Products formed during the interaction of seawater droplets with zinc surfaces: I. Results from 1- and 2.5-day exposures

Abstract

This paper reports on a study in which fine seawater droplets were placed on zinc surfaces for periods up to 2.5 days. At the end of each exposure, the droplets were either extracted from the specimens or allowed to evaporate in laboratory conditions. Scanning electron microscope-energy-dispersive spectroscopy, X-ray diffraction studies (including mapping), and Fourier transform infrared and Raman spectroscopy were carried out on the specimens after exposure and on the dried extractions. It was observed that soon after the droplets were placed on the surface, a thin moisture layer (termed the secondary spreading film) spread out from the central droplet. X-ray analysis indicated the presence of gordaite and simonkolleite in the center of the drop when the droplet volume was maintained. However, during droplet evaporation a wider range of species was detected, including boyleite, zinc chlorate hydrate, and sodium zinc chloride hydrate. In the secondary spread region, an amorphous phase with a composition and morphology consistent with zinc hydroxy carbonate was present. The phase analysis is supported by electrochemical data and by analysis of changes in droplet chemistry with time. A synthesis of the data permits a description of the processes leading to the phase development. The implications to atmospheric corrosion are discussed.