High Temperature Corrosion Testing
Thickness Measurements up to 550 °C
Ultrasonic thickness measurement is a common industrial method used to measure the thickness of solid materials, with access only from one side of the test object. Its main field of application is to check the remaining wall thickness of objects which are subject to corrosion or erosion, like piping or vessels. Most measurements are made at temperatures up to 60 °C, where standard equipment and procedures can be used.
In the refining and chemical industry, it is often necessary to check the wall thickness of plant components which may reach temperatures of 60 °C to 550 °C and cannot be cooled down during corrosion measurement. Temperature is also one of the factors that accelerates the corrosion process, so the measurement must be performed more frequently, compared to colder objects. Many refineries and chemical plants carry out regular spot checks of remaining wall thickness in order to properly assess the corrosion rate and predict necessary replacements. These measurements are typically performed manually, using specially designed equipment and modified test techniques.
To describe the measurement procedure, it is helpful to analyze possible sources of error during the measurement:
Reduction of the speed of the ultrasonic wave as the temperature increases
In carbon steel, it is estimated at around 1% per 55° Celsius. This means that the return time of the ultrasonic echo will increase and in the absence of speed correction the result will be artificially high. This error increases with the thickness of the test material and the temperature increase. To avoid this error, it is important to perform a calibration procedure using a test block of the same material and temperature as the tested material. The best practice is to heat the calibration block on the measurement spot until it reach the same temperature. If this is not possible, appropriate sound velocity correction should be applied. It is most convenient to do this in the software of the device because it provides immediate feedback to the operator for correct evaluation.
Increased material attenuation as the temperature increases
In carbon steel, it is estimated that attenuation of a 5MHz longitudinal wave at room temperature is 2dB per 100mm distance. This mean that for ultrasonic measurement, which is based on returned echo, a correction of 4dB per 100mm has to be done (round trip). At 500°C the attenuation is as much as 30dB (approximately 20 times higher). Significant gain adjustment is needed to obtain a decent signal.
Shifting of the probe zero with temperature change
When in contact with a hot object, the probe`s delay line heats too. The time of flight of the ultrasonic wave between the piezoelectric material and the probe‘s face is longer. Thickness readings
become artificially high. This error is difficult to correct due to the lack of precise information on the temperature of the delay line, as it does notheat up uniformly and immediately upon contact.
SONOTEC offers a high temperature corrosion kit for the SONOWALL 70 A-/B-Scan thickness gauge. The complete kit consists of the following components:
- SONOWALL 70 - High-End A-/B-scan thickness gauge with powerful pulser up to 400 V and 60°C ambient operating temperature
- Temperature compensation - Function for automatic adjustment of the sound velocity of hot test objects
- SONSOCAN TS5H ultrasonic probe – for precise high temperature measurements up to 550°C and 350°C with no duty cycle, including armored cable and handle extension
- SONOGRID Corrosion Management Software - for efficient collection of linear/2D/3D matrix data
- Heavy Duty impact protector set - for maximum protection against shocks and drops
Do you have any questions?
Feel free to contact us! We will be happy to help you.
