The Role Of Micro-droplets Phenomenon In Atmospheric Corrosion Process

The atmospheric corrosion of metals is usually electrochemical corrosion process, and which is the interaction result of water vapour, oxygen and corrosive media contained in the atmosphere. Air contaminant such as hygroscopic particles deposited on metal surface could absorb water molecules and promote formation of thin electrolyte layers. The formation of thin electrolyte layer on metal surface could accelerate the development of atmospheric corrosion.The micro-droplets formation on metal surfaces, an important phenomenon closely related to atmospheric corrosion, can be described as follows. Under suitably relative humidity, salt particles deposited on metals surface can absorb water molecules to form electrolyte droplet (primary-droplet) and when this primary-droplet can corrode the metal substrate, then, there will be a large amount of tiny water droplets with 1～10μm in diameter formed (micro-droplets) around primary-droplet, which have a closely relationship to the atmospheric corrosion initiation.In this paper, the behavior characteristics of micro-droplets were studied in the climate chamber, and the role of micro-droplets in the atmospheric corrosion process were also investigated using microscope observation method and electrochemical polarization techniques.It was found out that micro-droplets could only form in a corrosive combination of metal and electrolyte droplet, namely, the metal substrate could be corroded for the primary-droplet, and appeared easier at higher relative humidities (RH), while lower RH and oxygen-free atmosphere were helpless for the micro-droplets formation.Formation power of micro-droplets was systematically studied by electrochemical polarization method. The results show that electrochemical polarization of the three-phase boundary was the critical condition for micro-droplets formation. Cathodic polarization accelerated micro-droplets formation and anodic polarization reduced the formation of micro-droplets, independent of metal type, electrolyte species, composition of air, and environmental humidity. That is to say that the electrochemical interface state was the key condition for micro-droplets formation on metal surfaces.Phenolphthalein test proved that the micro-droplets came 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. Contact angle test reflected electrochemical polarization could lead to the decrease of liquid-metal interfacial energy.Based on the above-mentioned results, negatively charged three-phase boundary could caused a decrease in local surface tension of liquid/metal interface and cooling of the metal surface near to the three-phase boundary, which led to the local liquid evaporating and condensing again on the nearby metal surface were regarded as the mechanism of micro-droplets formation.In addition, the in situ observations indicated that micro-droplets could intensify metal surface wetting capacity and promote formation and spreading of thin electrolyte layer. Meanwhile, micro-droplets could accelerate thin electrolyte layers distributed on various locations merging together to form continuous thin film. The formation of thin electrolyte layer on metal surface means that atmospheric corrosion steps in the rapid development stage.