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.
Chap 3 Par 301 43
7/25/91 AC 43-4A
(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
CHAPTER 4. CORROSION PRONE AREAS AND PREVENTATIVE MAINTENANCE most aircraft. GENERAL. This chapter lists corrosion problem areas that are common to Corrosion prone areas should be cleaned, inspected, and treated more frequently than less corrosion prone areas. The list is not necessarily complete for any specific aircraft, but could be used to set up a maintenance inspection program. Also included in this chapter are factors in corrosion control and preventative maintenance.
EXHAUST TRAIL AREAS. Both jet and reciprocating engine exhaust gas deposits are very corrosive. Inspection and maintenance of exhaust trail areas should include attention to the areas indicated in Figure 4-1. Inspection should also include the removal of fairings and access plates located in the exhaust gas path.
a. Gaps, seams, hinges, and fairings are some of the exhaust trail areas where deposits may be trapped and not reached by normal cleaning methods.
b. Exhaust deposit buildup on the upper and lower wing, aft fuselage, and in the horizontal tail surfaces will be considerably slower and sometimes completely absent from certain aircraft models.
UNDER FAIRINGS- SAROUND RIVET HEADS & IN SKIN CREVICES FIGURE 4-1. EXHAUST TRAIL AREA CORROSION CONTROL POINTS BATTERY COMPARIMENTS AND BATTERY VENT OPENINGS. In spite of protective paint systems and extensive sealing and venting provisions, battery compartments continue to be corrosion problem areas. Fumes from overheated battery electrolyte are difficult to contain and will spread to internal structure.
Unprotected surfaces will be subjected to corrosive attack. For lead-acid Chap 4 Par 400 51
batteries, frequent cleaning and neutralization of acid deposits with sodium bicarbonate solution will minimize corrosion. If the battery installation includes external vent openings on the aircraft skin, these areas should be included in the inspection and maintenance procedure. If aircraft batteries with electrolytes of either sulfuric acid or potassium hydroxide are in use, their leakage will cause corrosion. Consult the applicable maintenance manuals for the particular aircraft to determine which type battery was installed and the recommended maintenance practices for each.
compartments should be done with ammonia or boric acid solution, allowed to dry thoroughly, and then painted with an alkali-resistant varnish.
behind lavatories, sinks, and ranges, where spilled food and waste products may ollect if not kept clean, are potential trouble spots. Even if some LAVATORIES, BUFFETS, AND GALLEYS. These areas, particularly deck areas ontaminants are not corrosive in themselves, they will attract and retair located under galleys and lavatories. Clean these areas frequently and maintain moisture and, in turn, cause corrosive attack. pay attention to bilge areas the protective sealant and paint finishes.
FIGURE 4-2. TYPICAL DAMAGE TO FLOOR AROUND LAVATORIES, BUFFETS, AND GALLEYS Chap 4 52 Par 402
7/25/91 AC 43-4A BILGE AREAS. On all aircraft, the bilge area is a common trouble spot.
The bilge is a natural sump or collection point for waste hydraulic fluids, water, dirt, loose fasteners, drill chips, and other odds and ends of debris.
Residual oil quite often masks small quantities of water which settle to the bottom and set up a hidden potential corrosion cell. With the exception of water displacing corrosion preventative compounds, keeping bilge areas free of all extraneous material, including water and oil will insure the best protection to clean such areas. against corrosion. A good vacuum cleaner and clean wiping cloths are necessary more punishment than any other area of the aircraft. It is exposed to mud, 405. WHEEL WELLS AND LANDING GEAR. water, salt, gravel, and other flying debris from runways during flight The wheel well area probably receives operations (see Figure 4-3).
aircraft wheels and on wheel well areas. Because of the many complicated a. Frequent cleaning, lubrication, and paint touch-up are needed on shapes, assemblies, and fittings in the area, complete coverage with a protective paint film is difficult to attain. Thus, preservative coatings tend to mask trouble rather than prevent it. Because of the heat generated from braking, preservative coatings cannot be used on aircraft landing gear wheels.
to the following trouble spots: During inspection of this area, particular attention should be given
(1) High strength steel.
hardware (bolts, pins, etc.). (2) Exposed surfaces of struts, oleos, arms, links, and attaching
(3) Axle interiors.
equipment. (4) Exposed position indicator switches and other electrical are typical water and debris traps. (5) Crevices between stiffeners, ribs, and lower skin surfaces which wheel web areas. (6) Magnesium wheels, particularly around bolt heads, lugs, and clamps and tubing identification tapes. (7) Exposed rigid tubing, especially at "B" nuts and ferrules under Chap 4 Par 404 53
- TYPICAL DEBRIS TRAPS
SWITCHES T -STRUTS
TIE DOWN POINTS
- UNPLATED HIGH
INSIDE AXLES STRENGTH STEEL
SPRINGS
WATER TRAPS
DISSIMILAR METAL
CONTACTS
UNDER FERRULES AND
IDENTIFICATION TAPES
- WHEEL BEARING RACES FIGURE 4-3. LANDING GEAR AND WHEEL CORROSION POINTS Chap 4 54 Par 405