Study Of The Electrochemical Characteristics Of Metal Under An Electrolytic Drop

Droplet corrosion is an important phenomenon of atmospheric corrosion. Undersuitable relative humidity, salt particles deposited on metals surface can absorb watermolecules to form electrolytic droplets or liquid film. Then it will accelerate theformation of corrosion cell on local area of metal surface. At the same time, thecorrosion current will flow from local anodes to cathodes on the metal surface. Theoccurence of corrosion current can accelerate the development of atmosphericcorrosion. Investigations of the influence of droplet properties, environmental relativehumidity and the wettability on the droplet corrosion of metals, can deepen theunderstanding of the interaction pattern bettween the metal surface and thesurrounding environment. Achievements can enrich the basic theoretical research ofatmospheric corrosion under thin electrolyte layer.Droplet corrosion has the heterogeneous electrochemical characteristic, so it isimportant to get the distributions of galvanic current and corrosion potential. In thispaper, wire beam electrod (WBE) technique, electrochemical impedance spectroscopy(EIS), combined with microscopic observation technique, are employed to investigatethe influence of the droplet size, droplet concentration, relative humidity andwettability on corrosion behavior of metals (stainless steel and carbon steel).Firstly the electrochemical distributions of carbon steel under different size ofsodium chloride droplets are investigated. The results show that carbon steel exhibitesheterogeneous electrochemical characteristics, and the electrochemical distributionsand the corrosion degree of carbon steel are influenced by the TPB zone. With theevolution of time, the main anodes first locate at the centre, and then tend to appearnear the cathodic zone. In addition, the average anodic current density and thecorrosion degree decrease with the increase of droplet size.Secondly, the influence of droplet concentration on corrosion of carben steel isinvestigated. The results show that, under low concentration (0.0006M and0.06M) conditions, the cathodes distribute at the edge of the droplet, and the anodes arepreferred to locate near the cathodes. At the same time the average anodic currentdensity incrcrease firstly and then tend to be stable with the evolution of time. Whileunder high concentration (1M and2M), the anodes locate at the centre and thecathodes distribute at the fringe. The average anodic current density decrease firstlyand then tend to be stable with the increase of time.Thirdly, the influence of oxide film and relative humidity on the corrosionbehavior of carben steel is studied. Under the equilibrium humidity conditions, thecathodes distribute at the fringe of droplet, the anodes are preferred to locate near thecathodes. The specimen with fresh electrode shows high average anodic currentdensity value. Under dry conditions, the anodes locate at the centre and the cathodesdistribute at the fringe. The specimen with fresh electrode exhibits high averageanodic current density at the initial stage, and then tends to be the same. In addition,relative humidity has the similar influence on the specimens with different oxide filmthickness, the average anodic current density increase gradually under high humiditycondition, obtain the maximum value at the early stage of drying, and then reduce.At the end, the localized corrosion of stainless steel under a drop of NaClsolution is investigated, and the following conclusions are achieved. The distributionsof galvanic current are heterogeneous, and the heterogeneity increase firstly and thendecrease with the evolution of time. Moreover stainless steel suffers more severecorrosion with the increase of droplet size.