Corrosion & Light Aircraft

These pollutants come in a variety of compounds and can be acids, salts or alkalines, but all of them act as electrolytes: that is to say they allow the transfer of ions from one location to another. The final piece of the jigsaw is the presence of two different metals in close proximity of each other: in the presence of an electrolyte they will form the anode of a crude battery. As the electrochemical reaction takes place, the anode is sacrificed (as the more reactive element) and is converted to a compound to the benefit of the cathode, which remains intact. This process is put to good use by boat owners, who fix chunks of zinc to the hull beneath the waterline of their vessels. Zinc is very reactive and is eaten away, protecting other metal fittings within the hull. Not surprisingly, this is known as dissimilar metal corrosion!

Pure aluminium is not strong enough for structural purposes in aircraft components, so to obtain the required properties it is alloyed with a significant (up to about 10%) amount of other metals – copper and zinc being the· most common. Add an electrolyte and the scene is set.

The most common form of corrosion is pitting – the pits form as anodes and if unchecked the pits will deepen and spread until a considerable amount of metal has been eaten away. In severe cases compromising the structural integrity of the part. This is the usual form of attack on aluminium/copper alloys, such as 2024 (by far the most common material used in all metal aircraft) most of the external skins are usually 2024. In this instance, it is the grains of aluminium which are converted to other compounds, as aluminium is more reactive than copper. The worse form of corrosion is intergranular. It generally attacks 7000 series alloys – those with an appreciable amount of zinc – the stuff that spars, stringers and other high strength parts are made from. It’s not all that common: it seems to be partly caused by poorly controlled heat treatment during manufacture. Unfortunately, once you can see it, it’s too late – the part is scrap.

The corrosion attacks the grain boundaries of the metal crystal structure, the alloying agents in this case being more reactive than the base metal. The corrosion biproducts are less dense and less cohesive and tend to force the grain structure apart. In extreme cases, the metal puffs up into a multitude of fine sheets called exfoliation. It is usually only found in extruded items, as the grain structure tends to be linear.

Stress corrosion is a particular form of intergranular corrosion: in highly stressed parts like landing gear components, a crack may be started from a stress riser originating from a scratch or surface corrosion: it is then accelerated by electrochemical reaction which has a preference for the very end of the crack. This is often the form that crankshaft failures take.

Quite a large percentage of engine failures can be traced to corrosion starting the chain of events that leads to a heart stopping silence: despite the oily interior, condensation within an engine can combine with contaminants in the oil to form corrosive compounds. The only real way to keep it at bay is regular oil changes and making sure the engine gets properly hot to vaporise off any water present.

Each set of circumstances produces a particular form of corrosion. Poorly prepared aircraft that are painted with 2 pack polyurethanes tend to suffer from filiform corrosion. If the acid pretreatment is not completely washed off and neutralised before spraying, it produces fine worm like lines of corrosion under the paint that will eventually lead to bubbling and flaking. Under surfaces are the usual places for filiform to happen, although it can occur anywhere. Aircraft painted with acrylic paints don’t seem to suffer as much from this, as the acrylic is slightly porous and allows the acid to be washed away. Mind you, acrylics are nowhere near as good as polyurethanes at protecting the metal from external pollutants.

Fretting corrosion occurs when two pieces of metal in very close proximity have a degree of relative movement to each other: it usually affects rivets and occasionally lap joints in sheet metal: an electrolyte need not be present, even if there is a protective oxide film forming, the rubbing action scrapes it off the surface of the joint allowing fresh metal to be exposed and the debris (a smoky grey powder) makes a fine grinding compound. The only cure is to drill out the rivet and replace it with a properly fitting one, even if it means going up a size. Piano hinges, as fitted to some control surfaces, (with the added bonus of dissimilar metal components) are very prone to fretting corrosion. Even modern composite materials are not immune, to a lot of peoples disgust, carbon fibre laminates and aluminium alloys suffer from quite severe dissimilar metal corrosion together.

Jet aircraft have their own particular problems: some strains of microbes and plant life find the combination of darkness and Jet-fuel are very much to their taste: they live in the water suspended in the fuel and form a scum that covers the sides and bottom of the tank, holding water against the surface in a poul tice effect. It doesn’t take much damage to reach the ‘uneconomic to repair’ level when the main spar forms part of a wet wing fuel tank.

There are various areas in an aircraft where you can reasonably expect corrosion: the battery box is an obvious one, and it should be regularly cleaned out using a solution of bicarbonate of soda and water, followed by flushing with clean water. Any low areas where water can collect are good places. Manufacturers are not very good at putting in drain holes, or for that matter designing out water traps: stringers on the low side of a fuselage often form convenient puddling spots, as do fuselage formers: they should have gaps at the bottom to allow water to drain, but often they are plugged with fluff and debris.

Dirty surfaces also tend to hold water in contact with the skin allowing corrosion to get a good foothold sight unseen. Ground points and static bonding strap terminals are ideal places for corrosion to form: apart fromdissimilar metal contact flow will promote the rot, and lead to a whole range of and hard to trace noisy radio problems. Even stainless steel parts – exhausts in particular, are not immune: stainless steel is technically known as corrosion resistant steel, not corrosion proof, and the combination of high temperatures and corrosive gasses will eventually take their toll.

Apart from the danger of having to scrap an airframe due to corrosion, a great deal of time and money is wasted dealing with the effects. It’s a major problem (I’d say the major problem) with ageing airline fleets and the good news is that some of the more effective control technology is filtering down to the general aviation sector. Prevention and control falls into three main categories, with many products combining the properties of two categories.

The first is simply mechanical protection: stick a layer of paint over it. That’s all very well if the paint film is unbroken, but if a scratch or crack in the paint allows the electrolytes through it’s not much good. For that matter, a lot of paints are porous and to make matters worse, most paints are not particularly inclined to stick to an aluminium surface unless it has been etched with an acid wash, and is absolutely clean and dry. An alodyned surface is rather better, which is primarily the reason paint shops bother with the process.

Most early metal aircraft left the factory with only a small amount of paint- trim lines and registration, and were polished to combat the problem of corrosion. The external panels were made of Alclad sheet-the chosen alloy (usually 2024) that had a thin film of pure aluminium applied during manufacturing – usually around 2 thou thick.

Pure aluminium is actually pretty corrosion resistant and any surface corrosion tends to protect the metal underneath. Alodyne usually produces a golden brown or blue colour when it’s applied, but it’s now possible to buy a clear version to protect polished surfaces. The problem with Alclad is, like paint, it scratches through the surface and the recommended repair in some cases is a riveted patch! Somewhat drastic, I’d say. Steel parts tend to be plated to protect the surface, although a coat of paint or two works very well as paint will bond to steel rather better than aluminium alloys. The second range of protectors fall into inhibitors and sacrificial elements. Whilst nickel or chrome plate on steel parts forms a decorative air and water tight cover, zinc or cadmium plating provides a sacrificial element that is corroded in place of the steel – as in the zinc ingots on boat hulls.

Some primers also contain fine metallic particles to act as sacrificial elements. Zinc Chromate, the yellow or green primer that you can see on a great many aircraft parts, is not, as you might expect, sacrificial, but an inhibitor: it’s slightly porous and any moisture present allows chromate ions (the active ingredient in the alodyne treatment) to be released and held on the surface of the metal. This neutralises the corrosion salts and prevents electrolytic action from taking place. Eventually, of course, the protective elements become consumed and no longer provide protection.

The third method of protection is by using water displacing compounds (WDCs): for corrosion to take place, there must be water present to provide the electrolyte. This is why aircraft in very dry climates tend to be fairly corrosion-free: all that can happen is oxides forming, and they tend to be self limiting, at least in the case of aluminium alloys. Displacing the electrolyte removes one of the essential elements required for corrosion.

The problem with WDCs is that they’re no good except in the very short term on external surfaces and most of them only work for a month or so internally because they evaporate to leave a waxy protective film: which is easily scratched, allowing electrolytes access. Repeated applications just allow a build-up of gummy deposits in the long term, which attracts dirt, water and so on.

The one WDC product which really does seem to be totally effective is ACF-50. It doesn’t evaporate off; but remains as a thin self healing film that excludes water; even getting into lap joints and around rivet heads. It won’t last forever, so it needs to be reapplied periodically. The DS military approves it for everything, including plastics, rubber and electronics. A friend of mine who operates a floatplane in a marine environment (a worst case scenario) absolutely swears by it; internal corrosion is held at bay, and radios that gave constant problems now work reliably having been dunked in it.

No corrosion control system is perfect: some of the really effective ones like zinc chromate have serious drawbacks; it’s highly carcinogenic, although this is a problem only during application and all of them need renewing from time to time, but that’s a small prices to pay compared with renewing an entire aircraft.