INTERGRANULAR   CORROSION The third type of corrosion, intergranular, is not visible on the surface and is very dangerous. It spreads through the interior of the metal along the grain bounda- ries,  reducing  the  strength  and  destroying  the  ability  of the metal to be formed or shaped. Among the metals affected by this type of corrosion are stainless steel, certain magnesium alloys, and the copper-bearing alu- minum alloys. Intergranular corrosion occurs in certain grades of stainless steel when the steel is heated as in welding. Brittleness results, and later the metal cracks near the weld. For this reason, a post-weld heat treatment is needed before you reinstall stainless steel parts that have been welded. As an ABF, you are going to be concerned mainly with  the  first  two  types  of  corrosion,  surface  and galvanic. With this in mind, remember that rust on steel and the white powder on aluminum or magne- sium  are  produced  by  corrosion.  These  products, along with dirt and salt, pick up moisture from the air and hold it in contact with the metal, which speeds up the  corrosive  action. CORROSION  REPAIR There are many factors that affect the type, speed, cause,  and  seriousness  of  metal  corrosion.  Some  of these  corrosion  factors  can  be  controlled;  others  cannot. Preventive maintenance factors, such as inspection, cleaning, painting, and preservation, are within the con- trol of the operating activity. When you first find corrosion on equipment or a structure, the first step you take should be the safe and complete  removal  of  the  corrosion  deposits  or  replace- ment of the affected part. Whether you remove the corrosion  or  replace  the  part  depends  upon  the  degree of corrosion, the extent of damage, the capability to repair or replace, and the availability of replacement parts. Any parts that have been damaged by corrosion should be replaced if continued use is likely to result in structural failure. Areas to be treated to eliminate cor- rosion  deposits  must  be  clean,  unpainted,  and  free  from oil  and  grease.  Chips,  burrs,  flakes  of  residue,  and surface  oxides  must  be  removed.  However,  be  careful to  avoid  removing  too  much  of  the  uncorroded  surface metal. Corrosion deposit removal must be complete. Failure to clean away surface debris permits the corro- sion  process  to  continue  even  after  the  affected  areas have  been  refinished. When corrosion is present, any protective paint films must first be removed to ensure that the entire corroded area is visible. After you remove corrosion, the extent of damage must be assessed. It is at this point that you determine whether to repair or replace the affected part or to perform a corrosion correction treatment.  The  correction  treatment  involves  neutral- izing any residual corrosion materials that may re- main in pits and crevices, and restoring permanent protective  coatings  and  paint  finishes. CORROSION  PREVENTION Corrosion  can  be  controlled  by  maintaining  a  dry environment  using  suitable  moisture  barriers  or  drying agents.   CLEAN,   DRY   METALS   DO   NOT   COR- RODE. Therefore, when moisture and dirt are perma- nently removed from metal surfaces, the tendency of such surfaces to corrode is usually eliminated. Thus, it follows that the major problem in the prevention of corrosion  consists  of  adequately  removing  moisture  and dirt from the surface of the metal to be protected and covering these surfaces to prevent recontamination. Consistent preventive maintenance is the most practical  method  of  controlling  metal  corrosion.  Main- tenance  such  as  cleaning,  painting,  and  preservation shows great savings in labor and materials by eliminat- ing  costly  repairs  and  replacements  required  when  cor- rosion  has  been  permitted  to  go  unarrested. To effectively remove oil, grease, dirt, and other undesirable foreign deposits, you should use certain cleaning  agents,  such  as  soaps,  solvents,  emulsion compounds,  and  chemicals.  When  you  work  with these agents, you should follow the correct method and sequence of procedure in applying them. You also must follow the accepted safety regulations and health precautions in the use and handling of the various cleaning agents. The important factors bearing on the choice of cleaning materials are the type and surfaces to  be  cleaned,  such  as  painted  or  unpainted  surfaces, and whether they are exterior or interior parts. Uses of Paint To prevent corrosion of metal (or deterioration of wood surfaces), you should repaint damaged or worn surfaces as soon as practical. Repaint no more often than is necessary for preservation. In the Navy, paint is used primarily  for  the  preservation  of  surfaces.  It  seals  the pores of wood and steel, arrests decay, and helps prevent the formation of rust. Paint also serves a variety of other purposes. It is valuable as an aid to cleanliness and sanitation,  both  because  of  its  antiseptic  properties  and because it provides a smooth, washable surface. Paint also is used to reflector to absorb light or to redistribute 2-33