3.3. EIS Analysis
According to the tensile strength, strength loss rate, and ∆Y values of the composites, the best strength and UV aging resistance were achieved with 6 wt.% spherical aluminum. Therefore, the composite prepared with 6 wt.% spherical aluminum was selected for EIS analysis to further investigate the impact of UV light on the protective properties of the aluminum powder/epoxy resin coating.
Figure 6 shows the equivalent circuit diagram of different immersion stages, where
Rs is the solution resistance,
Cc is the coating capacitor,
Cd is the solution double-layer capacitance, and
Rct is the coating resistance. The impedance mode value |Z| at 0.01 Hz is used to characterize the barrier performance of the coating [
22,
23,
24].
The EIS results of the pure epoxy resin coating at varying immersion times are shown in
Figure 7, including the Bode plot, phase angle plot, and Nyquist plot. When the pure epoxy coating was immersed in 3.5 wt.% NaCl solution for just 0.5 days, two time constants were identified, corresponding to a low-frequency impedance mode value |Z|
0.01 of about 10,188.33 Ω·cm
2. The phase angle of the pure epoxy coating was close to 90° within a frequency, and the maximum value was the manifestation of the second time constant. This indicated that the corrosive media had begun to penetrate into the coating and reach the metal surface, which damaged the bonding between the metal substrate and the coating. The Nyquist plots consisted of two semicircles, indicating that the coating at this time had certain barrier properties. The coating was able to protect the metal substrate from corrosive particles in the solution.
With increasing immersion time, the |Z|0.01 value and phase angle of the pure epoxy coating were reduced, and the radius of the Nyquist plot semicircle also decreased. After 3 days of immersion, the value of |Z|0.01 reached 6952 Ω·cm2. After 10 days, |Z|0.01 reached 5167.94 Ω·cm2, and after 20 days, |Z|0.01 was reduced to 4099.70 Ω·cm2. After 30 days, |Z|0.01 was about 2063.00 Ω·cm2, and the Nyquist plot showed a straight line with a slope of approximately 1 in the low-frequency region. This indicated that diffusion occurred at this time. Therefore, after 30 days, the corrosion medium contacted the Q235 steel substrate, which began to corrode. When the immersion time was increased to 60 days, the value of |Z|0.01 dropped to 579 Ω·cm2, which was a decrease of about two orders of magnitude compared to the value of the sample immersed for 0.5 days. This showed that the protective effect of the epoxy resin coating on the substrate significantly decreased. After 60 days, this pure epoxy coating lost most of its protective ability.
The EIS results of the composite coating with 6 wt.% spherical aluminum powder are shown in
Figure 8. When this coating was immersed in the 3.5 wt.% NaCl solution for 0.5 days, the Nyquist plot showed a semicircle. The Nyquist and phase angle plots of this composite coating demonstrated that it only had one time constant. The |Z|
0.01 value of this coating was 61,746.48 Ω·cm
2 in the low-frequency region, which was about 6 times higher than that of the pure epoxy resin after 0.5 days of immersion. Moreover, the phase angle was close to 90° across a very wide frequency range, which showed that the composite coating had good protective properties and could effectively prevent the corrosive medium from penetrating through the coating to the metal substrate surface.
As the immersion time of the composite coating was extended to 3 days and 10 days, its |Z|0.01 value gradually decreased to 55,168 Ω·cm2 and 51,842 Ω·cm2, respectively. When the composite coating was immersed for 20 days, a maximum value was observed in the phase angle plot, and the Nyquist plot showed two semicircular arcs. This indicated the appearance of the second time constant and that the corrosive media had reached the surface of the metal substrate through the coating at this time. Thus, the bonding between the substrate and the coating was damaged. At this time, the |Z|0.01 value of the composite coating was 48,060.27 Ω·cm2. After 30 days, the value of |Z|0.01 was 43,806.00 Ω·cm2; After 60 days, the value of |Z|0.01 was 25,791.33 Ω·cm2, demonstrating a significant decrease compared to 0.5 days of immersion. This showed that the protective effect of the composite coating has decreased with the longer immersion.
Compared with the pure epoxy coating, the composite coating prepared with 6 wt.% spherical aluminum powder provided better protection for the metal substrate during the NaCl immersion process. This composite coating effectively blocked corrosion medium penetration and inhibited substrate corrosion. However, with the extension of the immersion time, the protective performance of the composite coating did eventually decline.
The EIS results of the pure epoxy resin coating after 100 days of UV aging followed by immersion in 3.5 wt.% NaCl are shown in
Figure 9. The phase angle and Nyquist plots exhibited two time constants at different immersion times. When the coating was immersed in solution for 0.5 days, its |Z|
0.01 value was 4902.13 Ω·cm
2, which was 0.5 times that of the unaged coating after 0.5 days of immersion. This indicated that UV light aging significantly reduced the barrier properties of this pure epoxy coating. At this time, the coating resistance obtained by fitting the Nyquist plot was 3044 Ω·cm
2, which was also much lower than that of the unaged coating immersed for 0.5 days. After 3 days of immersion, the radius of the Nyquist semicircular arc decreased, the |Z|
0.01 value of the pure epoxy coating was 4346.77 Ω·cm
2, and the fitted coating resistance was 2110 Ω·cm
2. After 10 days of immersion, the Nyquist plot showed a straight line with a slope of approximately 1 in the low-frequency region, indicating a diffusion phenomenon. This showed that the corrosion media significantly penetrated the coating, reaching the surface of the metal substrate and causing metal corrosion. At this time, the |Z|
0.01 value of the pure epoxy coating was 2982.33 Ω·cm
2, and the coating resistance value obtained by fitting was 1142 Ω·cm
2. After 20 days of immersion, the radius of the Nyquist semicircular arc was further reduced. This indicated that corrosion continued to intensify. Meanwhile, the |Z|
0.01 value was 2288.59 Ω·cm
2, and the coating resistance value obtained by fitting was 620.4 Ω·cm
2. After 30 days of immersion, the |Z|
0.01 value was 1200.60 Ω·cm
2, and the coating resistance value obtained by fitting was 108.6 Ω·cm
2. This was significantly lower than that of the coating after 0.5 days of immersion, which indicated that the pure epoxy coating lost most of its protective effect at this time. After 60 days of immersion, the |Z|
0.01 value was 852.12 Ω·cm
2, and the coating resistance value obtained by fitting was 58.72 Ω·cm
2. At this time, the pure epoxy coating completely lost its protective effect, and the metal substrate was completely corroded.
The EIS results of the composite coating prepared with 6 wt.% spherical aluminum powder after 100 days of UV aging followed by immersion in 3.5 wt.% NaCl solution are shown in
Figure 10. During the initial stage of immersion, the Nyquist plot presented two semicircles of different sizes. Combined with the phase angle plot, this showed that the composite coating had two time constants during this period. The |Z|
0.01 value at this time was 51,690.12 Ω·cm
2, and the coating resistance value obtained by fitting was 3773 Ω·cm
2. After 3 days of immersion, the |Z|
0.01 value was reduced to 42,650 Ω·cm
2, and the coating resistance value obtained by fitting was 35,660 Ω·cm
2. After 10 days of immersion, the |Z|
0.01 value was further reduced to 38,700.52 Ω·cm
2, and the coating resistance value obtained by fitting was 32,754 Ω·cm
2. After 20 days of immersion, |Z|
0.01 further declined to 34,950.85 Ω·cm
2, and the coating resistance was 29,639 Ω·cm
2. After 30 days of immersion, the |Z|
0.01 value was 31,750.08 Ω·cm
2 and the coating resistance was 25,630 Ω·cm
2. When the immersion time reached 60 days, a straight line with a slope of approximately 1 was visible in the low-frequency region, which indicated that a large amount of the corrosion media penetrated the coating, reached the surface of the metal substrate, and caused metal corrosion. At this time, the |Z|
0.01 value decreased to 8103.27 ohms·cm
2, which was lower by an order of magnitude than that of the unaged composite coating immersed for 0.5 days.
The coating resistance and the coating capacitance of the pure epoxy coating and the composite coating prepared with 6 wt.% spherical aluminum powder before and after UV aging are shown in
Figure 11 and
Figure 12.
Figure 11 shows that the coating resistance decreased with increasing immersion time. However, the coating resistance of the unaged composite coating was much higher than that of the unaged pure epoxy coating at equivalent immersion times. After UV aging for 100 days, the coating resistance of both the pure epoxy coating and composite coating declined, but that of the aged composite coating was still higher than that of the aged pure epoxy coating.
Figure 12 shows that the coating capacitance increased with increasing immersion time. The capacitance value of the unaged composite coating was lower than that of unaged pure epoxy coating. This was also true after UV aging.
According to
Figure 11 and
Figure 12, the coating immersion process can be divided into four stages. In the first stage, the coating has high resistance and low capacitance. This stage is the initial period when the electrolyte enters the coating. However, with increasing immersion time, the coating capacitance gradually increases and the coating resistance gradually decreases. This stage is the second stage of the immersion process. During this second stage, water molecules penetrate the interior of the coating, meaning that this insulating coating now has a degree of electrical conductivity. A greater coating resistance means that the coating is a better barrier to water, oxygen, and other electrolytes. When the coating becomes saturated by water adsorption, it enters the third stage of the immersion process. At this time, because the coating is saturated with water molecules, its resistance value and capacitance value tend to be a certain value. With a further extension of immersion time, the corrosive particles in the electrolyte successfully penetrate through the coating and reach the metal surface. At the surface, they react with the metal at the interface between the coating and the metal surface, leading to metal corrosion. The generated corrosion products reduce the bonding force between the coating and the metal substrate. At this time, the coating resistance sharply drops and the capacitor value sharply increases. This is the fourth stage.
Comparing the unaged pure epoxy coating and the unaged composite coating shows that the pure epoxy coating reached the second immersion stage after 0.5 days of immersion. This coating then rapidly saturated in the third stage. On the 30th day of immersion, the corrosion medium penetrated the coating and reached the metal surface, causing the coating to corrode. Consequently, the change in the resistance and capacitance of the pure epoxy coating was almost a straight line. No obvious characteristics of the third immersion phase were observed in these plots. For the composite coating with the aluminum powder, the characteristics of the first, second, and third stages were clear within 60 days of immersion, but the characteristics of the fourth stage were not visible. This was because the addition of aluminum powder extended the pathway of the corrosion media into the composite coating, which enhanced the protective effect of the coating on the metal and prolonged its service life.
After UV aging, the rate of change of the resistance and capacitance values of the pure epoxy coating and the composite coating were both greater than those of the unaged coatings. This was because the UV light destroyed the three-dimensional network structure of the epoxy resin, leading to an increased number of holes and cracks in the coating. These holes and cracks enhanced the penetration of the corrosion medium through the coating to the surface of the metal substrate. Therefore, the penetration rate of the corrosion medium was significantly higher after UV aging, leading to higher coating resistance and capacitance change rates. However, the addition of aluminum powder significantly reduced the change rate of the coating resistance and capacitance caused by UV light aging. This shows that the addition of aluminum powder effectively improved the protective performance of the coating after long-term UV light illumination. This was because the aluminum powder effectively reflected the UV light, which reduced the UV light absorbed by the coating and protected the epoxy resin from UV light damage. In addition, the aluminum powder also blocked the penetration of the corrosion media. Moreover, the composite coating still maintained a good protective performance after 100 days of UV light illumination.