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This work considers the magnetic induction method on rough surfaces (ABNT NBR 10443) to determine the thickness of the patina layer formed on weathering steel (WS) sheets exposed for up to two years at São Paulo atmosphere and on WS sculptures exposed for sixteen and twenty-four years at the same site. A good agreement was found with SEM measurements. It was also found that patina thickening followed a two slopes bilogarithmic law, with a steeper slope for short exposure periods, in agreement with increased corrosion rate. For long-term exposure, it was found a low thickening rate (

Weathering steel (WS) is a low alloy steel particularly suitable for outdoor exposures. When exposed to wet and dry cycles, they develop a protective patina layer with complex composition, effectively reducing the corrosion rate. However, several years are necessary to completely develop this layer. Aiming to investigate the seasonal dependence on the climatic parameters and concentration of pollutants in the atmosphere in the patina development at early stages, the present study investigates the corrosion behavior of patina layers formed on WS exposed to the atmosphere of the Sao Paulo metropolitan area during a period corresponding to each of the four seasons, employing open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and electric equivalent circuit (EEC) fitting. SEM, Raman spectroscopy and thicknesses evaluation were used to characterize the patinas. The results showed that, while the patina compositions were independent of the exposure period, their thicknesses and corrosion resistances were extremely dependent on this variable. Excessive rain and wetting periods, as well as the presence of pollutants, proved to be deleterious, leading to the development of non-protective patinas. Based on the EEC fitting procedure, a model was proposed to explain the EIS responses of the patina layers based on the porous electrode theory initially developed by de Levie.