generally  occurs  at  a  stress  far  below  the  fatigue  limit  in a  laboratory  environment,  even  though  the  amount  of corrosion   is   very   small.   For   this   reason,   protection   of all  parts  subject  to  alternating  stress  is  particularly important   wherever   practical,   even   in   environments   that are   only   mildly   corrosive. PREVENTION  AND  CONTROL Much  has  been  done  over  the  years  to  improve  the corrosion   resistance   of   newer   Navy   warships. Improvements   include   the   selection   and   combination   of materials  of  construction,  chemical  surface  treatments, insulation  of  dissimilar  metals,  and  protective  paint finishes.   All   these   improvements   are   aimed   at   reducing maintenance   as   well   as   improving   reliability.   Despite refinements   in   design   and   construction,   corrosion control   is   a   problem   that   requires   a   continuous maintenance   program. With  this  idea  in  mind,  the  NAVSEASYSCOM  has developed  some  excellent  ship  class  corrosion  control manuals  for  you  to  use  as  reference  tools  aboard  ship. There  are  two  manuals  a  GS  supervisor  should  read. They  are  the  Corrosion   Control   and   Prevention   Manual f o r D D - 9 6 3 c l a s s ships, N A V S E A S9630-AB-MAN-010,   and   the   Standard   Corrosion Control   Manual,   NAVSEA    S9630-AE-MAN-010. Cleaning As  a  leading  petty  officer  or  work  center  supervisor, one  of  your  most  important  aids  in  the  prevention  and control   of   corrosion   is   an   adequate   cleaning   program. The  term  clean  means  to  do  the  best  job  possible  using the   time,   materials,   and   personnel   available.   A   daily wipedown  of  all  machinery  is  better  than  no  cleaning  at all.  The  importance  of  frequent  cleaning  cannot  be overemphasized.   Any   cleaning   procedures,   however, should   be   in   the   mildest   form   possible   to   produce   the desired   results.   For   example,   spraying   water   around multipin   connectors   can   cause   electrical   shorts   or grounds,  with  a  possible  loss  of  control  functions  or equipment   damage. In   general,   gas   turbine   engines   and   enclosures should  be  cleaned  as  often  as  necessary  to  keep  surfaces free  of  salt,  dirt,  oil,  and  other  corrosive  deposits.  A thorough  inspection  and  cleaning  of  gas  turbine  intakes and   enclosures   should   always   be   done   in   conformance with   PMS   requirements. These   cleanings   and inspections   should   be   done   before   getting   underway, after  an  extended  stay  in  port,  and  after  returning  to  port from  an  extended  time  at  sea. Since   marine   gas   turbines   are   more   subject   to internal   corrosion   than   engines   used   in   other   types   of applications,   internal   cleaning   is   of   particular importance.  This  is  accomplished  by  means  of  water washing. A   mixture   of   B   &   B   3100   water-wash compound  and  distilled  water  is  injected  into  the  engine air  inlet  while  it  is  being  motored  and  then  rinsed  with distilled  water  in  the  same  manner.  It  is  then  operated for   about   5   minutes   to   remove   all   liquid.   For   more detailed   information   on   this   procedure,   consult   the applicable   PMS   card. Characteristics  of  Metals As   a   GS   supervisor,   you   should   have   a   thorough knowledge   of   the   characteristics   of   the   various   metals used  throughout  the  engineering  plant,  as  well  as  the engines   themselves. To  some  extent,  all  metals  are  subject  to  corrosion To   keep   corrosion   to   a   minimum,   corrosion-resistant metals  are  used  to  the  fullest  extent  possible  consistent with   weight,   strength,   and   cost   considerations.   On exposed   surfaces,   the   major   preventive   for   providing relative   freedom   from   corrosion   is   a   coating   of protective  surface  film.  This  film  can  be  in  the  form  of an   electroplate,   paint,   or   chemicaI   treatment,   whichever is   most   practical. Most   of   the   metals   used   in   the   engineering   plants require   special   preventive   measures   to   guard   against corrosion.  In  the  case  of  aluminum  alloys,  the  metal  is usually   anodized   or   chemically   treated   and   painted. Steel  and  other  metals  such  as  brass  or  bronze  (with  the exception   of   stainless   steels)   use   cadmium   or   zinc plating,   protective   paint,   or   both.   In   all   cases,   the protective   finish   must   be   maintained   to   keep   active corrosion   to   an   absolute   minimum. PRESERVATION   AND DEPRESERVATION  OF  GAS  TURBINE ENGINES The  main  purpose  of  engine  preservation  is  to prevent  corrosion  of  the  various  types  of  materials  that make   up   the   engine   and   its   accessories.   Preservation also   ensures   against   gumming,   sticking,   and   corrosion of  the  internal  passages. Engine   preservation   and   depreservation   is   vital because  the  corrosion  of  engine  structures  can  and  does have   a   great   effect   on   the   operational   and   structural integrity  of  the  unit.  Therefore,  it  is  important  that  you know   about   methods   of   preservation,   materials   used, and  depreservation  procedures. 2-38