ON THE PASSIVE FILMS AND ANODIC BEHAVIOR OF SAMARIUM IN VARIOUS AQUEOUS-SOLUTIONS

Anodic polarization curves of Sm have been measured in solutions of phosphate (pH 1.53-6.86), borate (pH 8.45 and 10.4), 0.1 kmol . m-3 NaOH (pH 13.1), 0.005 kmol . m-3 Na2SO4 (pH 5.3), and 0.005 kmol . m-3 NaCl (pH 5.3). Surface films formed on Sm in these solutions have been examined by using in situ ellipsometry, x-ray photoelectron spectroscopy (XPS), and infrared absorption spectroscopy (IRS). It was found that in the phosphate, borate, and NaOH solutions Sm passivates spontaneously at corrosion potentials even at acidic pH and shows wide passive ranges until the transpassive dissolution occurs at higher potentials. In the Na2SO4 and NaCl solutions, however, Sm dissolves actively at potentials higher than corrosion potentials and does not passivate. The thickness and optical constants of passive films on Sm, which were determined by ellipsometry, depends on the composition and pH of the solutions and the electrode potential; the thickness of passive films increased from 4.6 to 15.9 nm in phosphate (pH 6.86), from 9.0 to 170 nm in borate (pH 8.45), and from 2.1 to 9.0 nm in NaOH (pH 13.1), respectively, with increasing potential. In an acidic phosphate solution (pH 2.98), thicker films (>150 nm) were formed at low potentials in the passive region. The transpassive dissolution caused a sharp increase in the film thickness and a decrease in the real part of the complex refractive index. From XPS and IRS analyses, it is suggested that the passive and transpassive films formed in phosphate solutions contain Sm(III) phosphate, and those in borate and NaOH solutions consist of Sm(III) oxide and hydroxide.