The corrosion of structural steel is an electrochemical process that requires the simultaneous presence of moisture and oxygen. In the absence of either, corrosion does not occur. Essentially, the iron in the steel is oxidised to produce rust, which occupies approximately 6 times the volume of the original material consumed in the process. The general corrosion process is illustrated here.As well as general corrosion, there are various types of localised corrosion that can also occur; bimetallic corrosion, pitting corrosion and crevice corrosion. However, these tend not to be significant for structural steelwork.The rate at which the corrosion process progresses depends on a number of factors relating to the ‘micro-climate’ immediately surrounding the structure, principally the time of wetness and the atmospheric pollution level. Because of variations in atmospheric environments, corrosion rate data cannot be generalised. However, environments can be broadly classified, and corresponding measured steel corrosion rates provide a useful indication of likely corrosion rates. More information can be found in BS EN ISO 12944-2 and BS EN ISO 9223