The term weathering steels describes a class of low-alloy structural steels that develop an adherent protective rust layer, called patina, during exposure to the atmosphere. As a result of this, weathering steels, unlike ordinary plain carbon steels, do not require painting, which means important advantages: avoiding associated costs of initial painting, periodic repainting, containment and disposal of blasting debris during repainting. The development of the protective patina depends on the steel microalloying elements which it contains, consequently, the corrosion process destroys a certain amount of steel in order to supply the microalloying elements. However, only part of all the Fe involved in this process gets into the patina. In fact, a percentage of Fe in the form of soluble corrosion products is swept away by the rainwater or dew, staining other materials with rough surfaces on the structures. The completion of the protective patina takes at least several years. In order to analyse the influence of non-metallic inclusions morphology and composition on the initial stages of the corrosive atmospheric attack, an experimental steel called PA (P: patina-A: acceleration) was designed. A great quantity of non-metallic inclusions with spherical morphology, great chemical reactivity and very small size was chosen. Considering the structural behaviour, this morphology is advisable since it produces a greater isotropy of the mechanical properties. Experimental samples of PA steel were tested in outdoors exposure producing, in only one year, a very effective passive patina. Consequently it can be observed that these tests clearly showed a vanishing effect of all staining contaminant products, which means an outstanding advantage towards the environmental protection. This aim was achieved through an increase of a non-metallic inclusions density with spherical morphology, great chemical reactivity and very small size of them. Non-metallic inclusions modified by rare earths additions, particularly Ce, were considered to join: weatherability and better mechanical properties. Experimental samples of PA steel were tested in two-outdoors exposure which produced, in only one year, a very effective passive patina. Consequently these tests clearly showed a vanishing effect of all staining contaminant products, meaning an outstanding advantage towards the environmental protection.
