*3.2. Electrochemistry*

#### 3.2.1. Inductive Coupled Plasma (ICP) Test

The amounts of aluminum ions (Al3+) detected from the 3.5% NaCl solutions using ICP technique after each test condition for the three coated samples are presented in Table 2. It can be seen that no ions were found in the absence of any potential applied, except at high temperature for PSC sample only. After the 80 ◦C free corrosion tests, the samples were immersed in the solution for another 24 h and very small amounts of Al3+ were released for both HA and PSC coatings. However, PEO coating has no charge transfer under all test conditions.

**Table 2.** Al3+ (in grams) detected from 3.5% NaCl solution after 24 h free corrosion test.


#### 3.2.2. Mass Loss (Al3+) from Polarization Tests

The amount of aluminum alloys released after polarizing the samples up to 400 mV from the OCP in 3.5% NaCl solution for 24 h are determined using the ICP method. Figure 5 shows the potential current versus time plots to calculate charge transfer at 400 mV for HA, PEO and PSC. There was a sharp increase in the current for the first 1200 s for HA then the current stabilized and steadily increased with some fluctuation in the rest of the test period. However, the PEO coating showed a gradual increase in current density throughout the test period without any sudden increase of the current which indicates a low rate of ion transfer through the coating. Also, the current values were

an order of magnitude lower. There was no stability of the current in the case of the PSC sample as a high variation of the curve occurred during the 24 h polarization test and the current was significantly high compared to the other two coatings. The first increase of the current occurred at about 1000 s from 1.5 × 10−<sup>2</sup> A/cm<sup>2</sup> to 3.0 × 10−<sup>2</sup> A/cm<sup>2</sup> which could be attributed to high movements of Al3+ movement through the coating due to its high porosity. However, the current increased from about 2.5 × 10−<sup>2</sup> A/cm<sup>2</sup> to 5.0 × 10−<sup>2</sup> A/cm<sup>2</sup> around 7000 s where it is expected that the PSC coating has completely removed from the substrate. The current fluctuation on the polarization curves for all coatings could be attributed for electrochemical activities taking place on the surfaces such as localized pitting corrosion in the substrate/coating interface region.

**Figure 5.** Current density versus time plots to calculate charge transfer for coating by (**a**) HA, (**b**) PEO and (**c**) PSC at +400 mV (Ag/AgCl).

#### 3.2.3. Open Circuit Potential (OCP) Measurements

The Open Circuit Potential (OCP) measurements were carried out for the 30 s to find the starting OCP values for all the three coating. Many measurements were made and the average values are presented in Figure 6 for all the samples. This represented the initial OCP measurements when the samples were immersed in 3.5% NaCl without applying any potential. Firstly, it can be seen that the PSC sample has the most negative OCP value of about −0.76 V followed with the Al substrate with −0.69 V. It is expected that Al has a low negative value of *Ecorr* since it can release three electrons per atom and consequently make the Al to be used as an anode in power sources applications. Also, it has been reported that in chloride solutions it has an *Ecorr* value of −0.75 V/SCE [52]. However, PEO coating has the highest starting OCP value with −0.048 nV while HA has around −0.4 V. According to [53], a high OCP value refers to better corrosion protection, which indicates the enhancements of these two coatings in the anticorrosion performances.

**Figure 6.** Initial Open Circuit Potential (OCP) measurements of the coating materials versus the Ag/AgCl reference electrode.

The OCP measurements have been extended for 5000 s, as shown in Figure 7 and it has been observed that the OCP value for the PEO sample dropped to the OCP value for the substrate after about 650 s. This sudden change in the OCP curve can indicate a rapid movement of ions through the coating part (insulator). However, the HA coating continued until approximately 3350 s then it decreased sharply to the OCP value for Al. The rapid decrease of the OCP value for PEO coating compared with the HA coating could be due to the difference in coating thickness where HA (42 μm) is higher than PEO (34 μm) which increase the resistance of Al3+ ions to penetrate through the coating [54]. However, the OCP value of the PSC coating keeps constant at a lower value than the OCP of the aluminum throughout the time of immersion.

In general, anodic areas are, at least in the early stages, much smaller than cathodic areas. So, in the early stages, the corrosion potential is more positive. However, with an increase in anodic sites during immersion, the corrosion potential becomes more negative.

**Figure 7.** OCP measurements of the coating materials for 5000 s in Ag/AgCl reference electrode.

#### 3.2.4. Anodic Polarization (AP) Resistance

The results of the AP resistance for all materials Al, HA, PEO and PSC samples are shown in Figure 8. The breakdown potential (*Eb*) is the potential where the passive film of the surface breaks down. It is expected that the coatings would decrease the possibility of breaking down the aluminum passive film and consequently decrease the current density and improve the corrosion resistance. *Eb* can be obtained from the AP curve (E. vs. i) where the current density increases sharply after this point. If there is no crevice corrosion, *Eb* refers to the pitting corrosion. However, *Eb* for the ceramic materials indicates the penetration of the electrolyte ions through the coating defects to the substrate metal. As a result, material's failure can be formed as localized pitting corrosion. When the potential reaches a voltage of ±1000 mV from OCP or reaching a given magnitude of current density (referred to as the threshold current), the potential decreased towards the OCP value. The breakdown voltages for all the materials systems can be determined from the anodic polarization curves at the potential value where the current increased rapidly and deviated from the initial growing rate. The values of the breakdown potentials of the materials were determined from the plots and the red lines in these graphs are just to show the method and not indicating the exact *Eb* values. These values are summarized in Figure 9.

**Figure 8.** AP Measurements for (**a**) Al substrate, (**b**) HA, (**c**) PEO and (**d**) PSC coatings.

**Figure 9.** Comparison of breakdown potential from AP measurements for the tested materials against the Ag/AgCl reference electrode.

#### 3.2.5. Corrosion Current Density

The corrosion current densities for the materials tested in this study were determined from the logarithmic scale of the current density in the anodic polarization curves as shown in Figure 10. The potential was shifted from the OCP value of the material to 250 mV in the opposite direction to ensure that the cathodic and anodic currents are different to measure the corrosion current density on the sample by extrapolating the anodic branch line from OCP value. Also, a comparison of the corrosion current density (*icorr*) between all the coated materials is shown in Figure 11. The result of Al substrate was not included due to the huge difference values of *icorr* between the aluminum substrate and other coatings. It is clear form this comparison chart that the PEO coating showed the lowest corrosion current density followed by HA and then PSC coatings.

**Figure 10.** Determination of corrosion current density for (**a**) Al substrate, (**b**) HA, (**c**) PEO and (**d**) PSC coatings.

**Figure 11.** Comparison of corrosion current density values from AP measurements for the coated test specimens.

#### 3.2.6. Optical Images of the Surfaces after Polarization tests

Figure 12 shows the surface behavior of the tested materials after Polarization scan in 3.5% NaCl solution. The aluminum surface (Figure 12a) has wide corrosion attack in the form of pits (see arrows). However, HA and PEO samples (Figure 12b,c, respectively) have fewer corrosion defects on the exposed surfaces. Regarding the PSC sample, a significant number of white spots (Figure 12d) on its surface can be seen which correspond to the aluminum substrate. From all these figures, the aluminum surface had more corrosion products as large-scale pits were initiated on its surface after the polarization test.

Table 3 summarizes the main corrosion parameters of the materials that were determined by the DC electrochemistry plots (anodic polarization curves). Therefore, PEO coating has lower corrosion current density (1.7 × 10−<sup>8</sup> A/cm2) than the HA coating (3.5 × 10−<sup>7</sup> A/cm2) and PSC coating (2.6 × 10−<sup>7</sup> A/cm2) under static anodic polarization tests.

**Figure 12.** Optical images of coating surfaces after polarization tests for (**a**) Al, (**b**) HA, (**c**) PEO and (**d**) PSC coatings.

**Table 3.** Summary of corrosion parameters of the materials.


#### 3.2.7. AC Impedance Test

The Nyquist plots for all materials are presented (Figures 13 and 14) for 10 days of immersion the samples in 3.5% NaCl solution to study the stability of the materials (aluminum passive film of the substrate and coating part of the other materials) and observe the change in the total resistance during the long exposure period. Also, the impedance data of the materials were fitted with equivalent circuit models using ZView software (Ametrek, NC, USA).

**Figure 13.** Nyquist plots for (**a**) Al substrate, (**b**) HA, (**c**) PEO and (**d**) PSC coatings at different immersion times.

**Figure 14.** Bode plots for (**a**) Al substrate, (**b**) HA, (**c**) PEO and (**d**) PSC coatings at different immersion times.

The impedance of all spectra from Day 0 and Day 10 of the Al substrate exhibits a capacitive behavior pattern (single semicircle) as shown in Figure 13a. At initial immersion, the aluminum has the highest value of the total resistance (Rtot) which is corresponding to the large difference in the imaginary impedance after 24 h of immersion. The charge transfer resistance seems to increase as the exposure time increases and the maximum Rtot (maximum radius) is observed at day 10 (Figure 13a). Also, the highest peak in the Bode diagram is recorded on the last day as in Figure 14a. Therefore, it is consistent with the increase in the radius in Nyquist plots over the entire period. Many cavities with different sizes, some of them can be seen by eyes (Figure 15a), were formed on the passive aluminum surface due to the contact of the corrosive aqueous media (3.5% NaCl). Although the mechanism of the pitting corrosion is not fully understood, it can be explained by two consequent stages. Firstly, the pits are developed due to the adsorption of chloride ions Cl− on the oxide film which cracks at weak areas causing micro-cracks and these pits ge<sup>t</sup> re-passivated as in Figure 15b. The intermetallic phases under the oxide layer have a low oxygen level driving the aluminum to by highly oxidized at the film broken sites. Secondly, the pits propagate due to the oxidation at the anode site (inside the pit) and the reduction at the cathode suite (outside the pits).

The AC Nyquist spectra for HA for all the period exhibited capacitive behavior (Figure 13b). Also, the stability of the capacitive behavior of HA is related to the high corrosion resistance of the HA coating. However, the slight decrease of the capacitive response at Day 10 (Figure 14b) could indicate the dissolution of the coating thickness or high porous coating was formed to allow electrolyte ions movements through the coated part [54]. For the surface behavior of HA sample after AC impedance test, different sizes of pits have been initiated on HA surface after 10 days of immersion the sample in 3.5% NaCl as shown in Figure 16. Similar pits were found in an anodized aluminum surface after a polarization test in 0.5 M NaCl solution performed by Ren et al. [55], which indicates penetration defect in the anodic film.

**Figure 15.** Optical images of aluminum surface after 10 days of immersion in 3.5% NaCl solution at (**a**) low magnification, (**b**) enlarged magnification.

**Figure 16.** Optical images of HA coating surface after 10 days of immersion in 3.5% NaCl solution (**a**) low magnification, (**b**) enlarged magnification.

Regarding the AC impedance of the PEO coating, as shown in Figures 13 and 14, there is an arc at the high frequency side with a change to a diffusion tail at the low frequency side. The starting peaks of the impedance in the Bode magnitude plot is the highest for PEO sample in the initial immersion time. The drop of the starting peak after 10 days of the immersion (Figure 14c) indicates a change in the corrosion process taking place in the interface layer between the Al substrate and PEO coating during this period. The surface behavior of PEO coating after AC tests is shown in Figure 17. General corrosion products on the PEO surface is shown in Figure 17a while some materials degradation take place as shown in the magnified image in Figure 17b.

**Figure 17.** Optical images of PEO coating surface after 10 days of immersion in 3.5% NaCl solution (**a**) low magnification, (**b**) enlarged magnification.

The arc radius in the Nyquist plot of PSC coating was slightly increased after the full period of immersion test (Figure 13d). This corresponds to the minimal change in the total resistance from the Bode plot shown in Figure 14d. The aluminum substrate can be seen after the corrosion test (Figure 18) which indicates that a relatively high amount of PSC coating degradation has occurred due to this chemical reaction.

**Figure 18.** Optical images of PSC coating surface after 10 days of immersion in 3.5% NaCl solution (**a**) low magnification, (**b**) higher magnification.

The Nyquist and Bode curves for all tested materials have been combined in one plot to compare their AC impedance spectra. The Nyquist and Bode plots for all materials at the initial and after 10 days measurements are shown in Figures 19 and 20, respectively. It can be seen that both Al and PSC have depressed one semicircle, which means that only one-time constant has occurred in the system for both of them. This corresponds to one single peak in the Bode plot for Al and PSC. On the other hand, the PEO sample has two-time constants while HA has an infinite curve which is the behavior of a capacitive response. This capacitive form of HA indicates the stability of the coating passive film. After 10 days of immersion, the samples in the 3.5% NaCl solution, the Nyquist and Bode plots were obtained; and they are represented in Figures 19 and 20, respectively.

From the Bode magnitude plots, it can be seen that for all the material systems, the impedance resistance |Z| decreases as the immersion time increases; this indicates that material degradation is taking place in the protective nature of the coatings. This decrease makes the response less capacitive behavior which indicates that the solution ions penetrated through the pores in the coating to the substrate as the time of the immersion increased [54]. The sudden alteration of the |Z| versus frequency curve (Bode plot) is related to the coating degradation and significant changes in coating capacitance Cc and coating resistance Rc are noticed. The coating resistance slowly decreases when coating capacitance increases. This can be attributed to the higher porosity of the coating, which then creates heterogeneities in the coating and makes the water maneuver easier.

**Figure 19.** Nyquist plots for the materials after (**a**) 0 day and (**b**) 10 days.

**Figure 20.** Bode plots for the materials after (**a**) 0 day and (**b**) 10 days.
