Polarization methods pipeline

The most frequently used materials for buried metal structures are the carbon steels. For prevention of their corrosion the most recommended, economical, and effective method is cathodic protection (CP). The use of CP is now standard procedm-e for long-term corrosion protection of imderground pipelines, oil and gasoline tanks, and other structures. With a shift of the metal potential to more of a negative value of -0.85 V versus a C11/CUSO4 reference electrode, it is possible to make the metal surface a cathode, which ensures an immune (no corrosion) state of the carbon steel. Cathodic polarization is achieved by direct current, which can be supplied either by sacrificial anodes in galvanic contact with the steel structure, or by impressed current from a rectifier. 

One method of getting around these problems is to use a small working electrode coupon connected to the pipe. The size of the electrode is based on a reasonable estimate of the holiday sizes on the pipe. The polarization curve is obtained by observing the potential on the known electrode. Such data must be evaluated carefully, since it represents only one point. On a long pipeline, several test locations should be evaluated. For more details on electrochemical polarization, the reader is advised to consult Refs 39 to 41 and 42. 

The DCG V method enables the detection of defects in the insulation by the determination of zones of inflow and outflow of polarizing current. A potential gradient is measured in the ground with a very sensitive voltmeter and two CSE electrodes placed on both sides of the investigated pipeline at distances from each other of 1-2 m. Defects can be localized with an accuracy of 10-15 cm on pipelines placed at a depth of up to 6 m. The shape and extent of the defects are determined from potential gradient graphs and soil resistivity measurements in the vicinity of the epicentre. Dur-... 

Sometimes joint methods (e.g., CIPS and DCVG) are used. A worker walking along the pipeline records the distance as well as the on/off potential changes at small intervals of, e.g., 0.5 m, vs. a close CSE reference electrode. Measurements are supplemented with potential ON/OFF gradients in one or two directions perpendicular to the pipeline. The method requires synchronous switching off of all DC current sources polarizing the tested pipeline section. These types of measurements allow determination of the effectiveness of CP and detection of insulation leakiness (places of increased potential). Polaketal. (1997) point out that the applied methods of detection of insulation... 

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