Time-Dependent Corrosion Wastage Model for Wrought Iron Structures

Damage due to atmospheric corrosion on metal structures is a significant aspect for both the design of new construction and the maintenance of existing buildings. This problem is particularly felt for nineteenth-century wrought iron constructions, because of both lack of proper maintenance and architectonic value. The main objective of the current paper, framed within a more comprehensive research project, is to provide a time-dependent model able to predict the corrosion wastage thickness on historical metal structures as a function of the protection coating life variability and its renovation cycles. Average damage curves, calculated on a sample of 20 buildings experimentally monitored for 20 years, were taken as literature references to calibrate the model, based on the hypothesis that the durability, due to phosphorus content, of historical wrought irons are between that of recent mild carbon steels and that of weathering steels. The reference damage curves for two environmental conditions (marine and urban-industrial) and two different materials (mild carbon steel and weathering steel) were interpolated, fitted, and extended to 125 years. A comparison was made between experimental damage curves and some significant models from literature, and the percentage error with respect to the tolerance and confidence intervals of the reference damage curves is discussed. Results definitely confirm a substantial difference between experimental values and those predicted by literature models. As an application example, the model was applied to estimate the remaining life of the metal structural elements of the Umberto I Gallery in Naples, one of the most significant monuments of the largest city in Southern Italy.