Corrosion barrier alumina/zirconia bilayer stack on low Cr steel by direct liquid injection metalorganic chemical vapor deposition

Numerous industrial fields necessitate the development of advanced metallic materials presenting high corrosion resistance in aqueous media and being less expensive than classical high chromium steels. In the present work, amorphous zirconia, alumina and alumina/zirconia coatings have been deposited on 2 wt% chromium steel coupons using an innovative direct liquid injection metal organic chemical vapor deposition process. Scanning electron microscopy and X-ray diffraction reveal that the alumina sublayer acts as an efficient barrier against steel oxidation occurring during the amorphous zirconia deposition performed under O2 at 400 degrees C. They also show that the steel oxidation occurring concomitantly to the zirconia coating modifies the film morphology by increasing its roughness and thickness, in comparison with the same zirconia coating on alumina. The anticorrosive properties of the thin films have been studied by an immersion test of the bare and coated coupons in deionized water at 80 degrees C during 6 days and by electrochemical impedance spectroscopy in aqueous sodium chloride solution for 48 h. They reveal that, in contrast to alumina and zirconia monolayers alone, the alumina/ zirconia stack acts as a corrosion protection coating in pure water through a synergetic barrier effect against (i) oxidation by alumina and (ii) aqueous corrosion by zirconia.