Other factors that can result in localized corrosion are contact between dissimilar metals and defects in coatings. Presence of chlorides and sulfates. 4. pH. Typically, the most corrosive soils are those with the lowest pH and highest concentration of chlorides, sulfates. Wet dry conditions are particularly difficult since the can result in concentration of corrosive species and mechanical action that tend to breakdown protective corrosion films and coatings on the metal surface. Localized corrosion is particularly a problem for buried structures since it makes monitoring and detection difficult.
Microbacterial action can also promote local anodic attack.
Such conditions are common particularly where sulfate reducing bacteria are present. These microbes reduce the commonly available sulfate ions in the soil to produce hydrogen sulfide. This, in turn, results in increased acidity and acceleration of anodic dissolution. Other factors that can result in localized corrosion are contact between dissimilar metals and defects in coatings.