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Fig. C-16. Galvanic Series in Seawater10

Fig. C-16. Galvanic Series in Seawater10

corrosion is aluminum in direct contact with carbon/epoxy. Since aluminum is more reactive than the carbon fibers, corrosion of the aluminum occurs. Therefore, to provide an electrical insulator, a layer of fiberglass is often bonded to the carbon/epoxy surface to prevent corrosion of the aluminum. Note the position of magnesium on the galvanic series. It is anodic to every other metal and therefore very susceptible to galvanic corrosion.

Other important types of corrosion encountered in aerospace alloys include pitting, exfoliation, and intergranular corrosion. Pitting, as the name implies, is the formation of small pits on the surface due to localized attack. In exfoliation, delamination of the surface grains, or layers, occurs under forces exerted by the corrosion products. High strength aluminum alloys are subject to both pitting and exfoliation. Iron and silicon impurities can contribute to the attack because they are cathodic with respect to the aluminum matrix.

Intergranular corrosion occurs preferentially along grain boundaries and is especially prevalent in stainless steels. In certain heat treatments, chromium carbides precipitate at the grain boundaries, depleting the areas right next to the grain boundaries of chromium, as shown in Fig. C.17. Since chromium, in amounts greater than 12%, is necessary to prevent corrosion of stainless steels, the chromium-depleted regions are susceptible to accelerated attack.

C.7 Hydrogen Embrittiement3,11

Hydrogen embrittlement has particularly been a problem in heat treated high strength steels. In general, the higher the strength level of the steel, the greater is the susceptibility to hydrogen embrittlement. Hydrogen embrittlement occurs primarily in BCC and HCP metals, while FCC metals are generally not susceptible.

Hydrogen embrittlement results in sudden failures at stress levels below the yield strength. It is normally a delayed failure, in which an appreciable amount

Chromium Depleted Areas Corroe (% Cr <

Matrix With % Cr> 12 Does Not Corrode

Carbide Particles Deplete Matrix of Chromium

Fig. C-17. Intergranular Corrosion of Stainless Steel10

Matrix With % Cr> 12 Does Not Corrode

Carbide Particles Deplete Matrix of Chromium

Fig. C-17. Intergranular Corrosion of Stainless Steel10

of time passes between the time hydrogen is introduced into the metal and failure occurs. Hydrogen embrittlement is a complex process, and different mechanisms may operate in different metals under different environments and operating stresses. However, hydrogen is a small molecule that can dissociate into monatomic hydrogen that readily diffuses into the crystalline structure. Very small amounts of hydrogen can cause damage, for example, as little as 0.0001% hydrogen can cause cracking in steel. Typical sources of hydrogen include melting operations, heat treatments, welding, pickling and plating. In addition, the cathodic reaction during corrosion in service can also produce hydrogen.

Characteristics of hydrogen embrittlement include a strain rate sensitivity, a temperature dependence, and delayed fracture. As opposed to many forms of brittle fracture, hydrogen embrittlement is enhanced by slow strain rates. In addition, it does not occur at low or high temperatures, but occurs at intermediate temperature ranges. For steels, the most susceptible temperature is near room temperature.

A comparison between hydrogen-free notched tensile specimens and ones charged with hydrogen in a static tensile test is shown in Fig. C.18. Note that there is a time delay before failure occurs, hence the term static fatigue. Also, below a certain stress level, failure does not occur. The higher the hydrogen content, the lower the stress level that can be endured before failure. There is also a large reduction in ductility associated with embrittlement. There is no single fracture mode associated with hydrogen embrittlement. Fracture can be transgranular, intergranular, and can exhibit characteristics of both brittle and ductile failure modes.

If the part is not under stress when it contains hydrogen, then the hydrogen can usually be safely removed without damage to the part by baking the part at

No Hydrogen

No Hydrogen

Fig. C-18. Hydrogen Effect on Static Tensile Strength elevated temperature. The use of a vacuum during baking is even more effective. High strength steels are usually baked at 365-385° F for at least 8-24 h to remove any hydrogen after chromium or cadmium plating operations.

C.8 Stress Corrosion Cracking311

Stress corrosion cracking (SCC) is the failure of an alloy due to the combined effects of a corrosive environment and a static tensile stress below the yield strength of the alloy. The stress for SCC can be either an applied stress or residual stresses in the alloy. Only specific combinations of alloys and environments result in SCC. For example, the high strength aluminum alloy 7075-T6 will readily crack in sea water, while Ti-6Al-4V is immune to sea water. As explained in Chapter 2 on Aluminum, the use of T7 overaged tempers greatly reduces the stress corrosion cracking susceptibility to the high strength aluminum alloys. If the right combination of stress and environment are present, almost every metal can be prone to SCC. Of the main aerospace alloys, SCC is a more serious problem in high strength aluminum alloys and high strength steels. Although titanium has been made to crack in laboratory tests, SCC of titanium alloys in service has not been a serious problem.

A simplified mechanism for SCC is shown in Fig. C.19. Stress causes rupture of the oxide film at the crack tip, which exposes fresh metal that corrodes and forms another thin oxide film. The oxide ruptures again, allowing more corrosion, and the crack slowly propagates through the alloy until the crack reaches a critical length, and failure occurs. Since the cathodic reaction during corrosion can often produce hydrogen, hydrogen can contribute to SCC, often making it difficult to distinguish between SCC and hydrogen embrittlement. In general, there is a threshold stress for stress corrosion cracking, denoted by KISCC, below which crack growth is not observed. The level of KISCC with respect to KI of a material gives a measure of its susceptibility to SCC, as shown schematically in Fig. C.20.

In SCC, failure can occur either by transgranular failure, intergranular failure or by a mixture of the two. Normally ductile metals fail in a brittle manner. Failures are often characterized by branched type failures with multiple cracking.

Protective Film

Fig. C-19. Stress Corrosion Cracking

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