3.2.1. Cyclic Potentiodynamic Polarization (CPP)

From the open circuit plots (Figure 6a) it can be seen that the surface of the samples was quite stable over the period of 1 h. The OCP values increase in the order: AZ31B < Al-coated AZ31B < Ti-coated AZ31B. It is evidently seen that bare Mg alloys with lower values of OCP are more susceptible to corrosion compared to coated Mg samples which showed higher values of OCP. But, Al-coated AZ31B is more active than Ti coated AZ31B.

**Figure 6.** (**a**) Open circuit potential (OCP) for 1h, and (**b**–**d**) cyclic potentiodynamic polarization (CPP) curves for bare and coated AZ31B Mg alloys in 3.5 wt % NaCl electrolyte.

Cyclic potentiodynamic polarization tests (Figure 6b–d) were carried out to determine whether coated and uncoated AZ31B alloys experience pitting corrosion in chloride containing solutions. This test can also help whether the passive film formed on the surface

has a tendency to heal itself (or repassivate) in the harsh environment. Moreover, CPP test was carried out three times to prove the repeatability of the obtained outcomes.

The corrosion parameters for the bare AZ31B samples as well as Al-and Ti-coated samples are given in Table 3. It is clear that Al-and Ti-coated AZ31B alloy samples have lower corrosion current densities (*i*corr) compared to bare samples, where lowest *i*corr and average corrosion rate (*P*i = 22.85*i*corr [43]) were observed for Ti-coated samples. The corrosion potential values were more noble for Al-coated and further higher for Ticoated samples. Overall, it could be suggested that both Al and Ti cold sprayed coating improved the corrosion behavior of AZ31B alloy, obviously much better in case of Ti-coated samples. In this research, HPCS Ti coating considerably lowered *i*corr to 0.049 μA/cm<sup>2</sup> from 2.504 μA/cm<sup>2</sup> and shifted *E*corr to more noble potential, i.e., −387.299 mVSCE from −1453.86 mVSCE for AZ31B Mg alloy in 3.5 wt % NaCl solution. Nonetheless, magnetron sputtered Ti coating could only lower *i*corr to 26.60 μA/cm<sup>2</sup> from 162.70 μA/cm<sup>2</sup> and *E*corr to −1525 mVSCE from −1570 mVSCE for AZ91D Mg alloy in 3.5 wt % NaCl solution [32].

**Table 3.** The *E*corr, and *i*corr, βa and βc for bare and coated samples calculated from the forward scan of cyclic potentiodynamic polarization (CPP) curves.


The difference between pitting potential *<sup>E</sup>*pit and *E*corr (*E*pit − *E*corr) can be used as a measure of the propensity to the pitting nucleation [22]. Moreover, the difference between repassivation or protection potential (*E*rp) and corrosion potential *E*corr (*E*rp − *E*corr) can be employed as a measure of the repassivation ability. Lager values of (*E*pit − *E*corr) and (*E*rp − *E*corr) signify enhanced resistance to pitting corrosion and higher repassivation ability, respectively [22]. The *<sup>E</sup>*rp − *E*corr values increase in the order: AZ31B < Al-coated AZ31B << Ti-coated AZ31B. On the reverse scan, AZ31B and Al-coated AZ31B showed a positive hysteresis loop, implying the further growth of pitting. It was reported that pitting corrosion can't ge<sup>t</sup> further expanded if reversed anodic curve shifted to lower current densities (as negative hysteresis loops) or the forward scan to be retraced by reversed curve. On the contrary, further pitting development is anticipated if reversed anodic curve shifted to higher current densities compared to forward scan (as positive hysteresis loops) [44]. The pits keep growing if *E*corr > *<sup>E</sup>*rp and vice versa. Ti-coated Mg alloy indicates negative hysteresis loop, depicting repassivation of pits, in contrast to AZ31B and Al-coated AZ31B with positive hysteresis loops where *E*corr > *<sup>E</sup>*rp; indicating irreversible growth of pits. As-cold sprayed Nb coatings (from group 5B) also showed negative hysteresis loops. The repassivation behavior of CS Nb coating was attributed to the stored energy in the CS coatings assisting to passivate quickly and simply [45]. Analysis of *<sup>E</sup>*pit − *E*corr values demonstrates that AZ31B and Al-coated AZ31B are most susceptible to pitting corrosion while Ti-coated Mg alloy indicates conspicuous resistance to pitting in chloride containing electrolyte.

The CPP tests also reveal that the anodic curves for AZ31B and Al-coated AZ31B had active current densities especially in case of AZ31B alloy. The cathodic current kinetics were highest for AZ31B compared to both Al-coated and Ti-coated alloys samples. The current density limit of 10 mA/cm<sup>2</sup> was reached at much lower potentials for AZ31B and Al-coated AZ31B alloy as well. Among all Ti-coated samples showed primary passivation followed by a secondary passivation (passive-like behavior) through a passive-activepassive (passive-like behavior) transition region. Interestingly, after the reaching the upper potential limit of 2.5 VSCE the reverse scan showed lower current densities suggesting that the surface is still passive and lower potential cause lower dissolution for the passivated Ti surface. This is a very good indication of highly stable passive film which did not deteriorate even at such potentials.
