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