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Rapidly flowing solutions can often disrupt adherent surface films and deposits that would otherwise offer protection against corrosion.  Thinning or removal of surface films by erosion from the flowing stream results in accelerated corrosion, called erosion-corrosion.  The attack is accelerated at elbows, tube constrictions, burrs, and other structural features that alter flow direction or velocity, and increase turbulence.

Erosion-corrosion takes the form of grooves, waves, gullies, teardrop-shaped pits, and horseshoe-shaped undercutting in the surface.  The effects of the hydrodynamic are not well understood.  Undercutting may occur in either upstream or the downstream direction.  As described in the schematic below, turbulent eddies thin the protective film locally to produce undercutting, which is seen in the accompanying photograph.


Figure 1.  Schematic of turbulent eddy mechanism for downstream undercutting of erosion-corrosion pits.


Figure 2.  Photograph of erosion-corrosion showing individual teardrop shaped pits with undercutting in the downstream direction.

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