3.5. Corrosion Resistance of Two Super-Hydrophobic Coatings with Immersion Time
According to Tafel’s law, which was empirically observed, the logarithm of the current density in an electrochemical reaction varies linearly with the electrode potential (at potentials removed from the open-circuit rest potential). For the electrode process involving a slow reaction step (activated polarization) at the electrode surface, the understanding of Tafel’s law about the corrosion reaction depends on the absolute reaction rate theory [
29], resulting in the well-known Butler–Volmer equation [
30]. This equation relates to the net current density, i, for a single electrode process.
where i
0 is the exchange current density (rate of either the forward or reverse half-cell reaction) at the equilibrium potential E
0, a is the transfer coefficient (usually 0.5) and n is the number of electrons transferred.
For the corrosion system consisting of a cathodic reaction and an anodic reaction, the application of the Butler–Volmer equation and the mixed potential theory resulted in the basic kinetic equation:
where E is the potential applied to polarize the corrosion system; i is the external current density; E
corr and j
corr are the corrosion potential and corrosion current density, respectively; b
c and b
a are the cathodic and anodic Tafel slope, respectively. When E is far away from E
corr, Equation (3) gives the famous Tafel law [
31]:
where a is a constant and b equals to b
c or b
a. The equation indicates that the logarithm of the external current density varies linearly with the potential at a high overpotential. The corrosion current density can be determined by extrapolating the straight line of E ~ log|i| back to the corrosion potential.
When the overpotential is large enough (E − Ecorr > 50 mv), the logarithm of the current density in the electrochemical reaction changes linearly with the electrode potential. The extension line of the anodic polarization curve and the straight area of the cathodic polarization curve (in accordance with the Tafel relationship) intersect at a point, and the current corresponding to this point is the current (jcorr) at which the metal corrosion reaches a steady state.
Figure 5 presents the potentiodynamic polarization curves of the bare substrate, two super-hydrophobic coatings and an enlarged example of the corresponding anode Tafel plots before and after immersion [
32,
33,
34]. For the polarization curves, the potential range of the cathodic branch selected to perform the linear fitting varied from E
op −50 mV to E
op −10 mV, the anodic branch varying from E
op +50 mV to E
op +110 mV [
35,
36]. In all cases, the polarization curves showed a good linearity in the selected potential range. The measurement results showed that the two super-hydrophobic coatings after the immersion test still had a certain protective effect for the bare carbon steel, but their anti-corrosion ability was significantly reduced compared with the coatings without immersion, which was proved by the negative shift of corrosion potential and the increase in current density (
Figure 5). The factor promoting the corrosion performance of the super-hydrophobic Ni-Co and Ni-Co/WC deposits was the decreased metallic area exposed to the corrosive media. As the immersion time increased, more corrosive media was trapped in the cauliflower-like structure, and the Co–Ni-based coating corroded first. The anodic and cathodic reactions were as follows:
However, due to the corrosion of the Co–Ni-based coating and the pores in the coating, the corrosive media penetrated the coating to cause the corrosion of the substrate. The anodic and cathodic reactions were as follows:
According to the Tafel extrapolation method, the corrosion parameters of the measured samples were shown in
Table 3 and
Table 4, respectively.
Compared to the unsoaked sample (6.29 × 10–7 A/cm2), the corrosion current density of the Co–Ni SHPB coating increased to 5.06 × 10–6 A/cm2 after immersion for 25 days, but was still lower than that of the bare substrate (1.45 × 10–5 A/cm2), which indicated that the loss of super-hydrophobicity reduced the corrosion resistance of the coating. Similarly, the corrosion current density of the Co–Ni/WC SHPB composite coating increased to 3.06 × 10–6 A/cm2 after immersion for 15 days, which demonstrated that it was easier for the Co–Ni/WC SHPB composite coating than the Co–Ni SHPB coating to lose its super-hydrophobicity.
Figure 6 shows the EIS plots and their fitted results for the bare carbon steel, unmodified Co–Ni coating and Co–Ni SHPB coating immersed for 0 days, 10 days and 25 days. As shown in
Figure 6a, the semicircle Nyquist plots of bare carbon steel show a typical behavior of solid metallic electrodes with the center below the real impedance axis. The Nyquist plot of the bare Co–Ni coating depicted a semicircle similar to the plot of bare carbon steel, and reflected a larger arc radius due to the barrier effect of the coating. In addition,
Figure 6a also revealed the appearance of only one capacitive loop for the Co–Ni SHPB coatings before and after immersion, and the size of the capacitive loop gradually decreased as the immersion time increased. Generally, the better anti-corrosion ability of a coating corresponded to the larger size of the capacitive loop. Compared to the bare Co–Ni coating, the corrosion resistance of the Co–Ni SHPB coating gradually halved as the immersion time prolonged to 25 days, but was still one order of magnitude larger than the base material. The equivalent circuit shown in
Figure 7a was proposed to fit the EIS plots of the bare Co–Ni coating, and the introduction of the Warburg impedance in
Figure 7a achieved a more satisfactory fitting result. The equivalent circuit shown in
Figure 7b was proposed to fit the EIS plots of the bare carbon steel, considering the influence of corrosion products, two parallel resistance and capacitance combinations of impedance were proposed to fit the EIS plots [
28].
The impedance module and phase angle were given in the Bode diagram (
Figure 6b,c). The impedance module Z (
Figure 6b) for all samples shows a nearly linear slope in the low and middle frequency ranges, where the Co–Ni SHPB coating before immersion was as large as about 100 kΩ·cm
2, higher than the Co–Ni SHPB coating after immersion and two orders of magnitude higher than that of the base material (1.21 kΩ·cm
2). The phase angle plot (
Figure 6c) shows only a one time-constant for the bare carbon steel (around 1 Hz) and Co–Ni coating without modification (around 10 Hz), which is associated with electrochemical activity and could be explained by the capacitance of the double layer on the solid/electrolyte interface. Obviously, the time-constant of the bare Co–Ni coating was shifted to a higher frequency than the base material, which could be attributed to the formation of the double-layer capacitance on the coating/electrolyte interface. Two time-constants could be observed for the Co–Ni SHPB coating after immersion for 0 days, 10 days and 25 days, and the time-constant at a low frequency was shifted to a lower frequency due to the decrease in the super-hydrophobicity as the immersion time was prolonged. The equivalent circuit in
Figure 7b was proposed to fit the EIS plots of the bare carbon steel and Co–Ni SHPB coatings during the immersion process, where R
ct and CPE
dl corresponded to the charge transfer resistance and double layer capacitance occurring at the substrate/solution interface within a very small, corroded area, and R
c and CPE
c corresponded to the coating resistance and double-layer capacitance at the substrate/coating interface, respectively. The symbol CPE indicates the possibility of a non-ideal capacitance, which is considered to be a description of the relaxation time associated with heterogeneities at the surface of the electrode. From the literature [
28,
37], the impedance of CPE is described as follows:
where Z
CPE represents the impedance of a CPE, Y
0 represents the modulus, ω represents the angular frequency and n is the exponent of the CPE varying between 0 and 1.
Based on the relevant equivalent circuits revealed in
Figure 7, the electrochemical parameters of all samples were shown in
Table 5. The solution resistance (R
s) remained stable for all tested samples after immersion in the NaCl solution, revealing a stable conductivity in the corrosive media. For the unmodified Co–Ni coating, the value of R
c was less than 10 Ω·cm
2 when analyzed. Thus, the equivalent circuit could be simplified as the circuit in
Figure 7a.
Before immersion, the Co–Ni SHPB coating displayed the highest charge transfer resistance value (Rct), representing the best corrosion resistance. The Rct value of the Co–Ni SHPB coating decreased from 648 to 42.8 and 9.45 kΩ·cm2 as the immersion time increased to 10 days and 25 days, corresponding to the decrease in non-wetting properties over the super-hydrophobic surface. Similarly, the Rc value of the SHPB Co–Ni coating decreased from 12.2 to 11.8 and 6.13 kΩ·cm2, with the increase in immersion time to 10 days and 25 days, which could be attributed to the degradation of the protective coating over the bare carbon steel substrate. In particular, when the immersion time reached 10 days, the Rct value dropped significantly, but the Rc value only dropped slightly, which indicated that the coating was not significantly damaged in the early stage of immersion. It was proven that the corrosion resistance of the super-hydrophobic coating was degraded due to the displacement of the air layer by the aqueous corrosive media.
Figure 8 shows the Nyquist plots and their fitted results of the bare carbon steel, bare Co–Ni/WC composite coating and Co–Ni/WC SHPB composite coating immersed for 0 days, 10 days and 15 days. The Nyquist plots of the bare Co–Ni/WC coating showed a weak Warburg impedance in the low-frequency range and a semicircle capacitive loop in the high-frequency range (
Figure 8a). The Bode phase angle plots (
Figure 8c) showed an asymmetric peak, and two peaks approximately around 1 Hz and 10
3 Hz were divided to correspond the composite coating and solution–substrate interface. Hence, an equivalent circuit (
Figure 9a) was proposed, which included two capacitances and resistance in parallel and combined with the Warburg impedance to fit the EIS of the unmodified Co–Ni/WC coating. In this circuit, R
c was added to correspond the resistance of the Co–Ni/WC composite coating.
Similar to the plots of the Co–Ni SHPB coating, only one semicircle capacitive loop appeared in the Nyquist plots of the immersed composite coating (
Figure 8a). The Bode module plot (
Figure 8b) halved the impedance drop for the Co–Ni/WC SHPB composite coating after immersion for 10 days, and about a seven-times drop for the immersion time of 15 days. According to the Bode phase angle plots, two time-constants could be observed with two corresponding peaks at approximately 10 Hz and 10
5 Hz, where the time constant observed in high frequency was ascribed to the existence of the super-hydrophobic composite surface and the time constant observed in the intermediate frequency could be related to the corroding interface. The equivalent circuit (
Figure 9b) characterized the corrosion principle of the super-hydrophobic coating before and after immersion in the NaCl solution, where R
s corresponded to the solution resistance, R
ct and CPE
dl were the charge transfer resistance and the double-layer capacitance occurring at the substrate/solution interface. The CPE
c and R
c corresponded to the double-layer capacitance at the substrate/coating interface and the resistance of the Co–Ni/WC coating, respectively.
The electrochemical parameters shown in
Table 6 clearly represented the surface conditions of the Co–Ni/WC SHPB coating during the immersion process. R
ct represented the charge transfer resistance at the solution/substrate and an obvious decrease from 67.0 kΩ·cm
2 to 44.5 kΩ·cm
2 and 18.8 kΩ·cm
2 as the immersion time increased to 10 days and 15 days. Similar results were observed from the CPE
dl, where the value increased from 1.67×10
−7 Ω
−1·s
n·cm
−2 to 4.86×10
−6 Ω
−1·s
n·cm
−2 and 2.22×10
−5 Ω
−1·s
n·cm
−2. The increase in the CPE
dl and decrease in the R
ct indicated the protective effect of the inner layer having dropped gradually with the immersion time. Generally, the CPE
dl reflects the dielectric behavior of the solution/coating interface, and CPE
dl becomes C for n = 1. Furthermore, C = C
0A, where C
0 and A are the normalized capacitance and the exposed coating area, respectively [
38]. The increase in the value of the CPE
dl as the immersion time increased showed that A was enlarged, which was consistent with the changing of the coating morphology in
Figure 2. In particular, the coating resistance (R
c) for the Co–Ni/WC composite coating without modification reached 760 Ω·cm
2, which was much more than the Co–Ni coating resistance due to the addition of second-phase particles (WC). Generally, the polarization resistance (R
p) always evaluates the total corrosion resistance, which was defined as a low-frequency limit of the electrode impedance and the frequency going to zero impedance of a capacitor to infinity. Therefore, R
p = R
c + R
ct for circuits 9c, R
p = R
c for circuits 9a. When the SHPB Co–Ni/WC composite coating was immersed for 10 days, the R
p value decreased slightly. However, as the immersion time reached 15 days, the R
p value decreased by more than one order of magnitude, which was close to the bare Co–Ni/WC composite coating.
The corrosion protection mechanism provided by the Co–Ni/WC SHPB coating was shown in
Figure 10. The air entrapped in the cauliflower-like structures could prevent the Co–Ni/WC composite coating from being wetted by the corrosive media through the limited solid contact area, endowing the coating with a larger charge transfer resistance and a lower corrosion rate, and then a better corrosion protection performance.
The long-term durability of the super-hydrophobic surface is a critical criterion in assessing their wide range applications. The stability of super-hydrophobic surfaces as engineering materials in corrosive media is crucial in many practical applications. Some literature has reported the stability of super-hydrophobic surfaces in different corrosive media, such as strong acid, strong alkali, pure water and 3.5% NaCl aqueous solutions. Zhang et al. fabricated super-hydrophobic (SHPB) surfaces by electrodeposition, and the prepared SHPB surfaces exhibited excellent chemical stability in pure water and corrosive water, such as acid and alkaline, conditions [
39]. Zhang et al. reported a handy and versatile fabrication of nonfluorinated giganteum-like super-hydrophobic aluminum surfaces. The super-hydrophobic surfaces maintained super-hydrophobicity after an 8-month air exposure and a 12-day immersion under 3.5% NaCl aqueous solutions, respectively [
40]. In our work, the as-prepared two kinds of robust coatings displayed a good long-term durability, whereas the Co–Ni SHPB coating and Co–Ni/WC SHPB composite coating lost their super-hydrophobicity over 10 days of immersion.