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
3-5
ALLOY TYPE OF ATTACK TO WHICH ALLOY IS SUSCEPTIBLE APPEARANCE OF CORROSION PRODUCT Silver Will tarnish in the presence of sulfur Brown-to-black film Gold Highly corrosion resistant Deposits cause darkening of reflective surfaces
3.3–3.99. RESERVED.
4-1 CHAPTER 4. CORROSION-PRONE AREAS AND PREVENTIVE MAINTENANCE
4.1 General. This chapter lists corrosion problem areas common to most aircraft. Be aware to clean, inspect, and treat corrosion-prone areas more frequently. The list is not intended for a specific aircraft, but it can be used to set up a maintenance inspection program. This chapter also includes factors in corrosion control and preventive maintenance.
4.2 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 areas indicated in Figure 4-1, Exhaust Trail Area Corrosion Control Points, and inspection should include removal of fairings and access plates in the exhaust gas path.
4.2.1 Gaps, seams, hinges, and fairings are areas where exhaust trail deposits may be trapped and normal cleaning methods cannot reach these areas.
4.2.2 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 on certain aircraft models.
Figure 4-1. Exhaust Trail Area Corrosion Control Points
4.3 Battery Compartments and Battery Vent Openings. In spite of protective paint systems and extensive sealing and venting, battery compartments continue to be corrosion problem areas. Fumes from overheated battery electrolyte are difficult to contain and can spread to internal structures where unprotected surfaces can come under corrosive attack. For lead-acid batteries, frequent cleaning and neutralization of acid deposits with sodium bicarbonate solution will minimize corrosion. If the battery
4-2 i nstallation includes external vent openings on the aircraft skin, these areas should be included in inspection and maintenance procedures. If aircraft have batteries with electrolytes of either sulfuric acid or potassium hydroxide, electrolyte leakage will cause corrosion. Consult applicable maintenance manuals to determine the battery type installed and recommended maintenance practices. Clean nickel cadmium compartments with ammonia or boric acid solution, dry thoroughly, then paint with an alkali-resistant varnish.
4.4 Lavatories, Buffets, Galleys, Doorways, and Cockpit Floorboards. These areas, particularly deck areas behind lavatories, sinks, and ranges, are potential trouble areas where spilled food and waste products may collect. Even though some contaminants are not corrosive, they will attract and retain moisture, and promote corrosive attack. Pay particular attention to bilge areas and aircraft structure located under galleys and lavatories. Clean these areas frequently and maintain protective sealant and paint finishes.
Figure 4-2. Typical Damage to Floor Around Lavatories, Buffets, and Galleys
4.5 Bi lge Areas. On all aircraft, the bilge area is a common trouble area. The bilge is a natural sump or collection point for waste hydraulic fluids, water, dirt, loose fasteners, drill chips, and other debris. Residual oil quite often masks small quantities of water which settle to the bottom and set up a hidden potential corrosion cell. Except for water displacing corrosion-preventing compounds, keeping bilge areas free of extraneous material, including water and oil, will insure the best protection against corrosion. A good vacuum cleaner and clean wiping cloths are handy tools to clean such areas.
4.6 Wheel Wells and Landing Gear. The wheel well area probably receives more punishment than any other area of the aircraft. It is exposed to mud, water, salt, gravel,
4-3 and other flying debris from runways during flight operations. See Figure 4-3, Landing Gear and Wheel Corrosion Points.
4.6.1 Aircraft wheels and wheel well areas should receive frequent cleaning, lubrication, and paint touch-up. Because of many complicated shapes, assemblies, and fittings in the area, complete coverage with a protective paint film is difficult, and preservative coatings tend to mask trouble rather than prevent it. Do not use preservative coatings on landing gear wheels because of the heat generated from braking.
4.6.2 During inspection of this area, pay particular attention to these trouble areas:
• Aluminum and high-strength steel.
• Exposed surfaces of struts, oleos, arms, links, and attaching hardware, such as bolts or pins.
• Axle interiors.
• Exposed position indicator switches and other electrical equipment.
• Crevices between stiffeners, ribs, and lower skin surfaces, typical water and debris traps.
• Magnesium wheels, particularly around bolt heads, lugs, and wheel web areas.
• Exposed rigid tubing, especially at “B” nuts and ferrules under clamps and tubing identification tapes.
Figure 4-3. Landing Gear and Wheel Corrosion Points
- TYPICAL DEBRIS TRAPS
SWITCHES -STRUTS
TIE DOWN POINTS
• UNPLATED HIGH SPRINGS- INSIDE AXLES• STRENGTH STEEL
- WATER TRAPS 1 DISSIMILAR METAL CONTACTS
UNDER FERRULES AND
IDENTIFICATION TAPES
- WHEEL BEARING RACES 4-4
4-5
4.7 External Skin Areas. External aircraft surfaces are ordinarily covered with protective finishes, and operators can also apply paint coatings. Affected external aircraft surfaces are readily visible or available for inspection and maintenance. Much emphasis has been given to these areas in the past, and maintenance procedures are well established. Even here, certain configurations or combinations of materials become troublesome and require special attention to avoid serious corrosion difficulties. Some common trouble areas are listed below, other than those attributed to engine exhaust deposits:
4.7.1 S teel, Titanium, Corrosion Resistant Steel (CRES), and Nickel Alloy Fasteners .
Fasteners, and areas around these fasteners, are trouble areas. These areas are subject to high operational loads, moisture intrusion, and dissimilar metal skin corrosion. See Figure 4-4, Intergranular Corrosion of 7075-T6 Aluminum Adjacent to Steel Fastener.
Figure 4- 4. Intergranular Corrosion of 7075- T6 Aluminum Adjacent to Steel Fastener
4.7.2 Faying Surfaces and Crevices. Similar to corrosion around fasteners, corrosion in faying surfaces, seams, and joints results from intrusion of moisture and other corrosive agents.
The effect of this type of corrosion is usually detectable by bulging of the skin surface.
4-6 Figure 4-5. Crack and Skin Bulging Caused by Corrosion
4.7.3 Magnesium Skins. Properly surface-treated, insulated, and painted magnesium skin surfaces give relatively little corrosion-related trouble if the original surface is maintained. However, trimming, drilling, and riveting destroy some of the original surface treatment, which may not be completely restored by touch-up procedures.
4.7.3.1 S ome aircraft have steel fasteners installed through a magnesium skin, with only protective finishes under the fastener heads and fillet sealant or tape over the surface for insulation. Paint coatings are inherently thin at abrupt changes in contour, such as at trimmed edges. With magnesium’s sensitivity to moisture, these conditions add up to a potential corrosion problem whenever magnesium is used.
4.7.3.2 Corrosion inspections should include all magnesium skin surfaces and magnesium fittings or structural components, with special attention to edges, areas around skin edges and fasteners, and cracked, chipped, or missing paint.
4.7.4 S pot-Welded Skins. Corrosion of this type is chiefly the result of the entrance and entrapment of moisture or other corrosive agents between layers of metal. See Figure 4-6, Spot-Welded Skin Corrosion.
4.7.4.1 S pot-welded assemblies are particularly corrosion prone, and corrosive attack causes skin buckling or spot-weld bulging and fracture. This type of corrosion is evidenced by corrosion products appearing at crevices through which the corrosive agents enter. Some corrosion may originate from fabricating processes, but its progress to the point of skin bulging or spot-weld fracture is the direct result of moisture or other corrosive agents moving through open
4-7 gaps or seams. Weld-through sealing materials should minimize this problem, but many in-service aircraft have unsealed spot-weld skin.
4.7.4.2 Corrosion may appear at either external or internal faying surfaces, but is usually more prevalent on external areas. More advanced corrosive attack causes skin buckling and eventual spot-weld fracture. Detect skin buckling in this early stage by sighting or feeling along spot-welded seams or by using a straight edge to show variations along the skin.
4.7.4.3 To prevent this condition, keep potential moisture entry points (such as gaps, seams, and holes created by broken welds) filled with a noncorrosive sealant.
Figure 4-6. Spot-Welded Skin Corrosion
4.7.5 Piano-Type Hinges. These are prime spots for corrosion due to dissimilar metal contact between the steel pin and aluminum hinge tangs. These hinges are also natural traps for dirt, salt, and moisture. Hinges tend to corrode and freeze in the closed position between inspections where they are part of access doors or plate assembly and actuated only when opened during an inspection. When the hinge is inspected, lubricate it and work it through several cycles to enhance lubricant penetration. See Figures 4-7, Hinge Corrosion Points, and 4-8, Hinge Failure Caused by Corrosion.