Pack rust corrosion evaluation on historical riveted steel bridges [Studio dei fenomeni di corrosione in fessura di ponti chiodati di valenza storica]

Several riveted steel structures were built around the world towards the end of the 19th century. One of the most famous is surely the Eifel Tower in Paris. In Italy, the San Michele bridge has undoubtedly a relevant historical and cultural interest. It was built in 1889 to connect Paderno d'Adda to Calusco d'Adda over the Adda river. Unfortunately, degradation phenomena occurring during its more than 130 years' service life have interested this infrastructure. Several research and studies have been carried out in the last years to assess its conservation, the residual structural strength and the restoration and maintenance operations efficiency. Corrosion is surely one of the damaging morphologies that has been affecting the bridge and its rate deserves particular interest. Corrosion attack mainly occurs on the "thin" elements of the bridge, inside the gaps between the riveted elements typically of width in the range between 0.1 to 3 mm sufficient to allow the penetration of water and to prevent its exit. At the same time, the access of oxygen restricted to the areas far from the fissure mouth. In this way, corrosion occurring inside the crevice sums to atmospheric corrosion that involves the external surfaces of the joint, thus producing deformation in the riveted elements due to the expansive action of the corrosion products. This corrosion form is known as "pack rust" and the structural response of the element modifies due to the simultaneous effects of thinning and deformation due to the accumulation of corrosion products. This autocatalytic mechanism is difficult to block with external interventions that are not able to act directly on the process. For this reason, the present work offers a general overview of the crevice corrosion phenomena found on metal historical infrastructures to better understand the damaging morphologies occurring at long-term and it examines the "pack rust" corrosion to evaluate and predict the evolution of the unique localized phenomenon. In this way, it is possible to evaluate any changes in the corrosion process that have occurred over the years, determine the average propagation rate of the corrosion attack and study most effective protective strategies for stopping it, while respecting the historical and architectural value of the structure.