time-consuming,  and  reduce  equipment  usage  time. These  problems  can  be  avoided  through  good preventive maintenance practices and procedures. To  have  good  preventive  maintenance  practices and procedures, you must know and be able to apply the  common  types  of  corrosion  prevention  and moisture protecting materials. Q1.    How does corrosion endanger aircraft or reduce the margin of safety? Q2. All maintenance personnel must be formally Q3. trained in what program? What is the primary factor to consider when selecting  materials  for  constructing  an  aircraft? on unpainted areas of working parts. Finally, shrouds, covers, caps, and other mechanical equipment provide varying   degrees   of   protection   from   corrosive mediums. However, none of these procedures will provide  100-percent  protection.  Weathering  causes paint to oxidize and decay. Sealants may be worked out by vibration or be eroded by rain and windblast. Preservatives  offer  only  temporary  protection  when used on operating aircraft. The mechanical coverings can be installed improperly or negligently. Control of corrosion begins with an understanding of the causes and the nature of corrosion. Corrosion is CORROSION THEORY in its most familiar form is a reaction between metal and water, and is electrochemical in nature. the process of electrochemical or direct chemical attack on metals. The reaction is similar to that which occurs when acid is applied to bare metal. Corrosion LEARNING OBJECTIVES: Define the theory of  corrosion  and  its  process.  Identify  the publications  and  materials  used  in  the prevention  of  corrosion. Metal corrosion is the decay of metals as they combine with oxygen to form metallic oxides. Corrosion is a chemical process that is the reverse of the process of smelting the metals from their ores. Very few metals are found in their pure state in nature. Most are found as metallic oxides. These oxides have other undesirable impurities in them. The refining process involves the extraction of the base metal from the ore. The base metal is then mixed with other elements (either metallic or nonmetallic) to form alloys. Alloying elements are added to base metals to develop a variety of useful properties. For instance. in aircraft structural applications, high strength-to-weight  ratios  are  the  most  desirable properties of an alloy. After the base metals are refined, whether alloyed or not, they have a potential to return to their natural state. However, potential is not sufficient in itself to begin   and   promote   this   reversion;   a   corrosive environment must also exist. The significant element of the corrosive environment is oxygen. The process of oxidation (combining with oxygen) causes wood to rot or bum and metals to corrode. Control of corrosion depends upon maintaining a separation   between   susceptible   alloys   and   the corrosive   environment.   This   separation   is accomplished in various ways. A good intact coat of paint provides most of the corrosion protection on naval aircraft. Sealants used at seams and joints prevent entry  of  moisture  into  the  metal.  Preservatives  are  used The electrochemical attack involves metals of different electrical potential. These metals do not have to be in direct contact. If one metal contains positively charged ions and the other negatively charged ions, all that is needed is an electrical conductor. When the conductor is present, current will flow between the two metals, as in the discharge of a dry-cell battery. In electrochemical corrosion, the electrical conductor may be any foreign material, such as water, dirt, grease, or any debris that is capable of acting as an electrolyte. The presence of salt in any of the foregoing mediums accelerates the current flow and increases the rate of corrosive attack. Once an electrical connection is made, the electron flow is established in the direction of the negatively charged  metal  (cathode).  This  action  eventually destroys   the   positively   charged   metal   (anode). Preventive   measures   include   avoiding   the establishment  of  the  electrical  circuit  and  removing corrosion as soon as possible to avoid serious damage. Figure 4-1 shows the electron flow in a corrosive Figure 4-1.—Simplified corrosion cell. 4-2