Corrosion of ferrous metals in acid soils

Abstract

Corrosion of ferrous metals in acid soils is influenced largely by the acidity of the soil. A definite correlation was obtained between the average rates of pitting of ferrous specimens exposed to acid soils for 8 and 10 years and the total acidity of the soils. A similar correlation was also obtained between the total acidity of soils and losses of weight of steel specimens in a laboratory test. The effect of acidity in inducing corrosion is attributed to its action in preventing the formation of protective layers of ferrous and ferric hydroxides. The pitting factor, defined as the ratio of the maximum depth of pits to the average depth of penetration, is shown to be roughly proportional to the ratio of the uncorroded to corroded areas on a metal surface. Measurements of several physical properties of soils which determine their permeability were correlated with the pitting factor, indi- cating that the more permeable the soil, exclusive of sands, the greater is the tendency for corrosion to take the form of pitting, other conditions being the same. Certain soils may be so impermeable that the rate of pitting in them is very low.