Long term corrosion tests have been carried out on Cor-Ten B weathering steel and mild steel at a number of locations in the UK. Test specimens were exposed under open and sheltered conditions on both real and simulated bridge decks. In some cases the test specimens were in close thermal contact with the bridge girders to check that small test specimens do give corrosion rate measurements representative of the steel in large structures. The results showed that small test specimens do provide relevant information and confirmed differences between corrosion under sheltered and open exposure conditions. Cor-Ten B corroded at a generally lower rate than mild steel but the difference was reduced under sheltered conditions.