Corrosion Corrosion  is  the  deterioration  of  metal  by Chemical   action.   When   dissolved   oxygen   is present in the boiler water, corrosion begins and continues until all metal has been transformed into iron oxide or, commonly stated, rust. When rust forms in the boiler, it may drop out as sludge or cling  to  other  metal  surfaces.  It  is  not  economi- cally possible to prevent at least some of the iron in  the  boiler  from  going  into  solution.  All  iron not  protected  by  a  coating  or  film  of  something that keeps out moisture and air is sooner or later going to become RUST. The idea is to slow down the  process  as  much  as  possible  by  KEEPING OXYGEN   OUT   and   by   maintaining   a   proper causticity residual. The  pH  level  of  boiler  water  is  also  a  factor in corrosion. The active agent in the corrosion of the  internal  water  surface  of  boilers  is  oxygen; however, the combined action of oxygen and the acid  action  of  the  water  are  required  for  the corrosion process. To suppress the acid action of the water, you can raise the pH value of the water by adding caustic soda. The lower the pH value, the  stronger  the  acid  concentration.  The  higher the   pH   value,   the   weaker   the   concentration. Economically,  acid  corrosion  cannot  be  stopped completely, but it can be suppressed by keeping oxygen  out  of  the  boiler  and  by  maintaining  a proper  pH  value  and  causticity  range. Prevention and Treatment for  Oxygen  Corrosion The chemical most commonly used in oxygen removal  is  sodium  sulfite,  and  it  is  quite  often referred  to  as  an  oxygen  scavenger.  It  is  an example  of  a  chemical  that  actually  reacts  with the  harmful  constituent.  It  reacts  with  oxygen, forming  a  neutral  compound—sodium  sulfate. When  enough  sodium  sulfite  is  fed  into  a boiler   so   that   a   surplus   of   the   chemical   is maintained,  any  of  the  oxygen  getting  into  the boiler water is taken up by the chemical, and the boiler water is kept virtually free of oxygen. By maintaining  a  suitable  residual,  little,  if  any, corrosion  due  to  oxygen  occurs.  Common  prac- tice  in  feeding  sodium  sulfite  is  to  maintain  a surplus  residual  of  about  20  ppm  to  50  ppm  in the boiler water. This is generally enough sodium sulfite  to  react  with  normal  amounts  of  oxygen that might get into the boiler. Higher concentra- tions  of  sodium  sulfite  are  unnecessary. 12-35 Sodium sulfite dissolves readily in water and must  be  fed  at  a  point  between  the  feed  heater and the boiler so that it is used to take up only the oxygen that gets by the deaerator or heater. If the sodium sulfite is fed through the feed lines by continuous feeding, it is always present in the feed lines and takes up oxygen in the feedwater in addition to maintaining a surplus in the boilers. Another advantage of using sodium sulfite is that  if,  for  any  reason,  a  feedwater  heater  or deaerator becomes inoperative or efficient opera- tion  is  temporarily  interrupted,  the  sodium  sulfite residual present in the boiler water can take up the  larger  amounts  of  the  oxygen  getting  in.  At the  same  time,  the  concentration  of  sodium  sulfite drops. This is shown by test analysis of the boiler feedwater.  This  test  gives  the  operator  ample warning  of  an  existing  malfunction  within  the boiler feedwater supply system. Immediate steps should be taken to correct this off-standard con- dition. Feedwater or makeup water tanks should be  heated  to  a  temperature  of  180°F  to  200°F. This heat alone helps to dispense of most of the dissolved  oxygen  before  it  can  enter  the  boiler. It also allows for more economical use of sodium sulfite. The  prevention  of  corrosion  in  the  boiler means  regulating  the  alkalinity  of  the  water, producing  protective  films,  and  removing  dis- solved  oxygen.  These  preventive  measures  are accomplished  by  maintaining  the  proper  chemical residuals  in  the  boiler  water  and  by  proper deaeration. Carryover—Foaming  and  Priming The  word  priming  is  used  rather  loosely  to express  the  action  of  the  water  and  steam  in  a boiler when an unusual amount of water is being carried  over  with  the  steam.  For  a  given  boiler installation, a certain amount of water or moisture in  the  steam  is  tolerated.  The  amount  depends upon  the  use  of  the  steam,  the  boiler  construc- tion,  and  the  facilities  for  removing  the  water from  the  steam.  The  mechanical  causes  include deficiency   in   boiler   design,   high   water   level, improper  method  of  firing,  overloading,  and sudden  load  changes.  A  poorly  designed  boiler may  have  insufficient  steam  disengaging  space. It  is  fairly  obvious  that  the  faster  the  steam  is produced in a given vessel, such as a boiler, the more  violent  is  the  boiling  effect.  But  when  the steam space above the water level is large enough, the  steam  leaving  the  boiler  does  not  show  any evidence   of   carryover.   The   size   of   the   steam