Effect of substrate surface modification on alumina composite sol–gel coatings

Chemically bonded composite sol–gel coatings have been recently proposed as a novel, low-cost method of protection of metals against corrosion and wear. However, the process operates in mildly acidic (pH = 3–4) conditions sufficient to initiate corrosion of mild steel during heat treatment of the coating at 300 °C. Therefore, there is a need to protect surface of steel from contact with the coating slurry. To achieve this, a protective film of iron phosphate was produced on mild steel substrate surface using commercial product (Crystcoat 747). The film also improved bonding between the substrate and the ceramic layer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterized the microstructure of the iron phosphate coating and the ceramic coating.