CHARACTERISTICS The   appearance   of   corrosion   will   vary   with   the metal   involved.   The   following   discussion   includes   brief descriptions   of   typical   corrosion   product   characteristics. These   descriptions   are   only   for   the   most   common materials   used   in   gas   turbine   propulsion   and   support equipment. Iron  and  Steel Possibly  the  best  known  and  most  easily  recognized of   all   forms   of   metal   corrosion   is   the   familiar reddish-colored  iron  rust.  When  iron  and  its  alloys corrode,   dark   iron   oxide   coatings   usually   form   first. These   coatings,   such   as   heat   scale   on   steel   sheet   stock and  the  magnetite  layer  that  forms  on  the  inside  of  boiler tubes,   protect   iron   surfaces   rather   efficiently.   However, if   sufficient   oxygen   and   moisture   are   present,   the   iron oxide  is  soon  converted  to  hydrated  ferric  oxide,  which is  conventional  red  rust.  Hydrated  ferric  oxide,  red  rust, does   not   protect   surfaces.   It   destroys   surfaces. Aluminum Aluminum  and  its  alloys  exhibit  a  wide  range  of corrosive   attacks,   varying   from   general   etching   of surfaces   to   penetrating   attacks   along   the   internal   grain boundaries  of  the  metal.  The  corrosion  products  of aluminum   are   seen   as   white-gray   powdery   deposits. Copper   and   Copper   Alloys Copper  and  its  alloys  are  generally  corrosion resistant,   although   the   products   of   corrosive   attack   on copper  are  commonly  known.  Sometimes  copper  or copper  alloy  surfaces  will  tarnish  to  a  gray-green  color, while  the  surface  will  remain  relatively  smooth.  This discoloration   is   the   result   of   the   formation   of   a fine-grained,  airtight  copper  oxide  crust,  called  a  patina. Patina   offers   good   protection   for   the   underlying metal   in   ordinary   situations.   However,   exposure   of copper   alloys   to   moisture   or   salt   spray   will   cause   the formation  of  blue  or  green  salts  called  verdigris.  The presence   of   verdigris   indicates   active   corrosion. Cadmium  and  Zinc Cadmium  is  used  as  a  coating  to  protect  the  area  to which  it  is  applied  and  to  provide  a  compatible  surface when  the  part  is  in  contact  with  other  metals.  The cadmium  plate  supplies  sacrificial  protection  to  the underlying  metal  because  of  its  great  activity.  During the  time  it  is  protecting  the  base  metal,  the  cadmium  is intentionally   being   consumed.   Zinc   coatings   are   used for  the  same  purpose,  although  to  a  lesser  extent.  Attack is  evident  by  white-to-brown-to-black  mottling  of  the surfaces.   These   indications   do   NOT   indicate deterioration   of   the   base   metal.   Until   the   characteristic colors  peculiar  to  corrosion  of  the  base  metal  appear,  the coating   is   still   performing   its   protective   function. Nickel   and   Chromium   Alloys Nickel   and   chromium   alloys   are   also   used   as protective   agents. They   are   used   as   electroplated coatings   and   as   alloying   constituents   with   iron   in stainless   steels   and   with   other   metals   such   as   copper. Nickel  and  chromium  plate  provide  protection  by  the formation   of   an   actual   physical   noncorrosive   barrier over   the   steel.   Electroplated   coatings,   particularly chromium  on  steel,  are  somewhat  porous.  Eventually, corrosion   starts   at   these   pores   unless   a   supplementary coating   is   applied   and   maintained. TYPES   OF   CORROSION As  stated  previously,  corrosion  may  occur  in  several forms,  depending  upon  the  metal  involved,  its  size  and shape,   its   specific   function,   the   atmospheric   conditions, and   the   corrosion-producing   agents   present.   Those corrosion   types   described   in   this   section   are   the   most common   forms   found   on   gas   turbine   engines   and machinery   structures. Direct   Surface   Attack The  surface  effect  produced  by  reaction  of  the  metal surface  to  oxygen  in  the  air  is  a  uniform  etching  of  the metal.  The  rusting  of  steel,  tarnishing  of  copper  alloys, and   the   general   dulling   of   aluminum   surfaces   are common   examples   of   direct   surface   attacks.   If   such corrosion   is   allowed   to   continue   unabated,   the   surface becomes  rough,  and  in  the  case  of  aluminum,  frosty  in appearance.   Direct   surface   attack   is   sometimes   referred to  as  uniform  etch  corrosion. Galvanic   Corrosion Galvanic   corrosion   is   the   term   applied   to   the accelerated  corrosion  of  metal  caused  by  dissimilar metals  being  in  contact  in  a  corrosive  medium. Dissimilar   metal   corrosion   is   usually   a   result   of faulty   design   or   improper   maintenance   practices   which result   in   dissimilar   metals   coming   in   contact   with   each other.  This  is  usually  seen  as  a  buildup  of  corrosion  at 2-36