Influence Of Droplet Characteristics On The Atmospheric Corrosion Of Carbon Steel

Atmospheric corrosion of metals is the most prevalent form, and the essence of atmospheric corrosion is the electrochemical corrosion under thin film. Due to the influence of surface energy, the electrolyte film adsorbed on the metal surface is usually not evenly distributed, but exists as droplets of different sizes. The formation of droplet is a common phenomenon, which provides conditions for the electrochemical corrosion, and is an important factor affecting the initial atmospheric corrosion behavior. Therefore, the study of atmospheric corrosion behavior of the initial droplet phenomenon needs a deep understanding.Research on atmospheric corrosion of metals usually used a large area of the metal electrode, which can not obtain electrochemical properties at different locations, especially the current distribution, thus limited the in-depth study. In this paper, wire beam electrode technology and dual microelectrode technique were used to examin the droplet spreading factor, the effect of pH and humidity. Meanwhile. the effect of environmental factors and the droplet characteristics on the mechanism was established. The electrochemical corrosion theory under thin film was developed.Corrosion behavior of carbon steel under the droplet was studied by using the wire electrode technique (WBE), and it was found that anodic and cathodic current value decreased gradually with the increasing time, then eventually kept stable. Furthermore, the current distribution over the WBE surface under the droplets of different sizes were studied. Under the smaller droplet, the carbon steel was more easily to form an asymmetric electrochemical area. In addition, the concept of droplet spreadability was defined. With the increasing of droplet spreadability, the average current density increased exponentially, the maximum value and the standard deviation of the anodic current density increased linearly, and the localized corrosion intensity index also increases, which means the corrosion intensity enhanced. At the same time, the corrosion intensity under the NaCl droplet is stronger than the seawater droplet.The current distribution at pH 4.64-10.22 all exhibited the typical form of the Evans model, with the anodes at the centre and the cathodes around the perimeter. The variation of average anodic current density at different pH with time were similar. At the first 90min, the average anodic current density decreased significantly, and became stabilized after 90min. With the decreasing of pH, the average anode current density increased, and the anodic and cathodic current density increased obviously with the pH into the acidic region. Moreover, with the decrease of pH, both the maximum anodic current density and the anodic current density standard deviation increased, which means that the addition of hydrogen ions into electrolytes greatly accelerated the corrosion occurring on iron surface.The corrosion behavior under NaCl droplets in different environmental humidity us different. In the early stage of the corrosion, the current value under low humidity is higher than the high humidity. In the late stage of the corrosion, the carbon steel corrosion in high humidity eventually become stable. However, in low humidity environment, corrosion of carbon steel is gradually stopping, and the current density on the electrode surface tends to zero. There is a small current peak at the end of dry period. The average anodic current density under different Ao all decrease with time. The decline degree of the corrosion current density is relatively large in the early stages, then gradually decreased and eventually to zero. There is a small current peak in the drying stage under different Ao, and the current peak occurs earlier and become larger with the larger A0.