Practical Cavitation Erosion Testing of Marine Bronze Coatings

Cavitation erosion testing is most useful when it connects a controlled laboratory exposure to a real engineering problem. A practical example is the evaluation of cavitation-resistant bronze coatings for maritime components such as ship rudders and propellers. These parts operate in zones where local pressure fluctuations can generate vapor bubbles that collapse near the surface, creating repeated high-intensity impact loads. Over time, this produces pitting, fatigue damage, coating failure, and material loss.

Cavitation Erosion Test of Bronze Coatings

In the study by Hauer et al., bronze coatings produced by cold spraying, warm spraying, HVOF spraying, and arc spraying were compared against cast nickel aluminum bronze and shipbuilding steel. The central question was simple: which coating process can produce a bronze surface that survives cavitation exposure long enough for marine service? To answer it, the researchers used a cavitation erosion test based on ASTM G32-16 with a vibratory apparatus, including a Hielscher UIP1000hdT ultrasonic vibratory system as test system.

Precise Control of Test Conditions and Automated Data Recording

The sonicator UIP1000hdT is well suited for this type of test because it delivers high-intensity, low-frequency ultrasound in the range used for cavitation erosion testing. The cavitation erosion test setup using the 1000-watt sonicator operates at 20 kHz, and allows for precise process monitoring, amplitude control, temperature measurement, and automatic protocoling of test data. These functions are important because cavitation intensity depends strongly on amplitude, liquid temperature, liquid pressure, sonotrode geometry, and the distance between the sonotrode and specimen.

(a) Cavitation erosion test according to ASTM G32-16 with sonicator UIP1000hd (indirect method). All test parameters are nominal values; the tolerances are listed in the standard.
(b) Schematic phases in the erosion-time curve and characteristic parameters in the test procedure.
Graphics and study: ©Hauer et al., 2021.

Ultrasonic Cavitation Erosion Test of Bronze Coatings

For the marine bronze coating example, the test was performed in the indirect ASTM G32 arrangement. In this configuration, the specimen is not attached to the vibrating horn. Instead, the ultrasonic sonotrode generates cavitation in distilled water, and the coated specimen is fixed beneath the sonotrode at a defined gap. Hauer et al. used a 0.5 mm distance between sample and sonotrode, a frequency of 20 kHz, and a peak-to-peak amplitude of 50 µm. The test liquid was distilled water, held at approximately room temperature, around 25 °C.

Specimen preparation is a critical step. Before cavitation exposure, the coated surfaces were stepwise ground and polished down to a fine diamond abrasive below 4 µm. This reduces the influence of loosely attached particles or surface irregularities that could otherwise detach immediately and distort the erosion curve. The goal is not to make the coating look good, but to create a reproducible starting condition so that measured mass loss reflects cavitation resistance rather than poor surface preparation.

The Procedure of Ultrasonic Cavitation Erosion Testing and its Results

The practical test procedure is straightforward. First, each specimen is cleaned, dried, and weighed on a precision balance. It is then mounted in the test cell beneath the sonotrode BS4d22 of the sonicator UIP1000hdT with the 0.5 mm gap set carefully and repeatably. The sonicator is operated at the defined amplitude and frequency, while the liquid temperature is controlled to prevent heating from changing cavitation intensity. After a defined exposure interval, the specimen is removed, cleaned, dried, and weighed again. This sequence is repeated over increasing, material-dependent exposure intervals until a complete erosion curve is obtained.
The raw measurement is mass loss. For engineering comparison, this mass loss is converted into volume loss using the material density. The volume loss is then divided by the exposed surface area to determine mean erosion depth. From the erosion-depth curve, the researcher can calculate characteristic erosion parameters such as maximum erosion rate, terminal erosion rate, and mean depth of erosion. Hielscher also notes that erosion can be reported as mass, volume, or penetration depth per time or per delivered ultrasonic energy, depending on the chosen protocol.

Mean erosion depths as function of adjusted coating quality parameters n. Powder annealing and thus reduced powder strength enables reaching high coating qualities. The inserts show the surface damage obtained after a cavitation testing time of 100min.
Graphs and study: ©Hauer et al., 2021.

One important lesson from the Hauer study is that early erosion rates can be misleading. Thermally and kinetically sprayed coatings often showed high initial material loss, followed by a lower, more stable erosion rate. For this reason, Hauer et al. used terminal erosion rate as a more representative indicator of long-term coating performance. In their 120-minute comparison, the terminal erosion rate was evaluated mainly from the second half of the test, above 60 minutes, to better capture the stabilized behavior.

The test results show why a controlled vibratory cavitation apparatus is valuable. Cast nickel aluminum bronze achieved a terminal erosion rate of about 0.40 µm/h. Optimized warm-sprayed bronze reached 0.57 µm/h, close to the cast reference. An optimized arc-sprayed coating on shipbuilding steel reached about 1.02 µm/h, while an optimized HVOF coating reached about 1.74 µm/h. Even when these coatings did not fully match cast propeller bronze, they dramatically outperformed shipbuilding steel; the study reports that arc-sprayed and HVOF-sprayed coatings achieved about 26 times and 16 times better cavitation resistance, respectively, than VL-A steel.

Use a Sonicator as Vibratory Apparatus for Your Cavitation Erosion Tests

The practical conclusion is that cavitation erosion testing with the UIP1000hdT sonicator as vibratory apparatus can do more than rank materials. It reveals how coating process, microstructure, oxide content, porosity, interface bonding, and post-treatment affect real erosion behavior. Hauer et al. concluded that HVOF and arc spraying can offer a strong performance-cost compromise for improving steel rudder surfaces, while cold and warm spraying are preferred when cavitation resistance close to bulk nickel aluminum bronze is required.

For laboratories and coating developers, the key to reproducible results is strict control of the test parameters: sonotrode amplitude, frequency, sonotrode-to-sample distance, liquid temperature, liquid chemistry, specimen preparation, weighing intervals, and erosion-rate calculation. With these conditions defined, the Hielscher UIP1000hdT provides a practical and repeatable way to translate ultrasonic cavitation into quantitative coating-performance data.

Foglio di lavoro campione per i test di erosione per cavitazione

 
You can find instructions for Cavitation Erosion Tests here!

ASTM G32 Cavitation Erosion Test Setup

The sonicators UIP500hdT, UIP1000hdT, UIP15000hdT and UIP2000hdT are suitable for ASTM G32 testing. We can supply each of these units with an accurate protocollo di misurazione dell'ampiezza of the mechanical amplitude at the sonotrode tip. We recommend using either of these devices with a sonotrode BS4d22 (22mm diameter) and a stand ST2.

Sonotrodo ASTM G32 -16 da 15,9 mm con punta sostituibile

Progettazione, produzione e consulenza – Qualità Made in Germany

Gli ultrasuoni Hielscher sono noti per i loro elevati standard di qualità e design. La robustezza e la facilità d'uso consentono un'agevole integrazione dei nostri ultrasuoni negli impianti industriali. Gli ultrasuonatori Hielscher sono in grado di gestire facilmente condizioni difficili e ambienti impegnativi.

Hielscher Ultrasonics è un'azienda certificata ISO e pone particolare enfasi sugli ultrasuonatori ad alte prestazioni, caratterizzati da tecnologia all'avanguardia e facilità d'uso. Naturalmente, gli ultrasuoni Hielscher sono conformi alla normativa CE e soddisfano i requisiti UL, CSA e RoH.

Letteratura / Riferimenti