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5.4.2.1 Fundamentals of Stress Corrosion Cracking

Stress corrosion cracking encompasses a variety of crack-inducing mechanisms (see Fig. "Role of hydrogen in corrosion processes") that only occur in case the following three conditions of the corrosion system are met:

  • Specific corrosion-sensitive material (see Table)

Depending on the corrosive, stressing, and material influences, SCC can be either intercrystalline or transcrystalline. SCC can be categorized into two main groups, according to the damage mechanism: tension-induced and expansion/strain-induced SCC. The damage symptoms can vary, depending on the surface behavior and the size of the loads (Fig. "Crack growth mechanism").

In stress-induced SCC, the crack forms under the influence of a static load (expansion speed = 0). The protective coatings are torn open (Fig. "Coating damage I"), allowing the corrosive media to act on locally exposed metal surfaces. Here, as well, expansion is ultimately the deciding cause of the local destruction of the protective coating (Fig. "Coating damage I"). The destruction occurs before the crack-initiating corrosive attack. Therefore, the creation of a protective coating at the phase boundary between the metal and the corrosive media is an important requirement for this type of SCC. This type of corrosion has characteristic damage symptoms (Fig. "Material behavior"). Threshold stresses (Fig. "Stress corrosion cracking") and threshold potential are typical parameters for SCC and must be determined with suitable testing procedures. No SCC occurs below these values. A distinction is made between anodic SCC (as described above) and cathodic SCC (“hydrogen-induced crack initiation”, also see Chapter 5.4.4), in which atomic hydrogen enters into the expanded metal grid during the corrosion reaction. This type of area is typically the bottom of a nick or a crack tip with high stress concentration. In these areas in austenite CrNi steels (for example), under the influence of Cl ions above 80°C, transcrystalline or intercrystalline (e.g. in heat-treated steels) crack growth occurs. With anodic SCC it is plausible, that the discharge of Hydrogen ions in the crack may create H-atoms, the diffusion of which has an embrittling effect and influences periodic crack growth (Fig. "Role of hydrogen in corrosion processes").

Strain-induced corrosion cracking (Ref. 5.4.3-1) can be expected when, as a result of the plastic deformations, the brittle reaction layer (Fig. "Coating damage I") cracks, and there is not sufficient time for the protective coating to regenerate. This is the case when the coating growth rate is lower than the expansion rate (Fig. "Tensile stress").This type of corrosion is common in high-strength titanium alloys. In stress-induced SCC there is no lower critical expansion rate.

Figure "Stress corrosion cracking" (Refs. 5.4.2.1-1.1 and 5.4.2.1-1.2): The diagram shows the typical behavior during stress corrosion cracking of two material families. If the static tensile stress in the CrNi steel is less than 150 MN.m-2 and in the Al alloy less than about 120 MN.m-2, then no crack will form with the given corrosive media. This shows that suitable tension-decreasing measures (such as heat treatment) and/or induction of compressive stress (e.g. through shot peening) can be effective in preventing SCC damage.



Illustrations 5.4.2.1-2 and 5.4.2.1-3 (Ref. 5.4.1.2-2): For every corrosion type, especially for SCC, damage to the passive layer (cathodic zone) is important for the creation of a corrosion-sensitive (anodic) area (also see Fig. "Role of hydrogen in corrosion processes"). Depending on how the damage in the grain boundary area is created through crack initiation (bottom left), such as along the grain boundary surfaces or from the inside of the grains to the surface in the shape of extrusions (top and middle diagrams), either intercrystalline SCC or transcrystalline SCC are promoted.
Extrusions are created in the micro-zone through dynamic changing loads. The surface deformation this causes is massive passive layer damage (singularity).

Figure "Crack growth mechanism": In stress-induced SCC the following damage mechanism takes place: at the surface of a normally corrosion-resistant material a brittle reaction layer forms (phase 1 in middle diagram), such as an oxide layer that prevents corrosion. Static expansion/strain of the material (expansion rate = 0) can cause this brittle coating to crack (phase 2). This can occur even due to relatively minor elastic macro-expansion, causing micro-slipping in the grains due to the different orientation of the grains (top diagram), which break through the protective coating as extrusions or expose the base material as intrusions. This process destroys the protective layer so that local corrosion can occur.
Once the protective surface coating is damaged once, the brittle reaction layer again forms in the base of the crack, which cracks again and causes crack growth. This crack mechanism leads to step-by-step crack growth that leaves characteristic progress lines in the micro-zone which, along with the reaction layers, indicate the damage mechanism of the SCC (see Fig. "Fracture of the spacers").

Strain-induced SCC (bottom diagram) on the other hand, occurs under increasing loads. The damage model is the formation of ductile, regenerative reaction layers, which plausibly explains why this SCC occurs increasingly in a limited expansion rate range. If the expansion rate is greater than the speed at which the reaction layer regenerates, it will not heal and corrosion will increase. If the expansion/strain rate is further accelerated, the SCC effect near the tough force fracture decreases. Titanium alloys exhibit typical behavior for strain-induced SCC (Fig. "Design of experiment").
Different expansion rates occur during engine operation during startup (due to, for example, increased centrifugal force in disks or pressure increases in the housings/casings, etc). The damage case described in Example "Apparently harmless changes" and Fig. "Special experiments" should be considered in connection with this phenomenon.

Figure "Material behavior": For SCC, the crack expansion/strain rate can be depicted in a similar way as with cyclical fatigue processes (top diagram). However, the dependency on the tension intensity at the crack tip is depicted, and not the nominal stress. This is understandable when considering the crack growth mechanism in Fig. "Coating damage II", which depends largely on the deformations at the crack tip.
Above a threshold value below which no crack growth occurs, the crack growth speed increases rapidly with the stress intensity (area “I”).
In zone “II” the crack growth speed remains constant over a large range of the stress concentration deflection. When the critical crack length is reached, ultimate fracture occurs.
The bottom diagram is a schematic depiction of a fractured surface with typical characteristics of SCC, such as steps and cracks in the brittle reaction layer (without signs of the influence of hydrogen; also see Fig. "Crack growth mechanism"). The markings of step-by-step crack growth are interesting, and must not be confused with the striations of a dynamic fatigue fracture. The cracked brittle reaction layer on the fracture surface clearly differentiates this damage from other similar crack types.

Figure "Tensile stress": The form, in which the corrosive mechanism attacks (top diagram), depends on parameters such as the sensitivity of certain structural components, the corrosive media, and the effective tensile stresses. For example, finely branching cracks indicate relatively low tensile stress levels, while deep single cracks indicate high tensile stress levels (middle diagram).
In threaded connections, the tensile loads on the thread base change at the first threads. This has a pronounced effect on the development of the corrosion cracks (bottom diagram).

Example "Apparently harmless changes" (Ref. 5.4.2.1-3, see also Fig. "Catastrophic consequences" and Fig. "Special experiments"):

Excerpt: “…In a helicopter turbine, after about 1000 flight hours, the axial compressor disk broke at 44,000 RPM during idling on the ground and was completely destroyed (see Fig. "Material behavior").

After hardening, (The) axial compressor disks made from turbine blade steel X15Cr13 were not tempered at the usual 725°C, but only at 540°C, in order to achieve the greatest possible strength. This treatment made the steel especially sensitive to corrosion, since a web of chrome carbides precipitates on the former austenite grain boundaries. The disks still achieve the required life span, since the high centrifugal force throws off the moisture from the incoming air that acts as a corrosive media. However, an apparently harmless constructive change -tightening of the balance ring- caused the moisture to build up, resulting in the disks failing due to SCC and corrosion fatigue.”

Comments: This damage case, which was investigated and described by Prof. G. Lange of the TU Braunschweig, shows the influences on the development of SCC in an exemplary manner. The increased hardness achieved by the lower tempering temperature did not increase safety against disk fractures, but led to a strength decrease and catastrophic part failure under corrosion conditions that are typical during operation. During engine startup, especially, conditions for strain-induced SCC (see Fig. "Corrosion in the area of the compressor") and corrosion fatigue were most evidently present.

Figure "Special experiments" (Ref. 5.4.2.1-3, Example "Apparently harmless changes"):

This example is an impressive depiction of the influences on SCC (compare with 5.4.4.2-2). Special tests are necessary to determine the SCC-sensitivity of a material. A sample test is shown in the bottom diagram.

Problems with corrosion tests during damage investigations:
If this type of test is intended to determine or verify the corrosion sensitivity of a system, then even minor deviations from the damage case should unallowably falsify the results.
If, for example, the material sample does not sufficiently correspond to the part, or if there is a difference in the structure or orientation relative to the direction of the tensile stress, the SCC behavior of the sample may be unallowably altered relative to the actual damaged part. For example, in cast parts, it must be assumed that, due to the casting and freezing conditions, different part areas may have very different grain sizes and that there are eliquation-dependent deviations among the alloy components. This behavior overlays with the operating similarity of the electrolyte and the tensile stress levels. If the part is coated, the coatings must be properly reproduced.
If, in a sample, a necessary threshold value of the stress concentration is not exceeded, as would be expected with smooth samples, then the sample is unsuitable for determining the SCC sensitivity of certain materials, such as titanium alloys (see Fig. "Design of experiment").
If the damage mechanism concerned is a strain-induced process, in which the induced expansion/strain rate is important, then a test such as this one, with static loading, is of little or no value (see Fig. "Design of experiment").
Therefore, the corrosion sample of an affected part should, if possible, be taken from the same production batch.
Surface hardening and induced compressive stresses can prevent SCC, while cold deformations with induced tensile stresses and/or structural changes (such as martensite formation) can promote SCC.
Therefore, in tests that are designed to permit conclusions as to the operating behavior of engine parts, it must be ensured that the surface condition (e.g. treatment) of the sample or test part is sufficiently similar to the original part.

Figure "Design of experiment": The problems associated with reproducing SCC damages in laboratory tests on samples is especially pronounced with titanium alloys.

For example, Ti-alloys show serious damage in laboratory tests with hot salt above about 300 °C, and especially above 450°C to 500°C (top diagrams). However, there has been no known engine damage that was determined to have been caused by hot salt corrosion, even though highly-stressed titanium parts in modern engines, such as bladings, are struck by marine atmosphere and often exhibit salt deposits. A satisfactory explanation for this behavior might be the deviation of laboratory tests from actual operating conditions.
Cases of SCC due to hot salt attack have been reported during the manufacturing process (Fig. "Fraction caused by hand sweat", Ref. 5.4.2.2-5). In one case, hand sweat reacted with a titanium alloy fan blade during heat-treatment.
Cases of SCC have also been reported in connection with reaction coatings containing Cl (such as in etching, cleaning, and degreasing baths), contaminants (synthetic wear products containing Cl), and labeling.

In laboratory tests on titanium alloys it is important to simulate a realistic tension intensity. During manufacture, after damage was observed on the untreated seam head of a electron beam weld seam (Fig. "Sensitization during operation"), even though smooth samples in the same media (hot perchloroethylene degreasing bath at about 90°C) showed no comparable damage, the SCC-sensitivity of a high-strength Ti alloy was finally determined (bottom diagram) with the aid of CT samples (bottom right diagram).
According to Ref. 5.4.2.1-4, chloride compounds, which are created, for example, through surface reactions in cleaning baths containing Cl (HCl splits off and reaction creates chlorides), can later (e.g. during following manufacturing processes or operation) lead to stress corrosion cracking in titanium materials at temperatures above 150°C.

References

5.4.2.1-1.1 W.Gruhl, “Zeitschrift für Metallkunde” 53, (1962) page 670.

5.4.2.1-1.2 H.Spähn, H.Steinhoff, periodical “Werkstoffe und Korrosion”, 20 (1969) page 733.

5.4.2.1-2 L. Engel, H. Klingle, “Rasterelektronenmikroskopische Untersuchungen von Metallschäden”, Gerling Institut für Schadensforschung und Schadensverhütung GmbH Köln, 1974, ISBN 3-9800043-0-9, page 182.

5.4.2.1-3 G.Lange, “Zerstörung von Hubschrauberturbinen durch Einsatz eines Stahls in korrosionsanfälligem Zustand bei gleichzeitig nicht werkstoffgerechter Konstruktion”, “Zeitschrift für Werkstofftechnik”, 5. Jahrg. 1974, Nr 1, pages 9 to 13.

5.4.2.1-4 H.Simon, “Oberflächenreaktionen an Titanwerkstoffen”, periodical “Metall Oberfläche” 5-1982, pages 211-217.

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