NAVAIR  01-1A-17  and  NSTM,  chapter   556, provides  information  on  low-lint  wiping  cloths. Rust  or  corrosion  initially  present  in  a hydraulic  system  can  usually  be  traced  to improper  storage  of  materials  and  component parts. Particles can range in size from large flakes to  abrasives  of  microscopic  dimensions.  Proper preservation  of  stored  parts  is  helpful  in  elimi- nating   corrosion. 2. Particles introduced from outside sources. Particles  can  be  introduced  into  hydraulic  systems at points where either the liquid or certain working parts of the system (for example, piston rods) are at least in temporary contact with the atmosphere. The   most   common   contaminant   introduction areas  are  at  the  refill  and  breather  openings, cylinder   rod   packings,   and   open   lines   where components  are  removed  for  repair  or  replace- ment.  Contamination  arising  from  carelessness during  servicing  operations  is  minimized  by  the use  of  filters  in  the  system  fill  lines  and  finger strainers   in   the   filler   adapter   of   hydraulic reservoirs. Hydraulic   cylinder   piston   rods incorporate wiper rings and dust seals to prevent the dust that settles on the piston rod during its outward stroke from entering the system when the piston rod retracts. Caps and plugs are available and should be used to seal off the open lines when a   component   is   removed   for   repair   or replacement. 3. Particles created within the system during operation.  Contaminants  created  during  system operation  are  of  two  general  types—mechanical and chemical. Particles of a mechanical nature are formed by wearing of parts in frictional contact, such  as  pumps,  cylinders,  and  packing  gland components. These wear particles can vary from large chunks of packings down to steel shavings that  are  too  small  to  be  trapped  by  filters. The   major   source   of   chemical   contami- nants   in   hydraulic   liquid   is   oxidation.   These contaminants  are  formed  under  high  pressure  and temperatures and are promoted by the chemical action of water and air and of metals like copper and  iron  oxides.  Liquid-oxidation  products  appear initially   as   organic   acids,   asphaltines,   gums, and  varnishes—sometimes  combined  with  dust particles   as   sludge.    Liquid-soluble   oxidation products  tend  to  increase  liquid  viscosity,  while insoluble  types  separate  and  form  sediments, especially   on   colder   elements   such   as   heat exchanger  coils. Liquids containing antioxidants have little tendency to form gums and sludge under normal operating  conditions.  However,  as  the  tempera- ture  increases,  resistance  to  oxidation  diminishes. Hydraulic  liquids  that  have  been  subjected  to excessively  high  temperatures  (above  250°F  for most  liquids)  will  break  down,  leaving  minute particles of asphaltines suspended in the liquids. The   liquid   changes   to   brown   in   color   and   is referred  to  as  decomposed  liquid.  This  explains the  importance  of  keeping  the  hydraulic  liquid temperature  below  specific  levels. The second contaminant-producing chemi- cal action in hydraulic liquids is one that permits these  liquids  to  react  with  certain  types  of  rubber. This  reaction  causes  structural  changes  in  the rubber, turning it brittle, and finally causing its complete   disintegration.   For   this   reason,   the compatibility  of  system  liquid  with  seals  and  hose material  is  a  very  important  factor. 4. Particles introduced by foreign liquids. One of  the  most  common  foreign-fluid  contaminants is  water,  especially  in  hydraulic  systems  that require  petroleum-based  liquids.  Water,  which enters  even  the  most  carefully  designed  system  by condensation  of  atmospheric  moisture,  normally settles  to  the  bottom  of  the  reservoir.  Oil movement in the reservoir disperses the water into fine  droplets,  and  agitation  of  the  liquid  in the  pump  and  in  high-speed  passages  forms  an oil-water-air  emulsion.  This  emulsion  normally separates  during  the  rest  period  in  the  system reservoir;  but  when  fine  dust  and  corrosion particles are present, the emulsion is chemically changed  by  high  pressures  into  sludge.  The damaging action of sludge explains the need for effective filtration, as well as the need for water separation  qualities  in  hydraulic  liquids. CONTAMINATION    CONTROL Maintaining hydraulic fluid within allowable contamination limits for both water and particu- late matter is crucial to the care and protection of  hydraulic  equipment. Filters  (discussed  in  chapter  9)  will  provide adequate control of the particular contamination problem  during  all  normal  hydraulic  system operations  if  the  filtration  system  is  installed properly   and   filter   maintenance   is   performed properly.  Filter  maintenance  includes  changing elements at proper intervals. Control of the size and  amount  of  contamination  entering  the  system from   any   other   source   is   the   responsibility 3-9