Abstract: Micro-droplet formation was reported recently that a large amount of tiny water droplets formed around a primary droplet on a corrodible mental surface under suitably higher relative humidity, which may have a close relationship to the atmospheric corrosion initiation. The effect of the property of the primary droplet on the micro-droplet formation and the forming approach of the micro-droplets was investigated by using microscope observation and electrochemical techniques. The results show that no micro-droplets could be observed for the acidic primary droplet with pH value lower than 1, and the higher the pH value of the main droplet, the faster the micro-droplets formed, confirming the characteristic of the micro-droplets area with a higher pH value, higher potential and intensive oxygen reduction. In addition, phenolphthalein test and surface tension test proved that the micro-droplets could come from the adsorption and condensation of the water vapor around the primary droplet through the gas phase, instead of through surface spread from the primary droplet. It is under work whether the water vapor for the micro-droplets formation comes form the primary droplet or from humid air directly.

Key words: Micro-droplets, Atmospheric corrosion, Polarization, Cathodic oxygen reduction