**3.5 Corrosion morphology**

The corrosion morphologies of as-received and as-processed specimens of AZ80 and AZ91 Mg alloys immersed in 3.5 wt.% NaCl solution observed through scanning electron microscopy and is shown in **Figures 16** and **17**.

From **Figures 16(a)**, **(b)** and **17(a)**, **(b)**, it was observed the adequate amount of corrosion attack on the surface of the as-received and homogenized AZ80 and AZ91 Mg alloys after potentiodynamic polarization test. The ECAP performed samples of Mg alloys after the corrosion test exhibited comparatively less localized pits on the surface of ECAPed AZ80/91 Mg alloys have shown in **Figures 16(c)** and **17(c)**. This obtained result showed that pitting corrosion resistance of ECAPed Mg alloys are significantly improved through grain refinement and this is due to the distribution of secondary phases [33–35]. It is worth to declare that this appeared improved for AZ80/91 Mg alloy was due to the grain refinement, distribution of secondary phases and formation of

#### **Figure 16.**

*Corrosion morphology of AZ80 Mg alloys (a) as-received (b) homogenized at 673 K-24 h (c) 4P-598 K and (d) XRD for a corroded specimen of as-received.*

#### **Figure 17.**

*Corrosion morphology of AZ91 Mg alloys (a) as-received (b) homogenized at 673 K-24 h (c) 4P-598 K and (d) EDX for corroded ECAP-4P specimen.*

magnesium hydroxides formed on their surfaces which proved through microstructure and X-ray diffraction analysis shown in **Figures 16(d)** and **17(d)** for AZ80 and AZ91 Mg alloys respectively. The existence of such metal oxide partially protects the Mg surface from further dissolutions under this circumstance the breakdown of the film and the consequential nucleation and growth of a pit become more difficult. Moreover, the higher Ecorr value for ECAPed Mg alloys at higher passes revealed that the surface of Mg alloys was more passivated against corrosion this is due to slow dissolution rate of fine grains structure [30, 31]. Finally, from the results, it was concluded that the severe corrosion attack was observed on as-received AZ80 and AZ91 Mg alloys and further continuous reduction in corrosion attack was observed for ECAPed samples. Similar kind of results and trends is reported by many authors in their studies [29, 36, 37].
