As atmospheric corrosion proceeds the rust that forms offers a limited degree of protection so that the corrosion rate falls with time. The proposed mechanism for the enhanced performance of weathering steels involves a protective rust layer being formed. Corrosion rates will depend on both the protection afforded by the existing rust layer and the corrosivity of the surrounding environment in which corrosion is occurring, including possible interactions between the two. In relatively constant environmental conditions, if the rust layer develops to the stage of offering a constant degree of protection then a steady state corrosion rate might develop. If the protective properties of the rust layer continue to increase then corrosion rates would continue to fall. These two approaches can be used to analyse corrosion-time curves so that predictions of long term corrosion behaviour can be made on the basis of relatively short term testing.