, the Icorr (60 min) acquired from polarization tests is smaller than Icorr (70 min). It can be seen in Fig. 5 that both the anodic and the cathodic current density of the sample with 70 min is higher than that of the sample of 60 min, this may imply that the film of 70 min is easier to pitting or cracking under over-potential. The sample of 80 min is even more abnormal; it is shown in Fig. 3 that the radius of the EIS of 80 min is close to that of 60 min and the potentiodynamic test shows that it is the least in corrosion resistance. From the above experimental results, we can know that, although the film thickness increases linearly with treatment time, thicker film does not definitely mean better corrosion resistance. When the processing time is not too long and the film thickness is not too high, the corrosion resistance of film formed on magnesium alloy increase with treatment time. But when the treatment time is too long, not only the energy consumption will increase but also the internal-stress of the film will increase, which will make cracks appear on the film, and the corrosion resistance of the film will become lower than before. It can be concluded that the treatment time of 60min is appropriate for acquiring high corrosion resistance film for ZK60 anodized in this solution at 20 mA/cm2.
Fig. 6. The relationship between potential and time at different current density.
constant current density of 5, 10, 20, 30, 40, 50 mA/cm2, respectively, and the treatment time is chosen as 40 min. The relationship between potential and time during anodizing at different current density is shown in Fig. 6. The behaviour of potential can provide valuable information on the anode. It can reflect directly the course of film growth and it was found that higher potential indicates thicker film while anodizing under the same condition [10]. It is shown in Fig. 6 that breakdown voltage and the terminal voltage of the sample anodized at 5 mA/cm2 and 10 mA/cm2 are lower than that of the samples anodized at other current densities. To learn the growth rate of the film with current density, the film thicknesses at different current densities are also measured and results are shown in Fig. 7. Like the results in Fig. 2, the relationship of the film thickness and current density is linear too. The EIS and potentiodynamic polarization tests of the samples with different anodizing current density are also measured; the results are shown in Figs. 8 and 9, respectively. The EIS features are similar to the EIS behaviour in Fig. 3, the high and medium frequency region in Fig. 8 can be explained by the equivalent circuit of Fig. 4. The Icorr values can be
3.2. Effect of current density To study the effect of current density on the properties of the anodized films, the anodizing processes were carried out at
Fig. 7. The relationship between film thickness and current density.
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Fig. 9. The potentiodynamic polarization tests with different current density.
current density would make the growth of the film too quick, which would result in coarser oxide grain and the internal stress of the anodized film will increase, resulting in micro-cracks on the films. As a result, the corrosion resistance of the anodized films will decrease. The above results indicated that optimum anodizing current density for ZK60 is 20 mA/cm2. 3.3. Morphology and XRD pattern of the anodized film
Fig. 8. EIS of the anodized film with different current density in 3.5 wt.% NaCl solution.
acquired from the potentiodyanmic polarization curves in Fig. 9 and the data are listed in Table 2. It is clearly shown in the EIS results (Fig. 8) and the potentiodynamic polarization tests (Table 2) that the optimum anodizing current density for the anodized film with best corrosion resistance is 20 mA/cm2. When current density is less than 20 mA/cm2, the corrosion resistance of the films increased with current density. This can be explained by the fact that the film growth rate is higher at large current density, the film formed at higher current density with the same anodizing time will be thicker and thus is more protective. However, when the current density exceeds 20 mA/cm2, the corrosion resistance of the anodized films begins to decrease with current density. It can be concluded that although high current density can increase the growth rate of the film and shorten the time to an optimum film thickness, there exists a contrary effect. High Table 2 The fitted result of Icorr in Fig. 9 Current density (mA/cm2)
Icorr (mA/cm2)
5 10 20 30 40 50
1.829 10 1.259 10 2.398 10 2.828 10 4.137 10 5.257 10
2 3 4 3 3 3
Surface and cross-sectional morphology of the anodized specimens were examined. Fig. 10 gives the SEM micrographs for the surface morphologies of ZK60 Mg alloy anodized for different time when 20 mA/cm2 constant current density was applied and Fig. 11 shows the cross-sectional morphologies of the anodized film with different processing time. In Fig. 11b–d, the substrates were etched by a mixture of 5 g picric acid + 5 ml glacial acetic acid + 10 ml distilled water + 100 ml ethanol to reveal the grain boundaries of the substrate alloy. In Fig. 10a, the SEM micrograph was taken when the sample was anodized for 30 s, in this situation, the potential just reached the breakdown potential, and many tiny sparks were just observed on the surface. As a result, the film was only formed on some localized locations on the surface; most of the surface of the alloy is still exposed. Fig. 10b is the morphology of the sample with anodizing time of 1 min, at this time, the potential rose to about 73 V, and the tiny sparks changed into big sparks whose life-span were longer than before, and the big sparks wandered everywhere all the time. It is shown in Fig. 10b that the surface is nearly covered by a porous film except for some small part, but the film was so thin that the grinding scratches formed during sample preparation were still seen. After 2 min anodizing, the surface was completely covered by the porous film (Fig. 10c). Fig. 10d shows that some cracks appear on the film at this time, which might be the result of the internal stress as the film thickness increases. The surface morphology of the samples for 20 min and 40 min anodization has no apparent difference, as can be seen from Fig. 10 (e and f). These photos show that the anodizing films thicken with time and the formation of the films is non-uniform. The crosssectional can afford more details about the film growth process.
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Fig. 10. SEM of the surface morphology of the samples anodized with 20 mA/cm2 at different time (a) 30 s, (b) 1 min, (c) 2 min, (d) 5 min, (e) 20 min, (f) 40 min.
It is shown in Fig. 11a that the thickness of the film with 10 min is still very thin, but the film with 20 min anodization is relatively thick and the thickness of the film becomes thicker when the treatment time is enhanced. It is clearly shown in Fig. 11b and c that the grain boundaries have no apparent effects on the microstructure of the anodization films, there are no cracks leading from the sites of the grain boundaries. It proves that the cracks on the surface of the anodized films are caused by the surface internal stress, not by the substrates. It can also be seen in Fig. 11 that the microstructures of the anodized films are porous, but the film of 60 min is the thickest and the pores are less than the others. This may explain why the sample with 60 min is the best corrosion resistant. The sample with 60 min is also examined by EDS (Energy dispersive spectroscopy) and XRD. The EDS results show that the composition of the anodized film is as follows: O 46.15; Na 1.14; Mg 44.72; Si 6.78 (wt%). It can be seen that the main component of the film are O, Mg and Si. To further learn the phase composition of the film, XRD pattern of the film
anodized for 60 min at 20 mA/cm2 was measured. Before the XRD examination, the anodized film was scratched down by a diamond knife to separate the interference from the substrate. The XRD result of the film with 60 min is shown in Fig. 12. It can be seen that there are two phases in the anodized film. The MgO, not hydroxide, is the main oxide existing in the film. The presence of Mg2SiO4 is also clearly identified in Fig. 12 which shows that silicon exists in SiO42 but not SiO2. 3.4. The comparison with HAE From the above experimental results, the appropriate processing parameters for ZK60 magnesium alloy anodizing in this alkaline solution should be 20 mA/cm2, 60 min. In order to evaluate the corrosion protection provided by anodic films in this new process, the corrosion resistance of the anodized film by this new process is compared with the film formed by HAE process. The HAE is a traditional magnesium anodizing process. The parameters of HAE in this experiment are as
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Fig. 11. SEM of the cross-sectional morphology of the samples anodized with 20 mA/cm2 at different time (a) 10 min, (b) 20 min, (c) 40 min, (d) 60 min.
follows: 120 g l 1 KOH, 34 g l 1 KF, 30 g l 1 Al(OH)3, 34 g l 1 Na3PO4, 19 g l 1 KMnO4, 20 mA/cm2(current density), 40 min (anodizing time). The EIS and polarization curves for ZK60 blank sample and samples with different anodic films are shown in Figs. 13 and 14. It is shown in Fig. 13 that the modulus of the anodized film of new process is higher than that of the anodized film by HAE. The polarization curves experiments in Fig. 14 show the same result. The fitted results of Icorr in Fig. 14 for the blank sample, HAE and the new process are 3.578 10 5 A/cm2, 5.345 10 6 A/cm2 and 1.379 10 7 A/cm2, respectively. This shows that the protective effect of the new process is more excellent than that of the
Fig. 12. XRD of the film anodized for 60 min at 20 mA/cm2.
Fig. 13. EIS plots for ZK60 covered or not covered by anodic films produced by different processes. (a) Bode plots of modulus vs. frequency, (b) Bode plots of phase angle vs. frequency.
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4. Conclusions (1) Both treatment time and current density have obvious influence on the corrosion resistance of anodized films. For ZK60 anodized in alkaline solution containing silicate, the optimum treatment time is about 60 min and the optimum current density is 20 mA/cm2. (2) Compared with HAE, the film anodized in 100 g/l NaOH + 20 g/l Na2B4O710H2O + 50 g/l C6H5Na3O72H2O + 60 g/l Na2SiO39H2O has much higher corrosion resistance. (3) The anodized film consists of MgO and Mg2SiO4, which can make up the volume shrinkage by the formation of MgO, resulting in a uniform and corrosion resistive film.
Acknowledgement Fig. 14. Potentiodynamic polarization curves for ZK60 covered or not covered by anodic films produced by different processes.
traditional HAE process. The excellent protection of the new process can be attributed to the effects of Mg2SiO4 in the anodized film. While the film covered on the surface of Mg or Mg alloy only consists of MgO, the ratio of the volume of MgO to the volume of the consuming Mg will be less than 1. Therefore, the film is loose and cannot offer an effective resistance to corrosion, such as the natural oxide layer on magnesium surface [5]. The protective effect of anodized film by the formation of Mg2SiO4 can be calculated as follows: The densities of Mg, MgO and Mg2SiO4 are 1.74, 3.58, 3.21 (g/cm3), respectively [17]. Then the molar volumes of Mg, MgO and Mg2SiO4 can be calculated and the results are 13.972, 11.260 and 43.835 (cm3/ mol), respectively. If 1 M of Mg is transformed into MgO, the volume ratio of new formed MgO to the consumed Mg is 0.806, there will be shrinkage in the film. However, 1 M of Mg can transformed into 0.5 M Mg2SiO4, the volume ratio of the newly formed Mg2SiO4 to the consumed Mg is 1.569; this clearly shows that the formation of Mg2SiO4 in the anodization film will make up the shrinkage. The result shows that the participation of Mg2SiO4 in the anodization film can make the film more uniform and anti-corrosive.
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