CAUSES Prevention and control of corrosion begins with an understanding of the causes and nature of this phenomenon. As stated earlier, corrosion is caused by an electrochemical or a direct chemical reaction of a metal with other elements. In the direct chemical attack, the reaction is similar to that which occurs when acid is applied to bare metal. Corrosion in its most familiar form is a reaction between metal and water and is electrochemical in nature.

In an electrochemical attack, metals of different electrical potential are involved and they need not be in direct contact. When one metal contains positively charged ions and the other metal contains negatively charged ions and an electrical conductor is bridged between them, current will flow as in the discharge of a dry-cell battery. In this type of reaction, the conductor bridge 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 media tends to accelerate the current flow and hence speed the rate of corrosive attack.

Once the electrolyte has completed the circuit (fig. 2-32), the electron flow is established within the metal in the direction of the negatively charged area (cathode). The positively charged area (anode) is eventually destroyed. All preventive measures taken with respect to corrosion prevention and control are designed primarily to avoid the establishment of an electrical circuit. Or secondly, to remove electron flow as soon as possible after its establishment before serious damage can result.

Figure 2-32.- Simplified corrosion cell. Electrochemical attack is evidenced in several forms depending on the metal involved, its size and shape, its specific function, the atmospheric conditions, and the type of corrosion-producing agent (electrolyte) present. A great deal is known about the many forms of metal deterioration that result from electrochemical attack. But despite extensive research and experimentation, there is still much to be learned about other more complex and subtle forms of metal deterioration. Descriptions are provided later in this chapter for the more common forms of corrosion.

Since there are so many contributing factors to the process of corrosion, selection of materials by the manufacturer must be made with weight versus strength as a primary consideration and corrosion properties as a secondary consideration. However, close attention during design and production is given to heat treating and annealing procedures, protective coatings, choice and application of moisture barrier materials, dissimilar metal contacts and accesses. Every logical precaution is taken by the manufacturers to inhibit the onset and spread of corrosive attacks.

There are many factors that affect the type, speed, cause, and seriousness of metal corrosion. Some of these factors can be controlled; others cannot. Preventive maintenance such as inspection, cleaning, painting, and preservation are within the control of maintenance personnel. Preventive maintenance offers the most positive means of corrosion deterrence.

The electrochemical reaction that causes metal to corrode is a much more serious factor under wet, humid conditions. The salt in seawater and in the air is the greatest single cause of corrosion. Hot environments speed the corrosion process because the electrochemical reaction develops faster in a warm solution. Warm moist air is usually sufficient to start corrosion if the metal surfaces are unprotected.

Another corrosion factor is the relationship between dissimilar metals. When two dissimilar metals come in contact, if the more active metal is smaller than the less active one, corrosive attack will be severe and extensive. Insulation between such contact will inhibit this process. If the area of the less active metal is small compared to the other metal, corrosive attack will be relatively slight (fig. 2-33).

Figure 2-33.- Effects of area relationships in dissimilar metal contacts.