ELLIPSOMETRY OF SURFACE LAYERS ON LEAD AND LITHIUM

Abstract

An automatic self-compensating ellipsometer was used to study anodic and corrosion films on lead exposed to sulfuric acid and lithium to water. Acid concentrations were 1.3, 3.1 and 5 M. Anodic current densities for lead ranged from 0.01 mA/cm/sup 2/ to 1.3 mA/cm/sup 2/, and mass transport conditions included free convection and forced convection. Scanning electron micrographs of the resulting surfaces were also made. A computer interpretation of ellipsometer measurements assumes the formation of up to six layers and applies a continuous mass balance. Seven to nine parameters describing the mechanism and structure of surface layers are derived by multidimensional fitting of the measurements. A solution mechanism of film formation on lead is indicated by the dependence of film properties on factors that influence the interfacial concentration of reaction products. A colloidal or polymerized phase of PbSO/sub 4/ suspended in the solution is postulated on the basis of observed mass balances and interfacial solution refractive indices. Kinetic parameters describing the corrosion of lead, evaluated from ellipsometer measurements, are consistent with those reported in the literature. The lead electrode was found to be protected from corrosion during an open circuit potential of -.6 V vs Hg/HgSO/sub 4/ (basic sulfate). Cathodic reductionmore » of PbSO/sub 4/ films terminates with the onset of H2 evolution before complete conversion of the sulfate. Changes in kinetic parameters of cathodic electrode reactions which result in preferred H2 evolution are indicated by this observation. The formation of optically absorbing films was observed in the reaction of lithium with water vapor. 44 figures, 16 tables.« less