*3.4. Corrosion Resistance Investigation*

The corrosion properties of the treated specimen were determined by the potentiodynamic polarization method. The obtained curve on the coated specimen was compared to the curve of the untreated and both are illustrated in Figure 6.

The obtained curves in Figure 6 are presented in semi-logarithmic coordinates. A higher corrosion resistance of the specimen is obtained when the corrosion potential is higher and the corrosion current density is lower. Therefore, it is clearly seen that the corrosion potential of the coated specimen shifted to be more positive and the current density to a more negative value, providing higher corrosion protection to the alloy. That may indicate the reduction of the anodic and cathodic processes due to the presence of a newly formed protective oxide layer on the metallic surface. The movement of the corrosion potential towards the anodic area also indicates the improvement of the treated specimen's resistance.

**Figure 6.** Potentiodynamic polarization curves for alloy Ti-6Al-4V (a) treated by MAO process and (b) untreated specimen. Both examined in 3.5 wt % NaCl.

Based on the corrosion currents and obtained slopes of cathodic and anodic curves, the polarization resistance (Rp) was calculated according to Equation (1):

$$R\_{\mathbb{P}} = \frac{\mathbb{A}\_{\mathfrak{a}} \times \mathfrak{z}\_{\mathfrak{c}}}{2.3 \times \mathrm{i}\_{\mathrm{corr}} (\mathfrak{z}\_{\mathfrak{a}} + \mathfrak{z}\_{\mathfrak{c}})} \tag{1}$$

The Tafel slopes, β<sup>a</sup> and βc, were calculated from the anodic and cathodic curves on the plot. Results of calculations that present the corrosion potentials (Ecorr), corrosion current densities (icorr), and the polarization resistance (Rp) are summarized in Table 2.

**Table 2.** Calculated corrosion test results of untreated alloy Ti-6Al-4V and treated by MAO process specimens. Both examined in 3.5 wt % NaCl.


The calculations presented in Table 2 show that the polarization resistance of the treated specimen is 213.76 kΩ/cm2, while the untreated specimen has a resistance of 10.98 kΩ/cm2. Those values show that the oxide protective coating on Ti alloy is almost 20 times higher than the untreated one. Our results, together with those of additional previous works [31,32], lead to the conclusion that MAO treatment can be applied to improve the corrosion resistance of metals.

The polarization resistance of an alloy treated in molten salt is higher than that of a similar alloy obtained in aqueous electrolyte [33]. This can be attributed to the lack of impurities in the coating and the presence of smaller pores, which conduct current and therefore reduce corrosion resistance.
