Influence of the Cerium Concentration on the Corrosion Performance of Ce-doped Silica Hybrid Coatings on Hot Dip Galvanized Steel Substrates

The aim of this work was to investigate the effect of the cerium concentration on the morphology and anticorrosion performance of cerium-silane hybrid coatings on hot dip galvanized (HDG) steel substrates. 3-glycidoxypropyltrimethoxysilane (GPTMS) and bisphenol A (BPA) were employed as precursors to prepare the sol-gel based silane coating. Cerium nitrate hexahydrate was added to the silane coatings as dopant in five different concentrations. The morphology of the coatings before and after the corrosion test was examined by scanning electron microscopy (SEM), indicating an effect of the cerium concentration. Very low and very high cerium concentrations deteriorate the corrosion inhibition in the sol-gel matrix and consequently, there is an optimum concentration of cerium nitrate. Accelerated salt spray testing showed that corrosion near an artificial scratch is blocked efficiently by high cerium nitrate contents, whereas uniform corrosion is inhibited effectively with comparatively low ceria contents. Electrochemical studies indicate a general beneficial effect of the incorporation of cerium nitrate, although the performance of the coated substrate depends on the cerium nitrate content. The results of electrochemical impedance spectroscopy (EIS) and electrochemical polarization confirmed that the corrosion resistance of the coatings initially increases and then decreases as the cerium concentration goes up. Optimal corrosion resistance was obtained at a cerium concentration of 0.05 M.