Microbiological corrosive action in the soil is due to physical and chemical changes in the soil caused by  the  presence  of  these  organisms.  Some  bacteria are  responsible  for  the  production  of  active galvanic cells. These bacteria are mostly found in highly   waterlogged,   sulfate-bearing,   blue   clay soils.  The  bacteria  concentration,  as  well  as  the corrosion  rate,  varies  considerably  with  the  dif- ferent  seasons  of  the  year.  Cast-iron  and  steel pipes are corroded mostly by sulfide production. Compositional  Corrosion Compositional  corrosion  alters  the  composi- tion   of   metals.   Some   of   the   specific   types   of compositional  corrosion  are  discussed  in  the following   paragraphs. .  Dezincification.  This  is  a  selective  type  of corrosion  that  occurs  in  copper  and  zinc  alloys. When alloys of this kind (brasses) are exposed to this  type  of  corrosion,  the  zinc  dissolves  out  of the  alloy  and  leaves  only  the  copper. .  Graphitization.  Another  type  of  composi- tional   corrosion   is   graphitization   or   graphitic softening.  It  is  a  peculiar  form  of  disintegration that attacks grey cast iron. Cast iron is an alloy made of iron and carbon, the carbon being in the form  of  graphite.  When  cast  iron  with  such  a composition  is  subjected  to  graphitization,  the iron  dissolves  out  and  leaves  only  the  graphite. This action leaves cast-iron pipes and other similar equipment   weakened   mechanically.   However, after  graphitization  corrosion  occurs,  the  graphite pipe may last for many years if it is not subjected to  any  mechanical  forces  or  sudden  pressures.  The action   of   this   type   of   corrosion   is   similar   to dezincification. .  Hydrogen  embrittlement.  This  is  a  term applied to metal that becomes brittle because of the action of some form of corrosion that causes the  formation  of  hydrogen  on  its  surface.  When hydrogen  forms  on  the  surface  of  steel,  the  ac- tion of the hydrogen may form blisters or actually embrittle the metal. Hydrogen liberated near the surface of steel in an electrolyte will diffuse into the metal quite rapidly. The hydrogen picked up by the steel is in an atomic state and causes the steel  to  become  brittle. When  the  production  of  atomic  hydrogen on  the  surface  of  the  metal  stops,  the  hydrogen leaves the metal in a few days and the metal again regains its original ductility. Stress Fatigue of Metals Corrosion  affects  metals  that  are  under  stress. The  action  caused  by  stresses  on  a  pipeline  or structure is due to the shifting of the various rocks and  soils  of  the  earth.  Usually  a  complete  pipeline is not under stress; certain sections are under stress while adjacent sections are not. Because of these pressures  and  strains,  localized  electrochemical action  takes  place.  The  section  of  the  pipe  or structure  under  stress  becomes  anodic,  whereas the unstressed sections become cathodic. In this way, the pipe under stress begins to corrode and weaken  because  of  the  action  of  corrosion. Corrosion Caused by Nonelectrolytes Nonelectrolytes  are  materials  that  will  not conduct   electricity.   These   materials   include nonelectrolytic  vapors,  liquids,  and  bacterial organisms. Since they do not conduct electricity, they  do  not,  in  themselves,  cause  corrosion. NONELECTROLYTE   GASES   AND VAPORS.—   Nonelectrolytic  gases  and  vapors usually must be subjected to high temperatures before corrosive action can take place. Hydrogen sulfide causes scaling of iron at temperatures from 1400°   to   2000°F.   High-chromium   alloy   steels resist  this  type  of  corrosion  best.  The  only  remedy for this type of corrosion is to keep the gases away from  the  metal  or  use  a  metal  that  can  resist corrosion. High-carbon steels are attacked by hydrogen at  temperatures  above  750°F.  This  hydrogen  com- bines with the carbon grains in the steel and causes the  metal  to  weaken  at  the  grain  boundaries between  the  iron  and  carbon. Oxygen  will  combine  directly  with  most  metals at high temperatures. The temperature at which oxygen  will  combine  with  the  metals  depends mostly upon the type of metal. In the process of cutting   iron   with   an   oxyacetylene   torch,   the oxygen  combines  with  the  iron. NONELECTROLYTIC   FLUIDS.—  Non- electrolytic  fluids  include  such  liquids  as  pure water,  lubricating  oils,  fuel  oils,  and  alcohols. These  fluids  do  not  cause  corrosion,  but  corro- sion does occur in storage tanks that contain these liquids  and  in  pipelines  that  carry  them.  The corrosion  is  not  caused  by  the  nonelectrolyte liquids, but by the foreign products in them. For example, if impure water is introduced into an oil pipeline,  the  water  will  cause  the  inside  of  the 7-22