Corrosion endangers the gas turbine and its supportequipment by reducing the strength and changing thestructural characteristics of the materials used in theirconstruction. All such materials are designed to carrycertain loads and withstand given stresses andtemperatures, as well as to provide an extra margin ofstrength for safety. Corrosion can weaken the structure,thereby reducing or eliminating this safety factor.Replacement or repair operations are costly, timeconsuming, and restrict the usage of the equipment.Corrosion in electronic and electrical components cancause serious malfunctions. These malfunctions reducethe effectiveness and reliability of the engineering plantand can often completely destroy these components.A thorough comprehension of the dangers ofcorrosion and the ability to recognize and cope with thevarious types of corrosion should be included in theobjectives of any maintenance training program. As awork center supervisor, you may find that corrosionprevention and control frequently turn out to be anall-hands evolution. To some extent you can avoid thissituation through frequent inspections, effective use ofavailable manpower, and proper training of yoursubordinates.CORROSIONThe problem of gas turbine engines and supportequipment protection is threefold: (1) prevention ofcorrosion of the metal parts; (2) control of deteriorationof nonmetallic materials; and (3) elimination of physicaldamage during replacement, repair, and maintenance.Of the three basic problems, corrosion of metals is themost difficult to control.Metal corrosion is the deterioration of a metal.When the metal is combined with oxygen, it formsmetallic oxides. This combining is a chemical processthat is essentially the reverse of the process of smeltingmetal from ore. Very few metals occur in nature in thepure state. For the most part, they occur as metallicoxides. The refining process involves the extraction ofrelatively pure metal from its ore and the addition ofother elements (both metallic and nonmetallic) to formalloys.After refining, regardless of whether or not they arealloyed, base metals possess a potential or tendency toreturn to their natural state. However, this potential isnot enough in itself to initiate and promote thisreversion.There must also exist a corrosiveenvironment in which the significant element is oxygen.It is the process of oxidation that causes metals tocorrode.It is a well-known fact that the tendency to corrodevaries widely between various metals. For example,magnesium alloys are very difficult to protect and havea very low corrosion resistance. Copper alloys haverelatively good corrosion resistance and are very easyto protect.Corrosion may take place over the entire surface ofa metal by having a chemical reaction with thesurrounding environment. Or corrosion may beelectrochemical in nature between two differentmetallic materials or two points on the surface of thesame alloy that differ in chemical activity. The presenceof some type of moisture is usually essential forcorrosion to exist.CAUSESPrevention and control of corrosion begins with anunderstanding of the causes and nature of thisphenomenon. As stated earlier, corrosion is caused byan electrochemical or a direct chemical reaction of ametal with other elements. In the direct chemical attack,the reaction is similar to that which occurs when acid isapplied to bare metal. Corrosion in its most familiarform is a reaction between metal and water and iselectrochemical in nature.In an electrochemical attack, metals of differentelectrical potential are involved and they need not be indirect contact. When one metal contains positivelycharged ions and the other metal contains negativelycharged ions and an electrical conductor is bridgedbetween them, current will flow as in the discharge of adry-cell battery. In this type of reaction, the conductorbridge may be any foreign material such as water, dirt,grease, or any debris that is capable of acting as anelectrolyte. The presence of salt in any of the foregoingmedia tends to accelerate the current flow and hencespeed the rate of corrosive attack.Once the electrolyte has completed the circuit (fig.2-32), the electron flow is established within the metalin the direction of the negatively charged area (cathode).The positively charged area (anode) is eventuallydestroyed. All preventive measures taken with respectto corrosion prevention and control are designedprimarily to avoid the establishment of an electricalcircuit. Or secondly, to remove electron flow as soon aspossible after its establishment before serious damagecan result.2-34
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