Effect of survey conditions on the reliability of coating flaw detection of buried pipe

摘要

External corrosion control of buried pipe can be achieved by the combination of barrier coating and cathodic protection. Cathodic protection by impressed-current or sacrificial anode methods applied the current flows to coating defects that exposed steel and through the undamaged coating. Coating damage and deterioration can be induced by many reasons; damage during handling and laying, enhanced failure at low temperatures, failure during commissioning and operation, disbanding due to inadequate surface cleaning, rock penetration during installation and service, lack of coating integrity at elevated temperature, disbanding through pipe movement and lack of adhesion etc. Therefore, several electrical surveys are used to examine the performance of both coating and cathodic protection detection where corrosion occurs. Generally, indirect assessment of external corrosion of pipe has been undertaken as a 2 step process using a close interval potential survey (CIPS) performed to determine the level of cathodic polarization and a direct current voltage gradient (DCVG) survey to determine the location of coating defects. Usually, two techniques are used separately, but there are several advantages to undertaking a combined CIPS and DCVG survey. Both surveys are performed at the same time by the same surveyors, under the same climatic and soil conditions. A further advantage can be obtained by two surveyors walking over the pipeline. However, in a power plant, the earth current measured between two reference electrodes placed on the ground will only indicate the relative magnitude of the DC moving in the ground between the two electrode positions. Where this current is going to, or coming from can't be determined. To solve these limitations, the area potential and earth current (APEC) survey was recently proposed. This method uses 3 reference electrodes. Three kinds of survey method have an advantage and a disadvantage. Therefore, this work focused the effect of survey conditions on the reliability of coating flow detection of buried pipes. 3 kinds of test bed were constructed, and cathodic protection system was real-time monitored and controlled via the internet or mobile phone. The materials of buried pipe were 'polyken' coated steel pipe, coal-tar coated cast iron pipe, and pre-stressed concrete cylinder pipe (PCCP). The buried depths of pipes were from 1 to 3 meters depending the design purpose. In every pipe, coating flaws were intentionally formed from 1 to several tens square centimeters. The surveyed results were discussed to enhance the reliability of coating flaw detection by the following factors; the buried depth, the size of coating flaw, the materials of buried pipe, the interference of other rectifiers, the direction of survey, and the temperature of the environment.