2-6
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.
2-7
Figure 2-8. Severe Exfoliation Corrosion of a Seat Track Figure 2-9. Severe Exfoliation Corrosion 2-8
2-9
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
CRACKING FOLLOWING GRAIN FLOW
CRACKING FOLLOWING
GRAIN FLOW GRAIN FLOW
POSITIONING OF PART IN
EXTRUSION ENVELOPE 2-12
2-13
2.6.2 Corrosion Fatigue. Cyclic stress and corrosion combine to form this type of stress in two stages. No metal is immune to cyclic stressing if the metal is in a corrosive environment.
However, damage from corrosion fatigue is greater than damage from both cyclic stresses and corrosion. Corrosion fatigue failure occurs in two stages. During the first stage, corrosion and cyclic stress severely damage the metal by pitting and crack formation.
Fracture by cyclic stressing will ultimately occur, even if the corrosive environment is completely removed. The second stage is propagation of the crack, often spreading from corrosion pits. Stress concentration effects and the physical properties of the metal primarily control the cracking. Fracture of a metal part by fatigue corrosion generally occurs at a stress level far below the fatigue limit in laboratory air, even though corrosion is relatively minor. For this reason, it is particularly important to protect all parts subject to cyclic stress, even in mildly corrosive environments.
2.6.3 Fretting Corrosion. Damage can occur at the interface of two highly-loaded surfaces held immobile against each other. However, vibration may cause the surfaces to rub together resulting in an abrasive wear called fretting. This rubbing action removes any protective film on the metallic surfaces. 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, as shown in Figure 2-15, Fretting Corrosion and Cracking from Cyclic Loading of Lug. Prevention measures include vibration dampening, tightening joints, applying a lubricant, or installing a fretting-resistant material between the two surfaces.
Figure 2-15. Fretting Corrosion and Cracking from Cyclic Loading of Lug
2-14
2.6.4 Heat Treatment. Rigidly control heat treatment of airframe materials to maintain corrosion resistance and improve essential mechanical properties. If heat treatment of a clad aluminum alloy is too long or at too high a temperature, this may diffuse the cladding, degrade the cladding’s inherent resistance to corrosion, and reduce its protection of the core aluminum alloy. Aluminum alloys with heavy amounts of copper and zinc are highly vulnerable to intergranular corrosion attack if not quenched rapidly during heat treatment. Stainless steel alloys are susceptible to carbide sensitization when slowly cooled after welding or after high temperature heat treatment. Post-weld heat treatments are normally advisable for reduction of residual stress.
2.6.5 Hydrogen Embrittlement.
2.6.5.1 Environmentally-induced failures may often be the result of hydrogen damage rather than oxidation. Atomic hydrogen is a cathodic product of many electrochemical reactions, forming during naturally-occurring corrosion reactions, as well as during many plating or pickling processes. Whether hydrogen is liberated as a gas or atomic hydrogen is absorbed by the metal depends on the surface chemistry of the metal.
2.6.5.2 Atomic hydrogen, due to its small size and mass, has very high diffusivity in most metals, and penetrates most clean metal surfaces easily and migrates rapidly to favorable sites where it may remain in solution; precipitate as molecular hydrogen to form small pressurized cavities, cracks, or large blisters; or react with the base metal or with alloying elements to form hydrides.
2.6.5.3 The accumulation of hydrogen in high-strength alloys often leads to cracking, which often occurs in statically-loaded components several hours or even days after applying a load or after exposure to a hydrogen source. This type of cracking is also called hydrogen stress cracking, hydrogen delayed cracking, or hydrogen-induced cracking. Similar fracture processes can occur in new and unused parts after heat treatments or machining then exposure to a hydrogen source. For this reason, processing must use well-controlled conditions to minimize the amount of hydrogen generated in processes such as pickling or electroplating.
2.7 Common Corrosive Agents. Substances that cause corrosion of metals are called corrosive agents. The most common of these agents are acids, alkalis, and salts. The atmosphere and water, the two most common media for these agents, may act as corrosive agents as well.
2.7.1 Acids. In general, moderately strong acids will severely corrode most airframe alloys.
The most destructive of these corrosive agents are sulfuric acid, present in battery acid;
halogen acids, such as hydrochloric, hydrofluoric, and hydrobromic acids; nitrous oxide compounds; and organic acids in human and animal wastes.
2-15
2.7.2 Alkalis. Although alkalis are generally not as corrosive as acids, alkali solutions aggressively attack aluminum and magnesium alloys unless they contain a corrosion inhibitor. Washing soda; potash, of which wood ashes are an example; and lime, with cement dust as an example, are particularly corrosive to aluminum. However, ammonia, another alkali, is an exception because aluminum alloys are highly resistant to it.
2.7.3 Salts. Most salt solutions are good electrolytes and can promote corrosive attack. Some stainless steel alloys are resistant to salt solutions, but aluminum alloys, magnesium alloys, and other steels are extremely vulnerable. It is harmful to expose airframe materials to salts or salt solutions.
2.7.4 The Atmosphere. Major corrosive agents existing in atmospheric air are oxygen and airborne moisture, both in abundant supply. Corrosion often results from direct action of atmospheric oxygen and moisture on metal, and additional moisture often accelerates corrosive attack, particularly on ferrous alloys. However, the atmosphere in industrial and marine environments may also contain other corrosive gases and contaminants which are unusually corrosive.
2.7.4.1 Industrial atmospheres contain many contaminants, but most common are partially-oxidized sulfur compounds. When these sulfur compounds combine with moisture, they form highly corrosive, sulfur-based acids destructive to most metals. At chemical industrial plants, other corrosive atmospheric contaminants may be present in large quantities, but such conditions are usually confined to a specific location.
2.7.4.2 Marine atmospheres contain chlorides such as salt particles or droplets of salt-saturated water. Since salt solutions are electrolytes, they corrosively attack aluminum and magnesium alloys, which are vulnerable to this type of environment.
2.7.5 Water. The corrosiveness of water depends on the dissolved minerals, organic impurities, and dissolved gasses, particularly oxygen, in the water. One characteristic of water which promotes corrosion is conductivity, or the ability to act as an electrolyte and conduct a current. Physical factors, such as water temperature and velocity, also have a direct bearing on corrosiveness.
2.7.5.1 The most corrosive of natural waters, seawater, and fresh waters are those that contain salts. Seawater is extremely corrosive due to the presence of chloride ions, but waters in harbors are often more corrosive because of industrial waste contamination combined with seawater.
2.7.5.2 The corrosive effects of fresh water vary from locality to locality due to the concentration of dissolved impurities in any particular area. Some municipal potable water supplies with chlorine and fluorides added can be quite corrosive. Commercially-softened water and industrially-polluted rain water are usually very corrosive.
2-16
2.8 Microorganisms.
2.8.1 Microbial corrosive attack comes from the actions of bacteria, fungi, or molds.
Microorganisms occur nearly everywhere, and bacteria and fungi cause the greatest corrosion problems.
2.8.2 Bacteria may be either aerobic, requiring oxygen to live, or anaerobic, surviving only when free oxygen is not present. Bacteria accelerate corrosion by oxidizing sulfur to produce sulfuric acid. Bacteria living adjacent to metals may promote corrosion by depleting the oxygen supply or releasing metabolic by-products oxidation of inorganic compounds such as iron, sulfur, hydrogen, or carbon monoxide. The resultant chemical reactions cause corrosion.
2.8.3 Fungi are the growths of microorganisms that feed on organic materials. While low humidity does not kill microbes, it slows their growth and may prevent corrosion damage. Ideal growth conditions for most microorganisms are: 1) temperatures between 68 °F and 104 °F, or 20 °C and 40 °C, and 2) relative humidity between 85 percent and 100 percent. Mold, bacteria, or other microbes attack some moisture-proofing coatings, especially if the coated surfaces are contaminated. This is counter to the former belief that operators could prevent fungal attack by either applying moisture-proofing coatings to nutrient materials, or by drying compartment interiors with desiccants. Microbial growth occurs at the interface of water and an aviation fuel, where the fungus feeds only on a fuel. Fungi produce organic acids, alcohols, and esters as a by-product of their growth, and these by-products provide even better growing conditions for the fungus. The fungus typically attaches itself to the bottom of a tank, looks like a brown deposit on the tank coating when the tank is dry, and may start growing again when water and fuel are present.
2.8.4 The spore form of some microorganisms can remain dormant for long periods while dry, and can become active when moisture is available. When desiccants become saturated and cannot absorb moisture passing into the affected area, microorganisms can begin to grow. Dirt, dust, and other airborne contaminants are the least recognized contributors to microbial attack. Unnoticed, small amounts of airborne debris may be sufficient to promote fungal growth.
2.8.5 As a rule of thumb, fungi nutrients are only those materials derived from plants or animals. Thus, wool, cotton, rope, feathers, and leather provide sustenance for microbes, while metals and minerals do not. This rule of thumb is mainly still valid, but the increasing complexity of synthetic materials makes it difficult or impossible to determine from the name alone whether a material will support fungus. Many otherwise resistant synthetics are susceptible to fungal attack by chemicals added to change a material’s properties.
2.8.6 Damage resulting from microbial growth can occur when any, or a combination of, three basic mechanisms comes into play. First, fungi are damp and have a tendency to hold moisture, which contributes to other forms of corrosion. Second, because fungi are living organisms, they need food to survive. This food is obtained from the material on which
2-17 the fungi are growing. Third, these microorganisms secrete corrosive fluids that attack many materials, including some that are not fungi nutrient.
2.8.7 Remove microbial growth completely to avoid corrosion. A common practice is to remove microbial growth by hand with a firm, nonmetallic bristle brush and water.
Microbial growth removal is easier if the growth is kept wet with water. Microbial growth may also be removed with steam at 100 pounds per square inch (psi) and steam temperatures not exceeding 150 °F, or 66 °C. Protective clothing should be worn when using steam for removing microbial growth.
Note: Steam at 100 psi may damage, lift, or remove some paints or sealants.
2.9 Metallic Mercury Corrosion on Aluminum Alloys. Spilled mercury on aluminum should be cleaned immediately because mercury causes corrosion attack, which is rapid in both pitting and intergranular attack and is very difficult to control. The most devastating effect of mercury spillage on aluminum alloys is the formation of an amalgam which proceeds rapidly along grain boundaries, causing liquid metal embrittlement. If the aluminum alloy part is under tension stress, this embrittlement will result in splitting with an appearance similar to severe exfoliation. X-ray inspection may be an effective method of locating the small particles of spilled mercury because the dense mercury will show up readily on the X-ray film.
2.10–2.99. RESERVED.