Molecular Structural Information of the Atmospheric Corrosion of Zinc Studied by Vibrational Spectroscopy Techniques: II. Two and Three-Dimensional Growth of Reaction Products Induced by Formic and Acetic Acid

Abstract

In an effort to obtain a more molecular view of atmospheric corrosion, we present experimental data from three complementary acting vibrational spectroscopy techniques that have been used in parallel exposures to follow the initial atmospheric corrosion of zinc during exposure in dry or humid air to which formic acid or acetic acid was added, with the primary aim to simulate indoor exposure conditions. The techniques used were vibrational sum frequency spectroscopy (interface sensitive), IR reflection absorption spectroscopy (near-surface sensitive), and confocal Raman microspectroscopy (bulk sensitive with submicrometer surface lateral resolution). The growth of two-dimensional interface species of zinc formate or zinc acetate could be monitored in situ and distinguished from three-dimensional growth of ZnO and zinc hydroxy formate or acetate. These interface species are believed to act as precursors of the dissolution of aqueous zinc carboxylate species that subsequently deposit at the surface and result in local growth of crystalline or amorphous ZnO and local zinc hydroxy carboxylate. Differences in the growth of corrosion products induced by acetic or formic acid were mainly attributed to differences in pH of the aqueous adlayer and in different deposition velocities of the acids into the aqueous adlayer.