Soil corrosivity is often classified by its resistivity.
| 0 - 1500 ohm.cm | very corrosive |
| 1500 - 2500 ohm.cm | corrosive |
| 2500 - 5000 ohm.cm | mildly corrosive |
| 5000 - 10000 ohm.cm | slightly corrosive |
| > 10000 ohm.cm | non-corrosive |
However, experience suggests that there is little likelihood of corrosion of 304 and 316 stainless steels with soil resistivities of 2000 ohm.cm and above if the pH is greater than 4.5 and there is clean drainage and backfill.
As with other environments, chloride levels in relation to pitting and crevice corrosion may also influence the performance of stainless steels in soils and where higher chloride levels are anticipated inland or for non-tidal coastal areas or just where a greater degree of confidence is required, type 316 would be preferred. For coastal areas and other situations where a greater degree of resistance may be required, 2205 duplex or super stainless steels can be considered.
If thought necessary, external protection such as appropriate protective casings or tapes (taking care to ensure an effective overlap to avoid crevice corrosion) may be used. Stainless steel can also be cathodically protected.
Source: http://www.bssa.org.uk/
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