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Builders guide to safe aircraft materials

Metal corrosion


Rev. 4a — page content was last expanded 22 January 2012
  
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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.
10.1 Uniform corrosion or weathering
Even relatively pure water is an electrolyte so when exposed to normal 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.
10.2 Pitting corrosion
Pitting corrosion is the most destructive form of aluminium corrosion, developing at localised weak spots in the oxide barrier when contaminants (like the salts in sea air and inland dust, or in spray thrown up from asphalted surfaces) produce an active solution sufficient to destabilise the protective aluminium oxide film and attack the metal. The reaction commences localised pitting which continues to accelerate; the depth of the subsequent cavities may be greater than the width and the cavity may be concealed by the powdery corrosion products. Although they may not be readily visible, pits in the very thin sheet (~ 0.016 inch/0.4 mm) used in light aircraft stressed skins act as stress raisers/concentrators and can suddenly evolve into stress fractures.

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.
10.3 Crevice or concentration cell corrosion
The process is similar to pitting corrosion except that it occurs in the very narrow gaps between 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.]
10.4 Galvanic corrosion
When two dissimilar metals (or different alloys of the same basic metal) are in close contact in the presence of an electrolyte, a galvanic cell may be formed in which spontaneous electrochemical corrosion occurs. Oxidation occurs at the more active metal (the anode) and electrons flow from the anode to the less active metal (the cathode). The anodic metal corrodes but the cathodic metal may not. In a galvanic couple, aluminium alloys are anodic to most other metals, and to a lesser degree to other aluminium alloys (see the following galvanic series table), so additional steps must taken to separate the two metals — by using a corrosion inhibiting compound [CIC], for example.

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.
  1. Magnesium
  2. Mg alloy AZ-31B
  3. Mg alloy HK-31A
  4. Zinc (hot-dip, die cast, or plated)
  5. Beryllium (hot pressed)
  6. Aluminium 7072 clad on 7075
  7. Al 2014-T3
  8. Al 1160-H14
  9. Al 7079-T6
  10. Cadmium (plated)
  11. Al 218 (die cast)
  12. Al 5052-0
  13. Al 5052-H12
  14. Al 5456-0, H353
  15. Al 5052-H32
  16. Al 1100-0
  17. Al 3003-H25
  18. Al 6061-T6
  19. Al A360 (die cast)
  20. Al 7075-T6
  21. Al 6061-0
  22. Indium
  23. Al 2014-0
  24. Al 2024-T4
  25. Al 5052-H16
  26. Tin (plated)
  27. Lead
  28. Steel 1020
  29. Iron (cast)
  30. Copper (plated, cast, or wrought)
  31. Nickel (plated)
  32. Chromium (plated)
  33. Tantalum
  34. Stainless steel 310
  35. Stainless steel 301
  36. Stainless steel 304
  37. Stainless steel 430
  38. Stainless steel 410
  39. Stainless steel 17-7PH
  40. Tungsten
  41. Brass, yellow, 268
  42. Brass, naval, 464
  43. Muntz metal 280
  44. Brass (plated)
  45. Nickel-silver (18% Ni)
  46. Bronze 220
  47. Copper 110
  48. Brass, red
  49. Stainless steel 347
  50. Molybdenum, commercial pure
  51. Copper-nickel 715
  52. Brass, Admiralty
  53. Stainless steel 202
  54. Bronze, phosphor 534
  55. Monel 400
  56. Stainless steel 201
  57. Stainless steel 321
  58. Stainless steel 316
  59. Stainless steel 309
  60. Stainless steel 17-7PH (passive)
  61. Silicone bronze 655
  62. Stainless steel 304 (passive)
  63. Stainless steel 301 (passive)
  64. Stainless steel 321 (passive)
  65. Stainless steel 201 (passive)
  66. Stainless steel 286 (passive)
  67. Stainless steel 316L (passive)
  68. Stainless steel 202 (passive)
  69. Titanium
  70. Silver
  71. Gold
  72. Graphite
Note: the passive forms of stainless steel alloys deter galvanic activity because passivation entails oxidising the surface, thus making it more inert.
10.5 Other corrosion forms
Intergranular corrosion: attacks the grain boundaries within the metal structure when the chemical differences between grains and the grain boundaries react with each other in the presence of an electrolytic solution. This usually occurs in areas where end grain is exposed; rivet holes for example.

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.
10.6 Effect on aluminium alloys
The 6061 magnesium-silicon, 3003 manganese and 5005 magnesium alloys have very high resistance to general corrosion. Copper, which reduces corrosion resistance more than any other element, is the major alloying element in the 2024 copper-magnesium-manganese alloy. This makes bare 2024 less resistant to general corrosion than 3003, 6061 and 7075, thus the Alclad form of 2024 is often preferred.

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.
10.7 Corrosion inhibition
Apart from keeping the aircraft clean and free from trapped moisture, the materials and methods used to protect metal against corrosion are either to provide an impermeable paint coating to prevent corrosion from starting; or an anodic coating, which is sacrificed if corrosion starts. In some circumstances a corrosion-inhibiting compound will inhibit galvanic corrosion. Painting is covered in the module '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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