Application: Use of dual element transducers ("duals") for measurement of remaining metal thickness in corrosion applications.

Problem: 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.

Theory Of Operation: It is the irregular surfaces that are frequently encountered in corrosion situations that give duals an advantage over single element transducers. All ultrasonic gauging involves timing the round trip of a sound pulse in a test material. Because solid metal has an acoustic impedance that differs from that of gasses, liquids, or corrosion products such as scale or rust, the sound pulse will reflect from the far surface of the remaining metal. The test instrument is programmed with the velocity of sound in the test material, and computes the wall thickness from the simple formula Distance = (Velocity) X (Time).

Most gauges designed for corrosion applications measure the round trip transit time interval to the first backwall echo. Some instruments can also measure the interval between successive multiple echoes. This technique can be very useful 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.

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 interface echoes that interfere with the near surface resolution of single element transducers. With a dual, the receiver element is unlikely to pick up this false echo. Finally, duals may be designed for high temperature measurements that would damage single element contact transducers.

Equipment: A number of small, hand-held microprocessor-based ultrasonic thickness gauges have been specifically designed for corrosion survey applications. Typically these gages will be used with a dedicated group of dual element transducers, covering various thickness ranges and temperature conditions. In some critical applications, especially at elevated temperatures, a user may require an ultrasonic waveform display to help verify that valid echoes are being detected. The Model 38DL PLUS is an advanced hand-held corrosion gauges with waveform display that is designed for such cases. For full details, see the Model 38DL PLUS datasheet. If a waveform display is not required, other instruments designed for this type of testing include the compact and simple Model MG2 Series gages. In corrosion applications, duals can be used effectively with flaw detectors as well. Digital flaw detectors such as the Panametrics-NDT EPOCH series (EPOCH XT, EPOCH LTC, EPOCH 600, and EPOCH 1000) provide a thickness measurement as well as a waveform display.

Procedure: The following general principles apply to all corrosion measurements with dual element transducers, whether used with a portable gauge such as the 38DL PLUS or a flaw detector. Keep in mind that in all cases the instrument must be properly calibrated for sound velocity and zero offset in accordance with the procedure found in the instrument's operating manual.
1. Transducer Selection: For any ultrasonic measurement system (transducer plus thickness gauge or flaw detector) there will be a minimum material thickness below which valid measurements will not be possible. Normally this minimum range will be specified in the manufacturer's literature. As transducer frequency increases, the minimum measurable thickness decreases. In corrosion applications, where minimum remaining wall thickness is normally the parameter to the measured, it is particularly important to be aware of the specified range of the transducer being used. If a dual is used to measure a test piece that is below its designed minimum range, the gauge may detect invalid echoes and display an incorrectly high thickness reading. The table below list approximate minimum measurable thickness in steel for the standard transducers used with the Panametrics-NDT Model MG2, MG2-XT, MG2-DL and 38DL PLUS gauges. Note that these numbers are approximate. The exact measurable minimum in a given application depends on material velocity, surface condition, and geometry, and it should be determined experimentally by the use.

In selecting a transducer for a corrosion application it is also necessary to consider the temperature of the material to be measured. Not all duals are designed for high temperature measurements. The chart above lists recommended temperature ranges for the Panametrics-NDT duals used with the 38DL PLUS or MG2 Series gauges. For other transducers, consult the manufacturer's catalog or data sheets. Using a transducer on materials whose temperature is beyond the specified range can damage or destroy the transducer.

2. Surface Condition: Loose or flaking scale, rust, corrosion or dirt on the outside surface of a test piece will interfere with the coupling of sound energy from the transducer into the test material. Thus, any loose debris of this sort should be cleaned from the specimen with a wire brush or file before measurements are attempted. Generally it is possible to make corrosion measurements through thin layers of rust, as long as the rust is smooth and well bonded to the metal below. Some very rough cast or corroded surfaces may have to be filed or sanded smooth in order to insure proper sound coupling. It may also be necessary to remove paint if it has been applied in thick coats, or if it is flaking off the metal. While it is often possible to make standard corrosion measurements through thin coats of paint (on the order of a few thousandths of an inch or 0.1 - 0.2mm), thick paint will attenuate signals or possibly create false echoes, and require special techniques such as echo-to-echo or ThruCoat measurement. Severe pitting on the outside surface of a pipe or tank can be a problem. On some rough surfaces, the use of a gel or grease rather than a liquid couplant will help transmit sound energy into the test piece. In extreme cases it will be necessary to file or grind the surface sufficiently flat to permit contact with the face of the transducer. In applications where deep pitting occurs on the outside of a pipe or tank it is usually necessary to measure remaining metal thickness from the base of the pits to the inside wall. There are sophisticated ultrasonic techniques utilizing focused immersion transducers that can measure directly form the base of the pit to the inside wall, but this is generally not practical for field work. The conventional technique is to measure un-pitted metal thickness ultrasonically, measure pit depth mechanically, and subtract the pit depth from the measured wall thickness. Alternately, one can file or grind the surface down to the base of the pits and measure normally. As with any difficult application, experimentation with actual part samples is the best way to determine the limits of a particular gage/transducer combination on a given surface.

3. Transducer Positioning/Alignment: For proper sound coupling the transducer must be pressed firmly against the test surface. On small diameter cylindrical surfaces such as pipes, hold the transducer so that the sound barrier material visible on the probe face is aligned perpendicular to the center axis of the pipe. See the illustration below. While firm hand pressure on the transducer is necessary for good readings, the probe should never be scraped along or twisted against a rough metal surface. This will scratch the face of the transducer and eventually degrade performance. The safest technique for moving a transducer along a rough surface is to pick it up and reposition it for each measurement, not to slide it along. Remember that an ultrasonic test measures thickness at only one point within the beam of the transducer, and that in corrosion situations wall thicknesses often vary considerably. Test procedures usually call for making a number of measurements within a defined area and establishing a minimum and/or average thickness. Ideally, data should be taken at increments no greater than half the diameter of the transducer, to insure that no pits or other local variations in wall thickness are missed. It is up to the user to define a pattern of data collection appropriate to the needs of a given application. It is possible that on some severely corroded or pitted materials there will be spots where readings cannot be obtained. This can happen when the inside surface of the material is so irregular that the sound energy is scattered rather than being reflected back to the transducer. Lack of a reading may also indicate a thickness outside the range of the transducer and instrument being used. Generally, an inability to obtain a valid thickness reading at a particular point on a test specimen could be a sign of a seriously degraded wall which may warrant investigation by other means.

4. High temperature Measurements: Corrosion measurements at elevated temperatures require special consideration. Keep in mind the following points:
• Be sure that the surface temperature of the test piece does not exceed the maximum specified temperature for the transducer and couplant that you are using. Some duals are designed for room temperature measurements only.
• Use a couplant rated for the temperature where you will be working. All high temperature couplants will boil off at some temperature, leaving a hard residue that is not able to transmit sound energy. Panametrics-NDT Couplant E-2A (Thermotemp) can be used at temperatures up to 900 deg F/470 deg C, although it will harden as the upper limit is reached. Maximum recommended temperatures for our couplants are as follows:

• Make measurements quickly and allow the transducer body to cool between reading. High temperature duals have delay lines made of thermally tolerant material, but with continuous exposure to very high temperatures the inside of the probe will heat to a point where bonds will fail, destroying the transducer.
• Remember that both material sound velocity and transducer zero offset will change with temperature. For maximum accuracy at high temperatures, velocity calibration should be performed using a section of the test bar of known thickness heated to the temperature where measurements are to be performed. The Panametrics-NDT MG2 Series gauges and the 38DL PLUS have a semi-automatic zero function that can be employed to adjust zero setting at high temperatures. See the instrument operating manual for details. For other gauges and flaw detectors, see their operating manual for information on how to compensate for zero drift at elevated temperatures.

5. Gages and Flaw Detectors: An ultrasonic corrosion gauge is designed to detect and measure echoes reflected from the inside wall of a test piece. It is possible that material discontinuities such as flaws, cracks, voids, or laminations may produce echoes of sufficient amplitude to trigger the gauge, showing up as unusually thin measurements at particular spots on a test piece. However, a corrosion gauge is not designed for flaw or crack detection, and cannot be relied upon to detect material discontinuities. A proper evaluation of material discontinuities requires an ultrasonic flaw detector such as the Panametrics-NDT Epoch XT used by a properly trained operator. In general, any unexplained readings by a corrosion gauge merit further testing with a flaw detector.