# Nanostructured Black Nickel Coating as Replacement for Black Cr(VI) Finish
## 3 Results and Discussion
### Corrosion Testing in 0.5 M NaCl  
The corrosion resistance of the new coating was tested in 0.5 M NaCl and compared with the Ni bare substrate. All electrochemical measurements were carried out in triplicate to evaluate the reproducibility of the tests. Figure 8a shows the polarization curves measured after 2 h of immersion. The shapes of both curves are not very different, with similar corrosion potential, E\({}_{\text{corr}}\), and a quasi-passive region with a high slope, \(\beta_{\text{a}}\), which means that the metal oxidation will take place with difficulty as the potential increases. The corrosion current rates, i\({}_{\text{corr}}\), were obtained by the Tafel extrapolation method (using the linear region between \(-0.125\) and 0 V for the Ni sample and \(-0.025\) and 0.1 V for the black Ni sample) and are presented in Table 4. The curves are shifted in the current coordinates, and i\({}_{\text{corr}}\) is higher in the black coating but this may be due to the high roughness of the surface, which makes the real area higher than the geometric area (the one corresponding to the 2 cm\({}^{2}\) of the working electrode). Admitting that the surface material is the same (nickel) then the reaction rate is the same and the variations in the measured current are just a consequence of the different areas. Based on this assumption, the ratio of i\({}_{\text{corr}}\) on the coated and bare samples means that the geometric area of the black surface is around 11 times larger than the bare surface. Figure 7: SEM images of the (**a**) nickel surface before deposition, (**b–d**) after deposition with the black coating at different magnifications, (**d**) nanostructured surface responsible for the black color. Considering uniform dissolution, the i\({}_{\text{corr}}\) values determined by Tafel extrapolation can be converted to penetration rate (\(PR\)), using Equation (2)  
\[PR=\frac{i_{corr}M_{Ni}}{nFd_{Ni}}\cdot\left(3.15\times 10^{11}\right) \tag{2}\]  
where \(i_{corr}\) is the corrosion current density (A cm\({}^{-2}\)), \(M_{Ni}\) is the Ni molar weight (58.693 g mol\({}^{-1}\)), \(n\) is the number of electrons involved in the oxidation reaction (2 in this case), \(F\) is the Faraday constant (96,485 C mol\({}^{-1}\)), \(d_{Ni}\) is the Ni density (g cm\({}^{-3}\)) of the metal, and \(3.15\times 10^{11}\) is the conversion factor of year in seconds and cm in \(\upmu\)m. The values are presented in Table 4 and show small corrosion rates, higher for the black coating probably due to the area effect. These corrosion rates are just indicative because two assumptions were made in their determination: uniform corrosion and Tafel extrapolation technique valid for these results. The impedance response of both samples is presented in Figure 8b with Bode plots of EIS the spectra measured after 4 h in 0.5 M NaCl. It is important to refer that the spectra of the black coating measured after 1 h, 120 h, and 144 h did not change with respect to the one shown in Figure 8b, which demonstrates its stability. In these experiments, the Ni substrate presented the highest impedance, with the response of the solution resistance at high frequencies down to 3 kHz, a capacitive response of either a passive film or the double layer capacitance, between 3 kHz and 20 mHz and finally, for low frequencies, a resistive response of about 200 k\(\Omega\) cm\({}^{2}\), attributed to the charge transfer resistance. The black nickel sample showed a similar spectrum but with smaller charge transfer resistance and higher capacitance. These differences can be explained by the higher surface area of the black coating (not considered in the data treatment which uses the geometric area of the working electrode).