Q:
What are Fretting and Inter granular Corrosion?



A:
    FRETTING CORROSION:

  • Small amplitude oscillation in a corrosive medium.

  • Examples are press fit wheel on a shaft, turbine
    blades in dovetail slots, railroad car wheel shrink fit to
    their axles.


  • Oxidation is the "corrosion" film removed by the
    mechanical motion, becoming an abrasive.


  • The illustration below shows fretting corrosion on
    the root surface of an aircraft power plant compressor
    blade. Fatigue cracks can initiate as a result of this
    fretting pitting damage.






    INTERGRANULAR CORROSION:

  • Selective dissociation/attack of grain boundaries with no
    significant attack of the grains. Exfoliation is an extreme
    example.

  • Galvanic cell forms between grain and drastically
    different composition of the boundary -- possibly due
    to precipitates under service conditions or heat treatment.


  • Many other examples. For instance, in 304SS-precipitation
    of Cr carbides, e.g., (Cr, Fe) 23 C6, at grain boundaries,
    with Cr depleted regions next to them. Al alloys also
    experience this effect.