Variability in long-term corrosion of cast iron bridge piers

In 2010 the disused railway bridge at St Lawrence on the Queensland Pacific Ocean coast was demolished after 90 years service across the St Lawrence Creek, Queensland, some 6 km inland from the Pacific Ocean and subject large (up to 9 m) seawater tidal movements. Detailed examination of samples about 300×300 mm square and larger cut from several of the bridge piers at various elevations showed corrosion loss varying on average between 2-3 mm for the immersion and tidal exposure zones. This is remarkably little for such aggressive environment and for such long duration of exposure. There was negligible corrosion in the atmospheric zone. The multiple samples allowed statistical analysis of the variability of corrosion losses at different elevations. The statistical results are unique for such long-term exposures and are of major importance for structural reliability analyses of similar old bridges. Also, the corrosion loss results were compared with results reported in the literature for which the longest period of observation is only a few years. Observations show that cast iron is not free from localized corrosion and pitting as sometimes assumed. The corrosion loss results were compared to patterns previously observed for corrosion of steel through the tidal zone. A pattern consistent with so-called accelerated low water corrosion was observed, suggesting that at some period of time in the history of the bridge the tidal seawaters suffered some degree of water pollution.