Ultrasonic NDT
High Temperature Corrosion Testing
Published on 27th November 2020
Ultrasonic thickness measurement (UTM) 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 degrees Celsius, where standard equipment and procedures can be used.
However, there are exceptions. In the refining and chemical industry, it is often necessary to check the wall thickness of plant components which may reach temperatures of 60-550 degrees Celsius 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.
MEASUREMENT METHOD
Measurements at high temperatures are not easy to take. Therefore, special care is required not to expose yourself to burns. Operators are accusto- med to wearing suitable protective clothing, whi- le working under conditions of strong heat radia- tion, often in places that are difficult to access. To ensure the safety of the operator and to prevent damage to equipment, measurements are taken at a duty cycle, not continuously.
CHALLENGES
Ultrasonic probe
The ultrasonic probe is naturally the element of the measuring system most exposed to heat. The construction of a standard probe is based on piezoelectric materials and backing materials which cannot withstand a continuous tempera- ture above 60°C. Around the Curie temperature, piezoelectric material becomes depolarized and disbonding starts between the piezo element and the backing material (damper). Due to the varying shrinkage of the materials used to build the trans- ducer under the influence of temperature, other damage (disbonding) may also occur. In all cases, the probe is permanently damaged, so special materials are required to build a high temperature probe. The most important factor in choosing the right ultrasonic probe is therefore the maximum operating time at the working temperature, which guarantees the safety of the probe.
Changes in examined materials under high temperatures
Correctly conducting measurements on hot ob- jects also requires knowledge of the changes that occur in the examined material under the influ- ence of temperature. An important factor is the influence of temperature on ultrasonic waves. An increase in temperature causes an increase in the material‘s attenuation, and a decrease in the ul- trasonic wave velocity. The molecules of the ma- terial begin to bounce rapidly in a random motion and as a result, the ultrasound signal is transmit- ted less and less effectively. In carbon steel, the decrease in sound velocity is about one percent for every 100 ºF or 55 ºC. Attenuation, or loss of energy, increases in high temperature materials for the same reason and it becomes more diffi- cult to obtain a strong recognizable signal.
Ultrasonic couplant
Ultrasonic coupling also plays an important role in the measurement process, as it removes the air gap between face of the transducer and the test object. It ensures effective transmission of the ul- trasonic wave to the test material. Standard low temperature couplant will fail in a high tempera- ture environment, as it evaporates quickly, losing its properties. For this purpose, special coupling pastes are used, which, under the influence of high temperature, change their state into fluids and thus allow effective testing.
Due to the inability of the couplant to retain a vi- scous form at high temperatures, contact time for even the best couplant is very short. The sig- nal appears briefly, then diminishes rapidly before disappearing completely. Therefore, a powerful A-scan display device with signal freezing func- tion is required to evaluate the amplitude when contact with the material ceases.
EQUIPMENT
In view of the above challenges, the following se- lection criteria should be applied when selecting measuring equipment (in addition to the require- ments for low temperature measurements):
Ultrasonic Probe
• Lowest possible duty cycle for exact tempe- rature for the application (maximum contact time plus cooling time before next use)
• Probe sensitivity at application temperature (many probes degrade in performance rapidly at high temperature, despite high temperatu- re rating)
Specialized ultrasonic probes offer the possibility of continuous operation up to 350 degrees Cel- sius, as well as significantly extended operation cycles up to 550 degrees. The maximum cooling time between measurements is only 1 minute as the probe housing must only cool to 350 degrees Celsius, which means that the operator does not have to worry about long cooling. This translates to minimum pause between the measurements.
Ultrasonic gauge
Ultrasonic measurement at high temperatures is difficult due to many factors. Therefore, the thick- ness gauge should have the following features:
- Powerful pulser: high power transmitter (400V rectangular transmitter which can pro- vide enough energy to drive the probe to compensate for high attenuation losses)
- High gain and low noise device with digital filters: The received signal, which usually only appears for a short time, needs to be a decent quality for accurate evaluation. Devices which include digital filters have an advantage.
- Display & freeze function: Furthermore, a reasonable size of the display for A-scan eva- luation as well as a freeze function which all- ows to adjust the gates while freeze are a big advantage.
- Maximum operation temperature: Since heat radiation will be an important factor, the maximum operating temperature of the de- vice is also important, as many devices can suddenly switch off during measurements due to battery overheating.
- Dedicated software functions such as auto- matic zeroing of the probe in the air, i.e. com- pensation of the delay line, or compensation of the wave velocity in the material increase the accuracy of the measurements obtained.
Ultrasonic Couplant
The working range of the couplant guarantees that it will not vaporize or boil before the actual measurement can be taken. It will also not solidi- fy too fast (this would create a sound barrier), and when it finally melts, it doesn‘t emit dangerous or noxious vapors. Prior to using the couplant on the test object it is recommended to check the concentration of the crucial elements, to avoid causing local corrosion if residue is not cleaned.
The following factors should be considered when using coupling agents for ultrasonic testing:
- Working range of couplant
- Chemical composition if local corrosion is an issue
- Stable transmission factor at high temperatu- res
- Vapor, smokes when melting
- Easy to clean and apply
MEASUREMENT PROCEDURE
To describe the measurement procedure, it is hel- pful 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 ab- sence of speed correction the result will be artifi- cially 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 ca- libration 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 tempera- ture 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 attenua- tion 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 ul- trasonic 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 not heat up uniformly and immediately upon contact. To avoid this error the best practice is to keep the probe on the test object until it reaches same temperature. With the TS5H probe this can be done up to 350°C; above this temperature, the error can be minimized by frequently performing the zero in air procedure in the device.
Applying the couplant
As mentioned earlier, the time of contact bet- ween the couplant and the object is usually short, so the correct procedure is to apply it directly to the probe`s face before each measurement, in- stead of using it directly on hot object. When cou- plant is applied on the face it is recommended to press the probe firmly to the test object without grinding or twisting, in order to maintain a decent layer of couplant underneath the probe, thus gua- ranteeing a good transmission.
High temperature corrosion kit for SONOWALL 70 A-/B-Scan ultrasonic gauge
RECOMMENDED EQUIPMENT: SONOWALL 70 A-/B-Scan thickness gauge
SONOTEC developed a high temperature corro- sion kit for SONOWALL 70 A-/B-Scan thickness gauge. The complete kit consists of the following components:
- SONOWALL 70 - High-End A-/B-scan thick- ness gauge with powerful pulser up to 400 V and 60°C ambient operating temperature
- Temperature compensation - Function for au- tomatic adjustment of the sound velocity of hot test objects
- Heavy Duty impact protector set - for maxi- mum protection against shocks and drops
- 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 and export function to Excel
Contact and support
SONOTEC GmbH | Nauendorfer Str. 2 | 06112 Halle (Saale), Germany
+49 (0)345 / 133 17- | sonotec@sonotec.de | www.sonotec.eu
