Erosion-corrosion cavitation, impact

The erosion effects of cavitation on solid surfaces have been extensively investigated both in terms of surface erosion [68] and corrosion [69]. The consequences of these effects on metal reactivity are important since passivating coatings are frequently present on a metal surface (e. g. oxides, carbonates and hydroxides) and can be removed by the impacts caused by collapsing cavitation bubbles. An illustration can be found with the activation of nickel powder and the determination of the change in its surface composition under the influence of cavitation by Auger spectroscopy (Fig. 3.6) [70]. 

Wear impact plastic deformation makes some constituents more susceptible to corrosion. Cracks brittle constituents, tears apart ductile constituents to form sites for crevice corrosion, hydraulic splitting. Supplies kinetic energy to drive abrasion mechanism. Pressurizes mill water to cause splitting, cavitation, and jet erosion of metal and protective oxidized material. Pressurizes mill water and gases to produce unknown temperatures, phases changes, and decomposition or reaction products from ore and water constituents. Heats ball metal, ore, fluids to increase corrosive effects. 

The solid material absorbs the impact energy, leading to elastic or plastic deformation or even fracture. This may cause localized deformation and/or erosion of the solid surface. It has been suggested that the bubble grows and collapses, resulting in high pressure for a few milliseconds (65). The local pressure observed may be about 4000 atmospheres with a temperature increase up to 800 C. These conditions of high pressure and temperature accelerate the corrosion rate. The cycle of exposure of fresh surfaces to corrosion, followed by reformation of protective films that leads to cavitation repeats itself (9, 60). 

Erosion damage by solid particle impact or cavitation bubble collapse to an oxide or passive film will reveal the underlying nascent surface inducing a higher activity (higher corrosion... 

This inherent property reduces the amormt of heat treating and postmachining. 6B has outstanding resistance to cavitation erosion. Steam turbine erosion shields fiem 6B have protected the blades of turbines for years of continuous service. 6B has good impact and thermal shock resistance, resists heat and oxidation, retains high hardness even at red heat (when cooled, recovers ffill original hardness), and has resistance to a variety of corrosive media. 6B is useftd where both wear and corrosion resistance are needed. 

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