Observation and micro-electrochemical characterisation for micro-droplets in initial marine atmospheric corrosion

Abstract

Micro-droplets formed around NaCl droplet on carbon steel surface were observed with a confocal laser scanning microscope. Micro-electrochemical characterisation of the micro-droplets zone was also performed by a scanning Kelvin probe (SKP) and the scanning vibrating electrode technique (SVET). Results show that the electric current density and potential distribution of carbon steel under the NaCl droplet are asymmetrical; the peripheral regions of the droplet are cathodic, whilst its centre is anodic. The potential difference between the anode and cathode is 0.36 V, and the cathodic current density reaches 2.02 μA cm⁻². This kind of asymmetrical distribution of electrochemical characterisation results in cathodic polarisation at the fringe of the NaCl droplet, thus inducing the formation of OH^–, which could promote water adsorption and subsequent formation of micro-droplets around the NaCl droplet. The electrochemical potential difference of the oxygen concentration cell formed between the central and peripheral regions of the NaCl droplet is the main driving force for micro-droplet formation on metal surfaces.