Influence of current intensity and process time on the corrosion of aluminum oxide coatings

In this article, the influences of the microstructures of an anodizing layer, which in turn are influenced by the current intensity and the process time, on the corrosion resistance are shown. The microstructures are visualized by SEM images of the surface and cross-section of the layers. The corrosion resistance is determined electrochemically. In addition, hydrogen permeation measurements through anodized layers are considered and the pre-treatment is presented.

The demand for aluminum is expected to increase by up to 40 % by 2030 [1]. This demand will also be boosted by climate protection efforts. Hydrogen technologies play an important role in the mobility and energy sectors. Due to the changed challenges that arise with the use of hydrogen, the materials used must be specifically selected. Instead of high-strength steels, aluminum alloys are often used here, as these are hardly prone to hydrogen embrittlement even without a hydrogen barrier layer [2]. In addition, the use of aluminum reduces energy requirements in the mobility sector through weight savings [3]. However, as aluminium is susceptible to corrosion even from minor environmental influences, aluminium is often anodized to increase its corrosion resistance [4-7]. Different electrolytes can be used for this purpose. However, corrosion resistance is mainly influenced by other process parameters such as current density and process time. It has been shown that the corrosion resistance decreases when dilute sulphuric acid (_H2SO4) is used at higher current densities in the anodizing process [6], but increases with citric acid [5]. According to Mohammadi and Afshar, the corrosion resistance is influenced by the microstructure, with a thicker barrier layer improving the corrosion resistance [8]. However, thicker anodization layers are not generally more resistant to corrosion, as the longer process time can result in defects in the layer [9]. When anodizing in sulphuric acid electrolytes, it was shown that a larger pore diameter, a higher layer porosity and an irregular pore structure reduce the corrosion resistance [10]. Other studies showed that oxalic acid electrolytes are more suitable than sulphuric acid electrolytes for the anodization of Al6061 in order to further increase the corrosion resistance [11,12].

Material and method

Blanks with a diameter of 24 mm and a thickness of 8 mm made of commercial Al6061 were used as sample material.

Anodization

Before anodizing the samples, they are first pretreated. In the first step, the samples are cleaned in a commercial hot degreasing agent (New Dimensions Supreme, MacDermid Enthone, Waterbury, USA) for 5 min at 60 °C, then pickled in a commercial pickling solution (Alumon AC 10, Alumon AC 70, MacDermid Enthone, Waterbury, USA) for 2 min at room temperature. In the final pre-treatment step, pickling residues are removed in a commercial solution (Alumon AC 10, Alumon MS, MacDermid Enthone, Waterbury, USA) for 30 s at room temperature. Between each step, the sample is thoroughly rinsed in demineralized (DI) water.

The anodization of the samples is carried out in a 160 liter bath. A 5 M-% solution of oxalic acid dihydrate in deionized water (equivalent to 3.6 M-% or 0.4 M oxalic acid) is used as the electrolyte. The electrolyte temperature is kept constant at 20 °C ± 2 °C by a recirculating cooler (FL4006 - Julabo, Seelbach, Germany). High-alloy steel electrodes (1.4301), which are 40 times larger than the samples, are used. A rectifier from Kniel (VE3PUI2 150.30, Kniel, Karlsruhe, Germany) is used as a direct current source.

The anodization process is current-controlled, whereby the current densities are varied between 0.5 ^A/dm2 and 15 ^A/dm2. So that the different anodization layers can be compared, each layer should achieve the same layer thickness. For this purpose, each process is carried out with the same amount of charge. This results in different process times for different current densities. They are varied between 50 min and 1 min 40 s. To compare different layer thicknesses, two different charge quantities are examined. The target layer thicknesses are 6 ± 1 µm and 13 ± 1.5 µm. The resulting process parameters are shown in Table 1.

After anodization, the sample is thoroughly rinsed as a final step so that residues of the electrolyte can diffuse out of the pores of the coating.

Characterization

To characterize the microstructures of the anodized layers, SEM images are taken of the surface and in the fracture of the layer. These can be used to determine the pore diameter and the thickness of the barrier layer using the image analysis software Fiji ImageJ 1.54f (U.S. National Institutes of Health, Bethesda, Maryland, USA). Each value corresponds to the average of ten measured values.

Corrosion resistance is determined using potentiodynamic polarization measurements. Potentiostats (VSP - BioLogic, Seyssinet-Pariset, France; Interface 1000 - Gamry Instruments, Warminster, USA) are used for this purpose. A three-electrode measuring cell (FlexCell - Gaskatel Gesellschaft für Gassysteme durch Katalyse und Elektrochemie mbH, Kassel, Germany) is used. The aluminum sample is connected as the working electrode, a platinum wire is used as the counter electrode and a standard hydrogen electrode (Mini-HydroFlex - Gaskatel Gesellschaft für Gassysteme durch Katalyse und Elektrochemie mbH, Kassel, Germany) is used as the reference electrode. The measurements are carried out in 3.5 M-% NaCl electrolytes (0.6 M NaCl). The polarization tests are performed ± 250 mV against the open-circuit voltage with a potential change rate of 1 ^mV*s-1.

The hydrogen permeation measurements are carried out using the Devanathan-Stachurski method. For this purpose, the FlexCell measuring cell is extended with an additional test chamber so that the measurements can be carried out and evaluated in accordance with DIN EN ISO 17081.

The results

Microstructure

When examining the microstructure, the thickness of the barrier layer is analyzed using the cross-sections of the layer and the pore diameters on the surface. It is noticeable that the structure differs from that expected from the literature. The uniform and continuous pores typical of an anodized layer are not present [13]. Figure 1 shows that the pores are not tubular, but rather have a "coral-like" structure. The pores are neither evenly spaced nor continuous from the barrier layer to the layer surface. There are interrupted pores and eroded pore walls. This structure can be seen above all in the layers produced with 0.5, 1 and 5 ^A/dm2. At 15 ^A/dm2 the pores become more tubular, but still differ from those predicted in the literature.

Fig. 1: SEM images of the structure of the surface (top) and the cross-section (bottom) of anodized layers deposited with current densities of (1) 0.5 A/dm2, (2) 1 A/dm2, (3) 5 A/dm2 and (4) 15 A/dm2

The barrier layer thickness behaves as described in the literature and increases in proportion to the voltage [14]. This in turn increases with increasing current density. Figure 2 shows such a proportional behavior between maximum process voltage and barrier layer thickness between 0.5 ^A/dm2 and 5 ^A/dm2. The barrier layer thickness increases significantly from 55 nm at 52 V to a thickness of 120 nm at 130 V. At 15 ^A/dm2 the barrier layer thickness decreases by 10 nm to 110 nm compared to 5 ^A/dm2 despite the same maximum process voltage. With the larger amount of charge(Fig. 2, unfilled markings), it is noticeable that the process voltage and the barrier layer increase from 122 V to 136 V and from 115 nm to 130 nm at 3 ^A/dm2. Due to the longer process time, the process voltage increases further with the result that the barrier layer thickness is increased, while at 5 ^A/dm2 the maximum process voltage is already reached within the shorter process time of the smaller charge quantity.

Fig. 2: Relationship between maximum process voltage and the thickness of the barrier layer. Filled markings have 900 C as charge quantity, empty markings 1800 C

The pore diameter on the surface of the layer varies between 15 nm and 40 nm depending on the applied current density. The lower the current densities, the higher the pore density and the smaller the pore diameters (Fig. 1 above). Figure 3 shows large error bars that increase at higher current densities. This is due to the decrease in the uniformity of the pores at higher current densities. The measured pore diameters vary up to ± 30 nm. The pores grow together due to the dissolution of the pore walls on the surface of the layer, which leads to an expansion of the pores and thus larger pore diameters. This can be seen particularly clearly in the coating produced with 15 ^A/dm2 and is also confirmed in the measurements, where the pore diameter is 40 nm ± 25 nm. The chemical re-dissolution of the aluminum oxide is intensified by local temperature increases, e.g. by higher current densities [15-17].

Fig. 3: Relationship between current density and pore diameter. Filled markers have 900 C as charge quantity, empty markers 1800 C

The comparison between the different charge quantities shows that at 3 ^A/dm2 the pore diameters only increase by 5 nm, i.e. there are hardly any differences on the surface of the layers, whereas at 5 ^A/dm2 the pore diameter doubles from 15 nm to 30 nm (see Fig. 3). A clear widening of the pores can also be seen here. This can also be explained by the increase in the chemical re-dissolution of the aluminum oxide due to stronger local temperature increases as a result of the longer process time.

Corrosion measurements

For comparability, both the anodized samples and an untreated Al6061 sample were examined in the polarization test. The corrosion rate was determined from the current-potential curves using the Tafel diagram. For this purpose, the potential was numerically fitted to the measurement data using the EC-Lab software (Biologic, V11.52) via the Stern method equation <1>, so that the corrosion current _Icorr can be determined; at the same time, the corrosion rate is determined using equation <2> using the aforementioned software.

<1>

<2>

I is the measured current, E the measured potential and_Ecorr the fitted corrosion potential. The coefficients _ßa and _ßc are the anodic and cathodic Tafel coefficients. CR is the corrosion rate, the unit of which is determined by the constant K. EW is the equivalent weight and d is the density of aluminum oxide. A is the surface area of the sample.

The corrosion rate is regarded as a measure of corrosion resistance. The lower the corrosion rate, the higher the corrosion resistance. Each anodizing layer reduces the corrosion rate compared to the raw material, from 1500 nm/y to less than 20 nm/y (see Table 2). The corrosion resistance of the layers produced with 0.5 and 5 ^A/dm2 remains the same, although the same amount of charge was converted in a shorter process time. The corrosion resistance decreases for the coating produced with a current density of 15 ^A/dm2.

In the literature, the barrier layer is described as the decisive factor for increasing the corrosion resistance of non-compacted anodizing layers [8]. However, these investigations show that the thickness of the barrier layer is not the decisive factor for increasing corrosion resistance. The 0.⁵ A/dm2 layer has the same corrosion rate as the layer produced with 5 ^A/dm2, despite a lower barrier layer thickness of 55 nm instead of 120 nm (see Fig. 2), while the ¹⁵ A/dm2 layer has a similarly thick barrier layer and the corrosion rate even increases. Therefore, the results presented here are also in contrast to a previously published study, which states non-tubular pores as the reason for reduced corrosion resistance[10], as the layers produced with 0.5 ^A/dm2 and 5 ^A/dm2 with the "coral-like" structures are more corrosion-resistant. Accordingly, another layer property must also influence the corrosion resistance.

The total layer thickness would be obvious, but a comparison of the different charge quantities shows that the corrosion rate of 3 nm/y remains the same at 3 ^A/dm2 despite the anodization layer being twice as thick (from 6 µm to 12 µm). The comparison also supports the assumption that a thicker barrier layer does not necessarily increase corrosion resistance. When looking at the two different layers produced with a current density of 5 ^A/dm2, it is noticeable that they differ mainly in the structure of the layers and in the pore diameter on the surface. In the coating with lower corrosion resistance (1800 C), the pores on the surface are larger and the structure is more tubular; the same coating characteristics can be seen in the coating produced with a current density of 15 ^A/dm2.

It can therefore be shown that not only the thickness of the barrier layer influences the corrosion resistance, but also other layer characteristics such as the structure of the pores and the size of the pores on the surface. Therefore, the barrier layer thickness alone cannot be regarded as an indicator of corrosion resistance.

Permeation measurements

DIN EN ISO 17081 specifies that palladium must be applied to the sample outlet side for permeation measurements [20]. Initially, the effective diffusion coefficient of Al6061 without an anodized layer was determined using permeation measurements. The measurement method was confirmed and the calculated effective diffusion coefficients agree with the literature [21]. The effective diffusion coefficients are in the range from 2.02 × ^10-6 to 4.40 × ^{10-6 cm2/s}. With anodized aluminium oxide, the challenge is not to change the layer by applying palladium. Therefore, the sample preparation must first be adapted in order to carry out permeation measurements on anodized layers. Tests are carried out on chemical palladium nucleation. However, the challenge here is that the solutions used are highly alkaline and attack the anodization layer even after a short dwell time. By adjusting the pH value and the process times, it is possible to germinate both aluminum and aluminum oxide with palladium without significantly attacking the surface.

Fig. 4: SEM images of the structure of the surface (top) and the cross-section (bottom). Left: Aluminum oxide layer, right: Palladium-germinated anodization layer

Figure 4 shows SEM images of the surface and the fracture of an aluminum oxide layer (left) and an anodized layer seeded with palladium (right). A modified structure can be seen on the surface due to the palladium. When looking at the cross-section, increased fractures in the pore walls can only be seen in an area up to 1 µm deep in the anodization layer in the case of palladium nucleation, but no longer in the further course to the barrier layer. The thickness of the barrier layer of 100 nm is constant compared to the unchanged aluminum oxide layer. The changes to the anodization layer caused by the palladium nucleation could be minimized so that the samples can be prepared and the permeation measurements can be carried out.

Initial tests show reduced hydrogen permeation through the anodization layer. In further, subsequent series of tests, it is planned to investigate the hydrogen permeation through differently applied anodizing layers. A correlation with the corrosion resistance results is expected, as the integrity of the layer and its structure can also be decisive for the permeation of hydrogen.

Photos and graphics: Bosch, HS Aalen

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