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For short-term exposures it is known that the relationship between corrosion and soil moisture is a highly non-linear function. This is because of the wetting properties of the metal surface relative to the moisture content of the soil. Tests over 6 months exposure by Gupta & Gupta in 1979 showed that little corrosion (surface wetting) occurs for low moisture contents but that it increases very quickly for just a small further increase in moisture content.
The Gupta & Gupta test results relate only to sandy soils and sandy loams. Clay soils are not considered. However, clay backfill is predominant in many Australian water utility networks.
A laboratory program was completed at The University of Newcastle to replicate and extend the Gupta & Gupta study for clay soils. It involved determining the corrosivity of the clay soil towards mild steel for various soil moisture contents and compaction levels for exposures ranging from 3 to 12 months. Gupta and Gupta did not consider the effect of timing and of compaction. The latter is particularly important for highly impermeable soils like clays. A previous paper presented the results of samples exposed for 3 months. This paper presents the results of samples exposed for 6 and 12 months.