Extensive research work has thrown light on the requisites for a protective rust layer to form on weathering steels (WSs) in the atmosphere, one of the most important is the existence of wet/dry cycling. However, the abundant literature on WS behaviour in different atmospheres can sometimes be confusing and lacks clear criteria regarding certain aspects that are addressed in the present paper. What corrosion models best fit the obtained data? How long does it take for the rust layer to stabilize? What is the morphology and structure of the protective rust layer? What is an acceptable corrosion rate for unpainted WS? What are the guideline environmental conditions, time of wetness (TOW), SO2, and Cl？, for unpainted WS? The paper makes a review of the bibliography on this issue. 1. Introduction Weathering steels (WS), also known as low-alloy steels, are mild steels with a carbon content of less than 0.2%？wt, to which mainly Cu, Cr, Ni, P, Si, and Mn are added as alloying elements to a total of no more than 3.5%？wt [1]. The enhanced corrosion resistance of WS is due to the formation of a dense and well-adhering corrosion product layer known as patina. Besides possessing greater mechanical strength and corrosion resistance than mild steel, the patina is also valued for its attractive appearance and self-healing abilities. The main applications for WS include civil structures such as bridges and other load-bearing structures, road installations, electricity posts, utility towers, guide rails, ornamental sculptures, fa？ades, and roofing. The recent introduction of high-performance steel, a new high-strength WS that does not require painting, has dramatically increased the number of steel bridges being built throughout the world, which has approximately trebled in the last ten years and now accounts for more than 15% of the market [2]. WS is an attractive material that reduces the life cycle cost of steel structures, which remain in service for long periods of time [3]. Extensive research work has thrown light on the requisites for the protective rust layer to form. It is now well accepted that wet/dry cycling is necessary to form a dense and adherent rust layer, with rainwater washing the steel surface well, accumulated moisture draining easily, and a fast drying action. Surfaces protected from the sun and rain (sheltered) tend to form loose and poorly compacted rust, while surfaces freely exposed to the sun and rain produce more compact and protective rust layers. The structures should be free of interstices, crevices, cavities, and other places where water
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