Products formed during the interaction of seawater droplets with zinc surfaces: II. Results from short exposures

Abstract

This paper reports on a study in which fine seawater droplets were placed on zinc surfaces for periods of 15 min to 6 h. After each exposure, droplets were either extracted from the specimens or allowed to evaporate in laboratory conditions. Scanning electron microscopy (SEM)-energy-dispersive spectroscopy, X-ray microdiffraction, and SEM-focused ion beam studies were undertaken on the specimens after exposure, as was in situ Raman spectroscopy on zinc covered with a saline droplet. Approximately 30 min after droplets were placed on the surface, a thin moisture layer (termed the secondary spreading film) spread out from the central droplet. As a function of position and time, oxide development in the central region was initially slow; however, after ∼30 min, a significant local attack of the underlying microstructure occurred. Correspondingly, precipitate phases developed on top of the oxide, which produced an oxide consisting of three bands: an irregular zone with a localized metal attack and significant voids at the metal/oxide interface, overlaid by a relatively void-free layer, which in turn was overlaid by a porous zone. At the edge of the drop (before the secondary spreading), oxide growth was much faster, and the deposition of phases that precipitate in solutions appeared critical. We discuss the chemistry and identification of the various phases and the implications of oxide formation to the electrochemical processes occurring in drops.