Investigation of Corrosion Resistance in Powder-Coated 6060 Aluminum Alloy: Effects of Powder Coating and Pre-Anodizing Followed by Powder Coating

Abstract

This study investigates the corrosion resistance of EN AW 6060 aluminum alloy powder-coated samples, with and without pre-anodizing treatment. The samples were exposed to a 3.5% NaCl solution, which is known for its strong corrosive effects, and their corrosion behavior was evaluated using two electrochemical techniques: Potentiodynamic Polarization and Electrochemical Impedance Spectroscopy (EIS). The aim was to assess the effectiveness of powder coatings in enhancing corrosion resistance and to examine the role of surface preparation and prior treatments. Polarization tests provided corrosion current densities and corrosion rates, while EIS data were analyzed using equivalent electrical circuits to evaluate the integrity of the protective layers. The results show that powder coatings significantly improves corrosion resistance compared to uncoated aluminum and the combination of pre-anodizing followed by painting offers the highest protection. These findings confirm the improved performance achieved through multilayer surface treatments and support the application of powder coatings acting as a durable barrier against environmental factors.

1. Introduction

Aluminum is widely known for its corrosion resistance, primarily due to the spontaneous formation of a thin, dense, and stable oxide layer (Al₂O₃) on its surface. This passive film acts as a protective barrier, significantly reducing direct metal exposure to aggressive environments, such as oxygen and moisture [1,2,3]. For architectural and construction applications, some surface treatment like powder coating must be performed on the aluminum profiles for aesthetic and protection reasons. However, under harsh conditions, especially in environments with high humidity containing chloride ions, localized forms of corrosion such as pitting and filiform corrosion can occur [4,5,6]. These localized corrosion phenomena spread on the aluminum profiles and accessories, leading to a degradation and structural weakening of the metal [7,8,9,10].

Understanding the corrosion behavior of aluminum and its mechanisms is critical for developing better protective strategies and ensuring long-term material reliability. Over time, various analytical methods have been used to characterize aluminum corrosion, including gravimetric weight loss measurements, hydrogen evolution tests, salt spray exposure, and electrochemical noise analysis [11,12,13,14]. While these approaches provide valuable insights, they often have limitations. Weight loss methods, for example, are cost-effective and simple but only provide an average measure of degradation over time without real-time data or information on corrosion mechanisms. Hydrogen evolution tests offer quantitative measures of corrosion rates but are susceptible to environmental factors such as temperature and pressure, which may affect repeatability. Salt spray testing, although widely used for accelerated corrosion simulation, sometimes fails to replicate the complexities of real-world exposure conditions. Electrochemical noise analysis, on the other hand, can detect small corrosion events but is hard to interpret because the signals are random [15,16,17,18].

Electrochemical Impedance Spectroscopy (EIS) has emerged as a powerful and non-destructive technique that addresses many of these limitations. EIS enables the in situ and real-time monitoring of corrosion processes by measuring the impedance of a material over a wide frequency range, revealing detailed information about the electrochemical properties and the integrity of the passive oxide film [19,20,21,22,23,24]. Modeling the impedance data with equivalent electrical circuits helps to extract quantitative parameters describing coating durability and effectiveness as well as differences in coating properties [25,26,27].

In this study, EIS and potentiodynamic polarization tests were performed on aluminum samples with different surface treatments. The purpose was to better understand how these treatments affect the creation, durability, and protective ability of the coatings on aluminum. This research aims to clarify how corrosion happens and to support the design of more effective methods for protecting aluminum from corrosion. Additionally, analyzing these effects helps in identifying which surface treatments provide the best long-term resistance, improving the lifespan and performance of aluminum components in various environments.

2. Materials and Methods

This work involved the examination of four different EN AW 6060 aluminum alloy specimens. An uncoated specimen was maintained as a baseline after being homogenized and aged, while the other three samples were treated with various surface processes, including two powder coating (referred as a white one and a black one) and a combination of pre-anodizing followed by a black powder coating.

The aluminum alloy samples were homogenized in two steps. In the preheating step, the samples were heated for 4 h to reach 585 °C, followed by a homogenization step with a 3.5-h dwell time at the same temperature. Subsequently, a two-step ageing process was followed. In the first step, they were held at 175 °C for 1.5 h, followed by a second step at 204 °C for 2.5 h.

Pre-anodizing and painted process was carried out as follows: The samples were treated in process tanks with the following steps: They were first subjected to alkaline degreasing at 70 °C, followed by rinsing in tap water. This was followed by alkaline etching in a caustic soda solution (75 g/L) at 65 °C, with subsequent rinses in tap water and deionized (DI) water. Desmutting was then performed in an 18 w/w nitric acid solution, followed again by rinses in tap water and DI water. The anodizing step was carried out in a 200 g/L sulfuric acid solution at 22 °C, using a current density of 1.4 A/dm² for 16 min, followed by rinsing in tap and DI water. With this method, one unsealed anodizing layer was created on the outer surface of the profiles with a thickness of 7 μm. The anodizing layer on aluminum was composed of a tough, porous layer of aluminum oxide (Al₂O₃) and is an integrated part of the metal, providing excellent durability and corrosion resistance. After the pre-anodizing process, the profiles were immediately transferred for applying the powder coating. The profiles were first dried at 90 °C for 45 min, then coated with polyester powder, and finally cured in an oven at 200 °C for 10 min to complete the powder polymerization. The thickness of the coating was around 90 μm and acts as a durable protective film alone or combined with the anodized layer that was mentioned above.

Polyester powder coatings are a type of thermosetting coating that utilize polyester resins and crosslinkers to create durable, protective films. The basic chemistry involves a condensation polymerization reaction between a polycarboxylic acid and a polyol, resulting in a polyester polymer. Furthermore, pigments and additives provide color, texture, and other desired properties like UV protection, flow control, and improved adhesion. The white powder coating was prepared using TiO₂ pigments, while the black powder coating was prepared using carbon pigments. Detailed information regarding the pigments is not disclosed, as it constitutes proprietary industrial information. These coatings were applied as a dry powder and then cured within a heat range from 170 to 200 °C, causing the crosslinking agents to react with the polyester resin, forming a durable, crosslinked network. Powder coating offers a wide range of colors and finishes, including smooth, textured, glossy, matte options and is an environmentally friendly option as it does not release harmful VOCs into the atmosphere and produces minimal waste [28,29,30].

The corrosion properties of the uncoated and coated aluminum specimens were examined using Potentiodynamic Polarization and Electrochemical Impedance Spectroscopy (EIS) (Gamry Instruments Inc., Warminster, PA, USA). At least three experiments were performed for each case (uncoated state and two powder coated state of AW 6060 aluminum alloy) in order to ensure repeatability of the electrochemical properties. Potentiodynamic Polarization tests provided important corrosion parameters, including corrosion potential (E_corr), corrosion current density (I_corr), and corrosion rate for each sample, calculated via Tafel equation. EIS analyses covered a frequency spectrum from 100 kHz to 100 mHz to explore the electrochemical characteristics of the samples by fitting the impedance data to electrical equivalent circuits. The experiments were conducted at ambient temperature (25 °C) in a 3.5% sodium chloride aqueous solution, employing a Gamry potentiostat/galvanostat. A three-electrode setup was used, featuring the aluminum specimen as the working electrode, a platinum rod as the counter electrode, and a saturated calomel electrode (SCE) as the reference. To further examine the aluminum’s microstructure and the morphology of the applied coatings, Scanning Electron Microscopy (SEM) (Phenom ProX desktop SEM, Thermo Fisher Scientific, Eindhoven, The Netherlands) was employed.

3. Results

3.1. Electrochemical Characterization

Electrochemical Impedance Spectroscopy (EIS) was used to evaluate the corrosion performance of aluminum samples with various surface treatments, all immersed in a sodium chloride (NaCl) solution. The data are presented through three standard plots, Bode phase angle (Figure 1a), Bode magnitude (Figure 1b), and Nyquist diagrams (Figure 1c), corresponding to aged and homogenized (initial), powder-coated (white and black), and pre-anodized plus powder coated specimens. In the diagrams (Figure 1), the straight line represents the fitting curve for generating the equivalent circuit modeling, while the data points correspond to the experimental measurements.

For the initial sample (uncoated sample), in the high-frequency region, a phase angle near 0° indicates that the impedance is dominated by the solution resistance, with minimal contribution from the surface film. This is typical for uncoated aluminum alloys where the native oxide is thin and may be defective, allowing the electrolyte response to predominate. Naturally formed or defective films allow the electrolyte to dominate the high-frequency response [31,32]. A phase angle approaching −90° in the mid-frequency region is characteristic of a capacitive response, typically associated with the presence of a protective oxide layer acting as a dielectric barrier. In the Bode plot of the uncoated aluminum alloy, the maximum phase angle is lower than −90° and the phase angle peak is broadened over a wide frequency range. Such broadening reflects a distribution of time constants, while the deviation from −90 indicates the reduced protective ability of the natural oxide film [31,32,33].

For the uncoated aluminum alloy, the Nyquist plot typically displays a depressed semicircle. The depression reflects surface heterogeneity and inhomogeneous current distribution, while the relatively small diameter of the semicircle corresponds to low polarization resistance. This behavior indicates that the native oxide film provides limited barrier properties, allowing for easier charge transfer and higher corrosion susceptibility. In the Bode magnitude plot, the impedance modulus (|Z|) at high frequencies is low and relatively flat, dominated by the solution resistance. In the mid-frequency region, |Z| presents a characteristic of capacitive behavior from the oxide layer. For the uncoated aluminum alloy, the Bode plot shows a loss of ideal capacitive behavior, with |Z| becoming flat and slightly decreasing at low frequencies. In this frequency range, an adsorptive process occurs in which ions penetrate through defects in the oxide layer. This mechanism is evidenced by the inductive response observed in the Nyquist plot. Such features are consistent with the reduced protective ability of the natural oxide film on uncoated aluminum [26,34].

The EIS results reveal distinct differences in corrosion protection performance between the black- and white-coated samples. The black-coated specimen exhibits a significantly higher impedance magnitude (|Z|) (Figure 1b) across the frequency range, especially pronounced at low frequencies, indicative of enhanced barrier properties against corrosive species ingress. This is corroborated by the Nyquist plot, where the black coating presents a larger semicircle diameter corresponding to increased polarization resistance (Rp), a direct measure of improved corrosion resistance.

Generally, intact painted coatings with excellent barrier properties exhibit a nearly ideal capacitive response, with the phase angle remaining close to −90° across the frequency range. For the examined powder-coated samples (both black and white ones), as electrolyte penetration initiates, pore resistance (R_por) develops, producing a frequency-independent plateau at low frequencies in the Bode plot. Under these conditions, the phase angle is frequency-dependent and approaches −90° only at high frequencies. The examined coatings maintained their barrier properties, as evidenced by the dominance of capacitive behavior (the AC current at high frequencies still prefers the capacitive path over the resistive one, and the phase angle remains close to −90°) and the absence of significant substrate corrosion [22,35]. The Nyquist plot of powder-coated samples displays the characteristic semicircle, where the high-frequency intercept corresponds to the combined contribution of polarization resistance (R_ct) and pore resistance (R_pore). The values of the R_ct and R_pore will be presented and discussed in detail in a later section of the manuscript. The pre-anodized and powder-coated samples show the largest semicircle diameter, representing superior corrosion resistance and higher impedance. The Bode phase angle diagram (Figure 1a) reveals that the sample treated with both pre-anodizing and powder coating exhibits a wide plateau near −90°, indicating strong capacitive behavior and a stable insulating layer. The pre-anodized and powder-coated sample demonstrates the most superior performance, characterized by a near-ideal capacitive response, the highest impedance across all frequencies, and a well-defined Nyquist semicircle. This indicates that the anodized oxide layer significantly enhances the protective function of the powder coating by increasing both the uniformity and stability of the interface, resulting in a highly resistant barrier against chloride-induced corrosion.

3.2. Surface Morphology and Corresponding Electrical Circuit Models

To understand deeply how EN AW 6060 aluminum alloy samples react after different surface treatments, the surface characteristics of the aluminum alloy samples, both untreated and after various treatments, were examined using Scanning Electron Microscopy (SEM). Additionally, different electrical circuit models were proposed for each treatment. These models show the unique features of each surface layer and help explain the results from impedance tests. The equivalent circuit used to model the corrosion behavior of the homogenized and aged aluminum sample (Figure 2a) includes the solution resistance (Rₛ), representing the electrolyte between the electrodes, a constant phase element (CPE₁) describing the non-ideal capacitive behavior of the metal/electrolyte interface, a second constant phase element (CPE₂) in combination with a pore resistance (R_por) representing the oxide layer and its barrier effect, and a charge transfer resistance (R_ct) representing electron transfer during corrosion reactions. Figure 2b depicts the uncoated sample that has clear grain boundaries and scattered Mg₂Si precipitates within the aluminum matrix [36].

Figure 3a shows an equivalent circuit suitable for powder-coated samples, which includes the solution resistance, resistance of pores within the coating, a constant phase element (CPE) capturing the coating’s capacitive behavior, and the charge transfer resistance at the metal/coating interface. Figure 3b,c include SEM images of the powder coated aluminum samples, which show that layers were well attached to the metal surface, showing that the coating process was successful. The coatings appeared compact and without major defects, which is important for good protection against corrosion. It was observed that the black coating was slightly thicker than the white one, which is consistent with the better EIS performance, described previously.

For the aluminum samples that underwent both pre-anodizing and powder coating, a more complex equivalent electrical circuit was applied to capture the behavior of the full protective system (Figure 4).

This model includes the solution resistance (Rs), representing the resistance of the electrolyte, and two sets of elements, one for the anodic oxide layer (CPE2 and R₂) and one for the substrate interface (CPE3, R_ct and Warburg element W), to account for charge transfer and diffusion effects. To reflect the influence of the powder coating, an additional branch composed of a constant phase element (CPE1) and resistance (R₁) was introduced. SEM observations (Figure 4b) show that the coated samples developed a well-organized multilayer structure. Despite the anodized layer exhibiting some surface voids and unsealed regions under magnification, the presence of the powder coating significantly improves overall protection by filling in imperfections and reinforcing the barrier against corrosive agents. Together, the two layers form an effective defense, improving corrosion resistance and extending the service life of the material.

The fitted values for these circuit elements, presented in

Table 1. Parameters of the equivalent circuit elements representing the corrosion behavior of uncoated, powder-coated, and pre-anodized plus powder-coated samples, after immersion in 3.5 wt% sodium chloride solution.

Al Samples NaCl solution	Rs	CPE1 S × sa	α1	CPE2 S × sa	α2	CPE3 S × sa	α3	Rpor	R1	R2	Rct	W S × s1/2
initial	11 Ω	2.2 × 10−6	0.98	8.9 × 10−7	1	-	-	618 Ω	-	-	25 kΩ	-
Powder coated (white colour)	0.1 Ω	8.9 × 10−10	0.92	1.1 × 10−8	0.91	-	-	2.4 ΜΩ	-	-	0.7 ΜΩ	-
Powder coated (black colour)	0.1 Ω	6.2 × 10−10	0.95	8.6 × 10−10	0.92	-	-	4.97 ΜΩ	-	-	9.65 ΜΩ	-
Pre-anodized and painted	0.1 Ω	1.74 × 10−9	0.95	1.8 × 10−9	0.93	1.4 × 10−9	0.9	-	7.5 ΜΩ	2 kΩ	0.47 ΜΩ	1.3 × 10−5

, show strong agreement with the experimental impedance data, confirming the models’ effectiveness in describing corrosion behavior in NaCl solution with a standard error less than 7%. Additionally, Figure 5 shows a comparative representation of the charge transfer resistance and double-layer capacitance values. The uncoated sample exhibited a low charge transfer resistance (25 kΩ), indicating poor corrosion protection. Powder-coated samples showed α values close to 0.9, indicative of relatively uniform surface coverage. The black powder-coated samples presented significantly higher pore resistance (4.97 MΩ) and charge transfer resistance (9.65 MΩ) than the white powder-coated samples, suggesting a more compact and effective barrier layer. The black-colored coating exhibits significantly greater resistance to corrosion compared to the white-colored coating. This is likely due to the fact that black paint forms a continuous, dense surface layer that effectively hinders the penetration of moisture, oxygen, and other corrosive agents toward the substrate. In contrast, the white-colored coating seems to be more porous and less uniform, allowing easier access for corrosive elements to reach and attack the material [28,29,30]. The pre-anodized and powder coated samples exhibited a complex multilayer structure with the highest resistance (R₁ = 7.5 MΩ), an intermediate pre-anodized layer resistance (R₂ = 2 kΩ) and improved α values (0.93–0.95). A low R₂ value indicates that the porous layer does not provide a good seal, which is also visible in the SEM images [37,38]. This is because the porous layer is not the only protective barrier, but an additional powder coating is applied on top to improve protection. Overall, the uncoated sample exhibited the lowest resistance, the high values of resistance indicate that the powder coating process was highly effective and pre-anodized and powder-coated samples show the most efficient protective behavior.

Polarization curves were also recorded for four aluminum samples in a 3.5% NaCl solution at 25 °C to further evaluate how surface treatments affect corrosion resistance (Figure 6). All coated samples display a characteristic polarization behavior featuring a small anodic region followed by a plateau where the current stabilizes over a range of applied potentials, indicating that the coating acts as a protective barrier limiting corrosion. In contrast, the uncoated initial sample shows an almost immediate increase in current at a nearly constant potential, which corresponds to the onset of localized corrosion. The flat initial part of the anodic branch indicates that the corrosion potential coincides with the breakdown potential, suggesting that localized corrosion had already initiated during the open circuit period [39]. Despite this, the uncoated sample eventually reaches a passive region, suggesting that a passive film is re-established after the initial localized corrosion event.

Both powder-coated samples, regardless of color, demonstrated significantly lower corrosion current densities than the untreated sample, highlighting the effectiveness of the coatings as protective barriers. Among them, the black-coated sample performed better, which is consistent also with the EIS results. The most notable result was observed for the sample that was pre-anodized before applying the powder coating, which exhibits the lowest corrosion current density, indicating the highest level of protection. This suggests that the combination of pre-anodizing and powder coating offers a dual-layer defense, combining a stable oxide film with a durable organic coating, to effectively reduce corrosion [40].

Table 2. Corrosion potentials (E_corr), corrosion currents (I_corr), anodic (β_a) and cathodic (β_c) Tafel slopes, and corrosion rates for aluminum samples with different surface treatments in a sodium chloride solution.

Sample	Ecorr [mV]	Icorr [A/cm2]	βa	βc	Corrosion Rate [μmpy]
Initial	−0.756	9 × 10−7	0.022	−0.52	30
Powder-coated (white color)	−0.693	1.11 × 10−7	0.47	−0.55	3.73
Powder-coated (black color)	−0.622	1.56 × 10−8	0.83	−0.68	0.508
Pre-anodized and painted	−0.729	9.72 × 10−10	0.9	−0.46	0.03

shows the corrosion potential (E_corr), corrosion current density (I_corr), and the anodic (β_a) and cathodic (β_c) Tafel slopes for aluminum samples with different surface treatments in a sodium chloride solution. The analysis results reveal significant differences in the electrochemical behavior of the samples depending on their surface treatment. The untreated samples show the highest corrosion current density (I_corr = 9 × 10⁻⁷ A/cm²), indicating relatively rapid corrosion, while the powder coatings significantly reduce the I_corr values. The sample combining pre-anodizing and powder coating exhibits the lowest corrosion current density (9.72 × 10⁻¹⁰ A/cm²). Regarding the corrosion rate, as seen from the values in Table 2 and Figure 7, the untreated samples show the highest corrosion rate (30 μmpy), the white powder-coated sample exhibits a corrosion rate of 3.73 μmpy and the black powder-coated sample an even lower one at 0.508 μmpy, demonstrating the effectiveness of the coating as a protective layer. The different corrosion rates between black and white powder coating are consistent with those of electrochemical impedance spectroscopy. The pre-anodizing and powder-coated samples exhibit the lowest corrosion rate (0.03 μmpy), confirming that this treatment provides the best corrosion protection. Overall, the data confirm that surface treatment especially the combination of pre-anodizing and powder coating dramatically improves the corrosion resistance of aluminum.

4. Conclusions

This research involved a systematic evaluation of the electrochemical behavior of aluminum samples with various surface treatments (untreated, powder-coated, and combined pre-anodized + powder-coated). Through electrochemical impedance spectroscopy (EIS) and equivalent circuit analysis, several findings were obtained, as follows:

• Even though the pre-anodized layer has some tiny holes and gaps, the powder coating helps considerably by covering those weak spots and making the surface tougher against corrosion. Together, these two layers provide strong protection and help the material last longer.
• The high values of resistance indicate that the powder coating process was highly effective.
• Black powder-coated samples showed better corrosion protection than white powder-coated ones, indicating that the coating forms a compact and uniform barrier that effectively inhibits corrosion.
• Surface modifications greatly enhance corrosion resistance. Black powder-coated specimens exhibit corrosion rates about 60 times lower, whereas samples that are both pre-anodized and powder-coated have the greatest protection, reducing corrosion rates by nearly 1000 times compared to untreated aluminum.