3.2.2. Electrochemical Impedance Spectroscopy (EIS)

For further elucidation of the influence of cold sprayed coatings on the corrosion behavior of AZ31B Mg alloy, electrochemical impedance spectroscopy (EIS) of coated and uncoated Mg alloys was measured at their OCP. Nyquist plots are shown in Figure 7a,b. The impedance spectra was fitted by the electrical equivalent circuits (shown in Figure 7c,d: models 1 and 2). For the Nyquist diagram of uncoated sample (Figure 7d), one inductive loop and one capacitive loop at low frequency and high frequency were considered, respectively. This assumption is in harmony with the previous researches [26,46]. EEC (electrical equivalent circuit) for uncoated Mg alloy is comprised of *R*ct (charge transfer resistance), *R*s (electrolyte resistance), *C*dl (CPE related to electrical double layer), adsorption inductance (*L*) and adsorption resistance (*R*L) elements.

**Figure 7.** Nyquist plots of (**<sup>a</sup>**,**b**) coated and uncoated AZ31B Mg alloy substrates at OCP, electrical equivalent circuits to fit the impedance spectra of (**c**) coated AZ31B Mg alloys (model 1), and (**d**) bare AZ31B Mg alloys (model 2).

Impedance spectra of the coated samples were fitted by using the electrical equivalent circuit, as shown schematically in Figure 8. This schematically EEC includes *R*s which is ohmic solution resistance at the working electrode/reference electrode interface; loop *R*ox-*C*ox which shows the resistance (*R*c or *R*ox) and capacitance (*C*c or *C*ox) of the oxide film; *R*ct and *C*dl; *<sup>R</sup>*po that is the electrolyte resistance (as additional resistance) in the localized corrosion sites (and/or the pores). The *R*c or *R*ox values are pretty high and any conduction of electrons through the oxide layer has been reported to be impossible [47,48]. Thus, Mansfeld and Kendig [49] proposed the removal of this circuit element from EEC and replacement of EEC1 with EEC2 [47,48]. This simplified EEC2 has been also reported in the previous researches [49–52], so a capacitive loop at high frequency and another capacitive loop at low frequency regions were assumed for the coated Mg samples. Likewise, with regard to the non-ideality of the systems, the capacitors were replaced with the constant phase elements (CPE) for all samples [51,53].

*R*ct can predominantly control the electrochemical processes rate at the interface between electrode and electrolyte (or across the electrical double layer) [54,55]. As a matter of fact, corrosion rate is reversely proportional to the *R*ct [56,57]. As presented in Table 4, Bare AZ31B Mg alloy with the lowest *R*ct showed the maximum corrosion rate between 3 samples. This could be attributed to the low protective performance of the formed corrosion surface film on the Mg alloy surface in corrosive solution. Nevertheless, values of *R*ct for Al- and Ti-coated samples were 164.707 kΩ·cm2, and 5580 kΩ·cm2, respectively. This indicates that Mg alloy could be protected by Al and Ti coatings. Nevertheless, *R*ct value for the Ti-coated Mg alloy is roughly 34 times the value of *R*ct for the Al-coated Mg alloy. The above-mentioned results reveal that the CS titanium coating can substantially declines the corrosion rate (1/*R*ct) of Mg alloy in chloride containing solutions.


**Table 4.** EIS fitted results for bare and coated samples in 3.5 wt % NaCl electrolyte.

Al-coated samples (per previous studies).

It is worth mentioning that the corrosion resistance could be characterized by the parameter of polarization resistance at the corrosion potential (Rpol). Values of Rpol for Al and Ti-coated samples were calculated using Equation (2) under EEC2 (Model 1) in Figure 7c [58–63].

$$R\_{\rm pol} = R\_{\rm po} + R\_{\rm ct} \tag{2}$$

 Ti and CS

AZ31B coated with Ti coating showed the much better corrosion resistance than Al-coated Mg alloy in 3.5 wt % NaCl solution. It is expected that this unprecedented development could be maintained even during longer immersion times.

### *3.3. Long Term Immersion Test in 3.5 wt % NaCl Solution for 11 Days*

The surface morphological characteristics of ground coated and uncoated AZ31BMg alloys after immersion test (in 3.5 wt % NaCl solution for 264 h) are shown in Figures 9–12. Uneven corrosion products with some micro-cracks [64] (Figure 9a,b,f) entirely covered the surface of the bare AZ31B Mg alloy surface. Corrosion products on the corroded bare AZ31B Mg alloy surface were mostly comprised of Mg, O, Cl as well as Al, and Na elements (Figure 9c–e). The presence of Mg, O and Cl elements (Figure 9d,e) in the corrosion products was related to the possible existence of MgCl2 and Mg(OH)2 phases [57]. Nonetheless, the lower corrosion current density, higher *R*ct, and *E*corr were all obtained for the AZ31BMg alloy when Al coating was applied on the AZ31B alloy (during short term electrochemical corrosion tests). Moreover, no significant corrosion was observed for the Al-coated AZ31B at the beginning of immersion test. However, deeper and even broad corrosion pits and also local net-cracks of corrosion products imply that the localized corrosion is worsened after 264 h of immersion (Figure 10a,b,f). In fact, the pitting density rises with uniform corrosion in the course of immersion time in chloride containing solutions. This behavior is also reported for the morphology of Al after corrosion [65,66].

Corrosion products on the corroded Al coating surface after 264 h of immersion were mainly constituted by Al, Mg and O as well as Cl and Na elements (Figure 10c–e). Mg element (from the AZ31B Mg substrate) was detected on the corroded Al coating surface. Al coating with low compactness could conduct chloride containing solution into the interior regions of the coating over immersion time. In fact, Al coating can't separate the AZ31B alloy surface from the corrosive electrolyte during long term immersion for 11 days.

High pressure cold sprayed Ti (from group 4B) coating (with high propensity to repassivation and also high pitting corrosion resistance) considerably mitigated the drawbacks associated with CP-Al coating (in this research). The corroded Ti coating surface didn't show any conspicuous corrosion pits and the other localized corrosions after long term immersion (Figure 11). It is interesting to note that, scratches (grinding tracks due to the coating surface preparation before the corrosion tests) are still distinguishable (Figure 11a,b,g) on the Ti coating surface even after 11 days of immersion.

Figure 11c–f show that corrosion products are primarily constituted by Ti and O elements which might correspond to existence of titanium-oxides [28] on the corroded coating surface. Likewise, Mg element (from AZ31B alloy substrate) wasn't detected on the corroded Ti layer surface. In fact, the Ti layer can considerably separate the AZ31BMg alloy surface from the corrosive electrolyte even during long-term immersion for 11 days.

On the contrary, galvanic cell formation between Mg alloy substrate and warm sprayed Ti coatings were observed by Moronczyk et.al [31]. This led to the corrosion products formation and their pile up at the interface between WS Ti coating and Mg alloy substrate. This finally caused the sudden rupture of the WS Ti coatings after merely 24 h of immersion test in 3.5 wt % NaCl electrolyte [31]. This behavior was also seen for magnetron sputtered Ti coating on AZ91D after 24 h of immersion in chloride coating solution [32]. This indicates that HPCS titanium coating not only modifies the hardness and wear behavior of Mg alloys, but also can exceptionally enhance the corrosion resistance of Mg alloys in aggressive solutions during long run immersion in chloride containing solution.

**Figure 9.** (**<sup>a</sup>**,**b**) surface morphology of ground surface of uncoated AZ31B Mg alloy after immersion test in 3.5 wt % NaCl solution for 11 days, EDS analysis of 1 (**c**), 2 (**d**), 3 (**e**) and (**f**) stereo microscope image at 50× magnification.

**Figure 10.** (**<sup>a</sup>**,**b**) surface morphology of ground surface of Al-coated AZ31B after immersion test in 3.5 wt % NaCl solution for 11 days, EDS analysis of 1 (**c**), 2 (**d**), 3 (**e**) and (**f**) stereo microscope image at 50× magnification.

**Figure 11.** (**<sup>a</sup>**,**b**) surface morphology of ground surface of Ti-coated AZ31B after immersion test in 3.5 wt % NaCl solution for 11 days, EDS analysis of 1 (**c**), 2 (**d**), 3 (**e**), 4 (**f**) and (**g**) stereo microscope image at 50× magnification.

**Figure 12.** Photos of uncoated AZ31B Mg alloy, Al-coated AZ31B Mg alloy, and Ti-coated AZ31B Mg alloy (from left to right) after immersion test in 3.5 wt % NaCl solution for 11 days.
