Modeling of atmospheric corrosion environments and its application to constant dew-point corrosion

In order to develop an accelerated corrosion test which simulates the corrosion behavior of stainless steels in a marine atmosphere, modeling of atmospheric corrosion environments were tried. From the measurement of the night and day variation of meteorological data, it was found that the dew-point of outdoor air is approximately constant and humidity depends on an air temperature. The mole fractions of Na⁺, Mg²⁺, and Cl^- ions in deposited salts on a stainless steel plate specimen exposed to a marine environment were the same as those in the seawater, and the Cl^- ion concentration of NaCl-MgCl₂ solution depends on the humidity of ambient air. Consequently, the Cl^- ion concentration of electrolyte layer or droplet formed on a metal surface, which is the critical factor of atmospheric corrosion in a marine environment, can be determined as a function of the dew-point of moist air and the temperature of metallic specimen. Then, the amount of electrolyte layer or droplet is determined by the total amount of chloride deposition on a metallic specimen. The determinative condition of atmospheric corrosion environments, therefore, can be described by the dew-point of moist air, the amount of chloride deposition, and the night and day variation of specimen temperature. An environment model described by the determinative condition was applied to simulate the corrosion behavior of Type 304 stainless steel in a subtropical marine environment in Okinawa. The generation and growth behavior of rust on the steel in the environment was well reproduced by using the constant dew-point test, that is, the accelerated corrosion test based on the model.