The high costs of maintenance painting make the concept of using slow rusting unpainted weathering steel particularly attractive. At several sites throughout Great Britain long term exposure trials have been carried out under both open and sheltered conditions with Cor-Ten B weathering steel and a special mild steel containing copper. These tests showed that open exposure corrosion tests were not generally applicable to steel sheltered by a bridge deck. Both the form and level of the corrosion-time curves, as well as the colour and texture of the rust, could differ considerably lower rate than the special mild steel, but severe pittinmg was apparent on Cor-Ten B specimens from sheltered marine environments. It is recommended some form of corrosion monitoring such as test specimens for periodic removal