Fig. 1: PEO characterisation.

Scanning electron microscopy images of (a) the top surface microstructure (scale bar = 20 µm), and (b) the cross-section view of the ZX31PEO sample (scale bar = 20 µm); (c) current-time diagram of the PEO process; (d) polarization test results of ZX31 and ZX31PEO samples.