Volume 17, Number 1, March 2012
Researchers at NRC are investigating the use of capacitive imaging sensors for determining the extent of corrosion on metallic pipe.
Corrosion is the main cause of deterioration in buried metallic pipes, typically showing up in the form of localized pits. Detection and quantification of this pitting corrosion is required to understand the condition of the pipe and its remaining service life. Capacitive imaging technology has the potential to provide a cost-effective way to acquire this information under harsh field conditions.
This non-contact and non-invasive technique was applied by researchers at the University of Warwick, U.K. to non-destructive inspections of objects made of various materials. A pair of capacitive electrodes is used to generate an electrostatic field within an object. The type of material and the geometry of the object determine its dielectric properties, which in turn influence the electrostatic field. Changes in material properties due to degradation, as well as the presence of defects and cracks, can be detected by moving the electrodes over the surface of the object. The penetration depth of the electrostatic field into the material is determined by the material’s dielectric properties as well as the sensor shape, configuration (Figure 1) and the frequency of the applied electrical field.
In laboratory experiments, NRC researchers used the capacitive imaging sensor to detect pits on a sandblasted section of ductile iron pipe. This pipe is heavily corroded in the area indicated by the arrow (Figure 2). For comparison purposes, the researchers also scanned the pipe with a laser scanner. The corrosion on the pipe was detected by both sensors and is displayed in Figure 3 in the area marked with arrows. From the two images, it can be seen that the laser technology is capable of highly detailed scans, better than those obtained with the capacitive sensor. However, the laser scanner is not practical for field use because of its size, and the necessity of removing a pipe from the ground for analysis.
Because the capacitive imaging technology has the potential to be used in the field, researchers are working to enhance its effectiveness. The next research phase will focus on improving the sensor design, identifying optimal operational parameters, and automating the signal processing to enable rapid in-situ detection and quantification of pitting corrosion on buried pipes, without the need for cleaning and sandblasting.
This technology is not limited to metallic pipes. In fact, in the scanning of metal objects, only surface defects are likely to be detected; in non-metallic objects the technique has the potential to detect sub-surface defects as well.
These NRC studies are intended to advance a technology that may help municipalities obtain critical information for the maintenance and management of their buried water mains.
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