The “galvanic series” allows us to rate metals from most anodic (the part that loses material during corrosion) to least anodic (most cathodic). At one end of the list are elements like gold, platinum and at the very bottom carbon (graphite). These are least likely to corrode away. On the other end are elements like aluminum, cadmium, zinc and on top is magnesium (being most anodic). It is obvious why cadmium and zinc are often used as sacrificial plating on steel parts. The thin coating will corrode before damage occurs to the main component. As anyone knows who does component overhaul and deals with magnesium bellcranks, transmission housings or on older helicopters some skins were magnesium, that stuff corrodes if you just look at it. In a simplified scheme of things, an anode gives up
electrons and gains oxygen, so it is the site of oxidation. The cathode of a galvanic cell gains electrons and gives up oxygen and is the reduction site. Oxygen is not required for corrosion but it readily accepts electrons and greatly accelerates the process. Metal corrosion is caused by an electrolytic (galvanic) reaction which is the result of the fact that when two different metals or areas within an alloy are in contact (having different electrical potentials) and are exposed to a salt solution (electrolyte), it causes a current to flow. This oxidation/ reduction or “redox” reaction is what metal corrosion is about. We can do several things to slow corrosion. By painting we seal the exposed surfaces from the electrolyte (water) and the atmosphere. By passivating we most commonly create a coating on the surfaces that is resistant to being further oxidized. A coating of pure aluminum on an aluminum alloy sheet allows aluminum oxide to form. Duraluminum or alclad is what we call this. We use chromate “conversion coatings” as passivators on aluminum and magnesium alloys. “Black oxide,” “parkerizing” (commonly called gun bluing) and phosphatizing are used to passivate steel alloys. Anodizing is another method of passivating or thinly coating metals by chemical reaction that slow corrosion of the part. Sacrificial (cad. or zinc) coatings tend to be thicker than passivating coatings. Although electrical bonding of structure serves the more important purpose of preventing static build- up, thereby protecting avionics and preventing arcing hazards (fire protection) it can also reduce electrical potentials between metal parts and thus slow down corrosion. So much for metals. One good thing about metals corrosion is that we pretty much know it when we see
it. We know how much is too much and we know how to slow it down. This is not always true for composite materials.
COMPOSITE CORROSION Composite structures also corrode. Whether you are looking at glass, aramid or carbon fiber impregnated fabric, they will absorb water (which can cause swelling) and are also subject to degradation by exposure to sunlight. Ultra violet rays are particularly harmful, but visible light can also cause damage. Water damage may be allowed by inadequate surface protection, small cracks in surface protection or other types of physical damage to surfaces that are normally water proof (e.g. dropped tools, stone or other debris chips etc.). Water within the matrix of a composite may expand when it freezes making matters worse. Rotor blade shops are well aware of the need to remove any trapped moisture prior to repairs. The longer the wavelength of light the less energy
it carries per photon. That is why ultraviolet is of more concern, because it has a shorter wavelength than the yellows or reds of the spectrum. In order for ultraviolet light to cause damage two things have to occur. First the material has to absorb UV light. Once this happens there are several outcomes possible. First the absorbed energy can be reradiated as heat. Second, the energy can be reradiated at a different wavelength or reflected. These first two pathways are how the sunblock we apply to our skin (if we’re smart) works to reduce damage from ultra violet radiation. Third, the energy can be absorbed by the chemical compounds in the matrix and this is the primary reason UV can degrade composite polymers of various types. UV being a wavelength that can get through the earth’s atmosphere can also contain enough energy to degrade the chemical bonds (more than the dissociation energy required to break a bond) and thereby weaken composite structure. We have all seen examples of fiberglass polyester resin or epoxy materials that have become discolored by sunlight. This is not as strong as when it was initially cured and will continue to degrade until it can fall apart. Heat also damages composites. Many helicopters
have heat shields to deflect exhaust gasses. It is also an issue for aluminum alloy, but not to the same extent. Part of the problem is that composites can lose up to half or more of their initial strength without showing visible signs of degradation. There is evidence that shows that in some cases exposure to moderately
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HelicopterMaintenanceMagazine.com August | September 2019
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