Table of Contents
5.4.3 Corrosion Fatigue
Corrosion fatigue is a corrosive influence that promotes fatigue crack initiation and growth under dynamic loads. This corrosion type is primarily dependent on loads, unlike stress corrosion cracking, which is primarily time-dependent.
5.4.3.1 Fundamentals of Corrosion Fatigue
Corrosion fatigue is a type of low-deformation crack initiation under alternating corrosion and mechanical loads that are independent of the three influences that make up the corrosion system.
Unlike stress corrosion cracking, corrosion fatigue is not tied to a specific system, metal/media. Therefore it has a far greater number of possible damages (Fig. "Corrosion influence on fatigue cracks"). Both actively corroding materials and materials that behave passively can be damaged. A possible reason for the “universal” appearance of this corrosion type is that, under dynamic loads, pronounced extrusions can form in the micro-zone (Fig. "Coating damage I"), which represent intense damage for protective coatings (passive coatings). If pitting corrosion or intercrystalline corrosion (Fig. "Phases of corrosion fatigue damage") occur during the crack initiation phase, the relationships become very complicated. For this reason, practical experience and close investigation of operating conditions are indispensable for successful countermeasures (Ref. 5.4.3-2). Unlike with pure dynamic fatigue, there is a clear influence of frequency on noticeable corrosion fatigue with an SCC influence (Ref. 5.4.3-2). The material, corrosive media, and loads (e.g. frequency, mean stress, stress deflection) determine whether corrosion fatigue cracks are transcrystalline or intercrystalline (Ref. 5.4.3-1). The influence of corrosion fatigue can typically be seen in heat-treated steels as an intercrystalline component in the crack progress. In exceptional cases, even with “real” corrosion fatigue, intercrystalline crack progress has been determined in martensite and austenite steels in zones with middle stress intensity amplitudes. The intercrystalline component increases along with increasing average loads.
The term “corrosion fatigue” is differentiated from dynamic fatigue fractures in some literature, if the fractures originate in corrosion marks without another corrosive media acting. This seems impossible to the author when considering that virtually all metals have better dynamic strength in a vacuum than in an atmosphere, and that the crack growth speed can be up to four times as great as that in a vacuum (Ref. 5.4.3-2). This shows that a corrosion fatigue effect can always be expected, and especially when the surrounding media has already lead to pitting or intercrystalline corrosion. However, this effect is often not noticed, since the characteristic values for the material`s fatigue strength are determined in air and/or the failure mechanism in sample tests is considered to be the first crack. For this reason, the crack initiation and growth influence does not usually stand out. However, the more dynamically loaded parts, in which the crack growth phase forms a conscious or unconscious factor of the life span concept, are used (such as in highly loaded rotors in modern engines), the more important it becomes to consider the influence of corrosion fatigue.
For example, it has become normal practice to determine the LCF life span of rotors in vacuum testing rigs due to the lower energy consumption. However, before conducting this type of test, one must be certain that the vacuum is not a falsifying factor, since this would otherwise mean that the determined usable life spans would be longer than those that could actually be realized in practice.
Behavior of different material groups:
Cr steels: in these heat-treated steels, corrosion fatigue usually occurs in connection with pitting corrosion. At first, crack progress is primarily intercrystalline, i.e. it runs along the former austenite grain boundaries that are still present in the heat-treated structure. This intercrystalline progress can already be seen under the influence of pure water. Even with high-frequency vibrations, such as are typical in compressor bladings, intercrystalline fracture components occur (Fig. "Blade fracture"). With “real” corrosion fatigue (no SCC component), intercrystalline fracture components are only to be expected at middle stress concentration amplitudes (re: Ref. 5.4.3-3). The sensitivity to corrosion fatigue (and SCC) increases with hardness. It is plausible that the damage is also promoted by the increased notch sensitivity at higher hardnesses. Above 40 HRC, these materials should not be used under corrosive conditions.
Cr-Ni steels: with these austenite alloys, which normally have pronounced passive corrosion behavior (Fig. "Phases of corrosion fatigue damage"), pitting corrosion and/or intercrystalline corrosion are often present in the crack origin. Even in the sensitized material, dynamic fatigue cracks are usually transcrystalline. However, high mean stress and low frequencies promote intercrystalline crack progress. Tests showed fatigue cracks that were transcrystalline in a vacuum, but had pronounced intercrystalline growth at low frequencies (10-3 Hz) under the influence of water (!)(Ref. 5.4.3-3). This was traced back to an increased SCC-influence (intercrystalline growth) during the fatigue process.
Mg- and Al-alloys: with these alloys, dynamic fatigue fractures often originate in corrosion pittings. Marine atmosphere is noticeable even at very low dynamic loads (Fig. "Corrosion influence on fatigue cracks").
Ti-alloys: these alloys show an influence of hot salt on the LCF and HCF behavior above about 250 °C. In both cases the life span/dynamic strength decreases considerably. The influence is especially noticeable at low-frequency loads in the LCF zone. The sensitivity to hot salt action depends on a multitude of other influences such as the structure of the material.
Ni-alloys: Corrosion fatigue effects were observed in certain media such as greases containing MoS2 (at high temperatures) or special air contaminants. For example, MoS2 in closed air (such as between mating surfaces with screws) can extremely accelerate LCF crack progress, whereby grain size seems to have an influence. Pronounced stage I fractures develop (crack surfaces), the size of which naturally corresponds to the grain size. With the large grains typical in cast parts, correspondingly large cracks develop. It can be assumed, that single crystal material may be especially to crack surface development, depending on the orientation of their crystals relative to the loads. Experience has shown that the same is true for fatigue processes in protective gases such as argon. Evidently, even small impurities in the gas (e.g. iron compounds) can cause accelerated crack growth.
Figure "Corrosion influence on fatigue cracks": Similar to marine atmosphere, salt water steam has a weakening effect on the dynamic fatigue strength of many materials (bottom diagram, Ref. 5.4.3-5), when compared with dry air. For example, a decrease of over 50% can be expected in Cr steels and high-strength Al alloys.
In the top diagram (Ref. 5.4.3-4), curves “1” and “3” show this effect schematically. This also shows that, if corrosion fatigue is suspected, testing must be done in the operating medium. Pre-corrosion (curve “2”) without dynamic loads in a corrosion chamber or in a submerged test is not sufficient and results in overly high fatigue strength relative to realistic operating conditions. Even the crack initiation phase is not satisfactorily simulated in this type of test. Fracture mechanics tests are necessary for considering the crack growth phase.
With complex operating damages, sufficiently safe conclusions are no longer possible from laboratory tests under standard conditions (Ref. 5.4.3-2).
Figure "Phases of corrosion fatigue damage" (Refs. 5.4.3-2 and 5.3.3-6): The diagram shows the typical phases of corrosion fatigue damage and gives an impression of the variety of damage symptoms.
Origin conditions: materials that have surfaces with active corrosion behavior (such as unalloyed and low-alloyed steels), as well as those with passive behavior (such as high-alloy steels; form protective coatings), can show corrosion fatigue.
Crack nucleus formation: glide planes, corrosion-sensitive phases, inhomogeneities at grain boundaries (e.g. selective attack of Cl ions on sensitized Cr-Ni steels), weaknesses in the protective coating (such as flaws at structural inhomogeneities and impurities), and crack initiation in non-metallic inclusions such as carbides are attack zones for corrosion fatigue.
The protective passive layer is broken through and damaged due to slippings in the surface zone (compare with Fig. "Coating damage I"). This both exposes unprotected metal surfaces, and also creates notches in this zone with increased mechanical stresses that promote the corrosion process. In the passive state, the cracks often remain unfilled by corrosion products.
In the active state, first local pittings develop.
Intermediate stage: In the active state of a material group, the pittings expand, whereas in the active state the pittings become deeper, and their bases create new protective coatings and become passive. These, in turn, then crack open again during further load cycles (compare with Fig. "Coating damage I"). The process of the passive layer formation in the base of the pittings is a reason for the frequency-dependency of corrosion fatigue. If the grain boundaries are especially sensitive, i.e. suitable structural zones are present, then an initial attack begins here. The main growth direction of this attack is through the protective passive layer into the material.
In the active state, no passive coating is created, and the corrosion spreads out to the sides across a larger area.
Crack initiation: from the cracks and nicks that formed in the intermediate stage, relatively sharp cracks form with no notable expansion due to eroding corrosion.
Crack propagation: depending on the size of the loads, high stresses result primarily in straight single cracks that run into the material and low stresses result in branching at the end of cracks.
Figure "Liquid water in compressor": Almost all compressor parts are subject to more or less high dynamic loads. This is especially true for the blading. Because the simultaneous presence of a corrosive media is necessary for corrosion fatigue damage to occur, the temperature, pressure, and air speed in the working compressor play an important role.
The increasing pressure allows liquid water to be present even at about 140°C (top diagram), so stages with this compression temperature can be affected by it.
The typical salts of ocean water can release chemically combined water (bottom diagram) at higher temperatures (up to over 250°C), creating aggressive moisture in existing cracks at least for a short time.
As shown, damp marine air is sufficient for fatigue corrosion in Cr steels under dynamic loads (compressor blades in older engine types). The probability of corrosion fatigue crack initiation is lower in the hotter rear compressor zones than in the forward areas, where moisture can act even during operation. During startup and acceleration of the engine, however, there may temporarily be sufficient moisture in the entire compressor. This moisture can be in the form of condensation water that collects in structural hollow spaces (Fig. "Failure of variable vane adjuster"), gaps (such as the blade root area, constructed V-stator vanes with hollow boxes - Fig. "Corrosion in the area of the compressor", variable vane adjusters), corrosion notches, roughness, and porous deposits or coatings, and only dries out during operation, when its concentration is increased. Experience has shown that dynamic fatigue cracks and fractures of compressor blades made from Cr steels and Al alloys often show signs of corrosion fatigue.
5.4.3.2 Damage due to Corrosion Fatigue
Even though, judging by the damage mechanism, dynamic fatigue damage would seem to indicate an atmospheric influence and corrosive environments would seem to decrease dynamic strength and increase crack growth rates, there are very few cases in which damage is explicitly traced back to corrosion fatigue.
This is true both for the damages shown in Figs. "Corrosion as cause of a disk burst" and "Fracture of the spacers", which have a causal relationship with pitting corrosion, and also for Figs. "Local corrosive attack" and "Blade fracture".
Figure "Local corrosive attack" (Ref. 5.4.3-7): This compressor of a small helicopter engine contains cast bladed disks (blisks) made from a precipitation hardened high-strength steel (17-4 PH). In an unprotected state, this material tends to pitting corrosion under the influence of chlorine ions, which are a typical component of the salts in marine air (Fig. "Liquid water in compressor"). Salt deposits on the blades are hygroscopic and absorb water from the surrounding air. With (NaH4)2SO4 and NaCl deposits this occurs above 80% relative humidity, with MgCl2 ,above a mere 32%.
Corrosion protection for the compressor blading:
Regular washing of the compressor with a suitable detergent has proven to be a suitable measure against the corrosive attack.
Coating the compressor with NiCd provides sufficient corrosion protection only when the coating is not damaged all the way to the base material, which experience has shown cannot be ensured under realistic operating conditions. Al powder-filled inorganic lacquer systems, however, provide active corrosion behavior with cathodic corrosion protection of the base material that is effective even in case of local coating damage that exposes the base material (e.g. scratches or FOD).
Hard erosion coatings with noble corrosion behavior such as TiN, which build up a potential relative to the base material, dangerously promote corrosion pittings through cell action. Because FOD damage or erosion notches can always be expected to break through or chip off hard protective coatings and expose the base material, these protection systems are unsuitable in cases where corrosion-sensitive base materials would be subject to erosive and corrosive loads.
Figure "Blade fracture": The compressor blading of older engine types is often made from Cr steels. Only in a polished state do these steels (so-called knife steels) have sufficient corrosion resistance to Cl ions in watery media. In practical operation, however, experience has shown that after a relatively short operating time, erosion roughs up the blades and causes pitting corrosion in the base material. The sensitivity to cracking corrosion types (SCC and corrosion fatigue) usually increases with the hardness of these heat-treated materials.
In the case at hand, a fighter aircraft crashed after the fracture of a forward compressor rotor
blade due to a fatigue fracture that originated in a pitting corrosion notch. The hardness of the blade was clearly to high (45 HRC, rather than the maximum allowable hardness of 40 HRC). Since inspection of a large number of other blades did not reveal any more similar deviations, this case was classified as an isolated flaw, which has been borne out by following years of experience.
The isolated case showed the intercrystalline structure typical of pitting formation in Cr steels. The fatigue fracture has several intercrystalline regions (white arrows), which indicate the corrosive influence during crack growth typical for corrosion fatigue.
5.4.3.3 Measures for Preventing Corrosion Fatigue Damage
In general, the same rules for preventing pure dynamic fatigue fractures hold true for measures for preventing corrosion fatigue:
- lowering the dynamic loads and, if possible, also the mean stress
- avoiding stress peaks
- measures to prevent corrosive attack (i.e. the presence of corrosive media) during operation (i.e. during dynamic loading). This includes suitable protective coatings, but also means that constructive design must avoid gaps and hollow spaces that open outwards, and in which corrosive media can collect. Sufficiently large openings can also prevent the accumulation of the media during operation and contribute to corrosion protection.
Compressive internal stresses from rolling or peening do not prevent corrosion fatigue damage in an active state (material decomposition, pitting corrosion). With passive surfaces (without pitting corrosion), however, improvement is possible.
- Material selection must be supported by suitable, realistic testing methods. For example, testing parameters such as the load cycles and frequency must be considered. These must realistically correspond to the expected operating conditions.
- Sufficiently early and practical overhauling of at-risk parts must be planned and undertaken. This can, for example, prevent dangerous pitting corrosion that would result in a corrosion fatigue-promoting stress concentration.
- Material parameters must be determined under conditions that simulate those during operation as closely as possible (media, loads, time spans).
- The maximum attainable hardness values should not be taken advantage of, if this would dangerously increase the risk of corrosion fatigue.
References
5.4.3-1 M. Karim-Khani, “Ermüdungsverhalten randschichtwärmebehandelter Stähle ohne bzw. mit Korrosionseinfluß”, Mensch & Buch Verlag, Berlin 1999, ISBN 3-9333-46-55, pages 78 to 83.
5.4.3-2 H.Speckhardt, “Werkstoffverhalten bei Überlagerung von Schwingungsrisskorrosion und anderen spezifischen Korrosionserscheinungen”, Bruchuntersuchung und Schadensklärung, Allianz 1976, pages 75-82.
5.4.3-3 M.O.Speidel, “Interkristalline Korrosionsermüdung in Stahl”, Bruchuntersuchung und Schadensklärung, Allianz 1976, pages 83-87.
5.4.3-4 Kh.G.Schmitt-Thomas, Th.Kirner, “Form und Mechanismen von Rissen und Brüchen unter gleichzeitiger korrosiver und dynamischer Beanspruchung mit unterschiedlichen Frequenzen”, periodical Werkstofftechnik 9, Verlag Chemie GmbH, D-6940 Weinheim 1978, page 407.
5.4.3-5 A.Tross, “Der Mechanismus des Dauerbruchs und seine Beeinflussung durch Korrosion”. periodical “Werkstoffe und Korrosion”, Volume 20, Issue 11/1969, pages 954 to 962.
5.4.3-6 H.Spähn , G.H.Wagner,“Corrosion Fatigue of Steels”, Bruchuntersuchung und Schadensklärung, Allianz 1976, pages 59 to 74.
5.4.3-7 H.J.Kolkmann, “Gas Turbine Compressor Corrosion and Erosion in Western Europe”, AGARD-CP-558, proceedings of the conference “Erosion, Corrosion and Foreign Object Damage Effects in Gas Turbines”, Rotterdam, The Netherlands 25-28 April 1994, chapter 30 page 30-3.