Alumina coatings from aluminium electrodeposited in molten salts

In this study, a new process has been performed to generate a ceramic alumina coating on a metallic surface with the purpose of improving its mechanical and corrosion resistance at high temperatures. The alumina coating was obtained by dry oxidation of an aluminium layer which had been deposited on the metallic substrate (i.e. : structural steel) by electrolysis from a molten salts bath. The used electrolytic melt was a ternary mixture AlCl3-NaCl-KCl heated at a temperature below 200degreesC. The oxidising operation was a thermal treatment carried out under a dry oxygen atmosphere at a temperature of 550degreesC. This double procedure was original because it created alumina coatings strongly linked to their substrates in contrast to the coatings, obtained by classical deposition processes like sputtering, physical vapour deposition or chemical vapour deposition. This strong bonding was due to a diffusion phenomenon between aluminium and iron which occurs by cathodic aluminium electrodeposition The electrolytic conditions of deposition on steel were responsible for the structure of the coating and for the formation of the interdiffusion layer Al-Fe. The presence of this layer was proved by GDL-OES (glow discharge luminescence-optical emission spectroscopy) analysis.