Study of the Microstructure, Corrosion and Optical Properties of Anodized Aluminum for Solar Heating Applications

Abstract

Humans are increasingly required to harvest green solar energy in order to reduce energy bills and save the environment from the excessive use of fossil resources. In this article, the microstructures of both commercial non-colored anodized Al and commercial blackened anodized Al were studied using optical and scanning electron microscopy in order to interpret the results of their use as solar absorbing surfaces. Microscopic examination showed that the thickness of the anodization layers of the non-colored anodized Al and the blackened anodized Al were approximately 11 µm and 14 µm, respectively, and they were perfectly adhered to the mother Al. The corrosion rate of all studied Al surfaces was investigated using the potentiodynamic polarization technique in 3.5% NaCl as the corrosive medium. The blackened anodized Al surface exhibited the highest corrosion resistance, which made it the best surface for solar heating systems. Moreover, raw Al, matte black painted Al, and blackened anodized Al were tested as selective surfaces for solar radiation in different weather conditions. Our results demonstrated the superiority of the blackened anodized Al in terms of the ability to absorb solar radiation, in addition to its higher corrosion resistance properties. In experimental testing, temperature values higher than 90 °C were reached several times. A gain of an extra 5 °C was achieved when using a double-glazed cover in comparison with a single-glazed setup. In conclusion, we highly recommend using a commercial blackened anodized Al surface to manufacture solar absorbing heaters, owing to its similarity in solar radiation absorptivity with the commercial matte black painted Al, excellent corrosion resistance, superior endurance upon long-term exposure to solar radiation, light weight, low price, and availability. Additionally, the light reflectance % test demonstrated the characteristics of the used solar selective surfaces.

1. Introduction

The Sun is a tremendous renewable clean and free source of energy as long as there are non-fused hydrogen ions ready for nuclear fusion in its core [1]. This source of energy can be easily utilized for electricity generation via photovoltaic systems, industrial and domestic solar heating systems, concentrating solar plants using high temperature water evaporation at approximately 600–700 °C, and production of green hydrogen [2]. Every day that humankind does not use the available solar energy worldwide causes damage to the environment and thus, potentially harms everyone on the planet. In addition, using solar energy instead of normal electricity and natural gas probably reduces the cost of long-term usage, and supports the environment. Several recent articles have been published on the topic of solar water heating systems (SWHS), providing nearly comprehensive information associated with the development of solar collectors [3,4,5]. In fact, there is no logic in using traditional sources of energy in the presence of equivalent abundant competitive renewable energy sources when considering savings, renewability, eco-friendliness, and ease of use.

Solar energy can be obtained as thermal energy via various selective metal surfaces, augmented with the following surface treatments: matte black paint, composites of black paint with various metal powders, black paint with metallic oxides and compounds, and coating with metal oxides alone [6,7,8,9,10]. Black-painted surfaces tend to deteriorate due to their change in structure over time caused by long-term exposure to solar radiation. Thus, solar collectors must be characterized by good corrosion resistance, long operating time, and reduced susceptibility to the solar spectrum that reaches the Earth’s surface. Therefore, coatings using different techniques such as chemical routes, sputter coating, and anodizing selective surfaces are more reliable for solar heating system applications [11,12,13].

Practically, it is better to use blackened anodized aluminum as a selective surface to manufacture solar heating systems because of its good absorptivity for solar radiation as well as its excellent corrosion resistance. In addition, the texture of the anodized layer is almost perfect with regard to deterioration resistance over time. In 2021, Shaffei et al. investigated a comparison between black-dyed Al and blackened anodized Al [14]. Aluminum anodization and the blackening process of an anodized thin layer via chemical routes are well reported by many studies in this field [15,16,17]. However, there is not enough information regarding the use of blackened anodized commercial Al as an absorbent selective surface for solar radiation, or reporting on its microstructure and corrosion resistance in saline water.

Therefore, the present research emphasizes the following subjects regarding the utilization of anodized non-painted commercial Al, matte black painted Al, and blackened anodized Al surfaces: (i) Microstructural study of the used Al; (ii) Study of corrosion resistance of Al in highly salty water; (iii) Testing of blackened anodized commercial Al as a solar absorber compared with the non-painted Al and matte black painted Al; (iv) Measuring global solar intensity via solar heating experiments in conjunction with water and ambient temperature measurements simultaneously; and (v) Carrying out light reflectance %, transmittance %, and absorbance % tests for the used selective surfaces.

2. Experimental Apparatus and Equipment

Two identical wooden boxes with dimensions of 62 cm (length), 32 cm (height), and 15 cm (depth) were used in all experiments to house the aluminum systems. They had double-glazed front sides with air gaps of approximately 1 cm between two commercial glass sheets of 3 mm thickness for better insulation against the ambient environment. Each box was divided into two rooms using a wooden partition of approximately 3.5 cm thickness for good thermal insulation. The thickness of the wood used for the box’s construction was approximately 1.7 cm. The cover glasses were removable, so that the boxes could be used for single- or double-glazing tests, and also to facilitate the insertion and removal of water containers intended for solar heating. Six water containers of commercial Al with dimensions of 8 cm width, 4 cm depth, and 28 cm height were inserted. The Al tubes were rectangular in shape, corresponding to that usually used in building construction.

The designed water containers were as follows: two anodized commercial Al, silvery color, denoted (Al); three matte black painted silvery commercial Al (Glance spray paint, matte black 20 (UAE)), denoted (B. P. Al); one commercial blackened anodized Al, denoted (B. A. Al). All Al sheets were purchased from the local market in Amman, Jordan. Black painting of Al was performed by spraying twice to obtain a homogeneous layer with a thickness suitable to absorb solar radiation with high efficiency. The thickness of all Al containers was approximately 1 mm. Essential information regarding the anodization process is given in Section 4.2. Figure 1 shows the three types of water containers, Al, B. P. Al, and B. A. Al.

A digital LED light meter (type: MASTECH, MS6612T series) with a range of 0~200,000 LUX and accuracy of ±3% was used for measuring the Global Solar Intensity (G. Solar Intensity). A TESCAN-VEGA3 Scanning Electron Microscope (SEM) and a LEICA DM 300 P optical microscope (OM) (Germany) were used for microstructure study. Two JR-1digital thermometers, with range of −50–300 °C and accuracy ± 0.1 °C, were used for measurement of water temperature (T_w) and ambient temperature (T_a). Emery grinding papers with different grades were used for mechanical grinding, and diamond paste (2.5 µm) and γ-alumina fine powder were used for final polishing. A potentiostat (Voltalab, PGZ 100) and VoltaMaster 4 software were used for corrosion tests. A Filmtek-3000 instrument (USA) was used to measure surface percentage light reflectance (reflectance %) (R%). A SHIMADZU UV-1601 (Japan) instrument was used to measure matte black paint transmittance % (T%) and absorptance % (Abs.%).

3. Experimental Procedure

3.1. Microstructure Study

Microstructure study was carried out for the as-received anodized commercial Al via OM and SEM. The aim was to assess their microstructures and measure the thicknesses of the blackened anodization layer and the transparent anodization layer, which are major parts of the used Al. The process of metallography started with gentle cutting of three specimens; one was cut to 1 cm² Al, and two were cross-sections of the blackened and non-blackened anodized Al with dimensions of 1 × 0.5 cm. After that, the specimens were mounted in epoxy resin for easy preparation.

The process of metallography was initiated with mechanical grinding sequentially using emery papers of grades 220, 320, 400, 600, 800, 1200, and 2000 to prepare the surfaces for the subsequent polishing stage. The polishing process then began with 2.5 µm diamond paste for semi-final polishing to remove scratches from the mechanical grinding stage, and was followed by final polishing with γ-alumina fine powder to obtain mirror-like surfaces ready for the chemical etching step. The final step of the metallography preparation was the etching of the polished specimens using Keller’s solution (100 mL distilled water, 2.5 mL HNO₃ 69%, 1.5 mL HCl 33%, and 1 HF 48%) at approximately 50 °C. This solution is very useful for revealing the general microstructure of Al and its alloys. More information concerning metallography of aluminum and its alloys was reported by García et al. [18]. The cross-sectional prepared specimens were directly tested to measure the thickness of the anodization layers.

The microstructure obtained from OM and SEM is shown in the Section 4. SEM was employed for high magnification and resolution images and shows the transparent and blackened anodization layers that covered the used commercial Al.

3.2. Solar Heating System

The designed Al containers were used to heat tubs of water via different runs of solar heating. The containers were fixed in wooden boxes depending on the experiment’s requirements. Generally, the slope angle of boxes with a horizontal level is approximately the latitude of the measurement place (Amman, Jordan) plus 10°–15° (Latitude + 10°–15°) [19]. Therefore, the wooden boxes with the filled water containers were exposed to solar radiation at a slope angle of approximately 42°, because Amman, Jordan is at (31 57° North, 35 56° East) [6]. After that, water temperature (T_w), global solar intensity (G. Solar Intensity), and ambient air temperature (T_a) were measured as a function of time of day on different dates. The boxes were rotated manually to be approximately facing the Sun during the measurement period. Many solar water heating experiments were carried out for different cases, as follows:

Single- and double-glazing solar heating test

The first experiment was conducted on 26 September 2021 using single- and double-glazed boxes. For this purpose, two Al containers were filled with cold tap water with a temperature of approximately 9 °C and then fixed in the twin rooms of the constructed wooden boxes. One of them used single glazing and the other one used double glazing, as shown in Figure 2. The result of this attempt is given in the Section 4. After this experiment, all subsequent runs of solar heating were done in double-glazed boxes.

Using Al and B. P. Al water containers

On the same day above, Al and B. P. Al water containers were filled with cold tap water of approximately 9 °C and exposed to solar radiation from 9:00 to 17:00. T_w and T_a were measured hourly and are presented in the Section 4.

Using three types of water containers: Al, B. P. Al, and B. A. Al

The experiment using three types of water containers, Al, B. P. Al, and B. A. Al, was carried out on 7 October 2021 to assess solar radiation absorption performance in comparison with each other. The result of this trial is given in the Section 4.

Effect of weather conditions and starting time on T_w

Other experiments were run using different starting times to realize the effect of this factor on T_w relative to time of day. A sample of the obtained results is given in the Section 4. Experiments were carried out to illustrate the effect of weather change conditions, such as T_a and G. Solar Intensity, on hot water temperatures. Several cases of solar heating with different weather conditions were carried out. The maximum water temperatures and mean values of ambient temperature as a function of the mean value of G. Solar Intensity in some cases of solar heating were reported.

Measurement of normal incident light reflectance %(R%) for Al, B. P. Al, and B. A. Al, and transmittance % (T%) and absorptance % (Abs.%) for matte black paint

Three samples from the used solar selective surfaces, with an area of approximately 1.5 cm² each, were prepared. Normal incident light of wavelength 250–850 nm was applied using the Filmtek-3000 instrument for R% testing of non-painted Al, B. P. Al, and B. A. Al. The curves of the tested surfaces together with the curve fitting for the non-painted Al surface were produced using the Origin 8.6 graph drawing program.

Table 1. The parameters of Equation (2).

Parameters	Value	Standard Error
Residual Sum of Squares	24.25198	-
R-Square	0.99439	-
Intercept	−14.12885	0.33066
B1	0.11748	0.00275
B2	−2.88577 × 10−4	8.17811 × 10−6
B3	3.35354 × 10−7	1.03572 × 10−8
B4	−1.44851 × 10−10	4.73667 × 10−12

includes the details of the curve-fitting equation for the non-painted Al surface. In addition, T% and Abs.% curves were found for the matte black paint in the visible light range using the SHIMADZU UV-1601. The aim of this test was to compare the used matte black paint and the blackened anodized Al. The examined sample was prepared using a transparent glass slide that was sprayed twice with matte black to represent the same painting condition as the Al containers that were used in the research. The relationship between the T% and Abs.% of the matte back paint against the applied visible light wavelengths was measured.

3.3. Corrosion Resistance Study

Potentiodynamic polarization

Potentiodynamic polarization studies were conducted in a three-electrode cell consisting of a graphite counter-electrode, a saturated calomel electrode (SCE), and an aluminum specimen with exposed surface area of 2.1 cm² as the working electrode. The technique was used to investigate the corrosion behavior of Al in 3.5% NaCl at room temperature. The results were compared with the corresponding results from corrosion of pure Al in 3.5% NaCl. Polarization experiments were carried out using the potentiostat (Voltalab, PGZ 100) and VoltaMaster 4 software. Prior to each potentiodynamic polarization test, the working electrode was allowed to reach a steady state and the open circuit potential (OCP) was measured. Potentiodynamic polarization tests were carried out by applying an external potential to the Al samples with potential range (ࢤ1100 to ࢤ700 mV) relative the reference electrode, using 50 mV∙ min^ࢤ1 as the scan rate.

Potentiodynamic polarization measurements

Al is a very active metal in aqueous solutions and naturally produces a protective layer of aluminum oxide. However, in the presence of high concentrations of chlorides, it tends to corrode much more quickly and the oxide film becomes unstable and degrades, resulting in structural breakdown and pitting corrosion of the surface [20].

A Tafel plot (logarithm of current density vs. applied potential) was constructed for Al metallic samples in 3.5% NaCl solution. Electrochemical parameters such as corrosion potential (E_corr), corrosion current density (I_corr), solution polarization resistance (Rp), anodic Tafel slope (β_a), cathodic Tafel slope (β_c), corrosion rate and inhibition efficiency are all presented in

Table 2. Corrosion parameters for different aluminum surfaces in 3.5% NaCl solution at room temperature, extracted from the Tafel plot.

System	Surface Denotation	Ecorr (i = 0) mV	Icorr (mA/cm²)	Rp (kohm·cm²)	Ba (mV)	Bc (mV)	Corrosion Rate (µm/Y)
Backside of blackened anodized Al	-	−856.0	0.3703	42.60	30.2	−375.7	4.029
Blackened anodized Al	B. A. Al	−979.9	0.0807	65.01	47.4	−99.0	0.878
Non-painted anodized Al	Al	−761.4	0.1567	180.41	22.1	−211.5	1.705
Backside of non-painted anodized Al	-	−831.0	0.3921	54.79	24.2	−470.3	4.266
Matte black painted Al	B. P. Al	−775.7	0.0959	135.93	16.3	−120.6	1.044
Backside of matte black painted Al	-	−828.5	0.2795	34.87	27.1	−531.8	3.041

. The corrosion rate values were evaluated using Voltamaster 4 software.

4. Results and Discussion

4.1. Microstructure

Optical microscope images revealed that the thickness of the blackened anodization layer of the commercial Al was approximately 14 µm, as revealed in Figure 3, and the thickness was almost homogenous.

Furthermore, it seems that the anodization layer fully adhered to the mother Al sheet, as it was part of it before the anodization process. In addition, the study revealed a clear image of the microstructure of the used commercial Al, as shown in Figure 4, from which it could be concluded that there were some inclusions at the grain boundaries as well as in the matrix of the specimen.

As seen in Figure 5, the thicknesses of the transparent and blackened anodized layers were approximately 14 µm and 11 µm, respectively, as measured more precisely from the SEM images. These values matched well with the values measured from the OM images. This difference in the thickness of anodization was due to the blackening step that follows the anodization process of Al, as required.

4.2. Solar Water Heating

It is necessary to recall the definition of the thermal heat flow rate, which represents the power in Watts unit of heat transfer (P) [21], such that:

$$\mathrm{P}=\mathrm{A}\mathsf{\kappa }\frac{\Delta \mathrm{T}}{\mathrm{L}}=\mathrm{A}\mathsf{\kappa }\frac{\left({\mathrm{T}}_{\mathrm{h}}-{\mathrm{T}}_{\mathrm{c}}\right)}{\mathrm{L}}$$

(1)

where A (m²) is the area of the selected heating surface, κ (W/m.K or °C) is the thermal conductivity of the used material, L is the thickness (m) of the used sheet, and T_h and T_c (K or °C) are the temperatures of outer and inner surfaces of the manufactured solar heater that faces solar radiation if the above relationship is applied to the SWHS. The value of P is directly proportional with A, κ, and ∆T and inversely proportional with L.

In addition, it is important to give brief information regarding the anodization of Al. The anodization of Al and its alloys means the transformation of a part of the outer surface of an Al sheet into a transparent porous structure of aluminum oxide (Al₂O₃) by electrochemical means. This porous characteristic of the attained anodization layer is very important for subsequent sealing and coloring processes as needed. The Al sheet must be the anode in the electrochemical cell, and so, for this reason, it is called the anodization process. The thickness of the anodization layer depends on the process parameters, mainly reaction time, and can be in the range of 10–30 µm for commercial anodized Al. Moreover, the anodized layer is fully adherent to the original Al sheet. The goal of this process is to improve the corrosion resistance of the used Al and its alloys over the long-term, and to change the thermal and mechanical properties of the anodized Al surfaces. In addition, it may be conducted for decorative reasons, as the anodization procedure can be followed by a coloring step to obtain the required colors. Almost all information can be reviewed in the related references mentioned in the introduction [15,16,17].

It is obvious from Figure 6 that, generally, double glazing was more feasible to apply in the manufacturing of SWHS, as it collected more solar energy in comparison with the single glazing. As seen from the figure, the temperature difference between the single and double glazing was approximately 5 °C in favor of the double glazing. The logical reason for this result is that the air gap between the two panes in the double-glazed system provided better thermal insulation, as a result capturing more solar energy and losing less thermal energy during the duration of solar exposure.

Figure 7 reveals that temperatures of approximately 76 °C and 94 °C were reached for the Al and B. P. Al water containers, respectively. It is excellent to reach such high water temperatures using the simple solar heating system technique.

It can be seen from Figure 8 that the maximum water temperature of approximately 95 °C was achieved by the matte black painted and blackened anodized Al containers. The curves were almost in superposition with each other. This represents a superb result for blackened anodized selective surface absorbers for solar radiation, reaching high water temperatures for domestic use with excellent corrosion resistance. Furthermore, aluminum has very low density (2.7 × 10³ kg/m³) compared with other metals that are usually used in the manufacturing of SWHS, such as iron and copper (7.87 × 10³ kg/m³, 8.96 × 10³ kg/m³, respectively), which represents an additional advantage in using Al.

Figure 9 shows that even when varying the starting time of solar heating, the maximum temperatures for the three cases were very close, and they were almost equal at the end of the experiment. The starting T_w for all cases was approximately 13.5 °C, the mean value of T_a was approximately 21 °C, and the mean value of the global solar intensity was approximately 300 W/m² during the measurement period. This result is very important in terms of utilizing solar energy for water heating. Thus, it is possible to drain the hot water at a certain time when needed, and then obtain a high temperature later as well when the system is refilled again with cold water. Hence, the collected solar energy per unit of time could increase.

Generally, it can be seen from Figure 10a–d that the weather conditions affected the water temperature, but not to the degree of not gaining high water temperatures. In addition, the figure confirms that even when the ambient temperature is low, a maximum water temperature of approximately 80 °C could be reached, as seen in Figure 10d.

The effect of weather change on the outcome of the employed solar heating system is exposed in Figure 11. It can be concluded from the figure that the maximum water temperature was generally proportional with the global solar intensity when the system was well isolated from the external environment by front double-glazing. The ambient temperatures were mostly directly proportional to the global solar intensities. However, the figure revealed a special case in the valley point of approximately 333 W/m² global solar intensity, when the mean value of the ambient temperature was approximately 6 °C, which was due to the general weather conditions on that day. The research was conducted over two seasons (autumn and winter). We did not notice any performance change or material stability effects due to the variation in temperatures seen in the results. We also noticed that solar absorptivity did not change with time over many experiments.

Due to the reflectance % (R%) curves, it can be perceived from Figure 12a that the non-painted commercial Al (Al) was characterized by higher R% compared with the B. Al and B. A. Al surfaces. This result was expected, because black objects absorb light more than other colors, such as the silver color of commercial Al. In addition, it can be determined that the R% of Al clearly increased with increases in wavelength, and this result reflected the reflectivity property of the transparent anodization layer that covered the used commercial Al. This point needs to be fully studied with different thicknesses and colors of anodization.

$$\mathrm{R}\%=\mathrm{C}+{\mathrm{B}}_1 \mathsf{\lambda }+{\mathrm{B}}_2 {\mathsf{\lambda }}^2+{\mathrm{B}}_3 {\mathsf{\lambda }}^3+{\mathrm{B}}_4 {\mathsf{\lambda }}^4$$

(2)

where C is the intercept, λ is the wavelength, and B₁, B₂, B₃ and B₄ are the equation constants.

The intersect and the four constants that fulfill the above equation are recorded in Table 1.

The result in Figure 12a,b is congruous with the solar water temperature measurements of the surfaces that were tested experimentally. Thus, the Al surface absorbed much less solar radiation compared with the B. A. Al and B. P. Al, and the R% of B. A. Al was a bit higher than that of the B. P. Al.

Figure 13 reveals the relationship between the T% and Abs.% of the used matte black paint as a function of wavelength in the visible light range, from which it can be concluded that the thickness of the paint applied to the Al water containers was perfect for solar radiation absorption, as its T% was almost zero and Abs.% was nearly 100%. This test proved that the solar absorption of the blackened anodized selective surface was well comparable with the used matte black paint, as the obtained water temperatures were similar. Therefore, the blackened anodized Al is recommended to be used for SWHS manufacturing as a suitable solar selective surface with excellent corrosion resistance, as will be confirmed in the corrosion part of the discussion.

4.3. Corrosion Resistance Study

The results showed that most Al surfaces had low corrosion rates. The anodized and matte black painted surfaces exhibited better protection against corrosion in comparison to other surfaces following the order: blackened anodized Al > matte black anodized painted Al > Al (anodized) > bare Al surfaces. The lower corrosion rates for treated Al surfaces (blackened anodized and matte black painted anodized) makes them good choices for solar water heating applications. While the test environment was somewhat more aggressive than usual, the corrosion rates of all tested surfaces were in the range of a few microns or less. It is certain that the rates of corrosion will be much less than the values mentioned in Table 2 when testing is carried out in a natural context, such as tap water. As polarization resistance (Rp) increases, the flow of current (corrosion current, I_corr) during Al oxidation decreases, and hence a smaller corrosion rate is obtained. In conclusion, anodized aluminum surfaces can be used with both tap water and saline water.

5. Conclusions

Our results indicate that solar energy must be harvested in most countries that receive solar energy in order to reduce energy expenditure and ultimately support our environment. The best solar absorptivity and stability was recorded for the blackened anodized Al surface compared to all other studied surfaces, including the unpainted anodized Al and the matte black painted Al. The thicknesses of anodization layers were approximately 11 µm and 14 µm for anodized Al and blackened anodized Al, respectively, as measured from optical and scanning electron microscope micrographs. It is worth mentioning that the absorptivity of the blackened anodized Al for solar radiation was almost the same as that of the matte black painted Al. The use of blackened anodized Al in the solar heating system produced high water temperatures in different seasons (~90 °C in the autumn and ~80 °C in the winter). Corrosion testing in an extremely corrosive medium (3.5% NaCl solution) revealed that the blackened anodized Al had exceptional corrosion resistance in the micrometer range. Therefore, using ordinary tap water would be safe. The blackened anodized Al surface is highly recommended for use in the manufacturing of solar heating systems for several reasons. Its solar radiation absorptivity was similar to the commonly used matte black paint in industry. Blackened anodized Al is highly available, has good corrosion resistance, has high thermal conductivity, and is light in weight compared to many other metals used in the manufacturing of solar heating systems. It is excellent against surface degradation upon long-term exposure to solar radiation. In addition, the percentage reflectance test proved that the light reflectance behavior of the blackened anodized Al surface was very low, but slightly higher than the used matte black paint, which represents the main advantage of using it as an absorbent surface for solar radiation.