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 S tress -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 C orrosion 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 H ydrogen 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 S alts . 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 T he 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 come 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.
3-1 CHAPTER 3. EFFECTS OF CORROSION
3.1 General. Most metals corrode, but corrosion can be minimized by using corrosion-resistant metals and finishes. The principal material in airframe structures is high-strength aluminum alloy sheet coated, or clad, with a pure aluminum coating, called Alclad, which is highly resistant to corrosive attack. However, pitting of the Alclad will occur with an accumulation of airborne salts or industrial pollutants, and an electrolyte, such as moisture. Once the Alclad surface is broken, rapid deterioration of the high-strength aluminum alloy below occurs. Other metals commonly used in airframe structures require special preventive measures to guard against corrosion, such as non-clad, high-strength aluminum alloys, steel, and magnesium alloys.
3.1.1 Aluminum alloys typically have anodize or conversion coatings, which are primed or primed and topcoated. In some later generation aircraft, operators apply an aluminum plating using the ion vapor disposition (IVD) process, then prime and possibly topcoat the surface with paint.
3.1.2 Brass and bronze require cadmium plating, zinc plating, IVD aluminum coating, or a conversion coating. Most stainless steels do not need this protective coating.
3.1.3 Magnesium alloys are highly susceptible to corrosion attack, especially where airborne salts or industrial pollutants are present, and require special chemical and electrochemical treatments and paint finishes.
3.2 Effects of Corrosion on Metals. Below is a list of the characteristics and common properties of corrosion in commonly used aircraft metals:
3.2.1 Magnesium. This is a chemically active metal used in airplane construction, highly susceptible to corrosion, and difficult to protect. When a failure in the protective coating occurs, the coating should be promptly and completely repaired to avoid serious structural damage. Magnesium corrosion is possibly easiest to detect since, even in its early stages, corrosion products occupy several times the volume of the original magnesium metal. The beginning attack appears as a lifting of the paint film and as white spots on the surface which rapidly develop into snow-like mounds or whiskers.
Correction of damage involves completely removing corrosion and applying a chemical conversion coat and protective finish. Magnesium always requires protective coatings, and some magnesium parts in current aircraft have been initially protected by proprietary electrolytic processes, such as HAE and DOW 17 coatings. The HAE process can be identified by the brown or mottled gray appearance of the unpainted surface. DOW 17 coatings have a green to grayish-green color. Electrolytic coatings are thicker than those applied by immersion or brushing. Electrolytic finishes cannot be restored in the field.
Take care to minimize removal and repair of these coatings.
3.2.2 S teel . Corrosion of steel is easily recognized because the corrosion product is red rust.
When iron-based alloys corrode, dark corrosion products usually form first on the surface of the metal. These products are protective, but if moisture is present, this ferrous oxide coating is converted to hydrated ferric oxide, or red rust. This material will promote
3-2 further attack by absorbing moisture from the air. The most practical means of controlling corrosion of steel is complete removal of corrosion products by mechanical means and by maintaining the protective coating system, usually by a plating, and often combined with a paint system.
Note: Stainless steels generally do not exhibit red rust and should be assessed with the criteria in paragraph 3.2.7.
3.2.3 Aluminum. Aluminum and its alloys exhibit a wide range of corrosive attack, including uniform surface, galvanic, pitting, intergranular, exfoliation, crevice, stress, and fretting corrosion (see paragraphs 2.5 and 2.6). Where airborne salts or industrial pollutants are present, all aluminum alloys require protection. The corrosion product of aluminum is a white-to-gray powdery material which can be removed by mechanical polishing or brushing with materials softer than the metal. General surface attack of aluminum penetrates slowly but is accentuated in the presence of dissolved salts in an electrolyte.
Corrosion must be addressed when present, even in small amounts. Some alloys resist general corrosion and instead corrode by pitting or intergranular attack, which can affect the structural strength without appearing widespread. However, at least three forms of attack on aluminum alloys are particularly serious:
1. P enetrating, pit -type corrosion through walls of aluminum tubing.
2. Stress-corrosion cracking of materials under sustained stress and corrosive environment.
3. Intergranular attack characteristic of certain alloys where clearly defined grain boundaries differ chemically from the metal within the grain.
3.2.4 A nodized Aluminum. Some aluminum parts are protected with an anodized coating.
Aluminum oxide film on aluminum is naturally protective, and anodizing merely increases the thickness of the oxide film. When this coating is damaged in service, the coating can be partially restored by chemical surface treatment. As only partial restoration is possible, avoid unnecessary destruction of the anodized surface when performing any processing of the surface.
3.2.5 Titanium. Titanium is prone to cause severe corrosion of adjacent parts through galvanic corrosion. Titanium parts may be galvanically isolated from adjacent parts through barrier coatings or films. Frequent inspection for degradation of these barrier films and inspection for corrosion of adjacent parts is recommended. Contact with zinc and cadmium-plated parts can promote cracking of the titanium.
3.2.6 Cadmium and Zinc. Cadmium as a coating protects steel parts and provides a compatible surface when a part contacts other materials. Attack on cadmium is evidenced by a white-to-brown-to-black mottling of the surface. Zinc forms large amounts of white corrosion products. When cadmium and zinc plate show mottling and isolated voids or cracks in the coating, the plating is still performing its protective function. The cadmium plate on iron or steel is still protecting its core until actual iron rust appears.
3-3 Note: Any mechanical removal of corrosion products should be limited to metal surfaces from which the cadmium has been depleted.
3.2.7 S tainless Steels. These metals are iron -base alloys containing 12 percent or more chromium and other elements, with both magnetic and nonmagnetic properties. Magnetic steels are identified by numbers in the American Iron and Steel Institute (AISI) 400 series, such as 410 or 430. These steels are less corrosion resistant than the nonmagnetic alloys, which you identify by numbers in the AISI 300 series, such as 304 or 316. The AISI 300-series steels have nickel content ranging from 6 to 22 percent, while 400-series steels have nickel content of only 2 percent. The corrosion resistance of these steels lies in their ability to form a protective oxide film on the surface. This “passive” film can be reinforced by treatment in certain chemical solutions. However, such steels will also pit when exposed to harsh corrosive environments, such as airborne salts and industrial pollutants. Stainless steels are highly susceptible to crevice/concentration cell corrosion and stress-corrosion cracking in moist, salt-laden environments and can cause galvanic corrosion of almost any other metal with which they are in contact if proper techniques of sealing and protective coating are ignored.
Precipitation hardening stainless steels such as PH17-4, PH15-5, and PH13-8Mo. are now more widely used as compared to 400 series stainless. Corrosion can be prevented by keeping stainless steel clean.
3.2.8 Nickel and Chromium. These metals are protective coatings and alloying elements with iron in stainless steels. Chromium plating also provides a smooth, wear-resistant surface and reclaims worn parts. A nickel undercoat enhances corrosion resistance in a marine environment. The degree of protection is dependent upon plating thickness. Both metals form continuous oxide coatings that you can polish to a higher luster and still protect not only themselves but any underlying metal. Chromium coatings contain cracks, and corrosion originates in these separations.
3.2.9 Copper and Copper Alloys. These are generally corrosion resistant. However, the effects of minor amounts of corrosion on copper electrical components can appreciably degrade the performance of the components. The product of corrosion on copper is generally a bluish-green coating on the surface of the material. When coupled with most metals used in aircraft construction, copper is the less chemically active metal and greatly accelerates corrosion of other metals.
3.2.10 Silver, Platinum, and Gold. These metals do not corrode in the ordinary sense, although silver tarnishes as a brown-to-black film in the presence of sulfur. Gold tarnish is very thin and shows up as a darkening of reflecting surfaces.
3.2.10.1 W hen silver is plated over copper, accelerated corrosion of the copper through galvanic action can occur at pinholes or breaks in the silver plating. This corrosion is “red plague,” and presents as a brown-red powder deposit on the exposed copper.
3-4
3.2.10.2 “Purple plague” is a brittle gold-aluminum compound formed when bonding gold to aluminum. The growth of such a compound can cause failure in microelectronic interconnection bonds.
3.2.11 Tin. This material is common on radio frequency (RF) shields, filters, crystal covers, and automatic switching devices. Tin has the best combination of solderability and corrosion resistance of any metallic coating.
Table 3-1. Corrosion of Metals
ALLOY TYPE OF ATTACK TO WHICH ALLOY IS SUSCEPTIBLE APPEARANCE OF CORROSION PRODUCT Magnesium Highly susceptible to pitting White, powdery, snow-like mounds and white spots on surface Low Alloy Steel (4000–8000 series) Surface oxidation and pitting, surface, and intergranular Reddish-brown oxide (rust) Aluminum Surface pitting, intergranular, exfoliation stress-co rrosion and fatigue cracking, and fretting White-to-gray powder Titanium Highly corrosion resistant; extended or repeated contact with chlorinated solvents may result in degradation of the metal’s structural properties at high temperature No visible corrosion products at low temperature. Colored surface oxides develop above 700 °F, or 370 °C Cadmium Uniform surface corrosion; used as sacrificial plating to protect steel From white powdery deposit to brown or black mottling of the surface Stainless Steels (300-400 series) Crevice corrosion; some pitting in marine environments; corrosion cracking;
intergranular corrosion (300 series);
surface corrosion (400 series) Rough surface; sometimes a uniform red, brown, stain Nickel Alloy Generally corrosion-resistant; exposure to very high temperatures in air can lead to corrosion product formation on the surface of parts made from nickel;
exposure to stagnant seawater can lead to surface pitting due to crevice/concentration cell corrosion Green powdery deposit on the surface Nickel-Based (Inconel, Monel) Generally has good corrosion resistant qualities; susceptible to pitting in sea water Green powdery deposit Copper-Based Brass, Bronze Surface and intergranular corrosion Blue or blue-green powdery deposit Chromium (Plate) Pitting; promotes rusting of steel where pits occur in plating No visible corrosion products;
blistering of plating due to rusting and lifting