2-1 CHAPTER 2. CORROSION THEORY
2.1 Introduction. This chapter introduces corrosion theory, causes of corrosion, factors influencing its development, and descriptions of forms of corrosion and common corrosive agents.
2.2 Background.
2.2.1 Corrosion is a natural phenomenon which attacks metal by chemical or electrochemical action and converts it into a metallic compound, such as an oxide, hydroxide, or sulfate.
Corrosion is different from erosion, which causes destruction by mechanical action. The corrosion occurs because metals tend to return to their natural state. Noble metals, such as gold and platinum, do not corrode since they are chemically uncombined in their natural state. Four conditions must exist before corrosion can occur (see Figure 2-1, Simplified Corrosion Cell Showing Conditions which Must Exist for Electrochemical Corrosion):
1. Presence of an anode, or a metal that will corrode;
2. Presence of a cathode, a dissimilar conductive material which has less tendency to corrode;
3. Presence of an electrolyte, a conductive liquid; and
4. Electrical contact between the anode and cathode, which is usually by metal-to-metal contact, or through a fastener.
Figure 2-1. Simplified Corrosion Cell Showing Conditions which Must Exist for Electrochemical Corrosion
2.2.2 E limination of any one of these conditions will stop corrosion. An example would be a paint film on the metal surface (see Figure 2-2, Elimination of Corrosion by Application of an Organic Film to Metal Surface). Metals such as stainless steel and titanium can produce corrosion products so tightly bound to the corroding metal that they form an
2-2 invisible oxide, or passive, film, preventing further corrosion. When a film of corrosion products is loose and porous, such as those of aluminum and magnesium, an electrolyte can easily penetrate and continue the corrosion process, producing more extensive damage than surface appearance would show.
Figure 2-2. Elimination of Corrosion by Application of an Organic Film to Metal Surface
2.3 D evelopment of Corrosion.
2.3.1 All corrosive attack begins on the surface of the metal, and the corrosion process involves two chemical changes: 1) the metal that is attacked or oxidized undergoes an anodic change, and 2) the corrosive agent is reduced or undergoes a cathodic change. The tendency of most metals to corrode creates one of the major problems in aircraft maintenance, particularly where adverse environmental or weather conditions exist.
2.3.2 Paint coatings can mask initial stages of corrosion. Since corrosion products occupy more volume than the original metal, paint surfaces should be inspected often for such irregularities as blisters, flakes, chips, and lumps.
2.4 Factors Influencing Corrosion.
2.4.1 Some factors which influence metal corrosion and the rate of corrosion are the:
1. Type of metal;
2. Heat treatment and grain direction;
3. Presence of a dissimilar, less corrodible metal (galvanic corrosion);
4. Anode and cathode surface areas (in galvanic corrosion);
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5. Temperature;
6. Presence of electrolytes (such as hard water, salt water, or battery fluids);
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;
11. Time of exposure to a corrosive environment; and
12. Lavatory fluids with hard water, salt water, or battery fluids.
2.4.2 Most pure metals are not suitable for aircraft construction and are used to form alloys.
Most alloys are made up entirely of small, crystalline regions called grains. Corrosion can occur on the less resistant surfaces of these regions and at boundaries between regions, resulting in the formation of pits and intergranular corrosion. Metals have a wide range of corrosion resistance. The most chemically active metals (which tend to lose electrons easily, such as magnesium and aluminum) corrode easily. The most noble metals (which do not lose electrons easily, such as gold and silver) do not corrode easily.
2.4.3 High-temperature environments accelerate corrosion by increasing chemical reaction rates and moisture content in saturated air.
2.4.4 Electrolytes, or electrically conducting solutions, form when condensation, salt spray, rain, or rinse water accumulate on surfaces. Dirt, salt, acidic gases, and engine exhaust gases can dissolve on wet surfaces, increasing electrolyte conductivity and also the rate of corrosion.
2.4.5 When areas such as between faying surfaces or in deep crevices partially confine an electrolyte on a metal surface, that surface corrodes more rapidly than other metal surfaces outside this area. This area of corrosion is called an oxygen concentration cell.
The reduced oxygen content of the confined electrolyte causes adjacent metal to become anodic to other metal surfaces immersed in electrolyte and exposed to air on the same part.
2.4.6 Slimes, molds, fungi, and other living organisms, some of which are microscopic, can grow on damp surfaces. The area where such organisms become established tends to remain damp, increasing the possibility of corrosion.
2.4.7 Manufacturing 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.
2.4.8 In some cases, corrosion progresses at the same rate no matter how long the metal has been exposed to an environment. In other cases, corrosion can decrease with time because of a barrier formed by corrosion products, or increase with time if a barrier is breaking down.
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2.5 Forms of Corrosion. Different types of corrosive attack will vary with the metal concerned, corrosive media location, and the length of time of exposure. For descriptive purposes, the types are listed under what the FAA considers the most commonly accepted titles.
2.5.1 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, Uniform Etch Corrosion). On a polished surface, this type of corrosion may first appear as a general dulling of the surface. If the attack continues, the surface becomes rough and possibly frosted in appearance. However, it should be noted that the discoloration or general dulling of metal created by exposure to elevated temperatures is not uniform etch corrosion.
2.5.2 Pitting Corrosion. This is the most common effect of corrosion on aluminum and magnesium alloys, but can occur in other metal alloys (see Figure 2-4, Pitting Corrosion).
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 in the surface can be seen. The combination of small active anodes and large passive cathodes causes severe pitting. The principle also applies to metals passivated by chemical treatments or environmental condition.
Figure 2-3. Uniform Etch Corrosion
2-5 Figure 2-4. Pitting Corrosion
2.5.3 Galvanic Corrosion. This corrosion type occurs when two dissimilar metals make electrical contact in the presence of an electrolyte (see Figure 2-5, Galvanic Corrosion of Magnesium Adjacent to Steel Fastener). The rate of corrosion depends on differences in reaction activity between dissimilar metals. The greater the difference, the faster corrosion occurs. For example, aluminum skin panels and stainless steel doublers, riveted together in an aircraft wing, form a galvanic couple if moisture and contamination are present. The rate of galvanic corrosion also depends on the size of the 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. But if the corroding metal is larger than the less active metal, corrosion will be slow and superficial.
Figure 2-5. Galvanic Corrosion of Magnesium Adjacent to Steel Fastener
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2.5.4 Concentration Cell Corrosion or Crevice Corrosion. In this type, corrosion occurs in either a metal-to-metal joint, at the edge of a joint of identical metals, or on a spot on the metal surface covered by a foreign material (see Figure 2-6, Concentration Cell Corrosion). Below is a discussion on the three general types of concentration cell corrosion:
2.5.4.1 Metal Ion Concentration Cells. A liquid solution that may consist of water and ions of a metal in contact with water. A high concentration of the metal ions will normally exist under faying surfaces where the solution is stagnant.
A low concentration of metal ions will exist adjacent to the crevice created by the faying surface. An electrical potential will exist between the two points of connection, and the area of the metal in contact with the low concentration of metal ions will be anodic and will corrode. The area in contact with the high metal ion concentration will be cathodic and will not show signs of corrosion.
Figure 2-6 illustrates metal ion concentration cell corrosion.
2.5.4.2 Oxygen Concentration Cells. A solution in contact with a metal surface that normally contains dissolved oxygen. An oxygen cell can develop at any point where oxygen in the air does not diffuse into the solution, and creates a difference in oxygen concentration between two points where the solution and metal meet. Typical locations of oxygen concentration cells are under: 1) either metallic or nonmetallic deposits on the metal surface, 2) faying surfaces such as riveted lap joints, or 3) gaskets, wood, rubber, and other materials in contact with the metal surface. Corrosion will occur at the area of low oxygen concentration (the anode), as Figure 2-6 illustrates. Alloys such as stainless steel, which owe their corrosion resistance to surface passivity, are particularly susceptible to this type of crevice corrosion.
2.5.4.3 Active-Passive Cells. Metals which depend on a tightly adhering passive film, like an oxide for corrosion protection such as on corrosion resistant steel (CRES), are prone to rapid corrosive attack by active-passive cells. 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 a dirt particle.
Once the passive film is broken, corrosion will attack the active metal beneath the film. An electrical potential will develop between the large area of the cathode, or passive film, and the small area of the anode, or active metal, and rapid pitting of the active metal will result, as shown in Figure 2-6.
2.5.5 Intergranular Corrosion. This corrosion attacks grain boundaries of a metal. Each grain has a clearly defined boundary which differs chemically 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 and cause delamination (see Figure 2-7, Intergranular Cracking and Corrosion on a Wing Spar Chord). High-strength aluminum alloys, such as schedule 2014 or 7075, are more susceptible to intergranular corrosion if improperly heat-treated then exposed to a corrosive environment.
Figure 2-6. Concentration Cell Corrosion
- 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
SURFACE PROTECTS EXPOSED PREVENTS THE RE-FORMATION V OF PASSIVE FILM ACTIVE
METAL ACTIVE - PASSIVE CELL Figure 2-7. Intergranular Cracking and Corrosion on a Wing Spar Chord
2.5.6 Exfoliation Corrosion. Exfoliation 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. The lifting up or swelling is visible evidence of exfoliation corrosion. Exfoliation occurs on extruded, rolled, wrought, and forged high-strength aluminum and magnesium parts. See Figures 2-8, Severe Exfoliation Corrosion of a Seat Track, and 2-9, Severe Exfoliation Corrosion.
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Figure 2-8. Severe Exfoliation Corrosion of a Seat Track Figure 2-9. Severe Exfoliation Corrosion 2-8
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2.5.7 Filiform Corrosion. A special form of oxygen concentration cell corrosion or crevice corrosion occurring on metal surfaces with an organic coating system. You can recognize filiform corrosion by a worm-like trace of corrosion products beneath a paint film.
Figures 2-10, Filiform Corrosion, and 2-11, Filiform Corrosion Before and After Paint Removal, illustrate this corrosion. Filiform occurs when atmospheric relative humidity is between 78 and 90 percent and the metal surface is slightly acidic. Corrosion starts at breaks in the coating system and moves underneath the coating due to diffusion of water vapor and airborne oxygen through the coating. Filiform corrosion can attack steel and aluminum surfaces. The corrosion can lead to intergranular corrosion, especially around fasteners and at seams, if filiform corrosion is not removed, treat the area, and apply a protective finish. Prevention of filiform corrosion can involve 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, such as airborne pollutants, from the surface.
Figure 2-10. Filiform Corrosion
2-10 Figure 2-11. Filiform Corrosion Before and After Paint Removal
2.6 Corrosion and Mechanical Factors. Mechanical factors often aggravate corrosive attack by either residual loading inside the part, or by cyclic service loads applied to the part. The extent of the attack can be influenced by environmental conditions, alloy composition, erosion by sand or rain, and mechanical wear. These factors will combine to remove surface protective films and contribute to corrosive attack of underlying metal surfaces. Corrosive attack aided by a mechanical factor usually causes the part to degenerate faster than if it is only corroding. Examples of this kind of alliance between corrosion and mechanical factors are stress-corrosion cracking, corrosion fatigue, and fretting corrosion.
2.6.1 Stress-Corrosion Cracking. Stress-corrosion cracking is an intergranular cracking of a metal caused by combined stress and corrosion, and Figures 2-12 through 2-14 illustrate this. Stress may come from internal or external loading. Causes of internal stresses include nonuniform deformation during cold working, unequal cooling from high temperatures, or internal structural rearrangement involving volume changes. Internal stresses result when a structure is deformed during an assembly operation, such as during pressing in bushings, shrinking a part for press fit, installing interference bolts, or installing rivets. Concealed stress is more critical than design stress because stress-corrosion is difficult to recognize before it has overcome the structure’s design safety factor. The level of stress varies from point to point within the metal, and stresses near the yield strength generally promote stress-corrosion cracking. Specific environments which cause stress-corrosion cracking of certain alloys have been identified. Salt solutions and seawater may cause stress-corrosion cracking of
2-11 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, using a stress relief heat treatment, applying corrosion inhibitors, or controlling the environment. Shot peening a metal surface creates compressive stresses on the surface which help reduce stress-corrosion cracking and increase the threshold stress level.
Figure 2-12. Stress-Corrosion Cracking of 7079-T6 Fitting
Figure 2-13. Stress-Corrosion Cracking Starting at a Pit in CRES Material Figure 2-14. Stress-Corrosion Cracking of an Extruded Section