of vibration, tightening of joints, application of a lubricant, or installation of a fretting resistant material between the two surfaces can reduce fretting corrosion.
FRETTING DAMAGE ON
LUG HOLE WALL FIGURE 2-15. FRETTING CORROSION AND CRACKING FROM CYCLIC LOADING OF LUG
d. Heat Treatment. Heat treatment of airframe materials should be rigidly controlled to maintain their corrosion resistance as well as to improve their essential mechanical properties. For example, improper heat treatment of clad aluminum alloy may cause the cladding to incur excessive diffusion because the solution heat treatment is too long or at too high a temperature. This degrades the inherent resistance of the cladding itself, and reduces its ability to provide protection to the core aluminum alloy. Aluminum alloys which contair appreciable amounts of copper and zinc are highly vulnerable to intergranular corrosion attack if not quenched rapidly during heat treatment or other special treatment. Stainless steel alloys are susceptible to carbide sensitization when slowly cooled after welding or high temperature heat treatment. Post-weld heat treatments are normally advisable for reduction of residual stress.
e. Hydrogen Embrittlement.
(1) Environmentally induced failure processes may often be the result of hydrogen damage rather than oxidation. Atomic hydrogen is a cathodic Chap 2 Par 205 25
(2) Atomic hydrogen, due to its small size and mass, has very high diffusivity in most metals. It will therefore penetrate most clean metal surfaces easily and migrate rapidly to favorable sites where it may remain in solution, precipitate as molecular hydrogen to form small pressurized cavities, cracks or large blisters, or it may react with the base metal or with alloying elements to form hydrides.
(3) The accumulation of hydrogen in high strength alloys often leads to cracking, and this often occurs in statically loaded components several hours or even days after the initial application of the load or exposure to the source of hydrogen. Cracking of this type is often referred to as hydrogen stress cracking, hydrogen delayed cracking, or hydrogen induced cracking. Similar fracture processes can occur in new and unused parts when heat treatments or machining have left residual stresses in the parts, and have then been exposed to a source of hydrogen. For this reason, all processes such as pickling or electroplating must be carried out under well controlled conditions to minimize the amount of hydrogen generated.
calied corrosive agents. The most common corrosive agents are asies, alkalies, COMMON CORROSIVE AGENTS. Substances that cause corrosion of metals are and salts. The atmosphere and water, the two most common media for these agents, may act as corrosive agents too.
a. Acids. In general, moderately strong acids will severely corrode most of the alloys used in airframes. The most destructive are sulfuric acid (battery acid), halogen acids (i.e., hydrochloric, hydrotluoric, and nydrobromic), nitrous oxide compounds, and organic acids found in the wastes of humans and animals.
b. Alkalies. Although alkalies, as a group, are generally not as corrosive as acids, aluminum and magnesium alloys are exceedingly prone to corrosive attack by many alkaline solutions unless the solutions contain a corrosion inhibitor. Particularly corrosive to aluminum are washing soda, potash (wood ashes), and lime (cement dust). Ammonia, an alkali, is ar exception because aluminum alloys are highly resistant to it.
c. Salts. Most salt solutions are good electrolytes and can promote corrosive attack. Some stainless steel alloys are resistant to attack by salt solutions but aluminum alloys, magnesium alloys, and other steels are extremely vulnerable. Exposure of airframe materials to salts or their solutions is extremely undesirable.
d. Ihe Atmosphere. The major atmospheric corrosive agents are oxygen and airborne moisture, both of which are in abundant supply. Corrosion often results from the direct action of atmospheric oxygen and moisture on metal, and the presence of additional moisture often accelerates corrosive attack, particularly on ferrous alloys. However, the atmosphere may also contain other Chap 2 26 Par 205
7/25/91 AC 43-4А corrosive gases and contaminants, particularly industrial and marine environments, which are unusually corrosive.
(1) Industrial atmospheres contain many contaminants, the most common of which are partially oxidized sulfur compounds. when these sulfur compounds combine with moisture, they form sulfur-based acids that are highly corrosive to most metals. In areas where there are chemical industrial plants, such conditions are usually confined to a specific locality. other corrosive atmospheric contaminants may be present in large quantities, but
(2) Marine atmospheres contain chlorides in the form of 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.
e. Water. The corrosivity of water will depend on the type and quantity of dissolved mineral and organic impurities and dissolved gasses (particularly oxygen) in the water. One characteristic of water which determines its corrosivity is the conductivity or its ability to act as an electrolyte and conduct a current. Physical factors, such as water temperature and velocity, also have a direct bearing on the corrosivity.
because they are contaminated by industrial waste. those that contain salts. the presence of chloride ions, but waters in harbors are often even more so (1) The most corrosive of natural waters (sea and fresh waters) are Water in the open sea is extremely corrosive due to
(2) The corrosive effects of fresh water varies from locality to locality due to the wide variety of dissolved impurities that may be present in any particular area. Some municipal waters (potable water) to which chlorine and fluorides have been added can be quite corrosive. Commercially softened water and industrially polluted rain water are usually considered to be very corrosive.
207. MICRO-ORGANISMS.
Micro-organisms occur nearly everywhere. Those organisms causing the greatest corrosion problems are bacteria and fungi. a. Microbial attack includes actions of bacteria, fungi, or molds.
b. Bacteria may be either aerobic or anaerobic. Aerobic bacteria by depleting the oxygen supply or by releasing metabolic products. Anaerobic produce sulfuric acid. require oxygen to live. They accelerate corrosion by oxidizing sulfur to Bacteria living adjacent to metals may promote corrosion bacteria, on the other hand, can survive only when free oxygen is not present. The metabolism of these bacteria requires them to obtain part of their sustenance by oxidizing inorganic compounds, such as iron, sulfur, hydrogen, and carbon monoxide. The resultant chemical reactions cause corrosion.
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c. Fungi are the growths of micro-organisms that feed on organic materials. While low humidity does not kill microbes, it slows their growth and are temperatures between 68 and 104 °F (20 and 40 °C) and relative humidity may prevent corrosion damage. Ideal growth conditions for most micro-organisms between 85 and 100 percent. It was formerly thought that fungal attack could be prevented by applying moisture-proofing coatings to nutrient materials or by drying the interiors of compartments with desiccants. However, some moistureproofing coatings are attacked by mold, bacteria, or other microbes, especially if the surfaces on which they are used are contaminated. Microbial growth occurs at the interface of water and fuel, where the fungus feeds on fue'' These Organic acids, alcohols, and esters are produced by growth of the fungus. typically attaches itself to the bottom of the tank and looks like a brown by-products provide even better growing conditions for the fungus. The fungus deposit on the tank coating when the tank is dry. The fungus growth may start The spore form of some micro-organisms can remain dormant for long periods while dry, and can become active when moisture is available. When desiccants become saturated and unable to absorb moisture passing into the affected area, micro-organisms 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.
Fungi nutrients have been considered to be only those materials that have been derived from plants or animals. Thus, wool, cotton, rope, teathers, minerals were not considered fungi nutrients. To a large extent this rule of and leather were known to provide sustenance for microbes, while metals and thumb is 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 rendered susceptible to fungal attack by the addition of chemicals to change the material's properties.
f..
of corrosion. Second, because fungi are living organisms, they need food to survive. his tood is obtained from the material on which the fungi are growing. materials, including some that are not fungi nutrient.
g. Microbial growth must be removed completely to avoid corrosion.
Microbial growth should be removed by hand with a firm nonmetallic bristle brush water. Microbial growth may also be removed with steam at 100 psi and steam and water. Removal of microbial growth is easier if the growth is kept wet with temperatures not exceeding 150 °F (66 °C). Protective clothing must be used when using steam for removing microbial growth.
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METALLIC MERCURY CORROSION ON ALUMINUM ALLOYS. Spilled mercury on the formation of an almagem 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
209.-299. RESERVED.
Chap 2 Par 208 29 through 40
CHAPTER 3. EFFECTS OF CORROSION GENERAL. Most metals are subject to corrosion, but corrosion can be minimized by use of corrosion resistant metals and finishes. The principal material used in airframe structures is high-strength aluminum alloy sheet coated (clad) with a pure aluminum coating (alclad) which is highly resistant to corrosive attack. However, with an accumulation of airborne salts and/or alclad will occur. industrial pollutants, along with an electrolyte (moisture), pitting of the Once the alclad surface is broken, rapid deterioration of the high strength aluminum alloy below occurs. Other metals commonly used in airframe structure, such as nonclad high-strength aluminum alloys, steel, and magnesium alloys, require special preventive measures to guard against corrosion.
a. Aluminum alloys, for example, are usually anodized (a chemical coating), or in some later generation aircraft an aluminum applied plating (ion vapor disposition (IVD)), then primed and possibly topcoated with paint.
b. Steel (except most stainless steels) and other metals, such as brass conversion coating. and bronze, require cadmium plating, zinc plating, IVD aluminum coating and/or Magnesium alloys are highly susceptible to corrosion attack, especially where airborne salts and/or industrial pollutants are present, and require special chemical and electrochemical treatments and paint finishes.
301. EFFECTS OF CORROSION ON METALS. The characteristics of corrosion in commonly used aircraft metals (summarized in Table 3-1) are:
a. Magnesium. Magnesium is the most chemically active metal used in airplane construction and is highly susceptible to and difficult to protect from corrosion. Therefore, when a failure in the protective coating does occur, the prompt and complete correction of the coating failure is imperative if serious structural damage is to be avoided. Corrosion of magnesium is possibly the easiest type of corrosion to detect, since even in its early stages the corrosion products occupy several times the volume of the original magnesium metal. The beginning attack shows 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 the complete removal of corrosion and application protective coatings. of a chemical conversion coat and protective finish. Magnesium always requires Some magnesium parts in current aircraft have been originally protected by proprietary electrolytic processes, such as HAE and DOW 17 coatings. The HAE process can be identified by the brown to mottled gray green color. Coatings of the electrolytic type are thicker than those applied appearance of the unpainted surface. DOW 17 coatings have a green to grayishby immersion or brushing. Electrolytic finishes cannot be restored in the field. When failure occurs, corrosion products should be removed, the bare these coatings. magnesium should be touched up with chemical treatment solution, and the part repainted. Care should be taken to minimize removal of, and ensure repair of, Chap 3 Par 300 41
b. Steel. Corrosion of steel is easily recognized because the corrosion product is red rust. When iron-base alloys corrode, dark corrosion products usually form first on the surface of the metal. These products are protective.
However, if moisture is present, this ferrous oxide coating is converted to hydrated ferric oxide, which is red rust. This material will promote 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 a plating, often combined with a paint system).
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 Chapter 2, paragraphs
204. and 205.). Both bare and clad aluminum alloys resist corrosion in nonmarine areas. Where airborne salts and/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. Considerable attack can usually take place before serious loss of structural strength develops. However, at least three forms of attack on aluminum alloys are particularly serious:
(1) Penetrating 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
d. Anodized 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, it can be only partially restored by chemical surface treatment. Therefore, when performing any processing of anodized surfaces, unnecessary destruction of the anodized surface should be avoided.
e. Titanium. Although titanium is strongly corrosion resistant, electrical insulation between titanium and other metals is necessary to prevent galvanic corrosion of the other metal. Frequent inspection of such areas is required to ensure that insulation failure has not allowed corrosion to begin.
Under certain conditions, chlorides and some chlorinated solvents may induce stress-corrosion cracking of certain titanium alloys.
Cadmium and Zinc. Cadmium is used as a coating to protect steel parts and to provide a compatible surface when a part is in contact with other materials. Attack on cadmium is evidenced by white-to-brown-to-black mottling of the surface. Zinc forms voluminous white corrosion products. When cadmium and zinc plate show mottling and isolated voids or cracks in the coating, the Chap 3 42 Par 301
7/25/91 AC 43-4A plating is still performing its protective function. The cadmium plate on iron or steel is still protecting until such time as actual iron rust appears.
NOTE: Any mechanical removal of corrosion products should be limited to metal surfaces from which the cadmium has been depleted.
Stainless Steels. Stainless steels are iron-base alloys containing 12 percent or more of chromium (as well as other elements). They consist of two types, magnetic and nonmagnetic. The magnetic steels are identified by numbers in the American Iron and Steel Institute (AISI) 400-series, such as 410, 430, These steels are not as corrosion resistant as the nonmagnetic, which are identified by numbers in the AISI 300-series, such as 304, 316, etc. The AISI 300-series steels have nickel contents ranging from 6 to 22 percent, while the 400-series steels have nickel contents of only 2 percent. The corrosion resistance of these steels is due to 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 pit when exposed to harsh corrosive environments such as airborne salts and industrial pollutants.
Particularly susceptible are crevices and other areas in which foreign materials collect. Corrosion can be prevented by keeping stainless steel clean.
h. Nickel and Chromium. Nickel and chromium are used as protective coatings and as alloying elements with iron in stainless steels. Chromium plating is also used to provide a smooth, wear resistant surface and to reclaim worn parts. Where corrosion resistance in a marine environment is required, nickel undercoat is used. The degree of protection is dependent upon plating thickness. Both of these metals form continuous oxide coatings that can be polished to a higher luster and still protect not only themselves but any underlying metal. Chromium coatings contain cracks, and corrosion originates at these separations.
1. Copper and Copper Alloys. Copper and its alloys are generally corrosion resistant. However, the effects of minor amounts of corrosion on copper electric 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 corrosion of the other metals. aircraft construction, copper is the less active metal and greatly accelerates ordinary sense, although silver tarnishes in the presence of sulfur.
(1) When silver is plated over copper there can be an accelerated corrosion of the copper, through galvanic action, at pinholes or breaks in the silver plating. This corrosion is known as "red plague" and is identifiable by the presence of a brown-red powder deposit on the exposed copper.
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(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.
Tin is common on R-F shields, filters, crystal covers, automatic switching devices. Tin has the best combination of solderability and corrosion resistance of any metallic coating. The problem with tin is its tendency to grow "whiskers" on tin-plated wire and other plating applications.
Chap 3 Par 301 44
TABLE 3-1. CORROSION OF METALS
ALLOY TYPE OF ATTACK TO WHICH APPEARANCE OF
ALLOY IS SUSCEPTIBLE CORROSION PRODUCT Magnesium Highly susceptible to pitting white, powdery, snowlike mounds and white spots on surface Low Alloy Steel (4000-8000 pitting, surface, and intergranular Surface oxidation and Reddish-brown oxide (rust) series) Aluminum surface pitting, intergranular, white-to-grey powder exfoliation stresscorrosion and fatigue cracking, and fretting Titanium Highly corrosion resistant; extended or No visible corrosion products at low chlorinated repeated contact with solvents surface oxides temperature. Colored degradation of the may result in develop above 700 °F metal's structural (370 °C) properties at high temperature Cadmium Uniform surface to protect steel sacrificial plating corrosion; used as black mottling of From white powdery deposit to brown or the surface 400 series) Steels (300- Stainless cracking; intergranular pitting in marine Crevice corrosion; some environments; corrosion stain Rough surface; uniform red, brown, sometimes a corrosion (300 series) ;
surface corrosion (400 series) Chap 3 Par 301 45
Table 3-1. CORROSION OF METALS (CONTINUED) Nickel-base Generally has good Green powdery Monel) (Inconel, corrosion resistant to pitting in sea water qualities; susceptible deposit Brass, Copper-base Surface and intergranular corrosion powdery deposit Blue or blue-green Bronze Chromium Pitting (promotes (Plate) pits occur in plating) rusting of steel where No visible corrosion products; blistering of plating due to rusting and lifting Silver Will tarnish in the Brown-to-black film presence of sulfur Gold Highly corrosion Deposits cause resistant darkening of reflective surfaces Tin subject to Whisker-like deposit whisker growth
302.-399. RESERVED.
Chap 3 46 through 50 Par 301