Corrosion of varnished steel induced by sulphur dioxide and sodium chloride in the atmosphere

The behaviour of a steel varnish system in atmosphere polluted with SO2 and /or NACL is analysed with an emphasis on quantification of the under film corrosion of steel. Concentrations of 0.008% (80ppm) and 0.016% (160ppm) of SO2 and 0.017% and 5% NACL were employed in humidity-saturated conditions, running the tests for 500 hours. The effect of SO2 permeating through the varnish film is shown to be decisive for underfilm corrosion of the steel. The NACL, on the other hand does not react with the steel, given the low ionic permeabilities of the varnish. With both contaminants present, the underfilm corrosion increases with SO2 concentration and decrease with NACL concentration. Tables, graphs, Nyquist diagrams, references.