Evaluating Cathodic Protection Reliability on Steel Pipe in Microbially Active Soils

Cathodic protection (CP) has been recognized as the most effective method of preventing corrosion of coated-steel pipelines in soils. However, evaluation of CP reliability on steel pipe in microbially active soils has not yet been performed well. Primarily, this has resulted from lack of understanding of bacterial behavior under cathodically protected conditions. The reliability of CP of steel pipe in soils in the presence of bacteria, such as iron bacteria (IB) and sulfate-reducing bacteria (SRB), was evaluated using steel specimens exposed to bacteria-containing soils and potentiostatically polarized in the potential range from –0.65 V vs a copper-copper sulfate (Cu-CuSO₄) electrode to –1.3 $VCu−CuSO4$⁠. In the case of sandy soil containing IB, when the cathodic polarization potential was applied more negatively than the generally accepted protection potential of –0.85 $VCu−CuSO4$⁠, CP was achieved substantially by a decrease in the number of living IB as a result of environmental changes, including a decrease in the redox potential (E_h) and an increase in pH caused by cathodic reaction. In clay containing SRB, when the cathodic polarization potential was applied more negatively than the recommended protection criteria for active SRB clay of –0.95 $VCu−CuSO4$⁠, CP was achieved in the potential range from –0.95 $VCu−CuSO4$ to –1.1 $VCu−CuSO4$⁠. SRB thrived actively, resulting in formation of adherent iron sulfide (FeS) layers on the steel surface. CP was achieved through their temporary protective properties without ruptures. At potentials more negative than –1.1 $VCu−CuSO4$⁠, sufficient CP was achieved through formation of excessive hydroxyl (OH^–) ions generated by enhanced cathodic reaction.