—Never  attempt  to  take  readings  on  operating machinery. We have to understand that even with the best tools, it is the person behind the tool who makes things work. ABFs can take measurements accurately and new parts to be installed can be on hand. If the one who finally  assembles  the  pump  does  not  know  how  to torque a casing or pipe flange, he or she can destroy all the hard work and money that have been put into the job. For maintenance and repair on all equipment, use the appropriate technical manuals. CORROSION   CONTROL LEARNING OBJECTIVES: Describe the types of corrosion the ABF will confront. Identify their signs and explain the corrective action. The  most  effective  repair  protection  is  prevention. A thorough maintenance program continuously carried out  prevents  most  equipment  failure.  With  higher strength and closer tolerances being demanded of met- als, equipment would rapidly become inoperable with- out  regular  anticorrosion  maintenance. Corrosion endangers the equipment by reducing the strength  and  changing  the  mechanical  characteristics  of the  metals  used  in  its  construction.  Materials  are  de- signed  to  carry  certain  loads  and  withstand  given stresses as well as to provide an extra margin of strength for  safety.  Corrosion  can  weaken  the  structure,  thereby reducing or eliminating this safety factor. Corrosion may take place over the entire surface of a  metal  from  chemical  reaction  with  the  surrounding environment.  It  may  be  electrochemical  in  nature  be- tween two metallic materials or two points on the sur- face of the same alloy, differing in chemical activity. The presence of moisture is essential in both types of attacks. The most familiar example of corrosion is rust found on iron or steel. All metals are affected to some extent by the atmos- phere. Water and water vapor containing salt combine with  oxygen  in  the  atmosphere  and  produce  the  main source of corrosion. There are many forms of corrosion; the form of corrosion depends upon the metal involved, atmospheric conditions, and the corrosion-producing agents present. For this discussion, we may consider corrosion as three general types-surface, galvanic, and intergranular   corrosion. SURFACE  CORROSION The  effect  of  the  atmosphere  produces  a  corrosion that appears on the surface of a metal as a general roughening, etching, or pitting. Iron rust is the most common  example  of  surface  corrosion. Although  aluminum,  magnesium,  and  other  nonfer- rous metals do not rust, these metals are subject to surface  corrosion.  Surface  corrosion  on  unpainted  alu- minum alloy is evident as white or gray powdery depos- its on the metal surface. The condition is first indicated by the powdery residue deposited on the area of contact; later pitting and searing appear on the aluminum sur- face, and finally complete deterioration of the alumi- num. Corrosion on painted aluminum-alloy surfaces cannot  be  recognized  by  either  the  roughened  surface or  by  the  powdery  deposit.  Instead,  the  paint  or  plating appears to lift off the surface, indicated by a blistered appearance  and/or  discoloration  that  results  from  the pressure  of  the  underlying  accumulation  of  the  corro- sion  products. Surface corrosion on magnesium alloys can be rec- ognized by powdered or roughened surfaces. Magne- sium  corrosion  products  are  white  and  quite  large compared to the size of the base metal being corroded. The deposits have a tendency to raise slightly, and the corrosion spreads rapidly. When white, puffy areas are discovered  on  magnesium,  prompt  attention  is  required to  prevent  the  corrosion  from  penetrating  entirely through the structure. This can occur in a very short time. It has been generally established that surface cor- rosion is caused by moisture in the air. Since this type of corrosion is visible, it can be detected in its early stages by close visual inspection. Surface corrosion can  be  prevented  or  retarded  by  protecting  the  metal surface with a plating or paint and by keeping the plating  or  paint  in  good  condition. GALVANIC CORROSION Galvanic (or electrolytic) corrosion occurs when two  different  metals  are  connected  and  exposed  to  an electrolyte  such  as  water,  especially  salt  water.  When aluminum pieces are attached with steel bolts or screws, galvanic corrosion may occur between the aluminum and steel in the presence of moisture. An electrical potential is setup, current flows between the two metals, and an effect similar to that which occurs in batteries is produced.  Galvanic  corrosion  can  usually  be  recog- nized by the presence of a buildup of corrosion products at the joint between two metals. Preventive measures include  painting  and  plating. 2-32