Cathodic protection is being applied to buried steel pipelines and polarized potential of pipelines is kept less noble than the protection potential criterion,-0.85 V vs. Cu/CuSO 4. However, instantaneous anodic current has been infrequently observed in some fields even though polarized potential of pipelines is kept less noble than the protection potential criterion. In this study, the influence of instantaneous anodic current on corrosion protection of buried steel pipelines under cathodic protection was evaluated and the oxidation reaction causing anodic current was investigated to some extent. As a result, it was found that most of the anodic current was not attributed to corrosion reaction and the maximum value of corrosion rate of steel was as little as 1.8 m/year. Therefore, it was considered that there was no matter on buried steel pipelines even though the instantaneous anodic current occurred under the protection potential criterion. It was estimated that some part of the anodic current was attributed to the oxidation reactions of components in soil and of rust on the steel surface.

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Instantaneously noble potential caused by DC stray current has been observed in rare cases under cathodic protection of pipelines in service. Moreover, anodic current was simultaneously observed. In this study, the influence of instantaneously noble potential on corrosion protection of buried steel pipes under cathodic protection was studied in three kinds of soil by using IR-free probe. Furthermore, the relationship between corrosion weight loss of the steel and total electric quantity of anodic current caused by the noble potential was investigated. Results of this study indicate that the corrosion caused by noble potential below-0.75 V (vs. Cu/CuSO4 (sat.)) is negligibly small. Moreover, the results also indicate that the percentage of corrosion current to the anodic current is no more than 1%; most of the anodic current is estimated to be contributed by repeated oxidation reaction of iron rust on the steel surface.