Protective Nature of the Surface film Formed on the Weathering Steel

Abstract

Weathering steel containing Cu, P, etc., has been widely used for various steel structures such as, bridges, buildings, towers, etc., because of its maintenance free characteristics resulting from the formation of a protective and adhesive film during the exposure of the structure to the atmosphere. This feature facilitates the weathering steel to be used without any surface treatment. It is well known that rate of atmospheric corrosion dep-ends upon : i) the length of time, the moisture is in the contact of the surface ii) the extent of pollution in the atmosphere in which the material is exposed and iii ) the chemical composition of the iron and steels Here, i and ii both are of great importance . It is seen that sample of the same material exposed in a similar manner lost hundred times as much weight in the humid industrial atmosphere of England, as in the dry semi-tropical atmo-sphere of Khartown, Egypt.Variation in the composition of iron or steel (i.e., factor iii) may play as important role as atmospheric environment. For example, during exposure in an industrial atmosphere , a structural steel with 0.04% Cu lost 13.8g/sq.dm after 8 years of exposure, whereas a similar specimen of steel containing 11.4% Cr lost only 0. I6g/sq.dm'. Figure 1, is a time-corrosion plot2 of three steels in marine and industrial atmosp heres. It is seen from the figure that the loss in weight of copper steel during the first 0.5 and 1.5 years is considerably less in marine than in industrial atmos-phere , but from the shape of the curves it is probable that the corrosion losses in marine atmosphere will eventually be higher than in industrial atmosphere. Though the metal loss in 3.5% Ni steel is low it behaves in the similar manner as Cu steel.Opposite is true in case of more complex Ni- Cu-Si- P steel i . e., metal loss is less in industrial atmosphere to that of marine atmos-phere . It was suggested that such a reduced loss in weight was due to a very thin and adherent film formation on the latter surface