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Turbulent water, high velocity water (especially at bends in cooler tubing) and silt laden water; will all cause erosion of metals. Increased velocity inside a tube or across metal surfaces of boats in tidal water or drifting will also cause increased erosion.
Propellers have several special problems besides the normal problems of bronze (dezincification, galvanic, and stray current corrosion) in that they have large uneven surfaces exposed to tidal currents which cause electrical differences on different areas of surface and also differences in voltage due to the different speed between the hub and blades. Turbulence and pounding cause erosion also. Besides balancing the shaft and propellers, bonding using shaft straps and a zinc system will normally keep prop erosion to a minimum by evening out the voltage over the entire prop surface.
Water flowing across a metal surface can produce positive and negative areas on the same surface. Corrosion results. A zinc system will even out the voltage.