Q:
What are Exfoliation and De alloying?




A:
    EXFOLIATION:

  • Primarily affects Al alloys - inter granular
    attack parallel to the surface.


  • Corrosion products in grain boundaries
    wedge open the grains due to volume
    change (increase).


  • Grain boundary precipitation in alloyed and
    highly worked materials starts the process.
    1000 and 3000 series are resistant.


  • Specific tempers in 2000, 5000, and 7000
    series have problems.



    DEALLOYING:

  • De alloying is an attack of specific elements on
    an alloy.


  • Examples are:

    - Dezincification (attack of zinc) in brass

    - Graphitic corrosion (attack of iron) in gray
      cast iron, leaving a very weak structure.
      Graphite is cathodic to iron at low corrosion
      rates.