Study of the atmospheric corrosion of galvanised steel in a micrometric electrolytic droplet

The local corrosion rate of galvanised steel under a thin electrolyte layer comprising chloride and sulphate ions has been investigated. Localised measurements were achieved through the controlled deposition of an electrolytic droplet onto the surface of the sample, into which reference and counter electrodes were inserted to form an electrochemical microcell. The maximal height of the deposited droplet varied between 400 μm and 1100 μm, with a total diameter between 1.34 and 3.35 mm, respectively. Electrochemical polarization curves as well as electrochemical impedance spectroscopy data were recorded on the galvanised steel sample exposed to the electrolytic droplet to determine the local corrosion rate. Corrosion rates were greatly accelerated under electrolyte layers of thicknesses smaller than 800 μm. This increase in corrosion rate is a result of the decrease of the diffusion layer thickness leading to an enhanced rate of oxygen reduction. The corrosion rate of galvanised steel did not alter significantly for electrolytic droplets higher than 800 μm.