Commercially obtained aluminum sheets from Kenya Industrial Area, Kenmetals Ltd. (Nairobi-Kenya) with a data sheet showing 99.9% purity of 0.5 mm thick were cut into pieces of 4.0 cm × 1.5 cm. The pieces were cleaned in an ultrasonic bath of acetone for 5 minutes to degrease them. Degreased pieces were rinsed in distilled water to remove traces of acetone and left to dry in air.

Commercially obtained 70% pure undiluted perchloric acid (HClO₄) from RFCL Limited, India was added in small quantities to ethanol in a beaker. The reaction between ethanol and perchloric acid is exothermic. Therefore, ethanol in the beaker was kept at a temperature below 10˚C as perchloric acid was added. The ratio of perchloric acid to ethanol was 1:4 by volume.

Heat is generated as current passes through the electrolyte, and ethanol moderates the viscosity of the electrolyte and acts as a solvent for the molecules and ions. Perchloric acid ensures that required low pH favorable for the formation of Al³⁺ ions is maintained, and does not react to form other ions at the anode and also cools the electrolyte [7] .

Figure 1 shows the electrolytic cell used to electro polish the substrate. Graphite electrode was used for the cathode while the cleaned aluminum piece was made the anode. Voltage was applied gradually to the electrodes

that were immersed in the electrolyte hence the current drawn by electrolyte also varied. Since current generates heat, to ensure even temperature in the electrolytic solution continuous stirring using a magnetic stirrer was used. The temperature of the electrolytic solution was monitored using a mercury thermometer and kept below 25˚C. The low temperatures were observed to prevent rapid oxidation by perchloric acid that occurs at temperatures above 38˚C. During the electro-polishing the applied voltage and the current drawn were recorded stepwise. The temperature kept rising from 0˚C to 25˚C. The sample was removed and rinsed in de-ionized water to wash off remnants of the electrolytic solution.

A solution of 1.0 M of perchloric acid from RFCL Limited, India was mixed with ethanol in the ratio of 1:2 by volume. The mixture was cooled and maintained at temperatures below 10˚C by immersing the beaker with the mixture in an ice bath whose temperature was continuously monitored. Using graphite electrode as the cathode and polished aluminum as the anode a dc voltage of 30 V was supplied for 15 minutes. Figure 2 shows the set up for the texturing process.

Typical current density-applied voltage curves have been reported earlier by [8] that sets standards and safety guidelines for electro-polishing where electrolytes are basically classified in two categories; 1) perchloric acid in alcoholic electrolyte and 2) mineral acid in organic solution electrolyte. The typical current density-applied voltage curve for perchloric acid in alcoholic electrolyte takes the form in Figure 3 and electro polishing occurs

between B and C region [8] .

Figure 4 shows measure current-voltage characteristics during electro-polishing of aluminum samples. Ideally Figure 4 should mimic Figure 3, but it was noticed that the point where the gray colour of the sample begins to peel off was between 10 to 13 volts which indicated the onset of polishing. In Figure 4, it is observed that between 8 to 19 volts there is linear increase of current density with applied potential which is the trend for an ideal electro-polishing curve and this is the region where the samples were electro-polished.

The physical observation showed that, after electro polishing, the surface gained luster that resembled metallic mirror. The surfaces of the as cut aluminum piece was rough as shown in magnified picture taken using light microscope (Olympus BX81) at magnification of X50 as shown in Figure 5. After electro polishing, the surface looks smooth and does not show ruggedness as observed in as cut plate under same magnification (Figure 5(b)).

Ruggedness of the surface of as cut aluminum sheet before polishing is unfavorable for effective controlled texturing. The surfaces of the samples were also analyzed using Carl Zeiss SIGMA FE-SEM OXFORD X-ACT scanning electron microscope (SEM) equipped with EDS Detector. The micrographs shown in Figure 6 depict considerable change in surface smoothness whereby ridges and cracks of above 2 µm on the unpolished surface (Figure 6(b)) are considerably smoothened out on polishing (Figure 6(b)). However, sparsely distributed pores of less than 1 µm can be observed on the electro-polished surface.

Since the aluminum sheet was commercially obtained, analysis of as-cut samples was carried out using X-ray diffraction (XRD-Phillips, PW 3710 XRD machine) to confirm if the samples were pure aluminum sheet. The results of the analysis are shown in Figure 7. Figure 7(a) shows reference XRD spectrum for aluminum powder (99.99%) obtained from Gaithersburg, Maryland, USA while Figure 7(b) is the pattern obtained for the test aluminum sheets under study.

Figure 7 clearly shows that the peaks obtained for the aluminum substrates resemble very closely those of the reference sample. The intensities seems to differ but the crystallographic planes are similar with prominent peaks appearing at 38, 44, 65 and 78 degrees which corresponds the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes. These are consistent with observation by [9] for aluminum powder (99.99% purity) diffraction. The maximum 2θ angle scan done for the substrates was 95 degrees as opposed to the reference 2θ angle scan which was 140 degrees. This resemblance of peaks confirms that the samples used are indeed aluminum sheets.

In addition, during electro polishing, chemical reaction like oxidation could have taken place. In order to ascertain purity of the electro polished aluminum sheet, they were analyzed using EDX. The EDX analysis in Table 1 depicts the sheet as 99.66 at% pure Al containing 0.34 at% silver impurities. The silver impurities could have been contributed from the silver plated crocodile clips used to hold the samples during electrolysis. The sheet is largely pure aluminum. It is important to note that the absence of oxygen. The commonly formed aluminum oxide, from exposure to air was etched away and even after annealing at 400˚C in air the oxide was not formed.

Electro-texturing of the polished aluminum surface was done at different etchant concentrations with an aim of achieving different pore sizes. The concentration was changed by diluting fixed volume of perchloric acid with varying volume of ethanol. Four samples were textured, one in undiluted perchloric and the other three in ethanol diluted perchloric acid at volume ratios of 1:1, 1:2 and 1:3 of acid to ethanol. The SEM micrographs in Figure 8 show the resultant pores for the indicated concentrations.

The aluminum sheet etched in undiluted perchloric acid formed highly populated bigger pores of more than 20 µm wide for a fixed area of view. The sheets etched in diluted etchant concentration have fairly less density of pores of more than 20 µm width and had more pores of less than 2 µm size. This means that undiluted perchloric acid produces a rougher surface compared to the diluted perchloric acid as an etchant. For perchloric acid to ethanol ratios by volume of 1:1, 1:2, 1:3, there is no significant variation in pore population and sizes except that it took longer time to etch the aluminum surface as the acid concentration reduced.

The pores produced by electro-etching aluminum surface in perchloric acid are not uniform in size and do not form repeatable pattern but varies from nanometers range to tens of micrometers. Within the large micrometer sized pores other smaller pores of nanometre and micometre ranges are co-hosted. This means that electro etching the aluminum sheet in perchloric acid forms pores within pores shown in a SEM micrograph in Figure 9.