Advice on environmental classification The International Standards Organisation document ISO 9223 describes two methods for classifying atmospheric corrosivity, one based on environmental data and the second on corrosion rate tests.
Environmental data approach
This method combines measurements of the following three key factors to determine the overall corrosivity classification (C1-C5) of the environment.
• Time of wetness (T1-T5)
• Atmospheric sulphur dioxide pollution (P0-P3)
• Airborne salinity (S0-S3)
Corrosion rate approach
Corrosion rates are measured for standard metal specimens exposed for a year in the required location to a standard procedure given in ISO 9226. Unfortunately, both methods require measurements to be taken over a year to obtain representative average values. In most situations this will not be possible or practical at the design stage, so in the absence of a scientific approach, a judgement needs to be made. ISO 12944: Part 2 relates the classifications C1-C5 to descriptions of typical environments, and is a good starting point. Refer to the following table based on the ISO 12944 descriptions.
Further information to assist in making a judgement can be derived from a site visit to assess the nature of the bridge location and through contact with the local authority to determine future development plans.