Thickness Gage Tutorial

Ultrasonic Gaging - Corrosion

Corrosion survey

Just about anything that is made of common structural metals can be subject to corrosion. A particularly important problem that faces many industries is measurement of remaining wall thickness in pipes, tubes, or tanks that may be corroded on the inside surface. Such corrosion is often not detectable by visual inspection without cutting or disassembling the pipe or tank. Structural steel beams, particularly bridge supports and steel pilings, are also subject to corrosion that reduces the original thickness of the metal. If undetected over a period of time, corrosion will weaken walls and possibly lead to dangerous structural failures. Both safety and economic considerations require that metal pipes, tanks, or structures that are subject to corrosion be inspected on a regular basis. Ultrasonic testing is a widely accepted nondestructive method for performing this inspection, and ultrasonic testing of corroded metal is usually done with dual element transducers and dedicated corrosion gages.

The irregular surfaces that are frequently encountered in corrosion situations give duals an advantage over single element transducers. Dual element transducers incorporate separate transmitting and receiving elements, mounted on delay lines that are usually cut at an angle to the horizontal plane (the roof angle), so that the transmitting and receiving beam paths cross beneath the surface of the test piece. This crossed-beam design of duals provides a pseudo - focusing effect that optimizes measurement of minimum wall thickness in corrosion applications. Duals will be more sensitive than single element transducers to echoes from the base of pits that represent minimum remaining wall thickness. Also, duals may often be used more effectively on rough outside surfaces. Couplant trapped in pockets on rough sound entry surfaces can produce long, ringing surface echoes that interfere with the thin material resolution of single element transducers. With a dual, the receiver element is unlikely to pick up this false echo. Finally, most duals can make high temperature measurements that would damage single element contact transducers.

All gages designed for corrosion applications will measure the roundtrip transit time interval to the first backwall echo. Advanced instruments can also measure the interval between successive multiple echoes. This technique can be very useful for measuring metal thickness only in situations involving thick paint or similar coatings, however echo-to-echo measurement can be less effective at detecting pitting and measuring true minimum thickness of pitted pipe or tank walls.

For more detailed information, go to corrosion testing applications

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