No constant humidity If it is left in permanently wet or damp conditions, weathering steel will oxidise like any other unprotected carbon steel. A succession of wet and dry phases is required to form a stable oxide layer on the surface. Weathering steels should not be used in: •Sheltered locations with damp conditions •Permanent contact with water •Soil or covered by vegetation Weathering steels used in these locations should be protected with paint. The paint must extend above the level of the water, soil, or vegetation. No aggressive atmosphere High concentrations of chloride ions negatively affect patina adherence. According to EN ISO 9223, weathering steels should not be used within two kilometres of coastal waters unless airborne chloride levels do not exceed the S2 salinity classification (that is, Cl < 300 mg/m2/day). Direct contact between weathering steels and de-icing salts used on roads should be avoided. No atmospheric pollution Air-borne pollutants and industrial fumes can affect patina development. Corrosion is much higher if the metal surface is covered by solid particles such as dust or dirt. These particles can retain moisture and salts. In an industrial atmosphere, large amounts of sulfur dioxide (SO2) are detrimental to the compactness of the patina. EN ISO 9223 advises that weathering steels should not be used without protection in an environment above P3 (that is, SO2 > 200 mg/m2/day). 10 Pre-treatment Weathering steels are supplied un-weathered. Over time the patina will transform from an initial red-orange colour to a dark brown. Although the process normally takes up to two or three years, it can be accelerated using a well-tested pre-treatment procedure. In the pre-treatment, the surface of the weathering steel is degreased and then sandblasted to ensure homogeneous development of the patina. After this, the metallic sheets are exposed to a succession of accelerated wet and dry cycles. The water used may contain small amounts of oxidising agents such as vinegar or salt water to increase the rate of patina formation. After two months, this pre-treatment will result in a red-orange patina. The main reason this pre-treatment is used is to reduce the time needed to reach the final colouration. However, the pre-treatment will not stabilise the rust layer. It will reduce the amount of staining to nearby materials, but will not eliminate it completely. Clear varnish When touching weathering steels, some rust particles may linger on the hand which can be a problem for indoor applications. A clear varnish can be applied to the steel sheets. It will stop patina evolution, but it will protect users from rust and reduce dust. Suitable clear varnishes are available from your regular steel paint supplier. 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. When handling weathering steels onsite, care must be taken to avoid unsightly gouges and scrapes. The material should be kept as clean as possible and away from mud, grease, oil, paint, concrete, mortar splatter, and other foreign substances to minimise cleaning costs. Paint or crayon identification marks should be placed on parts of the weathering steels which will not be visible in the finished structure. If they are visible, the marks must be removed during the final cleaning operation. Storage in transit, at yards, or onsite should be minimised. When storage is unavoidable, uneven weathering can be minimised by positioning the material in an exposed area with good drainage. Blocking, to avoid contact with the ground, is essential. Cover cloths can be used to prevent water staining and dirt accumulation. They also reduce the chance that the steel will initially develop an uneven patina.