pipe to corrode. The water collects on the inside of  the  pipe  because  the  pipe  is  usually  cooler  than the oil. In a storage tank, the water will settle to the bottom of the tank because water is heavier than  oil,  and  will  cause  the  bottom  to  corrode. Hydrogen  sulfide  and  sulphur  dioxide  may  also be introduced into the pipeline to add to the cor- rosiveness of the water that collects on the metal. The   only   way   to   prevent   corrosion   from   this source is either to coat the inside of the pipeline and tanks with a protective film or to remove the water  from  them. Bacterial  Organisms Bacterial   organisms   may   also   cause microbiological   corrosion.   Colonies   of   bacteria that live close to the metal surface in stationary slimy deposits produce corrosive substances such as carbon dioxide, hydrogen sulphide, ammonia, and organic and inorganic acids. These corroding substances  are  found  only  in  the  locality  of  the colony and may be undetected in the surrounding water  or  soil.  Bacteria  that  cause  corrosion  in  this way need to produce only small amounts of cor- rosive  products  for  localized  attack.  However, colonies of bacteria that do not produce corrosive products may act as a protective film around the metal,  causing  unequal  distribution  of  electrical potential,  which  gives  rise  to  local  anodes  and cathodes. In this way, the production of local cells will  cause  increased  corrosive  action. Biological  corrosion  is  extremely  difficult  to control, since the organisms are very resistant to normal  methods  of  sterilization.  Probably  the most  logical  method  to  reduce  microbiological corrosion  is  by  the  use  of  some  barrier  coating between  the  environment  and  the  metal. Corrosion Caused by Electrolytes An electrolyte is any substance that conducts electricity. It conducts electricity because it con- tains  ions  that  carry  electrical  charges,  either negative  or  positive,  that  move  in  electrical  fields. Some   of   the   more   important   electrolytes   are discussed  in  the  following  paragraphs. ATMOSPHERIC   CONDITIONS.—   Cor- rosion  due  to  atmospheric  conditions  is  caused mainly by the water in the atmosphere. Pure water is a nonelectrolyte, but because water is a universal solvent, it is not found to be pure very often. Rain water is often considered to be pure, but this is not true. As rain falls to the ground, it dissolves 7-23 gases  out  of  the  atmosphere  and  becomes  impure. For  this  reason,  any  water  vapor  in  the atmosphere is also impure. If a piece of metal is exposed to atmospheric air, and the metal is cooler than the air, water vapor from the air will collect on  the  surface  of  the  metal.  The  layer  of  water on the metal maybe so thin that it cannot be seen; but there is enough of it, if impure, to start cor- rosion. In this case, when the gases dissolve into the  water,  the  water  becomes  an  electrolyte.  When metal is exposed to an electrolyte, galvanic cells are  produced  on  the  surface  of  the  metal,  since there are impurities in it. Each one of these cells starts  to  act  on  the  metal,  causing  corrosion  by electrochemical   action. WATER  AND  WATER  SOLUTIONS.—   If metal is exposed to water or water solutions, cor- rosion  is  likely  to  occur  if  the  water  or  metal  is impure.  If  the  water  or  metal  is  pure,  corrosion probably will not occur; however, these conditions seldom exist in nature. Impurities in the water and metal produce galvanic cells that cause corrosion. CHEMICAL  AGENTS.—   Chemical   agents such as acids and salts also cause corrosion. When these agents are present in the environment, direct chemical  attack  on  metal  is  the  result.  For example,   if   a   piece   of   zinc   is   exposed   to hydrochloric  acid,  a  definite  chemical  reaction takes place. The zinc and hydrochloric acid com- bine, producing zinc chloride and hydrogen. This action   continues   until   the   zinc   is   completely dissolved or the acid is too weak to act on the zinc. Corrosion  causes  the  zinc  to  dissolve. Another example that may be used to illustrate corrosion through the use of a chemical agent is to place aluminum in a lye solution. The lye will pit  (corrode)  the  aluminum  as  long  as  chemical action continues between the aluminum and lye. MATERIALS  LEAST  LIKELY TO BE AFFECTED BY SCALE  AND  CORROSION Whenever  installing  various  types  of  plumb- ing equipment in areas where corrosion is active, you  should  select  equipment  made  of  materials least  affected  by  corrosion.  To  prevent  elec- trochemical  action  in  plumbing  equipment,  the equipment should be made of materials that are not  affected  by  electrolysis.  Plastic  materials  such as polyethylene polyester and polyvinyl chloride are not acted upon by corrosion. Glass is another material that is not acted on by corrosion. (This