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