Development of a New Cavitation Test Designed to Evaluate Coatings

Allen Skaja (Bureau of Reclamation)

Damage caused by cavitation is observed on hydro-turbines, pumps, pipes, gates, draft tubes, and outlet conduits. Mitigating the effect of cavitation on structures will reduce cost in repairs and downtime. Traditionally, stainless steel weld overlays are used for cavitation repairs on metallic structures. Welding stainless steel to mild steel creates a galvanic corrosion cell, which causes the steel to corrode and delaminate the stainless steel, resulting in no cavitation protection. Cavitation resistant coatings could be used in combination with stainless steel weld overlays to reduce the galvanic corrosion cell, thus providing dual protection and extending the service life of the repairs. Unfortunately, most coatings cannot withstand moderate or severe cavitation, but only work under mild cavitation such as in draft tubes. This research developed a new cavitation test to evaluate coatings for moderate-severe cavitating environment. With this test, polyurethane elastomers showed good results as a mitigation technique. The ASTM G134 test standard is designed to evaluate metals to determine rates of material loss from a submerged cavitating jet. The new test procedure modified the ASTM G134, to incorporate greater operational flexibility such as greater standoff, and angled the sample to obtain a pure cavitation zone, with minimal influence of jet impingement. Since coatings are not as dense as metals weight loss is not measured, instead volume loss is monitored using an optical profilometer to obtain a 3D image and monitored periodically to determine a rate of material loss.