Types of Corrosion and Their Effects on Aircraft Structural Integrity Introduction
Today the vast majority of aircraft are made out of various types of metal. All metals are susceptible to corrosion which is why from the moment that any metallic aircraft component is manufactured it is treated to prevent corrosion. Certain elements are often added to base metals by manufacturers in order to create corrosion resistant alloys and metallic components are normally coated with corrosion resistant chemicals and paints before they are sold. Today some airplanes are made of composite materials which are not susceptible to corrosion however these planes have metal components which need to be properly treated and maintained in order to prevent corrosion as well. Types of Corrosion
Corrosion is the degradation of metal and can be caused chemically or electromechanically. Corrosion can take place on the surface of a metal as well as internally and is synonymous to the rotting of wood which can change the profile of a surface, weaken the interior and loosen or damage other components that are in contact with them. The combination of water, water vapour, oxygen and salt in the air all contribute to the most common forms of corrosion. Consequently aircraft that are operated in close proximity to bodies of water or that are located in industrial areas where chemicals are present in the air are very susceptible to corrosion. Corrosion is a major issue in aviation because it can jeopardize the safety of any flight. As previously mentioned there are two general types of corrosion chemical and electromechanical. Each form of corrosion converts some metal into compounds like oxides, hydroxides and sulfates. These compounds are created when oxidation and reduction take place simultaneously on a metallic surface. Oxidation is where electrons are removed from the metal and reduction is where electrons are used in order to convert oxygen and water into hydroxides. These reactions cause certain anodic regions where more oxidation has taken place and cathodic regions where there has been more reduction. These regions play a key role in determining the rate at which a metal will corrode because they affect the way electrons are exchanged between the substance and the surface. The main difference between electromechanical and chemical attacks is that in chemical attacks the anodic and cathodic areas form at the same points on a surface whereas electromechanical attacks happen when these regions are formed at a distance from each other. Figure 1 gives a basic visual representation of an electromechanical corrosion. Chemical attacks are often caused by exposure to corrosive liquids or gasses. For example aircraft often experience chemical attacks when exposed to battery fluid or fumes, residual flux deposits which are left behind when joints have been welded, brazed or soldered or when traces of corrosive cleaning fluids are left behind after washing. Electromechanical corrosion is the most common type of corrosion and it generally happens when some liquid that contains electrolytes, comes in contact with a metallic surface. Electrolytes are elements that become ionized when dissolved in an ionizing substance like water. When electrolytes come in contact with metals oxidation and reduction takes place which causes electromechanical corrosion to occur. Both chemical and electromechanical corrosion can take on various forms depending on the type of metal being used, the size and shape of the metal, the atmospheric conditions and the type of caustic agent that is present. The next section will outline some of the most common forms of corrosion in aviation. Forms of Corrosion
As previously mentioned the appearance of corrosion varies depending on the type of metal being used and its location on the aircraft. For example corrosion on aluminum and magnesium is characterized by pitting and etching on the...
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