Stability of Zn–Ni–TiO2 and Zn–TiO2 nanocomposite coatings in near-neutral sulphate solutions

Zn–Ni–TiO2 and Zn–TiO2 nanocomposites were prepared by galvanostatic cathodic square wave deposition. X-ray diffraction analysis and scanning electron microscopy revealed that the occlusion of TiO2 nanoparticles (spherical shaped with diameter between 19.5 and 24.2 nm) promotes the formation of the γ-Ni5Zn21 phase, changes the preferred crystallographic orientation of Zn from (101) and (102) planes to (002), and decreases the particle size of the metallic matrices. The stability of the nanocomposites immersed in near-neutral 0.05 mold m−3 Na2SO4 solution (pH 6.2) was investigated over 24 h. The initial open circuit potential for the Zn–Ni–TiO2 and Zn–TiO2 coatings were −1.32 and −1.51 V (vs. Hg/Hg2SO4), respectively, and changed to −1.10 and –1.49 V (vs. Hg/Hg2SO4) after 24 h of immersion. Data extracted from the steady state polarization curves demonstrated that the metal–TiO2 nanocomposites have, with respect to the metal coatings, a higher corrosion potential in the case of the Zn–Ni alloy composite; a lower corrosion potential in the case of Zn-based nanocomposite albeit the predominant (002) crystallographic orientation; and a lower initial corrosion resistance due to the smaller grain size and higher porosity in the Zn–Ni–TiO2 and Zn–TiO2 nanocomposites. Morphological and chemical analyses showed that a thicker passive layer is formed on the surface of the Zn–Ni–TiO2 and Zn–TiO2 deposits. After 24 h of immersion in the sulphate solution, the Zn–Ni–TiO2 coating has the highest corrosion stability due to the double-protective action created by the deposit’s surface enrichment in Ni plus the higher amount of corrosion products.