Characterisation of electrolytically deposited alumina and yttrium modified alumina coatings on steel

Conversion coatings modified by deposits of alumina added or not with yttrium have been studied in order to improve the resistance to thermal oxidation of steel. The proposed process leads to the formation of a coating characterised by strong interfacial adhesion with the steel. Conversion coating, characterised by a very porous morphology and strong interfacial adhesion to the substrate, facilitates the electrochemical deposition of ceramic layers and enhances its adhesion. After heating, the coatings present continuous composition gradients with refractory compounds at the surface. All the studied coatings increase the oxidation resistance of the steel at 750 °C; they reduce the oxidation parabolic rate constant and prevent a scale spallation phenomenon. The extent of beneficial effect depends on the nature of cathodic treatment bath. Yttrium improved the protecting character of alumina coating; reduces the rate of oxidation and increase the adhesion of deposits. The electrochemical behaviours, morphology, chemical composition and thermal stability of the different deposits are analysed.