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Tips for Improving High-Temperature Corrosion Thickness Measurements

Ultrasonic thickness gauging isn’t limited to testing materials at normal (ambient) temperatures. Measurements can be made on materials whose surface temperature approaches 932 °F (500 °C). When working with metals, there may be times when the thickness needs to be measured during operations at elevated temperatures. Oil refineries have requirements for periodic thickness measurements of high-temperature process piping, vessels, and tanks (primarily alloyed steel) using ultrasonic testing (UT) to monitor corrosion and other defects that can cause equipment failure. The refining process starts with heating crude oil and putting it into a distillation column (also known as a still). In the still, diesel, gas, and other products boil off and are extracted at different temperatures. The distillation temperature for petroleum products ranges from as low as 85 °F (29 °C) to greater than 1050 °F (566 °C). These high temperatures make inspecting refinery equipment challenging. Since the refining process runs continuously, the machinery can’t be cooled down for thickness inspections without shutting down the entire process. Heat can complicate the accuracy and efficiency of measurements. If the wrong transducer is used, the heat can damage it and shorten its useful life; many can only tolerate temperatures up to about 125 °F (52 °C). At higher temperatures, standard transducers will eventually suffer permanent damage due to internal disbonding caused by thermal expansion. Changes in temperature also affect sound velocity and attenuation in materials, and these factors need to be accounted for when taking measurements. Because sound velocity decreases as temperature increases, thickness measurements taken at a higher temperature than that used for calibration will result in thickness values that are larger than the actual part thickness. This can lead to a false sense of security when taking safety-critical thickness measurements. For this reason, it is essential to use correctly calibrated equipment when taking high-temperature thickness measurements. The following tips will help users overcome the challenges of testing materials at elevated temperatures.


ASTM, 2001, ASTM E797 – 95, Standard Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo Contact Method, ASTM International, West Conshohocken, PA.

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