Aerospace Corrosion Protection
Almost all metals used in aerospace are subject to corrosion. The attack may take place over an entire metal surface, or it may be penetrating in nature, forming deep pits. It may follow grain boundaries in its attack on metallic surfaces or it may penetrate a surface at random. It may be accentuated by stresses from external loads or existing in the metallic structure from lack of homogeneity or improper heat treatment. It is promoted by contact of the metals with materials that absorb water, such as wood, sponge, rubber, felt, dirt, surface film, etc.
Aerospace surface treatments
Corrosion can be prevented with aerospace engineered coatings and is categorised as follows...
a. Direct Surface Attack. The most common type of general surface corrosion results from direct reaction of metal surface with oxygen in the air. Unless properly protected, steel will rust and aluminium and magnesium will form corrosion products. The attack may be accelerated by salt spray or salt-bearing air, by industrial gases, or by the aircraft engine exhaust gases.
b. Dissimilar Metals Corrosion. When two dissimilar metals are in contact and are connected by an electrolyte (continuous liquid or gas path-salt spray, exhaust gas, condensate) accelerated corrosion of one of the metals may occur. The most easily oxidized surface becomes the anode and corrodes. The
less active member of the couple becomes the cathode of the galvanic cell. The degree of attack depends on the relative activity of the two surfaces; the greater the difference in activity, the more severe the attack. Magnesium and its alloys are quite active and corrode easily. They require maximum protection. Group IV materials in the list below are the least active and therefore require minimum protection. Except as noted below, whenever metals from two different groups are in contact with each other, special protection is required to assure that dissimilar metal corrosion does not occur.
Although aluminium alloys and tin are in different groups than magnesium, tin and the 5000 and 6000 series aluminium alloys may each be used in contact with magnesium without such protection. Tin may also be used with all aluminium alloys without special protection.
(1) Group I. Magnesium and its alloys.
(2) Group II. All aluminium alloys, cadmium, zinc.
1100, 3003, 5052, 6061, 220, 355, 356. All clad alloys.
2014, 2017, 2024, 7075, 195.
Under severe corrosive conditions, all these should be considered as dissimilar metals insofar as corrosion protection is concerned. This is particularly true when a large area of an alloy is in contact with a small area. Severe corrosion may be expected.
(3) Group III. Iron, lead, and tin and their alloys (except stainless steels).
(4) Group IV. Stainless steels, titanium, chromium, nickel, and copper and their alloys, graphite (including dry film lubricants containing graphite).
c. Pitting. While pitting may occur in any metal, it is particularly characteristic of passive materials such as the alloys of aluminium, nickel, and chromium. It is usually a localized breakdown of protection and may be due to a lack of homogeneity in the alloy surface, either from mechanical working or faulty heat treatment. It may also be due to an inclusion or rough spot in the metal surface or from localized contamination that breaks down the surface protection. Pitting takes place at random with no selective attack along grain boundaries. Isolated areas become anodic to the rest of the surface. Corrosion products formed accentuate the anodic characteristics in the pit area, and deep penetrating attack develops rather than a general surface attack.
Intergranular Corrosion prevention
d. Intergranular Corrosion. Selective attack along the grain boundaries of metal alloys is referred to as intergranular corrosion. It results from lack of uniformity in the alloy structure. It is particularly characteristic of precipitation hardened alloys of aluminium and some stainless steels.
Aluminium alloys 2024 and 7075 which contain appreciable amounts of copper and zinc respectively are highly vulnerable to this type of attack if not quenched rapidly during heat treatment or given other special treatment such as the T73 temper condition for the 7075 alloys. Aluminium extrusions and forgings in general may contain non-uniform areas, which in turn may result in galvanic attack along the grain boundaries. This type of corrosion is difficult to detect in its original stage although ultrasonic and eddy current inspection methods are being used. When attack is well advanced, the metal may blister or delaminate. This is referred to as "exfoliation."
Stress Corrosion cracking prevention
e. Stress Corrosion. This results from the combined effect of static tensile stresses applied to a surface over a period of time under corrosive conditions. In general, cracking susceptibility increases with stress, particularly at stresses approaching the yield point, and with increasing temperature, exposure time, and concentration of corrosive ingredients in the surrounding environment. Aluminium alloy bellcranks employing pressed-in taper pins, landing gear shock struts with pipe thread-type grease fittings, clevis joints, and shrink fits are examples of parts which are susceptible to stress corrosion cracking.
Fatigue protection
f. Corrosion Fatigue protection. Corrosion fatigue is a type of stress corrosion resulting from cyclic stresses on a metal in corrosive surroundings. Corrosion may start at the bottom of a shallow pit in the stressed area. Once attack begins, the continuous flexing prevents the repair of protective surface coating or oxide films and additional corrosion takes place in the area of stress. It is difficult to detect this type of attack in advance except as stress corrosion cracking develops.
Fretting prevention
g. Fretting fatigue. Fretting corrosion is a limited type of attack that develops when relative motion of small amplitude takes place between close fitting components. The rubbing contact destroys any protective film that may be present on the metallic surface and additionally removes small particles of virgin metal from the surface. These particles act as an abrasive and prevent the formation of any protective oxide film and exposes fresh active metal to the atmosphere. If the contact areas are small and sharp, deep grooves resembling brinnell markings or pressure indentations may be worn in the rubbing surface. As a result, this type of corrosion has also been called false brinnelling when developed on bearing surfaces.
