30-Year Development of a Cavitation-Resistant Elastomer
We review the testing and development of a next-generation coating for cavitation protection: a high-performance polyurethane.
For more than 30 years, my organization experimented widely with epoxies and polyurethanes to improve protection against cavitation damage. (To learn more about cavitation, see The Science of Cavitation: Diagnosis & Resistance Methods.) Throughout this process, cavitation resistance benchmarking was evaluated through four methods:
- Ultrasonic cavitation simulation, in accordance with ASTM G32
- Water jet cavitation erosion
- Cavitation tunnel testing
- Field testing
Ultrasonic Cavitation Simulation, in accordance with ASTM G32
This is a relatively simple laboratory test that simulates cavitation through ultrasonic means. It can raise the temperature of the water significantly in a localized area, and thus additional cooling is often needed. Its primary advantage is that it allows for the quick benchmarking of different materials.
Water Jet Cavitation Erosion
This test uses a cylinder subjected to a jet of water, which—on parting from the surface—creates two cavitation areas, and thus allows for the testing of cylindrical samples. The adhesion test capability is limited with water jet testing because the coating is continuous.
Figure 1. Water jet cavitation testing of bronze sample.
Figure 2. Water jet cavitation testing of semi-rigid coating.
Figures 1 and 2 show two samples, bronze and semi-rigid epoxy, following the completion of testing.
Cavitation Tunnel Testing
A cavitation tunnel is a continuous loop that can be used either to optimize flow modeling and geometry design (at the Emerson Tunnel in the UK), or for material testing at the Applied Research Laboratory (ARL) Tunnel at Pennsylvania State University. The latter test site has a reduced diameter test section, which allows for velocities of 60 m/s to be used and the creation of high-intensity cavitation. There are many other tunnels that simulate similar conditions, but the ARL Tunnel is perhaps the most widely known independent tunnel.
This is the most realistic test scenario as it also aims to meet operator needs. However it is limited in that operators are not always aware of the actual conditions of their equipment. For this reason, field testing is most successful when benchmarking different materials with identical operating conditions and durations. (For more on coating testing, read 4 Types of Tests that Measure the Strength of a Coating.)
Tracking Development through Testing
The initial development work was carried out using semi-rigid epoxies. These showed very high adhesion, good corrosion resistance to 140°F (60°C) immersed, and excellent cathodic disbondment and galvanic corrosion resistance. These epoxies suffered from erosion problems in high-cavitation scenarios.
Figure 3. Cavitation tunnel damage of semi-rigid coating.
Figure 4. Cavitation tunnel damage of Cr-Ni steel.
Figures 3 and 4 show the effects after 24 hours of cavitation tunnel testing using a Cr-Ni alloy as a benchmark, which was taken to 472 hours.1
Using rubber and urethane modifiers to make the epoxies more flexible showed promise in improving cavitation resistance. In the same benchmarking test, the flexibilized epoxy showed no wear after 500 hours.1
Field tests of this material showed adhesion problems with immersion performance and thus it was withdrawn from further testing. One of the problems of cavitation tunnel testing is that it does not always offer the duration to simulate the effects of immersion, which is why field tests are essential.
Initially, polyurethanes exhibited the best performance in Laboratory ASTM G32 ultrasonic tests, however longer testing showed two key problems:
- Although instantaneous, the high temperatures created during the cavitation process blistered the polyurethane.
- Polyurethanes suffered long-term immersed adhesion problems.
These observations helped in identifying two key properties to establish from a design perspective: adhesion and temperature resistance. Thus, further testing was approached in two ways:
- Hardness and Heat Resistance – The previous samples had hardness values ranging from 65 to 85 Shore A. To address this, a harder elastomer was developed, which was able to offer 87 Shore A; more resilience without becoming too hard to lose the benefit in cavitation. Further formulation adjustments increased the dry heat resistance from 149°F to 194°F (65°C to 90°C), with improvements in diffusion resistance. However, immersed resistance was still limited and most of the testing concentrated at 104°F (40°C) operating conditions.
- Adhesion – Addressed from two angles, a performance conditioner was used. This offered higher diffusion resistance and was optimized for adhesion to steel. In most instances, this conditioner was sufficient. To improve cathodic disbondment performance, a base coat of epoxy was shown to add benefit due to higher diffusion resistance and adhesion.
ASTM G32 ultrasonic-induced cavitation tests demonstrated these requirements, and Figures 5 through 8 show comparisons of different coatings.
Figure 5. G32 Test results rigid epoxy; erosion after 12 hours.
Figure 6. G32 Test results 85 Shore A “hard” elastomer; heat damage after 2 hours.
Figure 7. G32 Test results flexible epoxy; blistering after 12 hours.
Figure 9. G32 Test results flexible epoxy; erosion after 12 hours.
A resulting performance elastomer was developed based on extensive laboratory and field testing. Independent testing of this elastomer has confirmed:
- Resistance without damage after 500 hours of intensive cavitation testing, benchmarking with a Cr-Ni alloy (which was damaged within this time period).1
- Resistance without damage after 20 hours of 130 knots (60m/s) intensive cavitation testing.2 (Previous best polymer coatings had achieved under 15 hours before destruction.)
- ASTM G32 results significantly higher than 316L stainless steel.3
Next in the Series
The elastomer technology described above has been applied and in service for many years. Several case studies illustrating how this cavitation-resistant elastomer is faring in service are presented in the article Case Studies of a Novel Elastomer for Cavitation Resistance.
- M ENGELHARDT, “Cavitation Test Report Parts 1 and 3, 3rd Party (Belzona) Edition 2000/2001”, 2003-07-21-1E-TechBer-MEng-CAV-Testresults_3rdParty edition, Voith Siemens Hydro Power Generation, 2003
- D R METREY, “Evaluation of the Cavitation-Erosion Resistance of Belzona 2141”, File 01-071, PennState Applied Research Laboratory, 2001
- SAMSUNG HEAVY INDUSTRIES, “Cavitation Erosion Test Results of Belzona 2141” 2004