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Corrosion Severity Ratings

For design and corrosion risk assessment purposes, it is desirable to estimate the corrosivity of soils. One of the simplest classifications is based on a single parameter, soil resistivity. The generally adopted corrosion severity ratings are:

Soil resistivity (ohm cm) Corrosivity Rating
>20,000 Essentially non-corrosive
10,000 to 20,000 Mildly corrosive
5,000 to 10,000 Moderately corrosive
3,000 to 5,000 Corrosive
1,000 to 3,000 Highly corrosive
<1,000 Extremely corrosive

Sandy soils are high up on the resistivity scale and therefore considered the least corrosive. Clay soils, especially those contaminated with saline water are on the opposite end of the spectrum. The soil resistivity parameter is very widely used in practice and generally considered to be the dominant variable in the absence of microbial activity. (reference)