Fig. 1 XRD patterns of the (a) nano-tetragonal zirconia (b) nano-tetragonal zirconia recovered from the electroless Ni-P bath

Fig. 2 A SAED pattern and B high-resolution TEM image of nano-tetragonal zirconia, C the EDS spectrum of the synthesized nano-tetragonal zirconia, D FESEM image of the nano-tetragonal zirconia (magnification 100,000 ×)

Fig. 3 I XRD patterns of (a) nano-tetragonal zirconia as prepared and (b) nano-tetragonal zirconia after annealing at a temperature of 250 °C for 1 h, II TG-DTG curves of the nano-tetragonal zirconia, III UV-visible absorbance spectrum of (a) nano-tetragonal zirconia (b) nano-tetragonal zirconia from the electroless Ni-P bath, and IV FTIR spectra of (a) nano-tetragonal zirconia and (b) nano-tetragonal zirconia from the electroless Ni-P bath

Fig. 4 A XRD patterns of (a) bare Ni-P and (b) Ni-P-nano-tetragonal zirconia coatings, B EDS spectra of bare Ni-P coating, C EDS spectra of Ni-P-nano-tetragonal zirconia coating, D SEM image of bare Ni-P coating, E SEM image of Ni-P-nano-tetragonal zirconia coating

Fig. 5 A Anodic polarization curves of Ni-P-nano-tetragonal zirconia coatings in 3.5% NaCl solution at temperature 30 ± 2 °C [open circle—0 g/L, open triangle—1 g/L, open square—2 g/L, filled circle—5 g/L and filled triangle—10 g/L], B Tafel polarization curves of (a) bare Ni-P and (b) Ni-P-nano-tetragonal zirconia coatings, CSEM image of bare Ni-P after anodic polarization and D SEM image of Ni-P-nano-tetragonal zirconia coating after anodic polarization, E EDS spectra of bare Ni-P coating after anodic polarization and F EDS spectra of Ni-P-nano-tetragonal zirconia coating after anodic polarization

Fig. 6 A Nyquist plots of (a) bare Ni-P coating, (b) Ni-P-nano-tetragonal zirconia coating, B the circuit used to fit EIS data and C Bode plots (a) bare Ni-P coating and (b) Ni-P-nano-tetragonal zirconia coating