Influence of Corrosion Product on Corrosion Behavior of Carbon Steel in Atmospheric Environment

Abstract

Atmospheric corrosion proceeds under thin water film, and it is affected by many environmental factors such as temperature, relative humidity, rainfall, deposition of airborne sea salt. Most of the previous study of atmospheric corrosion of carbon steel has been focused on the corrosion behavior in initial corrosion process. However, corrosion products are formed on the surface at an early stage in case of carbon steels. Thus it is important to clarify the influence of corrosion products on atmospheric corrosion behavior.. While corrosion products may easily lead to wet condition with absorption of moisture, it is reported that corrosion products inhibit the corrosion physically or chemically. In this study, we focus on dew condensation which occurs on the surface of carbon steels with rust layer. The electrodes with corrosion products were prepared by wet/dry cyclic corrosion test. The remodeled concentric corrosion sensor was used as the electrodes. The temperature of electrode materials can be controlled from the backside of electrode because the backside of the remodeled sensor is exposed to atmosphere. The corrosion test was carried out in a chamber at 25℃ and 60％RH by adding the droplet of 0.1M NaCl solution or distilled water. During the corrosion test, the corrosion rate of the electrode was monitored by electrochemical impedance method. The electrode covered with corrosion products was cooled with a cooling equipment in the chamber at 25 ℃ and 60 ％ RH in order to measure the dew point. The dew point was evaluated by measuring solution resistance using electrochemical impedance method. The dew point of the electrode before corrosion test was measured. The solution resistance dropped sharply to a low value at around 17 ℃. The dew point was calculated at 17℃ under the 25℃ and 60％RH from the curve of saturation water vapor pressure and Tetens’s equation, indicating that the experimental and calculated values are in good agreement. The amount of corrosion products increased with an increase of the cycle number. The solution resistance of the electrode after corrosion test went down at the temperature higher than the calculated value. In addition, the deposition of corrosion products occurred an increase of the dew point of the electrode. This is attributed to the diameter of pore in rust layer. It is known that equilibrium water vapor pressure in the crevice increases with a decrease of the crevice size. This means that the humidity in crevice increases in comparison with relative humidity of atmosphere. It is predicted the deposition of corrosion products leads to the formation of smaller size of pores. The chloride ion is included in the rust layer, and thus the water absorption of chloride ion occurs even at higher surface temperature.