Corrosion of 316L Stainless Steel used for Building Applications in Marine-Urban Atmospheres: A Case Study from Genoa, Italy

This study concerns the corrosion behavior of a 316L stainless steel railing installed in an aggressive marine-urban atmosphere, located about 400 m as the crow flies from the sea, in Genoa, a coastal city in northwestern Italy. Despite the well-known relevance of surface finishes in determining the degree of resistance to atmospheric corrosion, the initial surface finish of the material used could not be ascertained. Only 3 months after it was erected, the railing showed, upon visual inspection, reddish-brown halos and spotted deposits distributed diffusely all over the structure. To investigate the likely causes of these sudden and unexpected corrosion phenomena, the morphology of the corrosion and the surface chemical compounds were examined using microscopic (optical and scanning electron, with an electron probe microanalyzer equipped with an energy-dispersive spectrometer) and profilometry (no-contact three-dimensional optical profiling) techniques. The morphology of the steel surface finish on the specific material used for the structure was found to be unsuitable for its designated use in such an aggressive atmosphere. Chloride occurring along the constituent grain boundaries of the rough surface finish triggered intergranular corrosion, leading to grain fall out and subsequent significant localized corrosion.