Fuselage, Wings and Stabilizing Surfaces — Page 33, Lesson 41

We're starting a new topic now: corrosion in aircraft structures. I want to walk you through the three main types we're going to look at — surface corrosion, intergranular corrosion, and stress corrosion — because corrosion is one of the most persistent defects found in aircraft. Rectification of advanced corrosion has been known to take thousands of man hours, so it's essential that corrosion is recognized at the earliest possible stage and effective preventative measures are taken. Let's start with surface corrosion. This is a fairly uniform attack which slowly reduces the cross-sectional thickness of the sound material, and so weakens the structure. The attack is recognized by etching or pitting of the surface. Now, the products of corrosion — the actual visible stuff that forms — are recognized differently depending on the metal. For steels: ferrous metals other than stainless steel become covered with reddish brown powder commonly known as rust. For aluminium and magnesium: corrosion produces powdery deposits, and the colour of which varies between white and grey. Corrosion of magnesium may take the form of deep pitting, or may be fluffy or granular. For copper alloys: copper corrosion in its most common form produces a blue-green salt deposit. Now here's the key point about surface corrosion: it is the least damaging form of corrosion, because there is evidence of the attack. That means it can be detected and rectified at an early stage. The very fact that you can see it is what makes it the least dangerous. Now let's move to the second type — intergranular corrosion, also called inter-crystalline corrosion. This one penetrates the core of the metal along the grain boundaries. Let me explain what that means. Think of the metal's internal structure as being made up of tiny grains, and between those grains are boundaries. As the material at the grain boundaries are usually anodic to the grain centres, the products of corrosion are concentrated at the boundaries. The rate of attack is not limited by the lack of oxygen, and it's accelerated if applied or residual stresses are present. Here's the dangerous mechanism: repeated fluctuating or tensile stresses cause separation of the grain boundaries, accelerating the spread of the corrosion. As a result, higher stress concentrations occur in the remaining sound material. This produces cracks, which spread, leading to complete failure. Now, why is this the most dangerous form? Because detection is difficult, and serious weakening may occur before any external evidence is visible. The only surface indication is a series of hairline cracks, and these are usually only visible through a magnifying glass. There is no effective method of determining or limiting the loss of strength that will occur, so that when detected, parts must be immediately rejected. That's a hard rule — no repair, no assessment, immediate rejection. Now the third type — stress corrosion. This is a combination of a steady tensile load and corrosive conditions, and it produces a form of metal fatigue known as stress corrosion cracking, abbreviated SCC. The stresses may be built in during manufacture of the part, or introduced during assembly, or may be due to operational or structural loads. Here's the mechanism: a metal under stress corrodes more rapidly than unstressed parts. Initially there is pitting of the surface. Loss of the metal at the corrosion pit intensifies the stress at this point, producing a crack which extends under the combined action of corrosion and load until failure occurs. And here's the insidious part — there is generally little visible evidence of corrosion and no apparent loss of metal. So let me tie these three together for you. Surface corrosion is visible, uniform, and the least damaging because you can catch it early. Intergranular corrosion attacks along grain boundaries internally, is nearly invisible, and demands immediate rejection of the part. Stress corrosion combines steady tensile load with corrosive conditions to produce cracking with little visible evidence. Each one has its own recognition features and its own danger level, and as a professional you need to know exactly which one you're looking at and how to respond.