Particle induced surface erosion - Tumbling and direct impact; a high-speed electrochemical, acoustic and visual study

A technique that monitors the impedance of a 250 mu m diameter aluminium electrode as a function of time with a 2 mu s resolution as sand particles (`slurry erosion') are impinged on the solid/liquid interface is reported using a submerged jet. The detection of individual particles as they approach an electrode, before any erosion/corrosion was registered, is demonstrated. This study shows that at least two types of erosion mechanisms are possible; direct or 'primary' impact and tumbling or 'scrape' events. The primary impact events are correlated to the acoustic emission from the environment which is shown to be significant for these events, whereas scrape events appear to produce far weaker acoustic emission signatures under the conditions employed. The velocities of the particles are reported and are of the order of 6-8 m s(-1) at the jet mouth. However, high-speed imaging of the particles as they strike the substrate indicates a significant deceleration prior to impact and an order of magnitude reduction in kinetic energy compared to that as they exit the jet.