The Analysis of Dew Corrosion Behavior of Stainless Steel based on the 'In-situ' Observation of 'Elementary Process' to Rusting

摘要

The rusting of stainless steel in an atmospheric environment is mainly caused by the drying of dew droplets. Sea salt particles, which naturally deposite on stainless steel in air atmospheric environment, solve in the dew droplets. By the drying of the droplets, they become concentrated chloride ion solution, which cause rusting. However, the details of the rusting behavior has not been fully clarified yet. In this study, the dew drying and rusting behaviors are analyzed by the approach of 'in-situ' observation. The drying behavior of the dew droplet and the rusting behavior of stainless steel were observed in controlled temperature and humidity which simulate the environment of Okinawa, Japan. The results shows that the drying process of the droplet is divided into three stages: pinning stage when the contact angle of the droplet decreases, de-pinning stage when the contact area of the droplet decreases, and self-pinning stage when the contact angle decreases furthermore. At the self-pinning stage, the salt precipitates inside of the droplet. The stainless steel rusted in the area under and near the precipitated salt. Based on the results, it can be implied that the rusting of stainless steel in atmospheric environment is caused by the crevice corrosion between the steel and the precipitated salts.