Water Permeability of Rust Layer Formed by Atmospheric Corrosion

Abstract

Synopsis:Water permeability of rust layers formed on a carbon steel and a low alloy steel by atmospheric corrosion was evaluated by means of impedance measurement of the rusted specimen electrode in 0.1 M sodium 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 resistance and also the time necessary to reach the steady state increase with increasing of the periods of atmospheric exposure. These values are larger for low alloy steels than those for the carbon steel at the same exposure 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. In case of low alloy steel the deviation of the final values from the linearity is found and the initial values come to agree with the line as exposure time becomes longer. It is supposed that corrosion rate depends on the time of wetness which determines how large is the area on the steel surface where the water reaches through the porosities and corrosion starts.