the  joint  between  the  metals.  For  example,  when aluminum   pieces   are   attached   with   steel   bolts   and moisture   or   contamination   are   present,   galvanic corrosion   occurs   around   the   fasteners. Pitting The  most  common  effect  of  corrosion  on  aluminum alloys  is  pitting.  It  is  caused  primarily  by  variations  in the  grain  structure  between  adjacent  areas  on  the  metal surfaces   that   are   in   contact   with   a   corrosive environment.   Pitting   is   first   noticeable   as   a   white   or gray  powdery  deposit,  similar  to  dust,  that  blotches  the surface.   When   the   superficial   deposit   is   cleaned   away, tiny  pits  or  holes  can  be  seen  in  the  surface.  These  pits may   appear   either   as   relatively   shallow   indentations   or as  deeper  cavities  of  small  diameters.  Pitting  may  occur in  any  metal,  but  it  is  particularly  characteristic  of aluminum   and   aluminum   alloys. Intergranular   Corrosion Intergranular   corrosion   is   an   attack   on   the   grain boundaries   of   some   alloys   under   specific   renditions. During  heat  treatment,  these  alloys  are  heated  to  a temperature  that  dissolves  the  alloying  elements.  As  the metal   cools,   these   elements   combine   to   form   other compounds.   If   the   cooling   rate   is   slow,   they   form predominantly   at   the   grain   boundaries.   These compounds   differ   electrochemically   from   the   metal adjacent   to   the   grain   boundaries.   These   altered compounds   can   be   either   anodic   or   cathodic   to   the adjoining  areas,  depending  on  their  composition.  The presence  of  an  electrolyte  will  result  in  an  attack  on  the anodic  area.  This  attack  will  generally  be  quite  rapid and   can   exist   without   visible   evidence. As  the  corrosion  advances,  it  reveals  itself  by  lifting up   the   surface   grain   of   the   metal   by   the   force   of expanding   corrosion   products   occurring   at   the   grain boundaries  just  below  the  surface.  This  advanced  attack is   referred   to   as   EXFOLIATION.   Recognition   and necessary   corrective   action   to   immediately   correct   such serious   instances   of   corrosion   are   vital.   This   type   of attack   can   seriously   weaken   structural   members   before the   volume   of   corrosion   products   accumulate   on   the surface   and   the   damage   becomes   apparent. Fretting Fretting  is  a  limited  but  highly  damaging  type  of corrosion   caused   by   a   slight   vibration,   friction,   or slippage  between  two  contacting  surfaces  that  are  under stress   and   heavily   loaded.   Fretting   is   usually   associated with  machined  parts  such  as  the  contact  area  of  bearing surfaces,  two  mating  surfaces,  and  bolted  assemblies. At  least  one  of  the  surfaces  must  be  metal. In   fretting,   the   slipping   movement   at   the   interface of  the  contacting  surface  destroys  the  continuity  of  the protective  films  that  may  be  present  on  the  surfaces. This   action   removes   fine   particles   of   the   basic   metal. The   particles   oxidize   and   form   abrasive   materials   that further  accumulate  and  agitate  within  a  confined  area  to produce  deep  pits.  Such  pits  are  usually  located  where they  can  increase  the  fatigue  potential  of  the  metal. Fretting   is   evidenced   at   an   early   stage   by   surface discoloration  and  by  the  presence  of  corrosion  products in  any  lubrication.  Lubricating  and  securing  the  parts so  that  they  are  rigid  are  the  most  effective  measures  for the  prevention  of  this  type  of  corrosion. Stress Stress,   evidenced   by   cracking,   is   caused   by   the simultaneous   effects   of   tensile   stress   and   corrosion. Stress  may  be  internal  or  applied. Internal   stresses   are   produced   by   nonuniform deformation   during   cold   working   conditions,   by unequal   cooling   from   high   temperatures   during   heat treatment,   and   by   internal-structural   rearrangement involving  volume  changes.  Stresses  set  up  when  a  piece is   deformed.   Examples   of   internal   stresses   include those  induced  by  press-and-shrink  fits  and  those  in  rivets and  bolts. Concealed  stress  is  a  more  dangerous  condition  than design   stress.   Concealed   stress   corrosion   is   difficult   to recognize   before   it   has   overcome   the   design   safety factor.   The   magnitude   of   the   stress   varies   from point-to-point   within   the   metal.   Stresses   in   the neighborhood  of  the  yield  strength  are  generally necessary   to   promote   stress   corrosion   cracking,   but failures  may  occur  at  lower  stresses. Fatigue Fatigue   is   a   special   type   of   stress   corrosion.   It   is caused  by  the  combined  effects  of  corrosion  and  stresses applied  in  cycles.  An  example  of  cyclic  stress  fatigue  is the   alternating   loads   to   which   the   connecting   rod   of   a double-acting   piston   in   an   air   compressor   is   subjected. During  the  extension  (up)  stroke  a  compression  load  is applied,  and  during  the  retraction  (down)  stroke  a  tensile or  stretching  load  is  applied.  Fatigue  damage  is  greater than  the  combined  damage  of  corrosion  and  stresses. Fracture   of   a   metal   part   due   to   fatigue   corrosion 2-37