UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

1220 IMMC 2018 | 19th International Metallurgy & Materials Congress

Eff ect of Voltage and Charge Densities on Nanoporous Oxide Pore Diameter on AA1050

Mustafa Kocabaş¹, İlyas Şavklıyıldız¹, Michele Curioni², Nurhan Cansever³

¹Selcuk University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Konya, Turkey²University of Manchester, School of Materials, Corrosion and Protection Centre, Manchester, United Kingdom

³Yildiz Technical University, Faculty of Chemical and Metallurgical Engineering, Department of Metallurgical and Materials Engineering, İstanbul, Turkey

Abstract

The aim of this work is to investigate the effect of applied voltage and charge densities on the nanoporous oxide pore diameter on electropolished AA1050 (1050 aluminium alloy) . Anodic oxidation was carried out in 2M sulfuric acid (H2SO4) solution at three different voltage values (12, 15 and 18 V) and charge densities (757, 3231, 6848 mC/cm2). The corresponding times for anodizing on each charge density were approximately 47, 181 and 383 seconds, respectively. All experiments were carried out under surveillance of computer control system. As a result of these treatments, oxide layers in porous structure were obtained at 411, 1751 and 3753 nm thicknesses. Surface and cross sectional morphology were investigated by using FE-SEM (field emission scanning electron microscope). For the cross sectional study, the anodised samples were prepared via ultramicrotomy with a glass knife cut. In addition, current density-time data obtained for all experimental conditions were represented on graphs drawn by using LabVIEW software. Conventional current density-time graphs were obtained under all conditions.

1. Introduction

Aluminum and its alloys are the most widely used for decades as a non-ferrous metal groups due to its outstanding characteristics such as lightness and the high strength. In addition, the existing advantages of aluminum and its alloys have been enhanced by improvements in surface properties (1,2). Anodic oxidation is an electrochemical method that consists of converting aluminum to its oxide (Al2O3) using an external current (2,3). Anodic oxidation process has been applied in order to improve the tribological and corrosion behaviors and also service life of aluminum and its alloys (1,2).

In sulfuric acid solution which is the most popular solution compared with other acid solutions (chromic and phosphoric acid), the treatment can be performed at lower voltage values (1,4–6).

The present study aimed for a better understanding of the effect of applied voltage and charge density on the anodic film morphology and pore diameter of anodized AA1050. For this purpose, sulfuric acid anodizing were processed in different conditions to investigate relationship between average pore diameter distribution and applied voltages with varied charge densities.

2. Experimental Procedure

In this study, AA1050 aluminum alloy (Al - 0.29Fe -0.15Si - 0.02Ti - 0.02Zn - 0.01Mg - 0.01Ni, in wt.%) was employed as substrate. In order to prepare sample surfaces for anodic oxidation, electropolishing were subjected in a solution containing ethanol and perchloric acid. Electropolishing process (20 V and 4 min) effectively removed the rolling lines and generated a surface with textured appearance.

There are many parameters that affect the anodic oxidation process. However, in this study, the effect of two parameters, voltage value and charge density, which are important parameters, are examined. The charge density (Q) is defined as the total area below the line in the current density-time graph, and the unit is C/cm2.

Anodic oxidation was carried out in 2 M sulfuric acid (H2SO4) solution at three different voltage values (12, 15 and 18 V), then for each applied voltages we aimed to reach different charge densities such as 757, 3231, 6848 mC/cm2 to observe evolution of the oxide layer. The solution temperature during experiment was kept constant at 20 ±2 °C and the solution was continuously mixed (200 rpm). AA1050 sample is used as anode where a pure aluminum piece is used as cathode material. LabVIEW software was used to control the experimental condition and collect the data. Current density-time plots were drawn during anodic oxidation process. First the max charge density was introduced to the program and the experiment was started at one of the selected voltages. When the system reached the max value, the program stopped the experiment automatically. Surface morphology and cross

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sectional micrographs were examined with the help of FE-SEM.

3. Results and Discussion

3.1. Current Density-Time Graphs

The current density-time graphs for each charge density in each voltage values are plotted as shown in Figure 1. The conventional four-stage graphs have been observed in all conditions during anodic oxidation process which confirms that the anodic oxidation process was done properly (1,5,7,8). In all experiment, the targeted charge density at higher voltage values has been achieved more quickly. So,the increased voltage caused the decrease in the duration of the anodic test.

Figure 1. Current density versus time graphs for different voltage values and charge densities.

Applying varied voltage values effect the current density and anodizing time records but, total charge density during experiment is constant which implies the anodizing process reproducibility.

3.2. Surface Examinations

In these images, the oxide pores formed by the process are clearly observed with homogeneous distribution throughout surfaces. Surface FE-SEM images given in Figure 2 a-f were selected to see voltage and charge density effect on morphology. Figure 2 a-b and c-d represent the coatings which are produced at same voltage (12V) with different charge densities (757 and 6848 mC/cm2, respectively). When we compare the morphology between Figure 2 a-b and c-d, increasing the applied charge density affect the pore diameter with a slight increment. Figure 2 a-b and e-f represent the coatings which are produced at same charge densities (757 mC/cm2) with different voltages (12V and 18V, respectively). Comparing these images shows us the increment on porous as increasing voltage.

Figure 2. FE-SEM images after the process at different charge densities and at 12V (a-b) 757, (c-d) 3231 and (e-f)

6848 mC/cm2.

Due to nature of alloying, the substrate material AA1050 contains small amount of secondary phases which were detected in the surface FE-SEM examinations after the anodic oxidation process (Figure 3). It is observed that the oxide layer was formed on the secondary phases too, but homogeneous pore distribution on secondary phases have not observed.

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

1222 IMMC 2018 | 19th International Metallurgy & Materials Congress

Figure 3. FE-SEM images of the secondary phases on the anodized surface in different conditions a) 15V-3231

mC/cm2 b) 15V-757 mC/cm2.

3.3. Measurement of Pore Diameters

In this study, oxide pore diameter which is one of the important parameter of anodic oxidation process, was measured with the help of the imageJ software. The pore diameter distribution of the (15V-6848 mC/cm2) condition are represented in Figure 4. Monolithic pore size range was observed around 10 nm. At least 500 pore diameters measurement was done to obtain average diameter for each condition. Table 1 represents the average pore diameter as anodizing results of AA1050 for all conditions. A slight increment in the average pore diameter due to the increased voltage value and the charge density was determined, as verified literature (1,9,10).

Figure 4. Pore diameter distribution of the 15V-6848 mC/cm2 condition.

Table 1. Anodizing results of AA1050 for different conditions.

Chargedensity

(mC/cm2)

Voltage value(V)

Time (s)

Averagepore

diameter(nm)

Film thickness

(nm)

757 12 115 8,51 411 ±12 757 15 60 8,98 411 ±12 757 18 47 9,22 411 ±12

3231 12 433 8,66 1751 ±16 3231 15 300 9,26 1751 ±16 3231 18 181 9,73 1751 ±16 6848 12 861 8,87 3753 ±61

6848 15 600 9,58 3753 ±61 6848 18 383 9,82 3753 ±61

3.4. Cross Sectional Examinations

In Figure 5, cross sectional SEM images of the anodic oxide of three different conditions are presented. For the cross sectional study, the anodized samples were prepared to conventional ultramicrotomy (11). The thickness of the anodic films, approximately obtained for all charge densities (757, 3231, 6848 mC/cm2) were 411 ±12, 1751 ±16 and 3753 ±61 nm, respectively. The micrograph of middle thickness (Fig. 5b) shows distinct morphology due to proper cutting direction. Normally, all samples should have the same pore characteristics and shapes irrespective of the cutting direction.

In this work, same oxide thicknesses were obtained in three different voltage values with same charge density which implies that oxide layer thickness is independent from applied voltage as shown Table 1. On the other hand, increasing charge densities results thicker oxide layer (Figure 5). However, the ratio of formed oxide layer thickness as a function of charge densities is acquired constant. Thus, three anodic oxide films, with similar morphology but different thickness, were produced.

Figure 5. The cross sectional micrographs on three different charges (a) 757, (b) 3231 and (c) 6848 mC/cm2.

Figure 6 shows cross section of the growing oxide layer on the second phase. It was observed that the oxide layer continued to growing thorough second phase, but growing rate is slightly decreased vicinity of these secondary phases.

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Figure 6. Cross sectional SEM images of growing the oxide layer on the substrate with second phase.

4. Conclusion

• It has been determined that increasing both voltage and charge density values resulted slight increase on average pore diameter.

• Three different thickness values were achieved with anodic oxidation at different charge densities (757, 3231 and 6848 mC/cm2).

• In the anodizing process, it was determined that oxide layer thickness is not depend on oxides voltage values, but charge density.

• Oxide layer growth is observed on second phase in substrate, but a minute decrement on growth rate around second phase regions was observed.

Acknowledgment

The authors expressed their thanks to EPSRC for the support of the LATEST2 Programme grant (EP/H020047/1) and TÜB TAK (The Scientific and Technological Research Council of Turkey) for its financial support (Scholarship number 1059B141401083 and Project number 114M063). This work was also supported by Research Fund of the Yildiz Technical University (Project Number: 2014-07-02-DOP02).

References

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[6] Macias G, Ferré-Borrull J, Pallarès J, Marsal LF. Effect of pore diameter in nanoporous anodic alumina optical biosensors. Analyst. 140(14), (2015), 4848–54.[7] Mohamed AK. Growth Mechanism of Porous Anodic Films Formed on Aluminium in Sulphuric Acid. PhD Thesis, University of Manchester; 2010, UK. [8] Nishinaga O, Kikuchi T, Natsui S, Suzuki RO. Rapid fabrication of self-ordered porous alumina with 10-/sub-10-nm-scale nanostructures by selenic acid anodizing. Sci Rep. 3, (2013), 2748.[9] Belwalkar A, Grasing E, Van Geertruyden W, Huang Z, Misiolek WZ. Effect of processing parameters on pore structure and thickness of anodic aluminum oxide (AAO) tubular membranes. J Memb Sci., 319(1–2) (2008), 192–8. [10] Bwana NN. Synthesis of highly ordered nanopores on alumina by two-step anodization process. J Nanoparticle Res. 10, (2008), 313–9.[11] Curioni M, Saenz de Miera M, Skeldon P, Thompson GE, Ferguson J. Macroscopic and Local Filming Behavior of AA2024 T3 Aluminum Alloy during Anodizing in Sulfuric Acid Electrolyte. J Electrochem Soc., 155(8), (2008), C387.