environment destroying the anodic area. Note that the surface of a metal may contain anodic and cathodic areas because impurities or alloying constituents may have different potentials than the base metal. Electrochemical attack is evident in several forms. The form you find depends upon the metal involved, its size and shape, its specific functions, atmospheric conditions, and type of corrosion-producing agent (electrolyte)  present. There are many factors that affect the type, speed, cause, and the seriousness of metal corrosion. Some of these factors you can control; others you cannot. Preventive maintenance factors, such as inspections, cleaning,  painting,  and  preservation,  are  within  the control of the operating squadron. They offer positive means of preventing corrosion. The electrochemical reaction, which causes metal to  corrode,  is  more  dangerous  under  wet,  humid conditions than under dry conditions. The salt in seawater and the salt in the air are the largest single cause of aircraft corrosion. Hot climates speed the corrosion   process   because   the   electrochemical reaction develops fastest in a warm solution. The warm moisture in the air is usually enough to start corrosion of the metals if they are uncoated. As expected, hot, dry climates usually provide relief from constant corrosion problems. Extremely cold climates will produce   corrosion   problems   when   a   salt-laden atmosphere is present. Melting snow or ice provides the  necessary  water  to  begin  the  electrochemical reaction. Thick structural sections are subject to corrosive attack  because  of  possible  variations  in  their composition, particularly if they were heat-treated during fabrication. Similarly, when large sections are machined  or  cut  out  after  heat  treatment,  thinner sections  have  different  physical  characteristics  than the thicker areas. Usually a difference in physical characteristics   provides   enough   difference   in electrical   potential   to   make   the   piece   highly susceptible to corrosion. Another factor relating to the size of materials is the relationship between dissimilar metals. (See figure 4-2.) If electrical contact develops between two dissimilar metals, the corrosion attack on the more active metal or anode (smaller size compared to the less active one) will be severe and extensive. See figure 4-2, bottom view. If the area of the less active metal is small compared to the other, anodic attack will be slight (fig. 4-2, top view). Corrosion on avionics equipment is a continuing process. The equipment does not have to be installed, operating,  or  exposed  to  a  particularly  harsh environment to corrode. The rate of the corrosion process is determined by the temperature, humidity, and chemicals in the environment. Moisture is the single largest contributor in avionics corrosion. It makes little difference whether the moisture is in the form of vapor or liquid. Its affects are detrimental to metals. A clean aircraft retains its aerodynamic efficiency and safety. Serious damage to the exterior and interior surfaces of aircraft can result from the lack of correct information about cleaning materials and equipment and   their   use.   Shipboard   procedures   are   not necessarily the same as procedures ashore, but the same materials are available to produce comparable results. A problem you may face when fighting corrosion is knowing what materials to use, where to find them, and  their  limitations.  You  should  use  only  those materials  that  have  military  specifications.  Corrosion control information can be found in many directives Figure 4-2.—Effects of area relationships in dissimilar metal contacts. 4-3