Formation and growth of micro-droplets during the initial stage of atmospheric corrosion

Abstract

A droplet of sodium chloride solution placed on carbon steel spread over the surface forming tiny water micro-droplets and an expanded wet area without changing the droplet shape. The formation of the micro-droplets depended on the cation of the salt and was related to the corrosion reaction of the steel substrate. The micro-droplets contained many sodium ions and very few chloride ions and showed high pH value. Potential distribution measured by Kelvin probe indicated that the primary droplet acted as an anode and the micro-droplets and the wet area became cathodic. Potentiostatic cathodic polarization accelerated their formation and growth for NaCl solution, and the cathodic current increased with time corresponding to increment of wet area. In the case of MgCl 2, the cathodic current decreased very slowly, and wet area and micro-droplets were not observed. In the cases of ZnCl 2 and AlCl 3, the cathodic current decreased for a while depending upon the salt concentration, then increased gradually showing simultaneous formation and growth of micro-droplets. Since precipitation of hydroxides as Mg(OH) 2 and Zn(OH) 2 restrained the increase in pH, the expansion of the wet area was explained by formation of a concentrated alkaline solution of high ionic strength at the edge of the droplet and wet area.