*3.4. Corrosion Resistance Investigation*

The corrosion properties of the treated samples were determined by a potentiodynamic polarization method. The obtained curves are illustrated in Figure 7.

**Figure 7.** (**a**). potentiodynamic polarization curves for alloy Ti-6Al-4V after MAO treatment with various current pulses (upper curves) and curve for base Ti-6Al-4V (lower curve). (**b**). enlarged curves for treated alloys. Examination was carried out in 3.5 wt.% NaCl.

According to theory, the higher the corrosion potential, the higher the corrosion resistance. Potentiodynamic polarization curves on Figure 7a clearly indicated a shift of the coated samples to the more positive potential. This may be attributed to the reduction of the anodic and the cathodic processes due to the oxide coating appearance.

The polarization resistance (Rp) was calculated according to Equation (1):

$$\mathbf{R\_P} = \frac{\boldsymbol{\beta\_a} \times \boldsymbol{\beta\_c}}{2.3 \times \mathbf{i\_{corr}} (\beta\_a + \beta\_c)}. \tag{1}$$

The Tafel slopes β<sup>a</sup> and β<sup>c</sup> were calculated from the anodic and the cathodic curves on the plot (Figure 7b). The corrosion potentials (Ecorr), the corrosion current densities (icorr) and the polarization resistance (Rp) are listed in Table 3.

**Table 3.** Calculated corrosion test results for alloy Ti-6Al-4V after MAO treatment with various current pulses and the base alloy.


Calculation results in Table 3 revealed that the corrosion rate of coated alloy was at least 40 times lower compared to the untreated alloy. The lowest value was obtained on Sam–1000 Hz, which exhibited 125 times higher corrosion resistance in comparison with untreated alloy. Thus, the rate of the corrosion was 0.000150 mm/year and 0.018644 mm/year for the Sam–1000 Hz and untreated alloy, respectively. Furthermore, the higher the applied current pulse in MAO process, the higher the corrosion resistance. It is well correlated with the morphology investigation of the coatings where Sam–1000 Hz was found to be more compact with lower porosity compared to other examined samples.
