Application of PM IRRAS to study structural changes of the magnesium surface in corrosive environments

Abstract

Light weight metals such as magnesium and aluminum have excellent engineering properties which makes them attractive for industrial applications. Poor corrosion resistance is the major drawback for their wide applications. Corrosion of metal surfaces is a common but complex phenomenon, which depends on the sample environment. Despite intensive studies the molecular aspects of magnesium corrosion are not fully understood yet.Infrared spectroscopy is an excellent analytical technique to study the chemical composition and structure of molecules. Reflection based infrared spectroscopy techniques are applicable to investigate the structure of molecules adsorbed on surfaces. In this paper we demonstrate that magnesium with its passive layer is suitable for infrared reflection absorption spectroscopy experiments. On the pure magnesium surface the IR light is strongly reflected and the highest enhancement of the normal component of the p-polarized light (probing the species present on the surface) is obtained at grazing angles of incidence. Growth of the passive layer leads to a gradual shift of the optimal angle of incidence to lower values (60° vs. surface normal). Polarization modulation infrared reflection absorption spectroscopy is used to study changes in the composition and structure occurring during magnesium corrosion in air and in aqueous solutions. In air and in aqueous NaCl solution Mg(OH)2 and magnesium carbonate minerals coordinating water and hydroxide ions are formed on the surface. IR spectra provide clear evidence that the coordination of the Mg2+ to carbonate ions changes in time from mono- to bi- and polydentate. In the presence of orthophosphate ions in the electrolyte solution, the Mg(OH)2 layer disappears from the surface while magnesium phosphate salts crystalize on the metal surface.