Analytical diagnosis of the conservation state of weathering steel exposed to urban atmospheres

Résumé

Weathering steel is a special kind of steel developed to be exposed outdoors. Its protection ability is based on the development of the so-called protective rust layer formed by different iron oxyhydroxides which act as a barrier for different stressors. However, this material does not react in the same way when it is exposed to different atmospheres. In this PhD manuscript several weathering steel sculptures made by Eduardo Chillida and Richard Serra exhibited in the urban-industrial atmosphere of Bilbao city are studied. Some of these artworks present an irregular surface which affects not only to the sculpture¿s appearance but also to the protective feature of the mentioned rust layer. Therefore, several analyses were carried out on these artworks in order to evaluate the cause for this rare development of the protective rust layer and its consequences. In this way, in-situ analyses have been very suitable due to their non-destructive and non-invasive nature. In this work, very novel spectroscopic techniques have been used during in-situ analyses and they have provided good quality results. Besides, SEM-EDS analyses have been carried out successfully to identify the composition and the deposition rate of atmospheric particles, finding a high rate of silicates and carbonates particles among others. Quantitative techniques have been also used for the assessment of the acid gases affection on the steel components, identifying sulphates as the most abundant compound in the steel surface. Biological degradation activity has been also detected in some sculptures. As an evaluating tool, a Protection Ability Ratio for SOx rich atmospheres has been developed in this PhD work. Finally, a thermodynamic study has been carried out in order to evaluate the proposed chemical reactions that take place on the steel surface between the atmospheric stressors and the components of steel.