| Atmospheric corrosion is the most widespread corrosion problem known to mankind, and can be defined as the corrosion of metal materials exposed to air and its pollutants, rather than immersed in a solution. The process of atmospheric corrosion is accelerated with the formation of an electrolyte droplet, which was named as main-droplet here, or a thin electrolyte layer through the adsorption of water vapor by the deposited salt particle at certain relative humidity. The formation of the electrolyte droplet or thin electrolyte layer electrochemically changes the surface situation of materials from homogeneity to heterogeneity. Therefore, a considerable potential difference or corrosion current is established in the electrolyte droplet or thin electrolyte layer, and the atmospheric corrosion is ignited and accelerated due to the existence of corrosion cell. Although much information has obtained about the atmospheric corrosion process, yet even today it is not fully understood. Recently, a lot of micro-droplets, 1~10μm in diameter, were observed around the edge of main-droplet formed by the deliquescence of salt particles deposited on metals in atmosphere. Some of characteristics of micro-droplets were that they were caused by the type of main-droplet and formed only when the metal can be corroded for the main-droplet. Obviously, the phenomenon of micro-droplets formation may be related to the atmospheric corrosion process. In this paper, the characteristics of the formation and spreading of micro-droplets were studied in detail based on the pH and potential distributions of micro-droplets in the humidity-controlled chamber. The relations between the micro-droplets formation and the atmospheric corrosion were also investigated with the scanning Kelvin probe, the electrochemical polarization and the surface analyses of SEM + EDS. The probable mechanism was also discussed for the formation of micro-droplets. The corresponding experimental results were listed as follow. (1) The phenomenon of micro-droplets formation was a universal experimental phenomenon during the deliquescence of salt particles in a corrosive environment. (2) The formation of micro-droplets needed two premises. One was that the relative humidity should be over the saturated relative humidity of main-droplet. The other was that the metal substrate could be eroded under the main-droplet. (3) Relative humidity was the vital factor for the formation and spreading of micro-droplets, and two critical values of relative humidity were found for the formation and spreading of micro-droplets. (4) Oxygen participated the formation of micro-droplets directly and a high value of pH was found in the areas covered with micro-droplets. Meanwhile, micro-droplets could only appear on high potential area in a corrosive combination of metals and salt particles. (5) The electrochemical polarization results proved that the current established by the potential difference between the central and peripheral area of main-droplet was the driving force for the formation of micro-droplets. (6) Without polarization, the spreading of micro-droplets showed little different compared with that under no wet-dry cyclic conditions. Under the electrochemical polarization, both polarization current and polarization resistance showed a similar oscillatory rule at a constant of polarization voltage. (7) According to the surface analyses, micro-droplets were mainly composed of hydroxyl ions formed by the oxygen reduction, cations migrating from the main-droplet and water molecules from air. Based on the above-mentioned results, the formation of invisible water clusters and water film, synergistic action of the oxygen reduction and the water molecule adsorption, and migration of cations in main-droplet were regarded as the main phases for the formation of micro-droplets.
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