Damage caused by cavitation and erosion is observed on hydro-turbines, pumps, pipes, gates, draft tubes, and outlet conduits. Mitigating the effects of this degradation on metallic hydraulic steel structures will reduce downtime and costly repairs. Traditionally, stainless steel weld overlays are used for cavitation repairs on these structures. Welding stainless to mild steel creates a galvanic corrosion cell in immersion conditions, which can cause delamination of the stainless steel and subsequent loss of cavitation protection. Cavitation resistant coatings could be used in combination with stainless steel weld overlays to eliminate the galvanic corrosion cell, thus providing protection and extending the service life of the repairs. This paper investigates the cavitation resistance and material properties of polymeric elastomer coatings. Several elastomers were selected for testing based on their mechanical properties such as percent elongation, tensile strength, tear resistance, and Shore hardness. Findings include the results of laboratory testing of cavitation resistant coatings. Elastomers are shown to provide better cavitation and erosion resistance than ceramic filled epoxies.
Cavitation occurs in localized areas where there is a pressure drop across a structure; the water goes through a phase transition and forms water vapor. These vapor bubbles implode, resulting in high velocity micro-jets which impact adjacent surfaces. These impacts release shockwaves of energy, which cause microscopic particles of the surface material to flake off.1 Repeated micro-jet impact causes microfractures in the affected surfaces and leads to pitting. Damage caused by cavitation is commonly observed on hydro-turbines, pumps, pipes, gates, draft tubes, and outlet conduits.
The Bureau of Reclamation's primary method for mitigating cavitation uses overlays of grades 308 and 309 stainless steel welded directly over mild steel.2 The interface between the mild and stainless steel in water creates a galvanic corrosion cell. Coatings can be applied to reduce the galvanic corrosion cell, but cavitation damages epoxy-type coatings within a few hours of operation, exposing the stainless/mild steel interface. The progression of galvanic corrosion between the two metals eventually delaminates the overlay material. Once disbondment occurs, cavitation begins degrading the exposed mild steel.2 The ideal coating for this application would resist cavitation for several years.
