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 industrial pollutants, along with an electrolyte (moisture), pitting of the alclad will occur.
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 and bronze, require cadmium plating, zinc plating, IVD aluminum coating and/or conversion coating.
c. 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 (see below) ) are:
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 snowlike mounds or whiskers. Correction of damage involves the complete removal of corrosion and application of a chemical conversion coat and protective finish. Magnesium always requires protective coatings. 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 appearance of the unpainted surface. DOW 17 coatings have a green to grayish-green color. Coatings of the electrolytic type are thicker than those applied by immersion or brushing. Electrolytic finishes cannot be restored in the field. When failure occurs, corrosion products should be removed, the bare 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, these coatings.
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 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 grain.
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.
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.
f. 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 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.
g. 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, etc. 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.
i. 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 aircraft construction, copper is the less active metal and greatly accelerates corrosion of the other metals.
j. Silver, Platinum, and Gold.
These metals do not corrode in the ordinary sense, although silver tarnishes in the presence of sulfur. The tarnish is a brown to black film. Gold tarnish is very thin and shows up as a darkening of reflecting surfaces.
(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.
(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 RF shields, filters, crystal covers, and automatic switching devices. Tin has the best combination of solder ability 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.
TABLE 3-1. CORROSION OF METALS
1 - ALLOY
2 - TYPE OF ATTACK TO WHICH ALLOY IS SUSCEPTIBLE
3 - APPEARANCE OF CORROSION PRODUCT
1 - Magnesium
2 - Highly susceptible to pitting
3 - White, powdery, snowlike mounds and white spots on surface
1 - Low Alloy Steel (4000-8000 series)
2 - Surface oxidation and pitting, surface, and intergranular
3 - Reddish-brown oxide (rust)
1 - Aluminum
2 - Surface pitting, intergranular, exfoliation stress corrosion and fatigue cracking, and fretting
3 - White to gray powder
1 - Titanium
2 - Highly corrosion resistant; extended or repeated contact with chlorinated solvents may result in degradation of the metal's structural properties at high temperature
3 - No visible corrosion products at low temperature. Colored surface oxides develop above 700 °F (370 °C)
1 - Cadmium
2 - Uniform surface corrosion; used as sacrificial plating to protect steel
3 - From white powdery deposit to brown or black mottling of the surface
1 - Stainless Steels (300-400 series)
2 - Crevice corrosion; some pitting in marine environments; corrosion cracking; intergranular corrosion (300 series); surface corrosion (400 series)
3 - Rough surface; sometimes a uniform red, brown, stain
Table 3-1. CORROSION OF METALS (CONTINUED)
1 - ALLOY
2 - TYPE OF ATTACK TO WHICH ALLOY IS SUSCEPTIBLE
3 - APPEARANCE OF CORROSION PRODUCT
1 - Nickel base (Inconel, Monel)
2 - Generally has good corrosion resistant qualities; susceptible to pitting in sea water
3 - Green powdery deposit
1 - Copper base Brass, Bronze
2 - Surface and intergranular corrosion
3 - Blue or blue-green powdery deposit
1 - Chromium (Plate)
2 - Pitting (promotes rusting of steel where pits occur in plating)
3 - No visible corrosion products; blistering of plating due to rusting and lifting
1 - Silver
2 - Will tarnish in the presence of sulfur
3 - Brown to black film
1 - Gold
2 - Highly corrosion resistant
3 - Deposits cause darkening of reflective surfaces
1 - Tin
2 - Subject to whisker growth
3 - Whiskerlike deposit
302 - 399 RESERVED.