ELECTROCHEMICAL FORMATION OF A-SITE SUBSTITUTED PEROVSKITE STRUCTURE LA1-XMXCRO3 OXIDE COATINGS

A method to form A-site substituted perovskite structure lanthanum chromium oxide coatings on metals has been developed. Precursor films having compositions of (LaOH)1-xMxCrO4.nH2O were formed on metals by cathodic polarization in chromate solutions (pH = 1.8-2.8) containing La(NO3)3 and M(NO3)2 [M = Ca or Sr]. The formation of the precursor films is explained by association and precipitation reactions of LaOH2+, CrO42- and M2+ ions which were formed at the cathode surface due to the rise in pH caused by hydrogen evolution and the reduction of NO3- ions. The formation conditions with Ca(II) were x < 0.1 and with Sr(II) x < 0.2, determined on chemically polished Ni. The conditions for precursor formation on type 304 stainless steel have also been surveyed. The precursors were converted into perovskite structure La1-xMxCrO3 by pyrolysis in a N2 atmosphere. The temperature and time of the pyrolysis depended on the substituted ion, M, and the x value.