Protective Characteristics of TiO₂ Sol-Gel Layer Deposited on Zn-Ni or Zn-Co Substrates

This study aimed to present the differences in the corrosion properties and protective ability of two bi-layer systems obtained on low-carbon steel in a model corrosive medium of 5% NaCl solution. These newly developed systems consist of Zn-Co (3 wt.%) or Zn-Ni (10 wt.%) alloy coatings as under-layers and a very thin TiO2 sol-gel film as a top-layer. Scanning electron microscopy (SEM) is used for characterization of the surface morphology of the samples indicating that some quantitative differences appear as a result of the different composition of both zinc alloys. Surface topography is investigated by means of atomic force microscopy (AFM), and the hydrophobic properties are studied by contact angle (CA) measurements. These investigations demonstrate that both sample types possess grain nanometric surface morphology and that the contact angle decreases very slightly. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used for characterization of the chemical composition and electronic structure of the samples. The roughness Rq of the Zn-Ni/TiO2 is 49.5 nm, while for Zn-Co/TiO2, the Rq value is 53.4 nm. The water contact angels are 93.2 and 95.5 for the Zn-Ni/TiO2 and Zn-Co/TiO2 systems, respectively. These investigations also show that the co-deposition of Zn and Ni forms a coating consisting entirely of Ni2Zn11, while the other alloy contains Zn, Co and the intermetallic compound CoZn13. The corrosion resistance and protective ability are estimated by potentiodynamic polarization (PDP) curves, as well as polarization resistance (Rp) measurements for a prolonged test period (35 days). The results obtained are compared with the corrosion characteristics of ordinary zinc coating with an equal thickness. The experimental data presents the positive influence of the newly developed systems on the enhanced protective properties of low-carbon steel in a test environment causing a localized corrosion—lower corrosion current density of about one magnitude of order (~10−6 A.cm−2 for both systems and ~10−5 A.cm−2 for Zn) and an enhanced protective ability after 35 days (~10,000–17,000 ohms for the systems and ~900 ohms for Zn)