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(1) Type of metal;
(2) Heat treatment and grain direction;
corrosion) ; (3) Presence of a dissimilar, less corrodible metal (galvanic
(4) Anode and cathode surface areas (in galvanic corrosion);
(5) Temperature;
(b) Presence of electrolytes (hard water, salt water, battery fluids, etc.);
(7) Availability of oxygen;
(8) Presence of different concentrations of the same electrolyte;
(9) Presence of biological organisms;
(10) Mechanical stress on the corroding metal; and
(11) Time of exposure to a corrosive environment.
Most pure metals are not suitable for aircraft construction and are used only in combination with other metals to form alloys. Most alloys are made surfaces of those regions which are less resistant and also at boundaries Corrosion can occur on between regions, resulting in the formation of pits and intergranular corrosion.
Metals have a wide range of corrosion resistance. The most active metals (those which tend to lose electrons easily), such as magnesium and aluminum, corrode easily. The most noble metals (those which do not lose electrons easily), such as gold and silver, do not corrode easily.
Corrosion is accelerated by higher temperature environments which in air. accelerate chemical reactions and allow greater moisture content at saturation
d. Electrolytes (electrically conducting solutions) form on surfaces when condensation, salt spray, rain, or rinse water accumulate. Dirt, salt, acidic gases, and engine exhaust gases can dissolve on wet surfaces, increasing the electrical conductivity of the electrolyte, thereby increasing the rate of corrosion.
e. When some of the electrolyte on a metal surface is partially confined (such as between faying surfaces or in a deep crevice), metal in this confined area corrodes more rapidly than other metal surfaces of the same part outside this area. This type of corrosion is called an oxygen concentration cell.
Corrosion occurs more rapidly than would be expected, because the reduced oxygen content of the confined electrolyte causes the adjacent metal to become anodic air. to other metal surfaces on the same part immersed in electrolyte exposed to the Chap 2 Par 203 13
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f. Slimes, molds, fungi, and other living organisms (some microscopic) can grow on damp surfaces. Once they are established, the area tends to remain damp, increasing the possibility of corrosion.
g. Manutacturing processes such as machining, forming, welding, or heat treatment can leave stresses in aircraft parts. This residual stress can cause cracking in a corrosive environment when the threshold for stress corrosion is exceeded.
h. Corrosion, in some cases, progresses at the same rate no matter how long the metal has been exposed to the environment. In other cases, corrosion can decrease with time, due to the barrier formed by corrosion products, or increase with time if a barrier to corrosion is being broken down.
FORMS OF CORROSION. There are many different types of corrosive attack and these will vary with the metal concerned, corrosive media location, and time exposure. For descriptive purposes, the types are discussed under what is considered the most commonly accepted titles.
a. Uniform Etch Corrosion. Uniform etch corrosion results from a direct chemical attack on a metal surface and involves only the metal surface (see Figure 2-3). On a polished surface, this type of corrosion is first seen as a general dulling of the surface, and if the attack is allowed to continue, the surtace becomes rough and possibly frosted in appearance. The discoloration or general dulling of metal created by exposure to elevated temperatures is not to be considered as uniform etch corrosion.
and magnesium alloys is called pitting (see Figure 2-4). b. Pitting Corrosion. The most common effect of corrosion on aluminum It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface.
When the deposit is cleaned away, tiny pits or holes can be seen in the surface.
Pitting corrosion may also occur in other types of metal alloys. The combination of small active anodes to large passive cathodes causes severe pitting. The principle also applies to metals which have been passivated by chemical treatments, as well as for metals which develop passivation due to environmental condition.
Chap 2 Par 203 14
FIGURE 2-3. UNIFORM ETCH CORROSION FIGURE 2-4. PITTING CORROSION Chap 2 Par 204 15
7/25/91 AC 43-4A metals make electrical contact in the presence of an electrolyte (see gold in a humid atmosphere, but aluminum would corrode very slowly in contact Figure 2-5). occurs. For example, magnesium would corrode very quickly when coupled with with cadmium. The rate of galvanic corrosion also depends on the size of the activities. c. Galvanic Corrosion. The greater the difference in activity, the faster corrosion The rate which corrosion occurs depends on the difference in the Galvanic corrosion occurs when two dissimilar parts in contact. If the surface area of the corroding metal (the anode) is smaller than the surface area of the less active metal the cathode), corrosion will be rapid and severe. When the corroding metal is larger than the less active metal, corrosion will be slow and superficial. For example, an aluminum severely, while a Monel bracket secured to a large aluminum member would result fastener in contact with a relatively inert Monel structure may corrode in a relatively superficial attack on the aluminum sheet.
CORROSION _ ELECTROLYTE
PRODUCTS (ANODE) (CATHODE) STEFL FASTENER • MAGNESIUM ALLOY FIGURE 2-5. GALVANIC CORROSION OF MAGNESIUM ADJACENT TO STEEL FASTENER
d. Concentration Cell Corrosion. Concentration cell corrosion is corrosion of metals in a metal-to-metal joint, corrosion at the edge of a joint even though joined metals are identical, or corrosion of a spot on the metal surface covered by a foreign material (see Figure 2-6). Another term for this type of corrosion is crevice corrosion. Metal ion concentration cells, oxygen concentration cells, and active-passive cells are the three general types of concentration cell corrosion.
(1) Metal ion concentration cells. The solution may consist of water and ions of the metal which is in contact with water. A high concentration of the metal ions will normally exist under faying surfaces where Chap 2 Par 204 16
7/25/91 AC 43-4A the solution is stagnant, and a low concentration of metal ions will exist adjacent to the crevice which is created by the faying surface. An electrical with the low concentration of metal ions will be anodic and corrode, and the potential will exist between the two points; the area of the metal in contact area in contact with the high metal ions concentration will be cathodic and not show signs of corrosion. Figure 2-6 illustrates metal ion concentration cell corrosion.
Oxygen concentration cells. The solution in contact with the metal surface will normally contain dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two
• Typical locations of oxygen concentration cells are under either such as riveted lap joints. Oxygen cells can also develop under gaskets, wood, metallic or nonmetallic deposits on the metal surface and under faying surfaces rubber, and other materials in contact with the metal surface. Corrosion will occur at the area of low oxygen concentration (anode) as illustrated in Figure 2-6. Alloys, such as stainless steel, which owe their corrosion resistance to surface passivity, are particularly susceptible to this type of crevice corrosion.
passive film, usually an oxide for corrosion protection, such as corrosion (3) Active-passive cells. Metals which depend on a tightly adhering resistant steel, are prone to rapid corrosive attack by active-passive cells.
The corrosive action usually starts as an oxygen concentration cell. As an example, salt deposits on the metal surface in the presence of water containing oxygen can create the oxygen cell. The passive film will be broken beneath the dirt particle. Once the passive film is broken, the active metal beneath the film will be exposed to corrosive attack. An electrical potential will develop between the large area of the cathode (passive film) and the small area of the anode (active metal). Rapid pitting of the active metal will result as shown in Figure 2-6.
e. Intergranular Corrosion. Intergranular corrosion is an attack along the grain boundaries of a material. Each grain has a clearly defined boundary which, from a chemical point of view, differs from the metal within the grain center. The grain boundary and grain center can react with each other as anode and cathode when in contact with an electrolyte. Rapid selective corrosion at the grain boundary can occur with subsequent delamination (see Figure 2-7).
High-strength aluminum alloys such as 2014 and 7075 are more susceptible to exposed to a corrosive environment. intergranular corrosion if they have been improperly heat-treated and are then Chap 2 Par 204 17
- LOW METAL ION CONCENTRATION - HIGH OXYGEN CONCENTRATION
HIGH METAL ION CONCENTRATION LOW OXYGEN CONCENTRATION
METAL ION CONCENTRATION CELL OXYGEN CONCENTRATION CELL
RIVETED LAP JOINT
• FOREIGN MATERIAL CREATES
PASSIVE FILM LOW OXYGEN REGION WHICH
PROTECTS EXPOSED PREVENTS THE RE-FORMATION
SURFACE OF PASSIVE FILM ACTIVE
METAL ACTIVE - PASSIVE CELL FIGURE 2-6. CONCENTRATION CELL CORROSION FIGURE 2-7. INTERGRANULAR CRACKING AND CORROSION ON A WING SPAR CHORD Chap 2 18 Par 204
Exfoliation Corrosion. Exfoliation corrosion is an advanced form of intergranular corrosion where the surface grains of a metal are lifted up by the force of expanding corrosion products occurring at the grain boundaries just below the surface. The lifting up or swelling is visible evidence of exfoliation corrosion (see Figures 2-8 and 2-9). Exfoliation is most prone to occur in wrought products such as extrusions, thick sheet, thin plate and certain die-forged shapes which have a thin, highly elongated platelet type grain structure. This is in contrast with other wrought products and cast products that tend to have an equiaxed grain structure.
FIGURE 2-8. SEVERE EXFOLIATION CORROSION OF A SEAT TRACK Chap 2 Par 204 19
FIGURE 2-9. SEVERE EXFOLIATION CORROSION Filiform Corrosion. Filiform corrosion is a special form of oxygen concentration cell corrosion or crevice corrosion which occurs on metal surfaces having an organic coating system. It is recognized by its characteristic worm-like trace of corrosion products beneath the paint film (see Figures 2-10 and 2-11). Filiform occurs when the relative humidity of the air is between 78 and 90 percent and the surface is slightly acidic. Corrosion starts at breaks in the coating system and proceeds underneath the coating due to the diffusion of water vapor and oxygen from the air through the coating. Filiform corrosion can attack steel and aluminum surfaces. The traces never cross on steel, but they will cross under one another on aluminum which makes the damage deeper and more severe for aluminum. If filiform corrosion is not removed and the area treated and a protective finish applied, the corrosion can lead to intergranular corrosion, especially around fasteners and at seams. Filiform corrosion can be removed using glass bead blasting material with portable abrasive blasting equipment and/or mechanical means such as buffing or sanding. Filiform corrosion can be prevented by storing aircraft in an environment with a relative humidity below 70 percent, using coating systems having a low rate of diffusion for oxygen and water vapors, and by washing aircraft to remove acidic contaminants from the surface, such as those created by pollutants in the air.
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FIGURE 2-10. FILIFORM CORROSION FIGURE 2-11. FILIFORM CORROSION BEFORE AND AFTER PAINT REMOVAL Chap 2 Par 204 21
CORROSION AND MECHANICAL FACTORS. Corrosive attack is often aggravated by mechanical factors that are either within the part (residual) or applied to the part (cyclic service loads). Erosion by sand and/or rain and mechanical wear will remove surface protective films and contribute to corrosive attack of underlying metal surfaces. Corrosive attack that is aided by some mechanical factor usually causes the part to degenerate at an accelerated rate compared to the rate at which the same part would deteriorate if it were subjected solely to corrosive attack. Environmental conditions and the composition of the alloy also influence the extent of attack. Examples of this kind of alliance are stress-corrosion cracking, corrosion fatigue, and fretting corrosion.
Stress-Corrosion Cracking. Stress-corrosion cracking is an intergranular cracking of the metal which is caused by a combination of stress and corrosion (see Figures 2-12 through 2-14). Stress may be caused by internal or external loading. Internal stresses are produced by nonuniform deformation during cold working, by unequal cooling from high temperatures, and by internal structural rearrangement involving volume changes. Internal stresses are induced when a piece of structure is deformed during an assembly operation,
(i.e., during pressing in bushings, shrinking a part for press fit, installing interference bolts, installing rivets, etc.). Concealed stress is more important than design stress, because stress-corrosion is difficult to recognize before it has overcome the design safety factor. The level of stress varies from point to point within the metal. Stresses near the yield strength are generally necessary to promote stress-corrosion cracking, but failures may occur at lower stresses. Specific environments have been identified which cause stress-corrosion cracking of certain alloys. Salt solutions and seawater may cause stress-corrosion cracking of high strength heat-treated steel and aluminum alloys. Methyl alcohol-hydrochloric acid solutions will cause stress-corrosion cracking of some titanium alloys. Magnesium alloys may stress-corrode in moist air. Stress-corrosion may be reduced by applying protective coatings, stress relief heat treatment, using corrosion inhibitors, or controlling the environment. Shot peening a metal surface increases resistance to stresscorrosion cracking by creating compressive stresses on the surface which should be overcome by applied tensile stress before the surface sees any tension load.
Therefore, the threshold stress level is increased.
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APP: 1 268.
65-55701-2
ADCN-WI FIGURE 2-12. STRESS-CORROSION CRACKING OF 7079-T6 FITTING FIGURE 2-13. STRESS-CORROSION CRACKING STARTING AT A PIT IN CRES MATERIAL Chap 2 Par 205 23
CRACKING FOLLOWING GRAIN FLOW
CRACKING FOLLOWING
GRAIN FLOW GRAIN FLOW
POSITIONING OF PART IN
EXTRUSION ENVELOPE FIGURE 2-14. STRESS-CORROSION CRACKING OF AN EXTRUDED SECTION Corrosion Fatigue. Corrosion fatigue is caused by the combined effects of cyclic stress and corrosion. No metal is immune to some reduction in its resistance to cyclic stressing if the metal is in a corrosive environment.
Damage from corrosion fatigue is greater than the sum of the damage from both cyclic stresses and corrosion. Corrosion fatigue failure occurs in two stages.
During the first stage, the combined action of corrosion and cyclic stress damages the metal by pitting and crack formation to such a degree that fracture by cyclic stressing will ultimately occur, even if the corrosive environment is completely removed. The second stage is essentially a fatigue stage in which failure proceeds by propagation of the crack (often from a corrosion pit or pits) and is controlled primarily by stress concentration effects and the physical properties of the metal. Fracture of a metal part, due to fatigue corrosion, generally occurs at a stress level far below the fatigue limit in laboratory air, even though the amount of corrosion is relatively small. For this reason, protection of all parts subject to alternating stress is particularly important, even in environments that are only mildly corrosive.
Fretting Corrosion. Damage can occur at the interface of two highly loaded surfaces which are not supposed to move against each other. However, vibration may cause the surfaces to rub together resulting in an abrasive wear known as fretting. The protective film on the metallic surfaces is removed by the rubbing action. The continued rubbing of the two surfaces prevents formation of protective oxide film and exposes fresh active metal to the atmosphere. Fretting can cause severe pitting (see Figure 2-15). Dampening Chap 2 24 Par 205