Corrosion of Carbon Steels in Soils

Typical steel structures used underground in contact with soils are pipelines, utility pipings and pilings. Pipelines are generally protected against corrosion with coatings and by cathodic protection. Driven pilings are commonly used in bare condition. 

The position of the water table determines the oxygen transport and hence the corrosion rate. The moisture content of soil greater than 20% is deemed to be corrosive (general corrosion of carbon steel) and the value of less than 20% was conducive to pitting corrosion.15 This observation is thought to be related to the diffusion rate of oxygen.16 The general effect of soil resistivity on the corrosivity may be denoted as  

The corrosivity of soils also depends upon the oxidation-reduction potential as classified by Booth et al.15 The classification scheme of the corrosivity of soils is given in Table 4.4b. Macrogalvanic cells are formed in underground pipelines due to foreign structure the combination of new and old pipe dissimilar metals (stainless steel and carbon steel) differential aeration dissimilar soils and stray currents. All these lead to localized corrosion of underground pipelines. 

Microbially influenced corrosion occurs in soil environment. The sulfate-reducing bacteria (SRB) reduce sulfate to sulfide and as a result iron sulfide is formed due to corrosion. The iron sulfide deposit on the steel surface and the steel form a galvanic couple, which is substained by the removal of electrons in the form of cathodic hydrogen, followed by the further formation of more iron sulfide.19,20 

The typical corrosion rates in soils range from 0.003 to 0.063 mm/yr with an average value of 0.020 mm/yr. The pitting rates range from 0.033 to 0.45 mm/yr with an average 

---

Levlin, E., Aeration Cell Corrosion of Carbon Steel in Soil In Situ Monitoring Cell Current and Potentitd," Corrosion Science, Vol. 38, No. 12, 1996, pp. 2083-2090. 

---