Effect of hydrodynamics on zinc anodizing in silicate-based electrolytes

The widespread use of zinc, both as a sacrificial coating on steel and as a stand-alone material, requires a high level of corrosion resistance in order to increase the product life cycle and to avoid environmental contamination by metal ions. This paper reports the synthesis of corrosion protective conversion layers on zinc by single step anodizing in silicate-based electrolytes. A rotating disc electrode is used to control the mass transport towards the electrode, and to study the effect of hydrodynamics on the structural characteristics and corrosion resistance of the surface layers formed. The dissolution of the zinc substrate is found to be required for the precipitation of silicate-rich surface layers. The correlation between the composition of the anodizing electrolyte and the surface layers indicates that an anodically induced deposition process takes place during anodizing. Electrochemical measurements and humidity chamber corrosion tests show that anodizing in silicate-based electrolytes increases the corrosion resistance of zinc substrates by forming a barrier layer that shields the base material and by inducing a galvanic protection.