than  it  can  be  removed,  even  with  the  best  of maintenance.  Another  lesser  known  but  major source of air is air that is sucked into the system past  actuator  piston  rod  seals.  This  usually  occurs when the piston rod is stroked by some external means  while  the  actuator  itself  is  not  pressurized. WATER  CONTAMINATION.—   Water is a serious   contaminant   of   hydraulic   systems. Hydraulic  fluids  are  adversely  affected  by dissolved,   emulsified,   or   free   water.   Water contamination may result in the formation of ice, which impedes the operation of valves, actuators, and other moving parts. Water can also cause the formation  of  oxidation  products  and  corrosion of  metallic  surfaces. SOLVENT  CONTAMINATION.—  Solvent contamination  is  a  special  form  of  foreign  fluid contamination   in   which   the   original   contami- nating substance is a chlorinated solvent. Chlori- nated  solvents  or  their  residues  may,  when introduced  into  a  hydraulic  system,  react  with  any water  present  to  form  highly  corrosive  acids. Chlorinated  solvents,  when  allowed  to  com- bine  with  minute  amounts  of  water  often  found in  operating  hydraulic  systems,  change  chemically into  hydrochloric  acids.  These  acids  then  attack internal   metallic   surfaces   in   the   system, particularly those that are ferrous, and produce a  severe  rust-like  corrosion.  NAVAIR  01-1A-17 and  NSTM,  chapter  556,  contain  tables  of solvents  for  use  in  hydraulic  maintenance. FOREIGN-FLUIDS  CONTAMINATION.— Hydraulic systems can be seriously contaminated by  foreign  fluids  other  than  water  and  chlorinated solvents. This type of contamination is generally a  result  of  lube  oil,  engine  fuel,  or  incorrect hydraulic  fluid  being  introduced  inadvertently  into the system during servicing. The effects of such contamination  depend  on  the  contaminant,  the amount in the system, and how long it has been present. NOTE:  It  is  extremely  important  that  the different types of hydraulic fluids are not mixed in  one  system.  If  different  type  hydraulic  fluids are  mixed,  the  characteristics  of  the  fluid  required for   a   specific   purpose   are   lost.   Mixing   the different  types  of  fluids  usually  will  result  in  a heavy,  gummy  deposit  that  will  clog  passages  and require a major cleaning. In addition, seals and packing  installed  for  use  with  one  fluid  usually are not compatible with other fluids and damage to the seals will result. ORIGIN  OF  CONTAMINATION Recall  that  contaminants  are  produced  from wear   and   chemical   reactions,   introduced   by improper  maintenance,  and  inadvertently  intro- duced  during  servicing.  These  methods  of  con- taminant  introduction  fall  into  one  of  the  four major  areas  of  contaminant  origin. 1.  Particles  originally  contained  in  the  system. These  particles  originate  during  the  fabrication and storage of system components. Weld spatter and  slag  may  remain  in  welded  system  com- ponents,   especially   in   reservoirs   and   pipe assemblies. The presence is minimized by proper design.  For  example,  seam-welded  overlapping joints  are  preferred,  and  arc  welding  of  open sections  is  usually  avoided.  Hidden  passages  in valve  bodies,  inaccessible  to  sand  blasting  or  other methods   of   cleaning,   are   the   main   source   of introduction  of  core  sand.  Even  the  most  carefully designed and cleaned casting will almost invari- ably free some sand particles under the action of hydraulic pressure. Rubber hose assemblies always contain  some  loose  particles.  Most  of  these particles  can  be  removed  by  flushing  the  hose before   installation;   however,   some   particles withstand  cleaning  and  are  freed  later  by  the action  of  hydraulic  pressure. Particles  of  lint  from  cleaning  rags  can cause  abrasive  damage  in  hydraulic  systems, especially  to  closely  fitted  moving  parts.  In addition, lint in a hydraulic system packs easily into  clearances  between  packing  and  contacting surfaces,  leading  to  component  leakage  and decreased  efficiency.  Lint  also  helps  clog  filters prematurely.   The   use   of   the   proper   wiping materials  will  reduce  or  eliminate  lint  contamina- tion. The wiping materials to be used for a given application  will  be  determined  by a. b. c. substances  being  wiped  or  absorbed, the   amount   of   absorbency   required, and/or the required degree of cleanliness. These   wiping   materials   are   categorized   for contamination  control  by  the  degree  of  lint  or debris  that  they  may  deposit  during  use.  For internal   hydraulic   repairs,   this   factor   itself will  determine  the  choice  of  wiping  material. 3-8