3.5.1. Potentiodynamic Polarization

The open circuit potential (EOC) evolution after 1 h of immersion and the potentiodynamic curves of the investigated systems are presented in Figure 4. During the immersion, the EOC of the TiCN and TiSiCN thin films showed a steady evolution, while the substrate slightly changed its value for half of the time, reaching a stable evolution at the end of the test. The TiSiCN coatings exhibited a positive EOC value (54 mV) compared with the TiCN coatings, indicating that the addition of Si had a positive effect on corrosion behavior.

**Figure 4.** (**a**) Open circuit potential evolution in time and (**b**) potentiodynamic curves of the investigated systems.

The main electrochemical parameters of the investigated specimens calculated based on the Tafel and potentiodynamic curves are presented in Table 4 (all the presented potentials are relative to the SCE value). Polarization resistance (*R*p) was determined from linear polarization measurements as the slope of the linear region of the Δ*E*–Δ*i* curve near *E*corr. Corrosion potential (*E*corr), anodic (β*a*) and cathodic (βc) slopes were estimated from Tafel plots. The corrosion current density was also calculated based on one form of the Stern–Geary equation (Equation (1)), based on previously determined parameters.

$$\frac{1}{R\_{\text{P}}} = \left(\frac{\Delta i}{\Delta E}\right)\_{E\_{\text{corr}}} = 2.3 \left(\frac{\beta\_{\text{a}} + \left|\beta\_{\text{c}}\right|}{\beta\_{\text{a}} \left|\beta\_{\text{c}}\right|}\right) i\_{\text{corr}} \tag{1}$$

considering the *E*corr parameter, the most electropositive value was demonstrated by the TiSiCN coating (*E*corr TiSiCN = −14 mV), as well as the smallest value of corrosion current density (*i*corr TiSiCN = 49.6 nA cm<sup>−</sup>2). Taking into account the polarization resistance, the highest value was observed in the case of the CoCr substrate, closely followed by the TiSiCN coating (*R*p TiSiCN = 425 <sup>k</sup>Ω·cm2).


**Table 4.** The main corrosion parameters of the investigated specimens.

3.5.2. Electrochemical Impedance Spectroscopy (EIS)

It was observed that the electrochemical performance of the investigated specimens was different as a function of their composition. For comparison, Nyquist and Bode plots for the investigated specimens are presented in Figure 5.

The electrochemical parameters were obtained by fitting the data with an equivalent circuit, which took into consideration the phenomenon at the interface of each investigated system with the testing electrolyte (inset Figure 5). *Rel* represents the electrolyte resistance, CPElayer represents the coating capacitance, Rpore represents the resistance associated with the current flow through the pores generated by the coatings' defects and CPEdl is a double layer capacitance in parallel with a charge transfer resistance - Rct. CPE was used instead of a capacitor due to the non-ideal character of the

working electrode. The physical interpretation of a circuit that has a constant phase element (for a better quality fit) depends on the value of α. If the α parameter is 1, then the CPE can be modeled as a capacitor. Since after the fitting, the α parameter showed values less than 1 in both cases (i.e., the α layer and αdl), a CPE was used. This can be due to possible deviations from the ideal dielectric behavior and it is usually related to the surface inhomogeneity [55]. According to Hirschorn et al. [56], these deviations arise either from different properties along the surface of an electrode (e.g., roughness), or properties normal for the surface (e.g., thickness).

**Figure 5.** (**a**) Nyquist plot (electrical circuit used for the fitting procedure included) and (**b**) Bode plot of the investigated specimens.

The electrochemical parameters of the investigated systems are presented in Table 5. It can be noticed the low value of the χ2 parameter, which is an indication of an excellent agreemen<sup>t</sup> between the experimental data and those simulated by the equivalent circuit.


**Table 5.** The fitting results of EIS curves for the investigated systems.

Taking into account the fitting results for the investigated systems, it can be noticed that the highest pore-associated resistance was obtained for TiSiCN, while the CoCr substrate showed low values. It was stated that CoCr alloys form a passive layer at the surface, which is mainly based on Cr(III), and smaller amounts of Cr(OH)3, Co and Mo oxides [57]. The fitted values associated to the CoCr specimen showed that the formed layer is not as compact and protective as the TiCN and TiSiCN coatings.

The *Qdl* parameter, which is representative of the substrate–electrolyte interface, indicated a better protection of the deposited/formed layer in the following order: TiSiCN > TiCN > CoCr. Thus, the best protection after immersion in 90 % DMEM + 10 % FBS was observed for the TiSiCN coatings, with the best capacitive character, indicated by the low value of Qlayer and the highest value of Rct.

Considering <sup>α</sup>layer, it can be observed that for CoCr, the CPE used for fitting the obtained data were the closest to a capacitor, since in this case, <sup>α</sup>layer = 0.96. This could be due to the low roughness measured before the corrosion as compared to the other investigated specimens (Table 3). Similar α layer values were obtained for the coatings and the time-constant dispersion is ascribed to the similar values of roughness. Going deeper, at the interface between the coating and the substrate, another double layer is formed. *Qdl* and αdl can give an indication of the compactness of the deposited/formed layer and the electrolyte ingress through the defects, which can create pathways for the electrolyte to

reach the substrate [58]. It can be observed that even though αdl was higher for TiCN, showing an almost defect-free structure, TiSiCN was the one showing better values of *Qdl* and *Rct.*

### *3.6. Morphology and Roughness after Corrosion*

SEM images after the corrosion tests are presented in Figure 6. It is worth noting that the uncoated substrate was more affected by the corrosion than the coated surfaces. The destruction of the protection layer can be seen on the coated samples, after performing the corrosion tests. Regarding the TiCN coatings, there were various corrosion products on the surface, indicating that this surface was affected by the corrosive solution. Moreover, the coating was partially destroyed in some areas, with the CoCr substrate being visible. The TiSiCN surface also had corrosion products, but there were less compared with the TiCN surface, indicating better anticorrosive properties. All surfaces were affected by the corrosive solution, but TiSiCN was less damaged.

**Figure 6.** SEM of the investigated (**a**) CoCr substrate and (**b**) TiCN and (**c**) TiSiCN coatings.

The main roughness parameters of the investigated surfaces are presented in Table 6. Comparing these results with the values obtained before the corrosion tests (Table 3), it can be said that all surfaces were significantly damaged after the corrosion tests. The Ra of the uncoated substrates increased from 46.9 ± 5.9 nm to 1342.9 ± 192.4 nm. A significant increase in Ra roughness was found for the TiCN coatings after corrosion (15584.3 ± 7462.8 nm), indicating a major deterioration of the coatings after the corrosion tests. The TiSiCN coatings were also affected by the corrosive process, but they have finer irregularities than TiCN, demonstrating that the addition of Si led to an enhancement of anticorrosive properties. All surfaces showed a negative value of Sk after the corrosion tests, signifying that the surfaces were characterized by many valleys formed during the corrosive processes. The TiSiCN surface had fewer valleys that the TiCN surface, as shown by the smaller absolute value of Sk.


**Table 6.** Main roughness parameters of the investigated specimens after corrosion tests.
