**1. Introduction**

Aluminum alloy 6082 has the highest strength of the 6000 series alloys and it used in many highly stressed applications such as aeronautics, trusses, bridges, transport applications, cranes and aerospace industries [1]. Aluminum is a metal which has a natural corrosion resistance due to the oxide layer that forms on its surface [1]. This dense layer is formed in a short time when it is exposed to the environment. However, under aggressive environments, aluminum is subjected to different types of corrosion such as pitting corrosion [2], intergranular corrosion [3] and stress corrosion cracking [4]. Pitting corrosion usually attacks aluminum surfaces causing localized holes in the protective film under chloride corrosive environments [5]. Regarding the atmospheric corrosion of aluminum, El-Mahdy et al. [6] studied the effects of aluminum corrosion rate under cyclic wet-dry environments. They found that aluminum corrosion rate can be affected by temperature, surface inclination and relative humidity. The corrosion rate increases with increasing temperature and decreases by increasing the angle of inclination [6]. Corrosion plays a significant role in human life and safety. Corrosion attacks the component and affects its function negatively and consequently, reduces its service lifetime. The economic costs of corrosion are obviously enormous in many industrial fields [7,8]. As a result, relatively poor corrosion resistance often decreases the lifetime of the aluminum alloy components. Thus, some surface engineering techniques on aluminum alloys would be indispensable to their applications.

Extensive researches on surface engineering that enhances the material resistance against corrosion, specifically, started in the 1980s and became a recognized manufacturing technology in the 1990s [9]. There are two main advantages of using surface treatments. Firstly, surface engineering techniques can enhance the tribological performances of the component's surfaces against the surrounding environments and consequently, increase service lifetime and reduce the cost of replacement and maintenance [10]. Secondly, they can give a wide range of options for selecting cheaper substrate materials for certain applications with surface engineering technology [11,12].

Surface engineering technology can be classified based on the change of the surface of the substrate [13]. The first group can enhance the surface without changing the chemical composition of the substrate using heat treatments or mechanical working. Enhancing the surface by the second group through changing the chemical composition of the substrate such as electroplating and thermal diffusion treatments, oxide coatings, anodizing and sulphur treatments. The third group can enhance the surface by adding a layer such as welding, laser alloying and thermal spraying [14–16].

Plasma Electrolytic Oxidation (PEO) is an electrochemical surface treatment that produces a protective surface coating on metals and alloys [17–19]. This surface coating, which is ceramic, is produced by passing a modulated electrical current through a path of the electrolyte solution. The applied electrical potential should be high enough to the plasma discharges and sparks to be formed and generate oxide films with relatively higher thickness [20–22]. The applied potential exceeds the breakdown strength of the growing oxide film. The coated surface produced is well adhered to the substrate and possesses good wear and corrosion resistance, in addition to a good surface barrier for thermal conductivity [23,24].

The Hard Anodizing (HA) technique is an electrolytic passivation process, which forms a thick oxide layer on the metal surface. It is considered to be a traditional coating method and it has been used since 1923 and commercially available since 1940 [25–27]. HA is relatively a cheap process and does not give good wear resistance due to low hardness. This process can give better surface corrosion resistance, which varies according to the substrate material, parameters of the process and treatments conditions [28]. The microstructure of the anodized aluminum is considered amorphous alumina. This porous layer slightly increases the corrosion resistance of the substrate, however, for significant corrosion protection, a sealing process is required [29,30].

Plasma Spray Ceramics (PSC) is used to produce a coating in which molten or softened particles are applied by impacting onto a substrate. It was first introduced by Schoop while studying the production of metallic particles from a molten metal for coating [31–34]. There are three main stages to form this coating. Firstly, plasma particles are created as small droplets stream [35]. Secondly, these particles are subjected to a high temperature using heat source generating thermal energy. The particles' composition is changed due to the chemical reaction between the droplet material and the flame. After that, they are flattened while striking a cold surface at high velocities. A common feature of lamellar grain microstructure is formed as a result of the rapid solidification and cooling processes [36].

The characteristics of the PEO coating interface on AA1060 aluminum alloy were investigated by Wang et al., as a function of PEO processing time using scanning electron microscopy and X-ray diffraction. Hundreds of coatings were detached from the substrate by an electrochemical method and ground into homogeneous powders [37]. Shifeng et al. [38] investigated the morphology and

protective properties of the PEO coatings produced on 5754 aluminum alloy in a mixed electrolyte. The current density on the sample surface increased, the coating grew faster, the thickness increased and the roughness gradually decreased. The coating corrosion resistance first increased and then decreased [38]. Cerchier et al. [39] used the Plasma Electrolytic Oxidation coating technique on samples of 7075 aluminum alloy and they obtained thick and adherent coatings [39]. Abdel-Gawad et al. [40] studied the corrosion behavior of the hard-anodizing coating formed on different groups of aluminum alloys in the sulphuric acid electrolyte. The corrosion resistance and the anodized coating layer are influenced by the type of the alloy and the anodizing conditions such as current density, acid concentration and time of duration [40]. The influence of the hard-anodizing process parameters such as H2SO4 concentration, electrolyte temperature, Al3+ concentration and current density for the AA2011-T3 was evaluated. Higher H2SO4 concentration and higher current density have improved coating hardness and defectiveness, however, potentiodynamic polarizations have revealed that they do not enhance corrosion resistance [41].

Many authors have evaluated the wear and corrosion behavior of surface coatings and most of them agreed that these coating materials increased the wear and corrosion resistance as compared to the uncoated ones [22,23,42,43]. The oxide film can be affected by changing treatment parameters depending on the purpose of the coating [44]. Also, it was found that as the thickness of the PEO coating increased, the corrosion resistance increased. For example, Qiu et al. [45] showed that the corrosion performances of the PEO coating on ZK60 Mg alloy could be improved by increasing the current density in the PEO process [45].

This work has investigated the enhancements of the three coatings; plasma electrolytic oxidation, plasma spray ceramic and hard anodizing on the performances of 6082 aluminum alloy surface against different corrosion experiments. The microstructure of the coating layer was detailed investigated and the comprehensive results are presented. In addition, the corrosion behavior of the coating layers was evaluated. The results are followed with the discussion of such behavior.

#### **2. Materials and Methods**

The substrate used in this study was 6082-T6 aluminum alloy with chemical composition listed in Table 1. T6 refers to the temper number which means solution heat-treated and artificially aged. The substrates were cut into discs with a diameter of 25.40 mm and a thickness of 10 ± 0.01 mm to be fitted in the holder for electrochemistry experiments suitable for the rig available in the lab. Three types of materials coatings were formed on the substrate, that is, Plasma Electrolytic Oxidation (PEO) and Plasma Spray Ceramic (PSC) and Hard Anodizing (HA). For PEO samples, a 3 mm diameter hole was drilled in the aluminum substrates for the anode to be inserted in the sample to allow for the anodization process and sent to Keronite International Ltd., Haverhill Suffolk, UK. For PSC samples, the aluminum substrate was sent to Bodycote Plc., Cheshire, UK where the Metco®101NS Grey Alumina Powder was used. The HA samples, coatings were provided by MP Eastern Ltd., Suffolk, UK.


**Table 1.** Chemical compositions of the 6082-T6 aluminum alloy; wt%.

A special sample holder for electrochemistry tests has been designed to allow testing the materials in a more convenient way as shown in Figure 1. This holder allows both faces of the sample to be tested without using resin. An Inductively Coupled Plasma (ICP) test was performed on all the coated samples together with the aluminum substrate to assess the number of ions (Al3+) released to the 3.5% NaCl solution after 24 h of immersing the samples into the solution at different test conditions. Then, the samples were polarized up to 400 mV against the reference electrode for another 24 h. A series of electrochemistry experiments was used to evaluate the corrosion performances of three types of coatings deposited on 6082 aluminum alloy in the electrolyte of 3.5% NaCl solution. The reference electrode is sliver/sliver chloride (Ag/AgCl). The first experiment was Open Circuit Potential (OCP) tests where the potential (in Volts) was recorded against time (in seconds) for 24 h. The second experiment was direct current (DC) anodic polarization measurements which involve changing the electrode potential from its OCP in a certain direction and a given scan rate. Anodic Polarization (AP) tests are involved in measuring the scan from OCP to more positive voltages (up to 1 V from OCP) to reveal more information about the kinetics of the corrosion and its type. The determination of the corrosion current density *icorr* will be done graphically from the plot of E. versus Log i, which is the intersection point of two lines. The third experiment was the AC impedance test which applied to the materials using an electrochemical measurement unit called Solarton (SI 1280B). The amplitude of the sinusoidal voltage was 10 mV which was selected to keep the system linear. The measurements were performed at frequencies ranging from the high value of 20 kHz to low-frequency value of 0.1 kHz to minimize the sample perturbation.

The microstructures of the coated layers and substrates were investigated using Leica optical microscope and Scanning electron microscope (Philips XL30 ESEM environmental SEM, Leeds, UK) equipped with Oxford Instruments INCA 250 EDX system analyzer after standard methods of metallography. The substrate and the coated layers were analyzed by X-ray diffractometer, (XRD, D8 Discover with GADDS system, 35 kV, 80mA, MoKα radiation, Leeds, UK) to identify the phases.

**Figure 1.** Designed sample holder for electrochemistry tests.

#### **3. Results and Discussion**

#### *3.1. Macrol Microstructure Analysis*

Figure 2 shows the microstructure of the substrate 6082-T6 aluminum alloy in polished and etched conditions. The micrographs revealed particles of different sizes distributed within the Al matrix. These particles may be the well-known intermetallics (β-Al5FeSi, Mg2Si, Al9Mn3Si, Mg2Si, α-Al(FeMn)Si) shown in this type of Al alloy [46,47]. These intermetallics support and strengthen the matrix. For that, the alloy 6082 is considered one of the highest strength of the aluminum alloys.

**Figure 2.** Microstructure of the substrate 6082-T6 aluminum alloy in (**a**) SEM polished micrograph and (**b**) Optical micrograph of the etched conditions.

The cross-sectional macrostructures of the coatings are shown in Figure 3. For all the three coatings, two main distinct layers can be seen in the cross-sectional view of the coating which is inner and outer layers. For HA (Figure 3a) the inner layer is dense and is composed of a thickness of 50 μm while the outer one is more porous with 10 μm in thickness. It can be shown that HA coating is less uniform and had more porosity compared with PEO coatings (Figure 3b). For PEO coating, the inner interface layer which is dense and well adhered to the substrate. This layer composes the major part of the coatings and it has a thickness of approximately 40 μm (Figure 3b). This interface coating layer has a uniform distribution with the substrate for both of them. From Figure 3c,d, PSC coating has the largest coating thickness of about 350 μm starting with a loose layer in the interface region of 85 μm. This layer is followed with the intermediate layer of approximately 250 μm and finally the top porous layer of 55 μm. The intermediate layer contains many laminar structures with white colors which could be the aluminum substrate. The dark layer (gap) at the interface is probably due to debonding of the PSC coating from the substrate. Although the deboning can be attributed to the metallographic preparation (sectioning/polishing); it also indicates a weaker bond with the substrate.

**Figure 3.** Optical cross-sectional micrographs of coating layers after polarization tests for (**a**) Hard Anodizing (HA), (**b**) Plasma Electrolytic Oxidation (PEO), (**c**) Plasma Spray Ceramic (PSC) and (**d**) SEM image of PSC.

The EDX spectra of hard anodizing reveal that the main element is oxygen followed with aluminum and sulphur which is due to the sulfuric acid (H2SO4) used in the electrolyte chemical composition during the HA process [48,49]. For PEO coatings, the main elements are aluminum, oxygen, manganese and magnesium as detected from the EDX analysis. These elements are due to the inclusion of the aluminum alloy elements. In addition, the EDX spectra showed titanium for PSC coating. The PSC coating was prepared using grey alumina powder (PSC Metco ®101NS, Cheshire, UK) which contains 2.5% of titanium dioxide according to the supplier [50].

For the XRD analysis, there are no phases detected for HA in XRD apart from aluminum element as shown in Figure 4 a,b. That is because the HA coating is amorphous in nature and is believed to be due to oxide hydration [51]. The main phases detected in the PEO coating are alumina phases

(α-Al2O3, γ-Al2O3) as is shown in Figure 4c. This result is consistent with the literature where α-Al2O3 was formed on the PEO applied on aluminum alloy on the inner layer of the coating due to the high temperature during the discharge stage of the PEO process. Also, the amorphous γ-Al2O3 alumina is abundant in the outer layer which is formed during the cooling stages because of the contact between the molten alumina and the electrolyte. Similar to PEO coatings, PSC coating has α-Al2O3 and γ-Al2O3 phases but with lower intensity peaks (Figure 4d).

**Figure 4.** XRD spectra for coating layer of (**a**) HA, for highlighting lower peaks zoomed XRD spectra for (**b**) HA, (**c**) PEO and (**d**) PSC.
