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Builders guide to safe aircraft materials
Rev. 4a — page content was last expanded 22 January 2012
Unprotected surfaces of low-alloy steels and aluminium alloys are subject to several forms of chemical or electrochemical attack collectively known as corrosion. Electrochemical corrosion generally involves oxygen, an electrolyte and electron transfer, converting metal to a metallic compound. When combined with fatigue it is a significant contributor to airframe ageing. Stainless steels are not immune to some forms of corrosion.
Electrolytes are electrically conductive solutions produced when atmospheric condensation, precipitation or moisture from other sources is contaminated with salts, acids or alkalies. Those contaminants normally exist in the atmosphere and surface soils in various concentrations or may be intentionally or unintentionally deposited on the airframe and then combined with moisture. Obviously coastal environments promote corrosion but there are also high salt levels in the airborne dust in many inland areas of Australia. The most common forms of corrosion are uniform corrosion, pitting, crevice corrosion and galvanic corrosion. atmospheric moisture and oxygen, bare steels like 4130 will soon acquire a thin, visible, all-over film of loosely adhering oxidised iron or red rust which, if left untreated and exposed to atmosphere moisture, will continue to oxidise (i.e. convert) more of the metal. If the member is a thin-walled tube the rate of uniform corrosion is such that the strength of the structure will continually reduce and will eventually reach a critical level. The rate of corrosion is affected by temperature; each 10° C rise in temperature will double the rate of oxidation. So AISI 4130 steel must be coated inside and outside with a non-conducting material that will also stop, or at least inhibit, the amounts of oxygen or moisture reaching the metal. Indeed it is sometimes advisable to part fill critical tubing components, such as a wing strut fuselage carry-through tube, with a corrosion-inhibiting oil.
Aluminium reacts to exposure to a normal clean atmosphere by developing a passive (i.e. non-reacting) aluminium oxide film over its surface, which protects the metal from further oxidation in similar atmospheric conditions and helps retain structural strength. The film has two layers, a very thin, tightly adhering inner barrier layer and a much thicker outer porous layer, which are integral to each other and to the base material; i.e. the film is not a 'coating' and in normal conditions will restore itself if damaged*. Thus most aluminium alloys don't need protective coating just to resist normal weathering if the pH of solutions contacting the metal is within the range 4.5–8. Lead and copper both form a protective passive film similar to aluminium.
(*Operating in conditions where the metal is exposed to dust/sand abrasion will still lead to accelerated loss of metal.)
By the way, the naturally occurring aluminium oxide — corundum — is not far behind diamond in the ranking of hardest minerals, which is why sanding pads/sheets coated with very fine aluminium oxide grit are a favoured tool in aircraft surface coating applications.
A thicker aluminium oxide film can be induced, in controlled electrochemical processes, on formed aluminium to produce a hard-wearing (but not impact-resistant) anodised surface that is highly resistant to corrosion. During processing the initially soft and porous outer film will also accept various dyes to add colour (black, blue, green, red, gold, etc) for an aesthetically pleasing appearance, which is particularly popular for architectural extrusions. The dye is sealed into the anodised layer when the layer hardens. Some aircraft kit manufacturers supply their completed wing spars anodised and dyed — gold seems a favourite. If incorrectly performed anodising can lead to hydrogen embrittlement of the metal, so it is a process that should be left to the experts.
Anodising should not be confused with alodining (a chromate conversion coating for aluminium to improve corrosion resistance) which imparts a yellowish colour to the metal. Alodine is a trade name.
Deep chloride corrosion pits have been found in stainless steel fuel injector lines and control cable/rod terminal fittings. Where such components are subject to tensile stress in normal operation over time, the pitting corrosion may initiate sub-surface stress corrosion cracking. faying surfaces where the solution concentrates by capillary or other action. Some moisture evaporates and the remaining solution trapped in the crevice becomes stagnant and increasingly corrosive. (Moisture deposited on metal will absorb carbon dioxide from the atmosphere and form a mild carbonic acid, which increases the electrolytic function.) Crevice corrosion is often brought about by poor sealing/protection practices when applying/rinsing off acid etch cleaning solutions prior to painting. If dust containing salt is allowed to remain trapped in joints, then hygroscopic action will eventuate in a corrosion cell. Water staining found between bundled and stored aluminium sheets is a form of crevice corrosion.
[Faying surfaces are surfaces that are closely connected and thus unable to be inspected without dismantling; e.g. sleeving, the surfaces within riveted joints, or between bolt or rivet heads and the metal being fastened, or between an attachment collar and a strut. Faying or fayed is a term that originated in wood shipbuilding indicating a very close joint between timbers.]
In the following table — MIL-STD-889 'Galvanic series in salt water environment' — the smaller the value, the more active is the material (i.e. the more anodic), and the greater the value, the more cathodic is the material. Thus for any two alloys in contact, the further apart they are in the table, the greater the possibility for — and the extent of — galvanic corrosion. The more anodic material (the smaller number) combines with oxygen atoms from any electrolyte present to form an oxide of the metal; i.e. the anodic metal will decompose while the cathodic metal is not affected.
However, the greater the mass of cathodic material relative to the anodic material, the greater the amount and rate of galvanic corrosion and vice versa. Thus an aluminium rivet in a stainless steel tube will corrode very quickly in galvanic cell conditions; i.e. it is not good practice for the anodic material to be the smaller mass in a potential galvanic cell.
Exfoliation corrosion, where grain layers are lifted away, is an advanced form of intergranular corrosion.
Stress corrosion cracking: occurs when high internal tensile stresses induced in the metal during manufacture are allowed to remain: i.e. the metal is not properly stress relieved. A high permanent stress can also be introduced during structure assembly if any of the welding, swaging or fastening processes are performed incorrectly. If this residual stress condition — or a stress condition introduced during normal operation — exists in association with pitting or other corrosion or (in the case of some stainless steel and aluminium alloys) chlorides, then a multi-branched form of intergranular cracking may occur within the metal, particularly when under tension. There may be surface evidence of the condition in the form of pits, rust deposits or cracks but there may be no surface evidence at all. Because of this the Australian Civil Aviation Safety Authority urges that all stainless steel flight control cables be replaced every 15 years, because of possible stress corrosion cracking within the cable terminal fittings.
Filiform corrosion: occurs in high relative humidity conditions under paint coatings such as polyurethane finishes, usually because contamination traces were not completely removed from the substrate or a treatment process was not allowed to fully complete before applying the next coating. Filiform corrosion can occur in electrical componentry where a threadlike deposit is built up across an insulator between two metal parts of the same material.
Fretting or friction corrosion: occurs when vibrations or in-flight loads cause oscillatory movement between faying surfaces.
A form of chemical corrosion will occur if incompatible surface coatings come in contact; for example, methyl ethyl ketone (used as a solvent in a number of fabric cements) will attack zinc chromate metal primers, leaving the metal open to corrosion under the fabric.
Also organic acids are present in bird and animal wastes, so keep mice out of the airframe (and by the way you've no idea the damage that mouse urine can do to electronics); and if you operate from a cattle paddock clean off any 'meadow mayonnaise' that accumulates.
Zinc is the major alloying element in the 7075 zinc-magnesium-copper. This makes 7075 most susceptible to stress corrosion cracking, and to intergranular corrosion if heat treatment has been improperly performed. The copper content reduces resistance to general corrosion so 7075 is less resistant to general corrosion than 3003 and 6061, but more resistant than 2024. It is often used in the Alclad form. Aircraft bolts manufactured from 7075 are usually anodised. Surface coatings and finishes'.
Sacrificial coatings entail plating or hot-dipping the structural metal with a metal that is anodic to it; cadmium and zinc are anodic to steel. so will inhibit corrosion of the steel substrate by acting as a sacrificial coating if surface corrosion develops. Cadmium is generally the standard plating for aircraft AN and MS hardware. Note in the table above that mild steel (line 28) is anodic to plated nickel and chromium (lines 31 and 32), so if a corrosion cell develops it will be the mild steel that decomposes.
The next module in this group is 'Hardware fittings in aircraft structures'
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Builders guide to aircraft materials – metals and hardware modules
| Guide contents | Properties of metals | [Metal corrosion] | Hardware fittings in aircraft structures |
| AN, MS hardware — rivets, bolts and locking devices | Safetying |
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