Water Permeability of Rust Layer Formed by Atmospheric Corrosion

Water permeability of rust layers formed on a carbon steel and a low alloy steel by at mospheric corrosionwas evaluated by means of impedance measurement of the rusted specimen electrode in 0.1 Msodium benzoate aqueus solution. Resistance of rust layer decreases with immersion time as the solution penetrates into the rust and finally attains a steady state. Initial and final values of the resistanceand also the time necessary to reach the steady state increase with increasing of the periods of atmosphericex posure. These values are larger for low alloy steels than those for the carbon steel at the sameexposure time. The linear relationship has been confirmed between atmospheric corrosion rates and reciprocals of the final resistances which provide a sort of measure for the porosity of rust layer. Incase of low alloy steel the deviation of the final values from the linearity is found and the initialvalues come to agree with the line as exposure time becomes longer. It is supposed that corrosionrate depends on the time of wetness which determines how large is the area on the steel surfacewhere the water reaches through the porosities and corrosion starts.