Corrosion Efficiency of Zn-Ni/ZrO2 and Zn-Co/ZrO2 Bi-Layer Systems: Impact of Zn-Alloy Sublayer Thickness

Abstract

The presented manuscript demonstrates the effect of the thickness of a zinc alloy sublayer on the corrosion resistance and stability of three types of bi-layer systems composed of Co- or Ni-modified zinc coatings (both as sublayers) and a top sol–gel ZrO2 film in a 5% NaCl solution. In order to obtain more detailed information, the alloy sublayers were electrodeposited with three different thicknesses (1, 5 and 10 µm, respectively) on a low-carbon steel substrate. Three consecutive dip-coated ZrO2 sol–gel layers were deposited thereafter on the individual zinc alloy sublayers. For comparison, an ordinary electrodeposited zinc coating was obtained and investigated. The aim of this study was to evaluate the effect of the thickness of the zinc-based sublayer on the protective characteristics of the bi-layer systems. The surface morphology features and the phase composition of the latter systems were examined using scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle (WCA) measurements and X-ray diffraction (XRD) analyses. The corrosion stability was evaluated by means of potentiodynamic polarization (PDP) curves and polarization resistance (Rp) measurements. The zirconia finish layers possessed an amorphous, dense and hydrophobic nature, while the sublayers were multicrystalline. The results confirmed the increased corrosion resistance of the protective system, which contains electrodeposited sublayer of Zn-Co alloy with a 10 µm thickness in a corrosive test medium.