Where to use Weathering Steel?
As with other forms of construction materials, there are certain environments which can lead to durability problems. The performance of weathering steel in these environments may not be satisfactory and its use in these environments should be avoided.
Marine Environment Exposure to high concentrations of depassivating chloride ions will greatly affect the patina formation. These can be deposited from airborne marine salts in aerosol originating from breaking waves at sea or on the shoreline, or from salt fogs, will greatly affect the patina formation. The hygroscopic nature of salt can prevent sheltered surfaces from fully drying when relative humidity is elevated and thus stop the formation of the rust patina, thereby resulting in the weathering steel continuing to corrode at a rate similar to mild steel. Evidence of a higher corrosion rate and a delayed, or even no, formation of the protective patina has been identified for unwashed and sheltered surfaces (i.e. microclimate effects), as well as in crevices, on weathering steel structures in coastal locations.
Therefore, when determining the suitability of using weathering steel in a given location, the atmospheric corrosivity assessment needs to take into account both the macroclimate, in accordance with AS 4312 and/or the CSIRO Corrosion Mapping and Model ; as well as the microclimate effects, as described in the Australian Steelwork Corrosion and Coatings Guide (ASCCG) . Details regarding humidity levels, wind strengths and wind directions, which assist in determining the macroclimate and microclimate for a specific location, can be obtained from the Australian Bureau of Meteorology site under climate data. Based on the findings of the Morcillo review of weathering steel performance data, it is recommended that weathering steel should only be used in areas where the maximum first year corrosion rate (taking into account both the macroclimate and microclimate effects on sheltered surfaces) of mild steel is less than 50 µm/yr. This is equivalent to a rain washed surface in atmospheric corrosivity category C3 (Medium) to ISO 9223. Therefore, if the determined atmospheric corrosivity is C4 (High) or C5 (Very High), weathering steel should not be used. Generally, weathering steel can be used in locations that are more than 2km from the open seacoast, where the maximum first year corrosion rate (taking into account both the macroclimate and microclimate effects on sheltered surfaces) of mild steel is less than 50 µm/yr. However, in some cases, this minimum safe distance may increase up to 40km, depending on prevailing wind strength and direction, ocean wave and coastal surf conditions, topography, obstructions to wind flow and the level of shelter near the site.