Aqueous corrosion characteristics of carbon steel and weathering steel in aerated acid-chloride solutions were studied by using immersion weight loss tests, crevice corrosion tests, electrochemical measurements, and analytical techniques. Neither carbon steel nor weathering steel showed passive behavior in these acid-chloride solutions. Corrosion morphology was examined using scanning electron microscopy (SEM). Surface morphology showed uniform attack with some shallow pits. Weathering steel was not susceptible to crevice corrosion because of the small potential difference between the steel within the crevice and the steel outside the crevice. Weathering steel was more resistant to acid-chloride solutions than plain carbon steel. The better corrosion resistance of the weathering steel is attributed to protectiveness of the surface layer. X-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) analyses revealed that the rust layer formed in acid-chloride solution was composed of hematite (Fe2O3) and magnetite (Fe3O4). From XPS analyses, however, there were some Cr and Cu compounds in the rust layer of the weathering steel. These compounds act as a factor for corrosion resistance in aqueous solutions.
ArcelorMittal strongly recommends that transparent coatings and varnishes are not used on outdoor applications. The varnish will behave like any other coating and protect the metal against corrosion. This stops patina formation, the main reason weathering steels are used. Another reason is that transparent coatings do not contain pigments and this makes them ineffective against ultraviolet (UV) radiation. The weathering steels must be re-painted every two years (depending on the level of solar exposure). Varnishes can be used for indoor applications, and applied to pre-patinated surfaces to avoid staining. In-use precautions Unlike other steel alloys which resist corrosion (such as stainless steel) weathering steels have a similar chemical composition to that of carbon steel. It can be produced and handled with the same equipment as carbon steel, resulting in relatively low transformation and assembly costs.