Relationship between corrosion rate of carbon steel and water film thickness under thin layer of artificial sea water

AC impedance method has been applied to atmospheric corrosion monitoring under thin film of artificial seawater. An atmospheric corrosion sensor, which consists of a pair of identical electrodes embedded concentrically in epoxy resin, was used for measuring the corrosion rates. The instaneous corrosion rates of carbon steel in the atmosphere of 60% to 95% Relative Humidity (RH) have been monitored by the continuous measurements of the impedance at 10 mHz and 10 kHz. The carbon steel under a thin water film including 0.1 kg/m² salt showed the highest corrosion rate at 60%RH. Above 70%RH, however, the carbon steel under a liquid thin film including 0.01 kg/m² salt gave the largest corrosion rate. The average thickness of a water film formed on the corrosion sensor was thermodynamically calculated by analyzing the drying and absorption processes of the artificial sea salt. The average thickness of water film formed on the corrosion sensor under film including 0.01 kg/m² salt was 10∼100 μm above 70%RH. The corrosion rate of carbon steel showed a maximum at the average water film thickness of several 10 s of micrometers.