Fuselage, Wings and Stabilizing Surfaces — Page 33, Lesson 39

Let’s start with the big picture. We’re looking at what happens to an aircraft’s metal structure over time — corrosion. This is a maintenance and airworthiness topic, and it matters because the airframe’s strength depends on the metal staying intact. I’m going to walk you through the conditions that cause corrosion, the evidence you’d look for, the types involved, and the environmental factors that drive the rate of attack. First, the fundamental rule. With one exception — oxidation — corrosion takes place when the metal is in contact with water, either as a liquid or as moisture in the atmosphere. So water is the enabler. The degree of corrosion is proportional to the impurities in that water. The impurities come from two main sources: industrial pollution, which has a high sulphur content, and airborne salt particles, which you get when operating over the sea. So the dirtier the water, the more aggressive the corrosion. Now, what actually happens chemically? The metal undergoes a chemical change. It is converted into a chemical compound — that’s the corrosion product, the stuff you see as rust or pitting — whilst the other metal remains untouched. That last phrase is important: in a corrosion reaction, one metal is consumed and changed, and the other metal stays as it was. That’s the essence of the process. Next, evidence of corrosion. The attack can take different forms. It may extend over the entire surface of the metal — that’s a general, even spread. Or it may penetrate locally, forming deep pits. Or it can follow grain boundaries inside the core of the metal — that’s the internal structure of the alloy, and corrosion tracking along those boundaries is particularly nasty because it works from within. Now, the weakening effect can be aggravated by stresses in the metal. Those stresses can come from external loads — the forces the aircraft experiences in flight — or they can be residual stresses left over from the manufacturing process or the method of assembly. So a part that’s already stressed corrodes faster and weakens more. Now the types of corrosion. The processes are complex, and the various types — oxidation and electrolytic, which is sometimes referred to as galvanic — seldom occur separately. One type frequently leads to another, so two or more types can exist simultaneously in the same piece of metal. So don’t think of them as cleanly separate; they cascade. Why does this matter so much in aviation? Because of the materials. In aeronautical engineering, the need to keep the weight of the aircraft structure to a minimum, commensurate with safety, has led to the development of high-strength alloys, most of which contain aluminium or magnesium. Those alloys suffer damaging corrosion unless they are effectively protected. And the rate of deterioration under unfavourable conditions can be very rapid. So the very materials we use to save weight are the ones most vulnerable. Finally, the environment. Aircraft operate under widely varying climatic conditions in all parts of the world, and some of those environments are highly conducive to corrosive attack. The rate of corrosion depends on the type of atmosphere. Let me give you the scale. Highly conductive to corrosion: tropical, industrial, and marine atmospheres. Moderate corrosion: temperate, suburban, and inland. Low rate of corrosion: arctic and rural. So a tropical coastal industrial area is the worst case — heat, salt, and pollution together. An arctic or rural setting is the mildest. And there’s a figure for you — Figure 1.28 shows oxidation, which is the one exception I mentioned at the start. That’s the case where corrosion happens without the water contact rule applying. So to tie it together: water plus impurities drives corrosion, the metal chemically changes into a compound, the attack can be surface-wide, pitted, or along grain boundaries, stresses make it worse, the high-strength aluminium and magnesium alloys are especially vulnerable, and the atmosphere type sets the rate — tropical, industrial, and marine being the worst. That’s the foundation for understanding how we protect and inspect airframe structures.