Fuselage, Wings and Stabilizing Surfaces — Page 33, Lesson 37

Right, let's pick this up. We've covered the mechanical fastening methods for joining aircraft structure, and now I want to finish that off with the last two methods before we move into a completely new subject: corrosion. So, the fourth method is bolting. This is employed where high shear or tensile loads are experienced. Now, shear load is a force that tries to slide one part sideways against another, and tensile load is a force that tries to pull the parts apart. Most applications use steel bolts. And here's the critical part: these bolts must be locked to make sure that they do not loosen in service. That locking can involve the use of locking wire, split pins, or special nuts. So it's not enough to just tighten a bolt — in aviation, we physically secure it so vibration can't back it off. The fifth and final method is pinning. As the name implies, this uses pins of various designs to hold the materials together. Simple enough — instead of a threaded bolt, you're using a pin to locate and hold the joint. Then we have adhesive bonding. Now, one of the common methods here is called Redux bonding. The process works like this: a sheet of adhesive is placed between the two materials, and then heat is applied to cure the adhesive, which produces a strong bond. Now, one big advantage of this method is that, compared to say riveted joints, it is easier to seal structures. And that makes it particularly useful for fuel tanks — because a fuel tank absolutely must not leak, and a bonded, sealed joint is much better at that than a row of rivets. Now, let's shift gears completely. We're leaving fastening methods and moving into a major maintenance and design concern: corrosion. Let me give you the formal definition first. Corrosion may be regarded as the slow destruction of a metal by electrochemical action — that's electrolytic corrosion. Considerable research by chemists and metallurgists is continually being carried out to find more effective methods of preventing this destruction, but corrosion remains a major problem. So even with all our modern science, we still fight this constantly. Now, why does it happen? Here's the fundamental idea. Most metals are unstable. Corrosion is the tendency of the metal to return to a stable state similar to that of the metallic ore from which it originated. Think about that — the metal ore was dug out of the ground, we refined it into pure metal, and now the metal "wants" to go back to being ore. With corrosive attack, the metal is converted into metallic compounds such as oxides, hydroxides, carbonates, sulphates, or other salts. So the metal is chemically changing into these compounds. Now, the mechanism. Corrosion is largely electrochemical in character, and it occurs in conditions that permit the formation of minute electrolytic electrical cells in or on the attacked metal, in the presence of an electrolyte. An electrolyte is a liquid that can conduct electricity — water with dissolved salts, for example. So you get tiny little electrical cells forming, like microscopic batteries, on the metal surface. It will also occur when a difference in potential exists between the different constituents of an alloy, or where dissimilar metals are in contact. So if two different metals touch each other, and there's an electrolyte present, you get a voltage difference, and that drives the corrosion. Now, here's the interesting part about what happens when a metal is exposed to the air. Oxygen reacts with the bare metal to form an oxide film which adheres to the metal surface. This oxide film forms a barrier between the air and the metal surface, which protects the underlying metal against further attack. So that thin oxide layer is actually a protective shield — it stops the air from getting at the fresh metal underneath. But — and this is the catch — this is all the protection required by some metals. However, the oxides may react chemically or combine with water to produce a film, an oxidation that is not impervious to the passage of further oxygen through it. So in some cases, that oxide film is porous — oxygen can still get through it. The oxide film may crack or flake, exposing the surface to further oxidation. Or, if the metal is subject to heat, the oxides may volatilize — meaning they evaporate or boil off. And in any of those cases, the protective barrier is lost, and the fresh metal underneath is exposed to attack all over again. So to tie it together: corrosion is an electrochemical process where unstable metal tries to return to its ore state, it needs an electrolyte and a potential difference to drive it, and while a protective oxide film can stop it, that film isn't always impervious — it can crack, flake, or volatilize, and then the corrosion continues. That's the core of why we inspect, seal, and protect aircraft structures so carefully.