Soil Parameters Affecting Corrosivity

Several variables are known to have an influence on corrosion rates in soil these include water, degree of aeration, pH, redox potential, resistivity, soluble ionic species (salts), and microbiological activity. The complex nature of selected variables is presented graphically in Fig. 10.2 [2]. 

The redox potential is essentially a measure of the degree of aeration in a soil. A high redox potential indicates a high oxygen level. Low redox values may provide an indication that conditions are conducive to anaerobic microbiological activity. Sampling of soil will obviously lead to oxygen exposure, and unstable redox potentials are thus likely to be measured in disturbed soil. 

Chloride ions generally participate in the dissolution reactions of many metals. Furthermore, their presence tends to decrease the soil resistivity. Chlorides may be found naturally in soils as a result of brackish groundwater and historical geological seabeds or come from external sources such as deicing salts applied to roadways. 

Sulfate ions are generally considered to be more benign in their direct corrosive action toward metallic materials than chlorides. However, concrete may be attacked as a result of high sulfate levels. The presence of sulfates also poses a major risk for metallic materials since these ions are nutrients to SRBs that convert these benign ions into highly corrosive sulfides. 

---

Soil acidity, or its pH, is the most important measured parameter. Details for conducting field measurements of pH in soils for use in corrosion testing are covered in ASTM Test Method for Measuring pH of Soil for Use in Corrosion Testing (G 51). Two electrodes, glass tind calomel, of a standard pH meter are pressed on undisturbed soil at a given site, and the vedue of their potential difference is linearly related to pH. Salts such tts fertilizers in the soil can affect the correct value. 

Soil resistivity The role of soil in the electrical circuitry of corrosion is now apparent. Thus the conductivity of the soil represents an important parameter. Soil resistivity has probably been more widely used than any other test procedure. Opinions of experts vary somewhat as to the actual values in terms of ohm centimetres which relate to metal-loss rates. The extended study of the US Bureau of Standards presents a mass of data with soil-resistivity values given. A weakness of the resistivity procedure is that it neither indicates variations in aeration and pH of the soil, nor microbial activity in terms of coating deterioration or corrosion under anaerobic conditions. Furthermore, as shown by Costanzo rainfall fluctuations markedly affect readings. Despite its short comings, however, this procedure represents a valuable survey method. Scott points out the value of multiple data and the statistical nature of the resistivity readings as related to corrosion rates (see also Chapter 10). 

---