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Effect of pulse current parameters on the mechanical and corrosion properties of anodized nanoporous aluminum coatings
Accepted Manuscript Effect of pulse current parameters on the mechanical and corrosion properties of anodized nanoporous aluminum coatings Iman Mohammadi, Shahab Ahmadi, Abdollah Afshar PII:
S0254-0584(16)30669-1
DOI:
10.1016/j.matchemphys.2016.09.006
Reference:
MAC 19152
To appear in:
Materials Chemistry and Physics
Received Date: 9 October 2015 Revised Date:
15 August 2016
Accepted Date: 4 September 2016
Please cite this article as: I. Mohammadi, S. Ahmadi, A. Afshar, Effect of pulse current parameters on the mechanical and corrosion properties of anodized nanoporous aluminum coatings, Materials Chemistry and Physics (2016), doi: 10.1016/j.matchemphys.2016.09.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of pulse current parameters on the mechanical and corrosion properties of anodized nanoporous aluminum coatings Iman Mohammadi*, Shahab Ahmadi, Abdollah Afshar
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Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11155-9466, Azadi Ave, Tehran, Iran *Corresponding Author, Tel: +98 21-6616-5204, Fax: +9821-6600-5717, E-mail address: [email protected]
Abstract
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In this study, the effects of pulse current parameters on corrosion resistance and mechanical properties of anodized coatings were evaluated. Hardness measurements, polarization and electrochemical impedance spectroscopy tests were employed to investigate the mechanical properties and corrosion behavior of these coatings. Also,
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field emission scanning electron microscopy (FE-SEM) was used to analyze the surface morphology and microstructure of the coatings. It was found that the properties of
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anodized coatings were dependent on various parameters, among which, time, temperature and pulse current parameters (current density limit, frequency and duty
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cycle) were optimized. Analysis of Variance (ANOVA) was conducted in order to optimize the results of designed experiments for predicting the hardness of anodic Al2O3 coatings. Experimental results showed that the temperature and the interaction of quadratic behavior of minimum current density with frequency and duty cycle were the most important terms influencing the hardness of these coatings. It was indicated that the highest hardness value of 642 HV was attained at the maximum and minimum
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current densities of 4.4, 1.27 A/dm2, respectively, a frequency of 82 Hz, procedure time of 27.2 min, duty cycle of 80.2% and the bath temperature of 13.5 ᵒC. In addition, the FE-
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SEM micrographs showed that the highest density is obtained through the mentioned optimum conditions. Moreover, the electrochemical tests revealed that the highest polarization resistance obtained at optimum conditions was more than 20 times greater
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than the other samples.
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Keywords: Anodizing; Aluminum; Pulse current; Hardness; Corrosion behavior
1. Introduction
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Aluminum and its alloys have a great potential to form an oxide layer on the own surfaces when exposed to medium atmosphere. Although this layer has a low thickness,
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it can cause an increase in mechanical properties and corrosion resistance of the aluminum alloy. Various methods have been used to increase the thickness of this layer
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among which anodized aluminum oxidation (AAO) is one of the most popular methods improving the corrosion behavior and mechanical properties of the aluminum alloy [13]. It is generally believed that the performance of the anodic layer depends on its structure and is influenced by various parameters such as electrolyte composition, current density and additives among which the current density and current waveform are the most important effective parameters on the properties of coatings [4-9].
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In traditional anodizing processes, the direct current mode has been typically applied to fabricate the oxide layers. The main drawback of this method is the formation of gas
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bubbles on the surface of anodized layer due to the significant difference between thermal conductivity coefficients of the evolved gas and oxide layer. These bubbles prod the localization of heat content on the surface of coating [10-11]. The heat
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localization leads to an increase in aggressiveness of electrolyte [12-13]. Among commonplace shortcomings for this method, the most important ones are increase of
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porosity, pore diameter and the propagation of defects which lead to degradation of hardness and corrosion behavior of anodized coatings [5, 11, 14-15]. Lee et al. [16] claimed that the hardness and corrosion properties of the anodized coatings improved significantly by applying the pulse current anodizing. In addition, it
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has been shown that the structure of anodized coating is improved using pulse anodizing [17]. It is due to that the heat amount generated in higher current densities
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applied at pulse anodizing is dissipated on the surface of coatings [5]. Therefore, higher current density and upper temperature resulting in a fast and inexpensive anodizing
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process could be conducted without any burning of the coating surface [18-19]. Hence, corrosion resistance enhancement of anodic layer, increasing the thickness and surface density of anodized coatings, and improving the efficiency of the film formation evolution are the main benefits of this method [5, 11, 19]. Hybrid pulse anodizing is a new approach fabricating anodic oxide coatings at a higher bath temperature and decreasing the imperfection of low temperature anodizing [20-21].
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As previously mentioned, using of pulse current leads to an increase in hardness of anodized coatings which their properties considerably depend on pulse current
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parameters. Therefore, due to various conditions of anodizing procedure, an optimization of the process parameters is required. Hence, to date, various methods are used for optimization [22-26]. Design of experiment (DOE) is one of the best methods
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for optimization of processes bearing a great potential for various purposes [27-31]. In this research, response surface methodology (RSM) and central composite design (CCD)
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are used to achieve the optimum values of hardness when using pulse current mode, sulphuric/oxalic acid electrolyte and an ambient bath temperature. 2. Experimental procedures
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2.1. Materials and pretreatment
Starting material in this study was AA 1050 with the dimension of 30×20×1. For obtaining a smooth surface, specimens were polished from the mechanical ground to
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P1200 grade paper. Prior to anodizing, the samples were formerly degreased by ultra-
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sonication in acetone for 10 min and latterly, electropolished in a mixture of HClO4 and ethanol in the volume ratio of 1:4 at 20 V for 30 s and at the room temperature. Finally, the specimens were rinsed immediately with distilled water and then dried. 2.2. Anodizing process
For anodizing process, a direct current plus pulse (DCP) waveform generated by pulse rectifier (SL 2/25 PCS, Iran) was used. As shown in Fig. 1, for this process, the following
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four parameters were important which could vary independently: maximum current density (Imax), minimum current density (Imin), toff and ton (In pulse current, duty cycle
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(C) = ton/ (ton + toff) and frequency (D) = 1/ (ton + toff)). Anodizing process in this study benefited a two-electrode method in which the sample was used as the anode and an aluminum sheet larger than anode size as cathode. For anodizing process, a
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sulphuric/oxalic electrolyte was employed as shown in Table. 1.
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2.3. Design of Experiments methodology
In this study, design of experiment (DOE) was evaluated by Design Expert 7 software. In order to optimize hardness of anodized coatings, six important parameters such as time, temperature and pulse current (current density limit, frequency and duty cycle)
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were considered. As shown in Table 2, a range of parameters was used for establishing the possible effects. For optimizing these factors, response surface methodology (RSM) based on central composite design (CCD) method was used. The experimental
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procedures produced by this method are shown in Table 3. As illustrated, 52 experimental procedures were designed by CCD method under various conditions
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based on performing pulse current at room temperature. As seen, the standard order of experimental procedure does not match run order since the software considers randomization of parameters for reducing the effects of unwanted parameters as well as noises on responses [32]. Also, as seen, 8 replicates were used due to the checking replicability of experiments. For obtaining maximum hardness of coatings, optimizing process was performed by ANOVA analysis based on finding the best model. For 5
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performing this approach, contour and surface plots (in contour and surface plot, two factors vary and other parameters are considered to be constant) were used.
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2.4. Characterization of coatings 2.4.1. Micro-hardness measurements
In order to evaluate hardness of anodized coatings under various experimental
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procedures, Vickers micro-hardness (Buehler model 1600-6100, USA) was performed.
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For this measurement, a load of 25 g was applied for 15 s based on ASTM B578 standard. The measurements were taken at five different points which were distributed on the coatings, and the average values were reported. 2.4.2. Surface morphology
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Morphologies of the surface coatings were inspected by high vacuum FE-SEM (model Mira 3-XMU, Czech). Before FE-SEM observation, Au was sputtered on the surface of
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the specimens.
2.4.3. Electrochemical measurements
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Electrochemical responses of the oxide layers were measured by using a potentiostat /galvanostat (Autolab model 302N) apparatus. The measurements were taken based on ASTM–G102-98 standard and an extrapolation method. A three-electrode cell was used in the condition of performed 3.5 wt% NaCl solution at room temperature. In this method, the anodized sample was placed as the working electrode, saturated calomel electrode (SCE) as reference electrode and a graphite sheet was chosen as the counter 6
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electrode. In order to study the potentiodynamic polarization of the samples, a scan rate of 1 mVs-1 starting at -400 mV below the open circuit potential (OCP) and finishing at
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800 mV above of the OCP was employed. In order to evaluate the electrochemical behavior of the anodized samples in more details, an electrochemical impedance spectroscopy (EIS) was used by EG&G (model
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273A) apparatus which worked at the same condition of potentiodynamic polarization measurement. To plot the EIS spectra, a frequency range of 10-2-105 Hz with an
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amplitude of 10 mV was used. It should be noted that in order to establish the open circuit potential, the samples were immersed for 30 min in the electrolyte solution before all electrochemical processes. Finally, the ZSimpWin V3.40 software was used for fitting the EIS result.
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3. Results and discussion
3.1. Effect of pulse current parameters on the hardness of anodized coatings
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Table 4 shows the results of analysis of variance (ANOVA) that are performed for
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analyzing the results of experimental design. According to this Table, ANOVA predicts that quadratic model is an appropriate model in the condition of the applied pulse current for forecasting the hardness of anodized coatings, which is due to the application of Fisher’s test (F test) by ANOVA (acceptable model contains appropriate F value and low P value [33]). This model was selected due to the lower P value, although the F value is greater than that for cubic model. In conformity with the results, ANOVA presents Eq. 1 which can predict the hardness of anodized coatings under the maximum 7
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current density (A), minimum current density (B), duty cycle (C), frequency (D), bath temperature (E) and anodizing time (F).
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Hardness (HV) = 524.74 – 13.18A – 14.02B + 67.09C – 57.50D – 69.18E – 18.78F – 42.5A×B – 2.38A×E – 29.63A×F – 7.8B×E – 13C×E – 7.75E×F – 17.75A2 – 50.61C2 – 46.73E2 – 91.65B2×C + 100.65B2×D (1) In order to study the normality of the data, normal probability plot (NPP) was
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performed, and the associated results are shown in Fig. 2. As seen, except some other residuals, the results are distributed along the predicted line (The residual means the
each experimental design [34]).
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difference between observed and predicted hardness values of anodized coatings in
The next step of this study was to investigate the effective factors resulting in obtaining maximum hardness for anodized coatings. For this purpose, the coefficient of Eq. 1 and
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contour and surface plots were used. Based on Eq. 1 and Fig. 3, the quadratic interactions of minimum current density with duty cycle and frequency were the most
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important factors influencing the hardness of anodized coatings. Fig. 4 illustrates the contour plots of this study. In this figure, the results with the same hardness are
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connected to each other to produce contour lines of a constant hardness. In this case, the response surface is viewed in a two dimensional plane. Generally, circular contour lines are the evidence of no interaction between two considered factors [34]. According to Fig. 4, temperature is one of the important parameters which has a negative influence on the hardness of anodized coatings (due to its negative coefficient in Eq. 1). It is related to this fact that the increasing of temperature on the surface of anodized
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coatings leads to greater aggressiveness of the electrolyte. Due to this effect, the dissolution rate of coatings increases (Eq. 2) which results in the formation of larger
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pores diameter and poor structure of the anodized coatings [11, 35]. Therefore, temperature increment leads to the decrease in the hardness of anodized coatings. + 6 → 2 + 3
(2)
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As shown in Fig 4, the maximum and minimum current densities have a negative effect on the hardness of anodized coatings. As shown in Fig. 4 (a), when both of these
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parameters are increased simultaneously, the hardness of anodized coatings is in its critical condition, according to the coefficient of the interaction between these factors. In fact, when using pulse current, the minimum current density has a recovery behavior for growth of the anodized coatings [5]. This means that when the samples are anodized
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with this current, some heat will be dissipated. In other words, using the pulse current causes the reduction of heat concentration on the surface and subsequently increases
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the hardness [18- 19]. On the other hand, as shown in Fig. 4 (b and c), the maximum and minimum current densities have interaction with temperature. It traces back to this fact
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that increasing of current density leads to higher rates for the formation of Al2O3 based on Eq. 3. As far both Joule effect and the Eq. 3 are exothermic and they are encouraged by current density, therefore, these two factors lead to the temperature increment on the surface of the oxide layer [11]. Therefore, this effect results to the increasing of the solution aggressiveness and dissolution rate of coatings [15, 35]. Fig. 5 shows the effect of current density increment on the morphology of anodized coatings. Comparison of
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Fig. 5 (a) and Fig. 5 (b) demonstrates that applying greater average current densities leads to increase in pore diameter. Therefore, using a greater current density increases
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porosity which subsequently reduces the hardness of anodized coatings. 2 + 3 → + 1424 /
(3)
As seen from Fig. 4 and Eq. 1, duty cycle improves the hardness of anodized coatings.
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When the duty cycle increases, the anodic oxidation of samples conduct with a greater current density which increases surface density. Due to the mentioned effect, hardness
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of coatings increased [17, 36]. Furthermore, when duty cycle reaches a maximum (i.e. direct current), temperature increases on the surface of these coatings since application of direct current mode conducts to releasing of gas bubbles (with the oxidation of O2ion reaction according to Eq. 4) on the surface of anodic oxide layer [10-11]. Due to these
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gas bubbles, voids and microcrack defects along the cell boundaries are created which result in a weak junction strength [16].
(4)
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(O2- → ½ O2 + 2e-)
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In other side, the difference between thermal conductivity of this gas and aluminum oxide, leads to the raise in heat content [11, 15] and increases the temperature (Fig. 4 (d)) on the surface of anodic layer. Hence, the aggressiveness of electrolyte and then the pore diameter are increased. This effect is shown in Fig. 5 (c). As seen, using direct current leads to higher pore diameter than the pulse one. Therefore, increasing of duty cycle finally decreases the hardness of coatings.
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Based on Eq. 1, frequency has a negative effect on the hardness of anodized coatings. When the frequency is increased, the mechanism of dissolution is activated in both of
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the appropriate and bad accumulated structure cells. As can be seen in Fig. 5, using a greater frequency conducts to the increasing of void and defect in the structure which decreases the hardness of coatings.
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Also, referring to Eq. 1 and Fig. 4 (e-f), coefficient of time and their interactions with maximum current density and temperature are negative because increasing in duration
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time causes powder coatings [37]. Furthermore, with the increase in film thickness, the anodizing electric resistance of the film also increases, which leads to an increase in the anodization temperature of the film surface [24, 25]. In addition, when anodizing time increases, due to larger surface area, gas bubble content increases which subsequently
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increases the temperature on the surface of anodized coatings. This increasing of temperature is shown in coefficient of interaction of time with temperature. Furthermore, as seen in Fig. 5 (e), when anodizing time is increased, larger pores are
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layer.
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created. Therefore, longer anodizing time leads to decreasing in hardness of the oxide
Based on the optimization of six parameters and Table 5, in order to obtain maximum hardness of coatings, the software calculated that anodized sample under six selected parameter levels will give a hardness of 642 HV by 95% confidence. Also, results showed that the interval parameter for this study is in the range of 495.15 to 790.18 HV. For the calculation of the error amount in this optimized level, confirmation run was
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evaluated in this point and were compared with the results predicted by Eq. 1 [32]. Results show that the hardness of coatings in optimal point is 628 HV which does not
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significantly differ from the predicted value. Fig. 6 shows the predicted hardness by CCD method in comparison with the actual values for the samples anodized at various
shows the accuracy of Eq. 1.
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process conditions. As seen, the actual points are close to the estimated line which
3.2. Effect of pulse current parameters on the corrosion behavior of anodized coatings
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3.2.1. Potentiodynamic polarization measurements
Potentiodynamic polarization tests were performed in 3.5 wt% NaCl solution in order to study the corrosion behavior of coatings synthesized under various conditions. Based
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on Fig. 7 and Table 6, corrosion potential of the coating anodized in direct current mode is -830.6 mV (vs. SCE). It is noteworthy that the corrosion potential enhances to a noble electrode potential of -642.5 mV (vs. SCE) when the anodizing conditions shift to the
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optimized one. Furthermore, as shown in Fig. 7, when the anodizing condition altered from the optimized conditions, the corrosion resistance of samples is decreased.
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Referring to the FE-SEM micrographs shown in Fig. 5, the surface density and thus corrosion resistance of the coatings synthesized under different process conditions deteriorate in comparison with the sample produced under the optimum parameters. Based on Figs. 5 and 7, the corrosion properties of the coatings firstly are affected by pore diameter. In other words, when the sample is anodized under DC current mode, various events can decrease the corrosion resistance of the coatings among which local 12
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heat amount as the main reason is concentrated on the surface of anodic film. This local heat amount leads to an increase in dissolution rate, pore diameter, defects and voids
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on the anodic film and thereby, reducing corrosion resistance of the samples. 3.2.2. Electrochemical impedance spectroscopy
In this section, EIS studies were evaluated to confirm the potentiodynamic polarization
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test. The spectra obtained from EIS experiments have been presented in Fig. 8. The impedance data were analyzed using the simplified equivalent circuit of Fig. 9 widely
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proposed in the literature [4, 35] as well as using ZSimpWin V3.40 software. As can be seen in Fig. 9, the equivalent circuit contains two constant phase elements of CPEwall and CPEbarr which are associated with the capacitive behavior of porous oxide structure and barrier layer, respectively. These parameters show non-ideal capacitive behavior in
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both CPE. Other elements of Rsol, Rwall and Rbarr present resistances corresponding to the solution, porous and barrier layers, respectively.
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Referring to Figs. 7 and 8, a quantitative comparison between the polarization data and the results pertinent to the nyquist plots show a similar corrosion behavior for the
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samples. Also, capacitance loop diameter (Fig. 8 (a)) and Z modulus (Fig. 8 (b)) of the coating formed at the optimized condition are substantially greater than that of the anodized sample in DC current mode. Furthermore, the results of pulse current show that the corrosion resistance of the anodized coatings deteriorates when the applied parameters are changed respect to the optimum conditions.
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The term of n was used in order to justify the lower polarization resistance of the sample anodized under DC current mode. The value of n corresponds to the linear
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slope modulus of bode plot (Fig. 8 (b)). In this case when n is near 1, the surface is uniform and smooth and lower values (in our case n=0.65 for the specimen anodized in DC current mode) shows deviation from ideal capacitive behavior (which has been
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attributed to the inhomogeneity of the surface) [38, 39]. Also, it is worth noting the lower n values show a deterioration in corrosion resistance. Due to increase of chemical
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dissolution on the surface of anodic layer which can increase the defects of structure of DC anodized sample, the specimen anodized by DC mode indicates a lower n value compared to the optimum condition one. Therefore, the corrosion resistance of the
4. Conclusion
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anodized coating fabricated by optimum condition is greater than other conditions.
In this paper, pulse anodizing process parameters were optimized by using response
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surface methodology based on central composite design method. Results showed that DOE is an effective method for this optimization because of the considerable interaction
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between the parameters. Also, analysis of variance estimated that temperature and quadratic behavior of the minimum current density with frequency and duty cycle are the most important factors influencing the hardness of coatings. Furthermore, it was found that the maximum hardness of anodized coating (i.e. 642 HV) was attained at the maximum and minimum current densities of 4.4, 1.27 A/dm2, respectively, a frequency of 82 Hz, time of 27.2 min, duty cycle of 80.2% and the bath temperature of 13.5 ᵒC.
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Results of FE-SEM showed that this optimized conditions lead to a dense surface, increasing the hardness.
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The electrochemical measurements indicated that the polarization resistance of the coatings obtained under optimized conditions is higher than those obtained under any other condition. Results showed that the polarization resistance of the coatings
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synthesized at the optimized condition obtained 6055 KΩ/cm2 which was more than 20 times higher than that for other samples. Moreover, the amount of pores of the coatings
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was the main factor affecting the corrosion behavior of the coatings. Therefore, the polarization resistance of the coatings was affected by both the amount and the size of pores.
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[33] E.R. Rene, M.S. Jo, S.H. Kim and H.S. Park, Statistical analysis of main and interaction effects during the removal of BTEX mixtures in batch conditions, using wastewater treatment plant sludge microbes, Int. J. Environ. Sci. Tech, 4 (2007) 177-182.
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[34] J. Antony, Design of Experiment for Engineers and scientists, Elsevier Science &
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Technology Books, New York, 2003.
[35] T. Aerts, J-B. Jorcin, I. D. Graeve, H. Terryn, Comparison between the influence of applied electrode and electrolyte temperatures on porous anodizing of aluminium, Electrochem. Acta., 55 (2010) 3957-3965.
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[36] X. Zhao, G. Wei, X. Meng, A. Zhang, High performance alumina films prepared by direct current plus pulse anodisation, Surf. Eng., 30 (2014) 455-459.
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[37] K. Yokohama, H. Konno, H. Takahashi and M. Nagayama, Advantage of pulsed Anodizing, Plat. Surf. Finish. 69 (1982) 62-65.
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[38] P.L. Cabot, J.A. Garrido, E. Perez, A.H. Moreira, P.T.A. Sumodjo, W. Proud, Eis study of heat-treated Al-Zn-Mg alloys in the passive and transpassive potential regions, Electrochim. Acta 40 (1995) 447-454. [39] Sh. Ahmadi, I. Mohammadi, S.K. Sadrnezhaad, Hydroxyapatite based and anodic Titania
nanotube
biocomposite
coatings:
electrochemical behavior, 278 (2016) 67-75. 20
Fabrication,
characterization
and
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Table captions Table 1. The chemical composition of anodizing electrolyte solution
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Table 2. The range of process parameters for pulse anodizing in order to obtain an optimum condition
Table 3. Experimental design for optimizing hardness of anodized coating under pulse current
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Table 4. ANOVA results for various models which predict the hardness of anodized coatings
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Table 5. Intervals range of the predicted point and 95% confident to obtain maximum hardness of anodized coatings under pulse current
Figure captions
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Table 6. Electrochemical parameters of the samples anodized in the various conditions
Fig. 1. Direct current plus pulse (DCP) waveform produced by pulse power supply.
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Fig. 2. Normal Probability Plot of residuals for hardness of the anodized coating
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synthesized by pulse current.
Fig. 3. Quadratic Surface plots corresponded to the effects of both a) minimum current density and duty cycle and b) minimum current density and frequency on the hardness of coating anodized via pulse current. Fig. 4. Contour plots of the hardness for the coating anodized under various conditions: (a) maximum and minimum current densities; (b) maximum current density and 21
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temperature; (c) minimum current density and temperature; (d) duty cycle and temperature; (e) maximum current density and time; (f) time and temperature.
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Fig. 5. FE-SEM micrographs of the surfaces anodized in the condition: (a) optimized, (b) Imax=4 A/dm2, Imin=3 A/dm2, f=2.5 Hz, t=45 min, D=75% and T=10 ˚C, (c) direct current mode by current density of 4 A/dm2 and the bath temperature of 10 ˚C, (d) at the same
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condition of (b) and with these differences that Imin=1 A/dm2 and f=333 Hz, (e) at the
cross section of optimized condition.
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same condition of (b) and with these differences that Imin=1 A/dm2 and t=60 min and (f)
Fig. 6. Predicted hardness by CCD method in comparison with the actual values for the samples anodized at various process conditions.
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Fig. 7. Polarization curves of the bare and the anodized sample fabricated at optimum condition, standard order 8, standard order 35 and direct current mode by current density of 4 A/dm2 and the bath temperature of 10 ᵒC.
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Fig. 8. (a) Nyquist spectra, (b) Bode and (c) Bode-phase diagrams of the optimum
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condition, standard order 8, standard order 35, bare sample and direct current mode by current density of 4 A/dm2 and the bath temperature of 10 ᵒC. Fig. 9. Schematic for anodized coating containing barrier and porous oxide layers with equivalent electric circuit of the anodized coating.
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Content 200 g/L 20 g/L 10 g/L 10 ml/L 3 g/L
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Composition H2SO4 H2C2O4 C7H6O6S.2H2O C3 H 8 O Al2(SO4)3
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Level
Parameter 3
B: Imin (A/dm2)
0
C: Duty cycle (%)
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D: Frequency (Hz)
2.5
E: Temperature (°C)
10
F: Time (min)
20
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0
1
2
3.5
4.5
5.5
6
0.5
1.5
2.5
3
33.5
57.5
81.5
95
74.3
201.3
328.2
400
13.6
20
26.4
30
27.2
40
52.8
60
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A: Imax (A/dm2)
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Run order
A: Imax (A/dm2)
B: Imin (A/dm2)
C: Duty cycle (%)
D: Frequency (Hz)
E: Temperatur e (°C)
F: Time (min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
41 52 27 3 13 36 2 37 45 48 10 20 9 42 5 29 47 24 8 19 30 39 16 7 26 25 11 28 22 44 32 50 18 17 31 34 38 23 12 21 33 6 49 1 51 15 43 4 40 14 35 46
3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.0 6.0 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5
0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 1.5 1.5 0.0 3.0 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
33.5 33.5 33.5 33.5 81.5 81.5 81.5 81.5 33.5 33.5 33.5 33.5 81.5 81.5 81.5 81.5 33.5 33.5 33.5 33.5 81.5 81.5 81.5 81.5 33.5 33.5 33.5 33.5 81.5 81.5 81.5 81.5 57.5 57.5 57.5 57.5 20.0 95.0 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5
74.3 74.3 74.3 74.3 74.3 74.3 74.3 74.3 328.2 328.2 328.2 328.2 328.2 328.2 328.2 328.2 74.3 74.3 74.3 74.3 74.3 74.3 74.3 74.3 328.2 328.2 328.2 328.2 328.2 328.2 328.2 328.2 201.3 201.3 201.3 201.3 201.3 201.3 2.5 400.0 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3
13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 10.0 30.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0
27.2 52.8 52.8 27.2 52.8 27.2 27.2 52.8 52.8 27.2 27.2 52.8 27.2 52.8 52.8 27.2 52.8 27.2 27.2 52.8 27.2 52.8 52.8 27.2 27.2 52.8 52.8 27.2 52.8 27.2 27.2 52.8 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 20.0 60.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0
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Standard order
Source
DF
Mean square
FValue
PValue
38 23 17 1 7
12126.38 13306.12 9827.34 4237.75 189.71
63.92 70.14 51.80 22.34
<0.0001 <0.0001 <0.0001 0.0021
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Linear 2FI Quadratic Cubic Pure error
Sum of squares 4.6E5 3E5 1.7E5 4237.75 1328
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Name
Level
A B C D E F Response Hardness/(HV)
Imax /(A/dm²) Imin /(A/dm²) Duty Cycle / (%) Frequency /(Hz) Temperature /(°C) Time /(min) (Prediction) 642
4.4 1.27 80.2 82 13.5 27.2 (95% PI low and 95% PI high) 495-790
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Low level
High level
3.54 0.54 33.54 74.26 13.61 13.61
5.46 2.46 81.46 328.24 26.39 52.78
(95% CI low) 561
(95% CI high) 723
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Factor
Polarization parameters specimen
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EIS parameters
Ecorr (mV vs SCE) -598.23
Rp (KΩ/cm2) 6055
0.954
CPEbarr (µF/cm2sn) 8.01
Standard order 8
41.39
-698.61
283.63
0.885
31.2
DC current mode
1132
-828.6
24.46
0.65
52.1
Standard order 35
56.6
-770.6
194.18
0.894
25.4
Bare
317
-954.35
11.4
0.98
21.6
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Optimized
Icorr (nA/cm2) 1.9
n
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Highlights Electrolyte temperature undesirably influences the hardness of anodized coatings.
Maximum hardness of coatings was evaluated by optimization of effective parameters.
The diameter of alumina nanotube considerably affects hardness of anodized coating.
RP of the sample formed at optimum condition was at least 20 times more than others.
Porosity is the main factor affecting the corrosion behavior of the coatings.
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S0254-0584(16)30669-1
DOI:
10.1016/j.matchemphys.2016.09.006
Reference:
MAC 19152
To appear in:
Materials Chemistry and Physics
Received Date: 9 October 2015 Revised Date:
15 August 2016
Accepted Date: 4 September 2016
Please cite this article as: I. Mohammadi, S. Ahmadi, A. Afshar, Effect of pulse current parameters on the mechanical and corrosion properties of anodized nanoporous aluminum coatings, Materials Chemistry and Physics (2016), doi: 10.1016/j.matchemphys.2016.09.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of pulse current parameters on the mechanical and corrosion properties of anodized nanoporous aluminum coatings Iman Mohammadi*, Shahab Ahmadi, Abdollah Afshar
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Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11155-9466, Azadi Ave, Tehran, Iran *Corresponding Author, Tel: +98 21-6616-5204, Fax: +9821-6600-5717, E-mail address: [email protected]
Abstract
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In this study, the effects of pulse current parameters on corrosion resistance and mechanical properties of anodized coatings were evaluated. Hardness measurements, polarization and electrochemical impedance spectroscopy tests were employed to investigate the mechanical properties and corrosion behavior of these coatings. Also,
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field emission scanning electron microscopy (FE-SEM) was used to analyze the surface morphology and microstructure of the coatings. It was found that the properties of
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anodized coatings were dependent on various parameters, among which, time, temperature and pulse current parameters (current density limit, frequency and duty
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cycle) were optimized. Analysis of Variance (ANOVA) was conducted in order to optimize the results of designed experiments for predicting the hardness of anodic Al2O3 coatings. Experimental results showed that the temperature and the interaction of quadratic behavior of minimum current density with frequency and duty cycle were the most important terms influencing the hardness of these coatings. It was indicated that the highest hardness value of 642 HV was attained at the maximum and minimum
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current densities of 4.4, 1.27 A/dm2, respectively, a frequency of 82 Hz, procedure time of 27.2 min, duty cycle of 80.2% and the bath temperature of 13.5 ᵒC. In addition, the FE-
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SEM micrographs showed that the highest density is obtained through the mentioned optimum conditions. Moreover, the electrochemical tests revealed that the highest polarization resistance obtained at optimum conditions was more than 20 times greater
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than the other samples.
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Keywords: Anodizing; Aluminum; Pulse current; Hardness; Corrosion behavior
1. Introduction
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Aluminum and its alloys have a great potential to form an oxide layer on the own surfaces when exposed to medium atmosphere. Although this layer has a low thickness,
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it can cause an increase in mechanical properties and corrosion resistance of the aluminum alloy. Various methods have been used to increase the thickness of this layer
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among which anodized aluminum oxidation (AAO) is one of the most popular methods improving the corrosion behavior and mechanical properties of the aluminum alloy [13]. It is generally believed that the performance of the anodic layer depends on its structure and is influenced by various parameters such as electrolyte composition, current density and additives among which the current density and current waveform are the most important effective parameters on the properties of coatings [4-9].
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In traditional anodizing processes, the direct current mode has been typically applied to fabricate the oxide layers. The main drawback of this method is the formation of gas
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bubbles on the surface of anodized layer due to the significant difference between thermal conductivity coefficients of the evolved gas and oxide layer. These bubbles prod the localization of heat content on the surface of coating [10-11]. The heat
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localization leads to an increase in aggressiveness of electrolyte [12-13]. Among commonplace shortcomings for this method, the most important ones are increase of
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porosity, pore diameter and the propagation of defects which lead to degradation of hardness and corrosion behavior of anodized coatings [5, 11, 14-15]. Lee et al. [16] claimed that the hardness and corrosion properties of the anodized coatings improved significantly by applying the pulse current anodizing. In addition, it
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has been shown that the structure of anodized coating is improved using pulse anodizing [17]. It is due to that the heat amount generated in higher current densities
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applied at pulse anodizing is dissipated on the surface of coatings [5]. Therefore, higher current density and upper temperature resulting in a fast and inexpensive anodizing
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process could be conducted without any burning of the coating surface [18-19]. Hence, corrosion resistance enhancement of anodic layer, increasing the thickness and surface density of anodized coatings, and improving the efficiency of the film formation evolution are the main benefits of this method [5, 11, 19]. Hybrid pulse anodizing is a new approach fabricating anodic oxide coatings at a higher bath temperature and decreasing the imperfection of low temperature anodizing [20-21].
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As previously mentioned, using of pulse current leads to an increase in hardness of anodized coatings which their properties considerably depend on pulse current
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parameters. Therefore, due to various conditions of anodizing procedure, an optimization of the process parameters is required. Hence, to date, various methods are used for optimization [22-26]. Design of experiment (DOE) is one of the best methods
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for optimization of processes bearing a great potential for various purposes [27-31]. In this research, response surface methodology (RSM) and central composite design (CCD)
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are used to achieve the optimum values of hardness when using pulse current mode, sulphuric/oxalic acid electrolyte and an ambient bath temperature. 2. Experimental procedures
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2.1. Materials and pretreatment
Starting material in this study was AA 1050 with the dimension of 30×20×1. For obtaining a smooth surface, specimens were polished from the mechanical ground to
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P1200 grade paper. Prior to anodizing, the samples were formerly degreased by ultra-
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sonication in acetone for 10 min and latterly, electropolished in a mixture of HClO4 and ethanol in the volume ratio of 1:4 at 20 V for 30 s and at the room temperature. Finally, the specimens were rinsed immediately with distilled water and then dried. 2.2. Anodizing process
For anodizing process, a direct current plus pulse (DCP) waveform generated by pulse rectifier (SL 2/25 PCS, Iran) was used. As shown in Fig. 1, for this process, the following
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four parameters were important which could vary independently: maximum current density (Imax), minimum current density (Imin), toff and ton (In pulse current, duty cycle
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(C) = ton/ (ton + toff) and frequency (D) = 1/ (ton + toff)). Anodizing process in this study benefited a two-electrode method in which the sample was used as the anode and an aluminum sheet larger than anode size as cathode. For anodizing process, a
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sulphuric/oxalic electrolyte was employed as shown in Table. 1.
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2.3. Design of Experiments methodology
In this study, design of experiment (DOE) was evaluated by Design Expert 7 software. In order to optimize hardness of anodized coatings, six important parameters such as time, temperature and pulse current (current density limit, frequency and duty cycle)
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were considered. As shown in Table 2, a range of parameters was used for establishing the possible effects. For optimizing these factors, response surface methodology (RSM) based on central composite design (CCD) method was used. The experimental
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procedures produced by this method are shown in Table 3. As illustrated, 52 experimental procedures were designed by CCD method under various conditions
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based on performing pulse current at room temperature. As seen, the standard order of experimental procedure does not match run order since the software considers randomization of parameters for reducing the effects of unwanted parameters as well as noises on responses [32]. Also, as seen, 8 replicates were used due to the checking replicability of experiments. For obtaining maximum hardness of coatings, optimizing process was performed by ANOVA analysis based on finding the best model. For 5
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performing this approach, contour and surface plots (in contour and surface plot, two factors vary and other parameters are considered to be constant) were used.
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2.4. Characterization of coatings 2.4.1. Micro-hardness measurements
In order to evaluate hardness of anodized coatings under various experimental
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procedures, Vickers micro-hardness (Buehler model 1600-6100, USA) was performed.
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For this measurement, a load of 25 g was applied for 15 s based on ASTM B578 standard. The measurements were taken at five different points which were distributed on the coatings, and the average values were reported. 2.4.2. Surface morphology
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Morphologies of the surface coatings were inspected by high vacuum FE-SEM (model Mira 3-XMU, Czech). Before FE-SEM observation, Au was sputtered on the surface of
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the specimens.
2.4.3. Electrochemical measurements
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Electrochemical responses of the oxide layers were measured by using a potentiostat /galvanostat (Autolab model 302N) apparatus. The measurements were taken based on ASTM–G102-98 standard and an extrapolation method. A three-electrode cell was used in the condition of performed 3.5 wt% NaCl solution at room temperature. In this method, the anodized sample was placed as the working electrode, saturated calomel electrode (SCE) as reference electrode and a graphite sheet was chosen as the counter 6
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electrode. In order to study the potentiodynamic polarization of the samples, a scan rate of 1 mVs-1 starting at -400 mV below the open circuit potential (OCP) and finishing at
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800 mV above of the OCP was employed. In order to evaluate the electrochemical behavior of the anodized samples in more details, an electrochemical impedance spectroscopy (EIS) was used by EG&G (model
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273A) apparatus which worked at the same condition of potentiodynamic polarization measurement. To plot the EIS spectra, a frequency range of 10-2-105 Hz with an
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amplitude of 10 mV was used. It should be noted that in order to establish the open circuit potential, the samples were immersed for 30 min in the electrolyte solution before all electrochemical processes. Finally, the ZSimpWin V3.40 software was used for fitting the EIS result.
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3. Results and discussion
3.1. Effect of pulse current parameters on the hardness of anodized coatings
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Table 4 shows the results of analysis of variance (ANOVA) that are performed for
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analyzing the results of experimental design. According to this Table, ANOVA predicts that quadratic model is an appropriate model in the condition of the applied pulse current for forecasting the hardness of anodized coatings, which is due to the application of Fisher’s test (F test) by ANOVA (acceptable model contains appropriate F value and low P value [33]). This model was selected due to the lower P value, although the F value is greater than that for cubic model. In conformity with the results, ANOVA presents Eq. 1 which can predict the hardness of anodized coatings under the maximum 7
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current density (A), minimum current density (B), duty cycle (C), frequency (D), bath temperature (E) and anodizing time (F).
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Hardness (HV) = 524.74 – 13.18A – 14.02B + 67.09C – 57.50D – 69.18E – 18.78F – 42.5A×B – 2.38A×E – 29.63A×F – 7.8B×E – 13C×E – 7.75E×F – 17.75A2 – 50.61C2 – 46.73E2 – 91.65B2×C + 100.65B2×D (1) In order to study the normality of the data, normal probability plot (NPP) was
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performed, and the associated results are shown in Fig. 2. As seen, except some other residuals, the results are distributed along the predicted line (The residual means the
each experimental design [34]).
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difference between observed and predicted hardness values of anodized coatings in
The next step of this study was to investigate the effective factors resulting in obtaining maximum hardness for anodized coatings. For this purpose, the coefficient of Eq. 1 and
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contour and surface plots were used. Based on Eq. 1 and Fig. 3, the quadratic interactions of minimum current density with duty cycle and frequency were the most
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important factors influencing the hardness of anodized coatings. Fig. 4 illustrates the contour plots of this study. In this figure, the results with the same hardness are
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connected to each other to produce contour lines of a constant hardness. In this case, the response surface is viewed in a two dimensional plane. Generally, circular contour lines are the evidence of no interaction between two considered factors [34]. According to Fig. 4, temperature is one of the important parameters which has a negative influence on the hardness of anodized coatings (due to its negative coefficient in Eq. 1). It is related to this fact that the increasing of temperature on the surface of anodized
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coatings leads to greater aggressiveness of the electrolyte. Due to this effect, the dissolution rate of coatings increases (Eq. 2) which results in the formation of larger
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pores diameter and poor structure of the anodized coatings [11, 35]. Therefore, temperature increment leads to the decrease in the hardness of anodized coatings. + 6 → 2 + 3
(2)
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As shown in Fig 4, the maximum and minimum current densities have a negative effect on the hardness of anodized coatings. As shown in Fig. 4 (a), when both of these
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parameters are increased simultaneously, the hardness of anodized coatings is in its critical condition, according to the coefficient of the interaction between these factors. In fact, when using pulse current, the minimum current density has a recovery behavior for growth of the anodized coatings [5]. This means that when the samples are anodized
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with this current, some heat will be dissipated. In other words, using the pulse current causes the reduction of heat concentration on the surface and subsequently increases
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the hardness [18- 19]. On the other hand, as shown in Fig. 4 (b and c), the maximum and minimum current densities have interaction with temperature. It traces back to this fact
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that increasing of current density leads to higher rates for the formation of Al2O3 based on Eq. 3. As far both Joule effect and the Eq. 3 are exothermic and they are encouraged by current density, therefore, these two factors lead to the temperature increment on the surface of the oxide layer [11]. Therefore, this effect results to the increasing of the solution aggressiveness and dissolution rate of coatings [15, 35]. Fig. 5 shows the effect of current density increment on the morphology of anodized coatings. Comparison of
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Fig. 5 (a) and Fig. 5 (b) demonstrates that applying greater average current densities leads to increase in pore diameter. Therefore, using a greater current density increases
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porosity which subsequently reduces the hardness of anodized coatings. 2 + 3 → + 1424 /
(3)
As seen from Fig. 4 and Eq. 1, duty cycle improves the hardness of anodized coatings.
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When the duty cycle increases, the anodic oxidation of samples conduct with a greater current density which increases surface density. Due to the mentioned effect, hardness
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of coatings increased [17, 36]. Furthermore, when duty cycle reaches a maximum (i.e. direct current), temperature increases on the surface of these coatings since application of direct current mode conducts to releasing of gas bubbles (with the oxidation of O2ion reaction according to Eq. 4) on the surface of anodic oxide layer [10-11]. Due to these
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gas bubbles, voids and microcrack defects along the cell boundaries are created which result in a weak junction strength [16].
(4)
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(O2- → ½ O2 + 2e-)
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In other side, the difference between thermal conductivity of this gas and aluminum oxide, leads to the raise in heat content [11, 15] and increases the temperature (Fig. 4 (d)) on the surface of anodic layer. Hence, the aggressiveness of electrolyte and then the pore diameter are increased. This effect is shown in Fig. 5 (c). As seen, using direct current leads to higher pore diameter than the pulse one. Therefore, increasing of duty cycle finally decreases the hardness of coatings.
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Based on Eq. 1, frequency has a negative effect on the hardness of anodized coatings. When the frequency is increased, the mechanism of dissolution is activated in both of
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the appropriate and bad accumulated structure cells. As can be seen in Fig. 5, using a greater frequency conducts to the increasing of void and defect in the structure which decreases the hardness of coatings.
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Also, referring to Eq. 1 and Fig. 4 (e-f), coefficient of time and their interactions with maximum current density and temperature are negative because increasing in duration
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time causes powder coatings [37]. Furthermore, with the increase in film thickness, the anodizing electric resistance of the film also increases, which leads to an increase in the anodization temperature of the film surface [24, 25]. In addition, when anodizing time increases, due to larger surface area, gas bubble content increases which subsequently
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increases the temperature on the surface of anodized coatings. This increasing of temperature is shown in coefficient of interaction of time with temperature. Furthermore, as seen in Fig. 5 (e), when anodizing time is increased, larger pores are
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layer.
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created. Therefore, longer anodizing time leads to decreasing in hardness of the oxide
Based on the optimization of six parameters and Table 5, in order to obtain maximum hardness of coatings, the software calculated that anodized sample under six selected parameter levels will give a hardness of 642 HV by 95% confidence. Also, results showed that the interval parameter for this study is in the range of 495.15 to 790.18 HV. For the calculation of the error amount in this optimized level, confirmation run was
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evaluated in this point and were compared with the results predicted by Eq. 1 [32]. Results show that the hardness of coatings in optimal point is 628 HV which does not
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significantly differ from the predicted value. Fig. 6 shows the predicted hardness by CCD method in comparison with the actual values for the samples anodized at various
shows the accuracy of Eq. 1.
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process conditions. As seen, the actual points are close to the estimated line which
3.2. Effect of pulse current parameters on the corrosion behavior of anodized coatings
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3.2.1. Potentiodynamic polarization measurements
Potentiodynamic polarization tests were performed in 3.5 wt% NaCl solution in order to study the corrosion behavior of coatings synthesized under various conditions. Based
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on Fig. 7 and Table 6, corrosion potential of the coating anodized in direct current mode is -830.6 mV (vs. SCE). It is noteworthy that the corrosion potential enhances to a noble electrode potential of -642.5 mV (vs. SCE) when the anodizing conditions shift to the
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optimized one. Furthermore, as shown in Fig. 7, when the anodizing condition altered from the optimized conditions, the corrosion resistance of samples is decreased.
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Referring to the FE-SEM micrographs shown in Fig. 5, the surface density and thus corrosion resistance of the coatings synthesized under different process conditions deteriorate in comparison with the sample produced under the optimum parameters. Based on Figs. 5 and 7, the corrosion properties of the coatings firstly are affected by pore diameter. In other words, when the sample is anodized under DC current mode, various events can decrease the corrosion resistance of the coatings among which local 12
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heat amount as the main reason is concentrated on the surface of anodic film. This local heat amount leads to an increase in dissolution rate, pore diameter, defects and voids
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on the anodic film and thereby, reducing corrosion resistance of the samples. 3.2.2. Electrochemical impedance spectroscopy
In this section, EIS studies were evaluated to confirm the potentiodynamic polarization
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test. The spectra obtained from EIS experiments have been presented in Fig. 8. The impedance data were analyzed using the simplified equivalent circuit of Fig. 9 widely
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proposed in the literature [4, 35] as well as using ZSimpWin V3.40 software. As can be seen in Fig. 9, the equivalent circuit contains two constant phase elements of CPEwall and CPEbarr which are associated with the capacitive behavior of porous oxide structure and barrier layer, respectively. These parameters show non-ideal capacitive behavior in
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both CPE. Other elements of Rsol, Rwall and Rbarr present resistances corresponding to the solution, porous and barrier layers, respectively.
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Referring to Figs. 7 and 8, a quantitative comparison between the polarization data and the results pertinent to the nyquist plots show a similar corrosion behavior for the
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samples. Also, capacitance loop diameter (Fig. 8 (a)) and Z modulus (Fig. 8 (b)) of the coating formed at the optimized condition are substantially greater than that of the anodized sample in DC current mode. Furthermore, the results of pulse current show that the corrosion resistance of the anodized coatings deteriorates when the applied parameters are changed respect to the optimum conditions.
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The term of n was used in order to justify the lower polarization resistance of the sample anodized under DC current mode. The value of n corresponds to the linear
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slope modulus of bode plot (Fig. 8 (b)). In this case when n is near 1, the surface is uniform and smooth and lower values (in our case n=0.65 for the specimen anodized in DC current mode) shows deviation from ideal capacitive behavior (which has been
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attributed to the inhomogeneity of the surface) [38, 39]. Also, it is worth noting the lower n values show a deterioration in corrosion resistance. Due to increase of chemical
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dissolution on the surface of anodic layer which can increase the defects of structure of DC anodized sample, the specimen anodized by DC mode indicates a lower n value compared to the optimum condition one. Therefore, the corrosion resistance of the
4. Conclusion
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anodized coating fabricated by optimum condition is greater than other conditions.
In this paper, pulse anodizing process parameters were optimized by using response
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surface methodology based on central composite design method. Results showed that DOE is an effective method for this optimization because of the considerable interaction
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between the parameters. Also, analysis of variance estimated that temperature and quadratic behavior of the minimum current density with frequency and duty cycle are the most important factors influencing the hardness of coatings. Furthermore, it was found that the maximum hardness of anodized coating (i.e. 642 HV) was attained at the maximum and minimum current densities of 4.4, 1.27 A/dm2, respectively, a frequency of 82 Hz, time of 27.2 min, duty cycle of 80.2% and the bath temperature of 13.5 ᵒC.
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Results of FE-SEM showed that this optimized conditions lead to a dense surface, increasing the hardness.
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The electrochemical measurements indicated that the polarization resistance of the coatings obtained under optimized conditions is higher than those obtained under any other condition. Results showed that the polarization resistance of the coatings
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synthesized at the optimized condition obtained 6055 KΩ/cm2 which was more than 20 times higher than that for other samples. Moreover, the amount of pores of the coatings
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was the main factor affecting the corrosion behavior of the coatings. Therefore, the polarization resistance of the coatings was affected by both the amount and the size of pores.
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[37] K. Yokohama, H. Konno, H. Takahashi and M. Nagayama, Advantage of pulsed Anodizing, Plat. Surf. Finish. 69 (1982) 62-65.
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[38] P.L. Cabot, J.A. Garrido, E. Perez, A.H. Moreira, P.T.A. Sumodjo, W. Proud, Eis study of heat-treated Al-Zn-Mg alloys in the passive and transpassive potential regions, Electrochim. Acta 40 (1995) 447-454. [39] Sh. Ahmadi, I. Mohammadi, S.K. Sadrnezhaad, Hydroxyapatite based and anodic Titania
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Fabrication,
characterization
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Table captions Table 1. The chemical composition of anodizing electrolyte solution
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Table 2. The range of process parameters for pulse anodizing in order to obtain an optimum condition
Table 3. Experimental design for optimizing hardness of anodized coating under pulse current
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Table 4. ANOVA results for various models which predict the hardness of anodized coatings
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Table 5. Intervals range of the predicted point and 95% confident to obtain maximum hardness of anodized coatings under pulse current
Figure captions
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Table 6. Electrochemical parameters of the samples anodized in the various conditions
Fig. 1. Direct current plus pulse (DCP) waveform produced by pulse power supply.
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Fig. 2. Normal Probability Plot of residuals for hardness of the anodized coating
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synthesized by pulse current.
Fig. 3. Quadratic Surface plots corresponded to the effects of both a) minimum current density and duty cycle and b) minimum current density and frequency on the hardness of coating anodized via pulse current. Fig. 4. Contour plots of the hardness for the coating anodized under various conditions: (a) maximum and minimum current densities; (b) maximum current density and 21
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temperature; (c) minimum current density and temperature; (d) duty cycle and temperature; (e) maximum current density and time; (f) time and temperature.
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Fig. 5. FE-SEM micrographs of the surfaces anodized in the condition: (a) optimized, (b) Imax=4 A/dm2, Imin=3 A/dm2, f=2.5 Hz, t=45 min, D=75% and T=10 ˚C, (c) direct current mode by current density of 4 A/dm2 and the bath temperature of 10 ˚C, (d) at the same
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condition of (b) and with these differences that Imin=1 A/dm2 and f=333 Hz, (e) at the
cross section of optimized condition.
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same condition of (b) and with these differences that Imin=1 A/dm2 and t=60 min and (f)
Fig. 6. Predicted hardness by CCD method in comparison with the actual values for the samples anodized at various process conditions.
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Fig. 7. Polarization curves of the bare and the anodized sample fabricated at optimum condition, standard order 8, standard order 35 and direct current mode by current density of 4 A/dm2 and the bath temperature of 10 ᵒC.
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Fig. 8. (a) Nyquist spectra, (b) Bode and (c) Bode-phase diagrams of the optimum
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condition, standard order 8, standard order 35, bare sample and direct current mode by current density of 4 A/dm2 and the bath temperature of 10 ᵒC. Fig. 9. Schematic for anodized coating containing barrier and porous oxide layers with equivalent electric circuit of the anodized coating.
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Content 200 g/L 20 g/L 10 g/L 10 ml/L 3 g/L
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Composition H2SO4 H2C2O4 C7H6O6S.2H2O C3 H 8 O Al2(SO4)3
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Level
Parameter 3
B: Imin (A/dm2)
0
C: Duty cycle (%)
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D: Frequency (Hz)
2.5
E: Temperature (°C)
10
F: Time (min)
20
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0
1
2
3.5
4.5
5.5
6
0.5
1.5
2.5
3
33.5
57.5
81.5
95
74.3
201.3
328.2
400
13.6
20
26.4
30
27.2
40
52.8
60
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A: Imax (A/dm2)
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Run order
A: Imax (A/dm2)
B: Imin (A/dm2)
C: Duty cycle (%)
D: Frequency (Hz)
E: Temperatur e (°C)
F: Time (min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
41 52 27 3 13 36 2 37 45 48 10 20 9 42 5 29 47 24 8 19 30 39 16 7 26 25 11 28 22 44 32 50 18 17 31 34 38 23 12 21 33 6 49 1 51 15 43 4 40 14 35 46
3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.5 5.5 3.0 6.0 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5
0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 0.5 0.5 2.5 2.5 1.5 1.5 0.0 3.0 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
33.5 33.5 33.5 33.5 81.5 81.5 81.5 81.5 33.5 33.5 33.5 33.5 81.5 81.5 81.5 81.5 33.5 33.5 33.5 33.5 81.5 81.5 81.5 81.5 33.5 33.5 33.5 33.5 81.5 81.5 81.5 81.5 57.5 57.5 57.5 57.5 20.0 95.0 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5 57.5
74.3 74.3 74.3 74.3 74.3 74.3 74.3 74.3 328.2 328.2 328.2 328.2 328.2 328.2 328.2 328.2 74.3 74.3 74.3 74.3 74.3 74.3 74.3 74.3 328.2 328.2 328.2 328.2 328.2 328.2 328.2 328.2 201.3 201.3 201.3 201.3 201.3 201.3 2.5 400.0 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3 201.3
13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 13.6 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 26.4 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 10.0 30.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0
27.2 52.8 52.8 27.2 52.8 27.2 27.2 52.8 52.8 27.2 27.2 52.8 27.2 52.8 52.8 27.2 52.8 27.2 27.2 52.8 27.2 52.8 52.8 27.2 27.2 52.8 52.8 27.2 52.8 27.2 27.2 52.8 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 20.0 60.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0
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Standard order
Source
DF
Mean square
FValue
PValue
38 23 17 1 7
12126.38 13306.12 9827.34 4237.75 189.71
63.92 70.14 51.80 22.34
<0.0001 <0.0001 <0.0001 0.0021
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Linear 2FI Quadratic Cubic Pure error
Sum of squares 4.6E5 3E5 1.7E5 4237.75 1328
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Name
Level
A B C D E F Response Hardness/(HV)
Imax /(A/dm²) Imin /(A/dm²) Duty Cycle / (%) Frequency /(Hz) Temperature /(°C) Time /(min) (Prediction) 642
4.4 1.27 80.2 82 13.5 27.2 (95% PI low and 95% PI high) 495-790
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Low level
High level
3.54 0.54 33.54 74.26 13.61 13.61
5.46 2.46 81.46 328.24 26.39 52.78
(95% CI low) 561
(95% CI high) 723
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Factor
Polarization parameters specimen
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EIS parameters
Ecorr (mV vs SCE) -598.23
Rp (KΩ/cm2) 6055
0.954
CPEbarr (µF/cm2sn) 8.01
Standard order 8
41.39
-698.61
283.63
0.885
31.2
DC current mode
1132
-828.6
24.46
0.65
52.1
Standard order 35
56.6
-770.6
194.18
0.894
25.4
Bare
317
-954.35
11.4
0.98
21.6
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Icorr (nA/cm2) 1.9
n
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Highlights Electrolyte temperature undesirably influences the hardness of anodized coatings.
Maximum hardness of coatings was evaluated by optimization of effective parameters.
The diameter of alumina nanotube considerably affects hardness of anodized coating.
RP of the sample formed at optimum condition was at least 20 times more than others.
Porosity is the main factor affecting the corrosion behavior of the coatings.
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