Study Of Alternating Current Interference On Cathodic Protection System Of Buried Pipeline

The dramatic development of buried oil & gas pipelines, high voltage alternating current (AC) transmission lines and AC electrified railway networks worsens the AC interference issue of the buried pipelines. Globally reported cases indicate that the traditional cathodic protection (CP) criterion "-0.85 VCSE (with respect to copper/copper sulfate reference electrode)" is no longer effective in the presence of AC, with the inefficacy mechanism remaining unclear. Besides, the selection of CP operation parameters under AC has not reached a consensus in the industry. In this work, the effects of AC on carbon steel CP system and the influencing mechanisms were investigated by combining laboratory experiments and theoretical analysis. The main conclusions reached are listed as follows:(1) The effect of AC on CP potential of carbon steel was studied by running simulative bench tests. The results showed that, the CP potential of carbon steel shifted toward the negative direction instantaneously at the moment AC was applied and gradually shifted toward the positive direction reaching a steady-state value ultimately when the applied CP level was relatively low. Comparatively, however, the CP potential shifted positively all the way through to a steady-state value in the application of AC under high CP level. Based on the AC oscillation theory and the polarization behavior of carbon steel, the CP potential shifting mechanism of carbon steel under AC application was proposed.(2) Corrosion behavior of cathodically protected carbon steel under AC interference was experimentally investigated. A criterion for assessing the risk of AC corrosion was proposed by taking CP potential, AC current density and the ratio of AC/DC current density as parameters that were all proven to relate to corrosion rate.(3) The electrolytic pH value played a significant role in AC corrosion of carbon steel. Without AC applied, passivation was observed on carbon steel when the pH value reached 12 or greater, and as a result, the corrosion rate was reduced dramatically. The application of AC, however, destroyed the passive film and hence increased the corrosion rate. Based on the test results, the mechanisms of AC corrosion of carbon steel in alkaline electrolyte or under overprotection level of CP were discussed. By additionally performing AC/DC unfiltered real-time potential measurements, the AC corrosion mechanisms of carbon steel in near-neutral electrolyte or under low CP level were also proposed.(4) The effects of AC on corrosion rate and polarization potential of carbon steel coupled with a dissimilar metal were investigated for magnesium alloy, zinc alloy and copper. The results showed that, the corrosion rate of carbon steel increased with AC when it was coupled with magnesium alloy or copper, while reduced with AC when coupled with zinc alloy. Meantime, it was found that the presence of AC caused the numerical and directional change of coupling current. Based on these observations, the AC corrosion mechanism of carbon steel in the presence of DC coupling was obtained.(5) The effect of AC on the performance of magnesium alloy sacrificial anode was studied. It was found that, the working potential of magnesium anode shifted positively under AC application and its offset, as well as the dissolution rate, increased with increasing AC. Polarity reversal occurred to magnesium anode immediately when the applied AC current density increased to 100 A/m2. Descriptively, the polarity reversal of magnesium anode disappeared after 24 hours of exposure to 100 A/m2 AC, while such phenomenon continued throughout the test duration of 96 hours when the AC current density reached 200 and 300 A/m2. Based on the variations of polarity and film effects of magnesium anode under AC conditions, the polarity reversal was mechanistically explained.(6) The effects of AC on the corrosion rate and surface film structure of zinc alloy substrate were experimentally studied. The results showed that the dissolution rate of zinc alloy increased with increasing AC current density. On such a basis, a calculation method was presented for predicting the corrosion rate of zinc alloy under AC condition. Besides, the AC corrosion mechanism of zinc alloy was discussed based on the electrochemical performance of the corrosion product film on the zinc substrate surface.