Atmospheric Corrosion Behavior of Mild Steel in the Initial Stage under Different Relative Humidity

The corrosion behaviour of mild steel in the initial stage was studied in simulated pollutant-free atmosphere with relative humidity (RH) of 65%, 75% and 95%. The mass gain has a positive linear relationship with the exposure time. It increases significantly with the increase of relative humidity. The initiation and propagation of corrosion in the initial stage have a feature of filiform corrosion. An aeration cell is formed in a water droplet. Fe is dissolved to form Fe 2+ in the center of the cell, and O 2 is reduced to produce OH - at the edge of droplet. They move toward each other and react to form Fe(OH) 2 . At this site, corrosion continues to generate cellular corrosion products. Some Fe 2+ species flow forward to form a new active site as an anode. The old site acts as a cathode. The small anode-big cathode promotes the development of filaments. In case of high relative humidity, the corrosion is accelerated. The width of filaments and the growth rate of cellular corrosion products increase. Under RH65%, the dominant corrosion products are α -FeOOH and γ -FeOOH; while under RH75% and 95%, the corrosion products are mainly composed of γ -FeOOH, α -FeOOH, γ -Fe 2 O 3 and Fe 3 O 4 .