Cavitation Erosion Testing
Why Use Cavitation Erosion Testing?
Ongoing erosion or corrosion can require a regular replacement of parts or the renewal of surface coatings. Material surface erosion due to mechanical or chemical influences is a slow process resulting in the gradual destruction of material surfaces. Therefore, the evaluation of the material erosion resistance or of the erosion effect of liquids and slurries, can be a very time consuming process.
Ultrasonic cavitation erosion testing exposes the material surface to controlled, intense, repeating stress cycles. This results in a significant erosion of the material surface in a short time. You can quickly measure the erosion resistance for regular quality control in production, for evaluation of incoming materials or during research and development.
Standard applications include metallurgical testing, coating formulation testing, coating application testing or the evaluation of erosion inhibitors in liquids.
Why Does Cavitation Cause Surface Erosion?
Ultrasonic devices, such as the UP400St (400 watts, 24kHz) or the UIP1000hdT (1000 watts, 20kHz) couple ultrasonic vibrations into liquids, such as water. The fast reciprocal movement of the vibration in the liquid produces and collapses cavitation bubbles. When the bubbles collapse, high localized mechanical stress occurs in the liquid and on exposed material surfaces. Liquid jets of up to 1000km/h and local pressures of up to 1000atm lead to rapid fatigue on the material surface. This can remove oxide or passivation layers, coatings or fouling. It can cause pitting of solid materials, such as steel, titanium, aluminum, plastic or glass. Hence, cavitation erosion testing is a destructive testing method.
How Does Cavitation Erosion Testing Work?
Cavitation erosion of material surfaces causes gradual material loss. You can measure the material loss easily by weighing the material on a precision scale before and after a defined cavitation erosion exposure. A typical weight change for a cavitation erosion test is between 1 and 30mg. For further standardization, you can calculate the volume loss by dividing the weight loss by the material density. The mean penetration depth (MDP) is calculated by dividing the volume loss by the specimen surface area. Alternatively, you can measure the pitting depth or the displaced volume. You can use microscopic analysis to gain additional qualitative information about the erosion pattern.
When you use a Hielscher ultrasonic device for the cavitation erosion testing, you can preset the temperature range and pressure range that you want to work at. You can adjust the sonication amplitude. All parameters are monitored, displayed and protocoled to an SD-card. You do not need any proprietary software installation. If you like, you can control and monitor the ultrasonic process from your regular web-browser, if you connect the ultrasonic device to your computer via the ethernet cable (included).
What Is The ASTM G32 Standard Method For Cavitation Erosion Using A Vibratory Apparatus?
The ASTM G32-16 standard describes a standardized method for cavitation erosion. It defines a simple, controllable and reproducible test to quantify and compare the cavitation erosion resistance of different materials. The ATSM G32-16 specifications are useful for comparison of your results with that of other publications. If you would like to implement cavitation erosion testing in quality control, we do recommend to adapt the cavitation erosion test protocol to your specific requirements. We will be glad to assist you with the design of a customized cavitation erosion test protocol. For more information on cavitation erosion testing in accordance with ASTM-G32, please click here!
Why Should I Use An Energy Limit Instead of A Time Limit?
Many publications and erosion test protocols specify a cavitation exposure time. In Hielscher ultrasonic devices, you can preset a sonication time and the system will stop after this time has passed. You can then calculate the resulting cavitation erosion rate in mm/hr or mm3/hr. A time limit is acceptable, only if you do not change any parameters, such as liquid level, amplitude, pressure, temperature, liquid composition or gap between sonotrode and material surface. If any of these parameters change, so will the power of sonication and the intensity of the cavitation. It is important, that the actual net power delivered to the liquid must not fluctuate during the duration of the test.
In Hielscher ultrasonic devices you can set an energy limit. In this case, the ultrasonic device will stop, after it delivered the specified ultrasonic energy. The Hielscher device will display and record parameters, such as actual net power, amplitude, pressure and liquid temperature. Fluctuations in power or deliberate changes in parameters will be compensated when using an energy limit. You can then specify the resulting cavitation erosion rate in mm/kWhr, mm3/kWhr or mg/kWhr.
If you weigh the specimen between cavitation erosion intervals, you can generate curve showing the marginal weight loss (weight loss rate in each energy interval) over the cumulative energy.
For more precise results, the device can perform an automated calibration (30 seconds). This measures the power for all amplitude setting in air at ambient pressure. The Hielscher device uses this calibration data to give very precise net power values in real-time.
What Influences Cavitation Erosion?
Ultrasonic cavitation results in cavitation erosion. The more intense the ultrasonic cavitation is, the faster is the erosion. A more intense cavitation may erode material surfaces, that a very soft cavitation cannot erode at all. So there may be a minimum intensity required for your material to be erosion tested.
The vibration amplitude is the most important parameter for the sonication intensity and the resulting cavitation intensity. Higher amplitudes produce a more intense cavitation. In ultrasonics, the amplitude is specified in micron as peak-peak. Hielscher ultrasonic devices allow you to adjust the amplitude in a wide range. Once adjusted, the device keeps the amplitude at the adjusted level under all load conditions. This is an important feature in order to have controllable and repeatable cavitation testing conditions.
Hielscher Ultrasonic devices allow you to perform cavitational erosion testing at amplitudes from as little as 2 micron to 200 micron or more.
Liquid Pressure During Sonication
Many standard protocols for cavitation erosion testing use ultrasonic cavitation at ambient pressure. Liquid pressure is the second most important factor for the sonication intensity. A 10% increase in ambient pressure will increase the sonication intensity by about 10%. More intense cavitation reduces the time needed to achieve a certain degree of cavitation erosion. Often single specimen test can take somewhere from 15 to 120 minutes. If you have many specimens to test, working at higher pressures can cut the time for each test significantly. Tests at 5 barg (73psig) require approx. 80% less time for each test.
Hielscher supplies pressure-tight test cells with a digital pressure sensor for cavitation erosion testing. Using a pressure-tight cell, you can control and maintain the pressure during each test. The ultrasonic generator monitors the pressure sensor constantly and protocols the actual pressure to an Excel-compatible CSV-file on an SD card (included). Hielscher supplies pressure regulators to set and maintain the operating pressure.
As a standard Hielscher pressure-tight test cells for cavitation erosion testing are rated for up tp 5barg (73psig). Higher pressures of up to 300barg (4350psig) are available on request.
In general, cavitation erosion testing uses low frequency high intensity ultrasonics in the 18-30kHz range. In this range the variation of the frequency has very limited effect on the cavitation intensity. All Hielscher devices work at a constant frequency.
Distance From the Sonotrode
The material to be tested can be mounted to the sonotrode or under the sonotrode. You can make a threaded material specimen and mount it to the end of the ultrasonic sonotrode. In this case, the specimen vibrates at the specified ultrasonic amplitude and produces cavitation on its surface. This requires precision machining and not all materials are suitable for this option.
Alternatively, you can fixate a part or specimen in close proximity under a titanium sonotrode. In this case, the titanium sonotrode produces the cavitation and the material surface is exposed to the cavitation. This is the more convenient option as you can place specimen of various sizes or shapes in the test cell. If you use a larger sonotrode, such as a 50mm or 80mm diameter sonotrode, you can expose multiple parts to cavitation erosion at the same time. This is very useful when you have to test many parts per day, e.g. for quality control.
In both cases, the distance between the ultrasonic sonotrode and the material surface next to it is very important. In general, the cavitation erosion is faster when using a smaller distance. Typical distances range from 0.2 to 15mm. For conclusive results, you should use the same distance for all tests.
Warmer liquid result in a lower ultrasonic cavitation intensity. The input of mechanical vibration energy into the liquid will cause the liquid to heat up. In order to maintain a constant temperature during each cavitation erosion test, the liquid needs to be cooled. Hielscher supplies jacketed containers and jacketed pressure-tight cells. Alternatively you can use a cooling coil in a beaker or you can put the beaker in an ice-bath. A coolant that runs through the jacket or through the cooling coil removes heat from the liquid.
Hielscher ultrasonic devices, such as the UP400St or the UIP1000hdT come with a PT100 temperature probe (included). The ultrasonic generator monitors the actual liquid temperature continuously and protocols the temperature to an Excel-compatible CSV-file on an SD card (included). You can set the generator to pause the cavitation erosion testing should the liquid temperature deviate too much from your set-point, e.g. due to insufficient cooling capacity. The generator can resume the sonication automatically when the liquid reached the specified temperature again.
In general cavitation erosion testing uses water, such as distilled water. Different liquids show different cavitation characteristics. If water is corrosive to your material, you may want to test alternative liquids, such as low viscosity silicone oils or organic solvents in order to eliminate or reduce the corrosive factor. Alternatively, you can make the liquid more corrosive, e.g. by changing the pH or more abrasive by adding abrasive particles. You can use cavitation erosion testing to evaluate the erosiveness and corrosiveness of liquids, such as drilling muds or to evaluate the effectiveness of corrosion or erosion inhibitors.
When you manufacture a part or a specimen, CNC machining, grinding or polishing cause damages to the grain structure near to the material surface. This reduces the erosion resistance.
Very often erosion and corrosion happen at the same time. Water, such as distilled, demineralized or de-ionized water can be corrosive to many materials. Ultrasonic cavitation promotes corrosion. Passivation layers, e.g. at anodized aluminium will increase resistance of a material surface to erosion and corrosion.
What Limitations does Cavitation Erosion Testing Have?
Some elastomers may require very intense cavitation exposure to show any cavitation erosion at all. In this case, sonication without a pressurized cell may not show any measurable effect.