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Corrosion fatigue is fatigue in a corrosive environment. It is the mechanical degradation of a material under the joint action of corrosion and cyclic loading. Nearly all engineering structures experience some form of alternating stress and are exposed to harmful environments during their service life. The environment plays a significant role in the fatigue of high strength structural materials like steels, aluminum alloys and titanium alloys. Materials with high specific strength are being developed to meet the requirements of advancing technology. However,their usefulness depends to a large extent on the extent to which they resist corrosion fatigue. Effects of corrosive environments on fatigue behavior of metallic materials have been studied as early as 1930 (1).

Contents 1 Characteristics 1.1 Effect of corrosion on S-N diagram 1.2 Crack propagation studies in corrosion fatigue 1.2.1 True corrosion fatigue 1.2.2 Stress corrosion fatigue 1.3 References

[edit] Characteristics

[edit] Effect of corrosion on S-N diagram

Effect of corrosion on smooth specimen S-N diagram is shown schematically in Fig.1.

. In this figure, curve A shows the fatigue behavior of a material tested in air. A fatigue threshold or limit is seen in A corresponding to the horizontal part of the curve. Curves B and C represent the fatigue behavior of the same material in two corrosive environments. In curve B, the fatigue failure at high stress levels is retarded and the fatigue limit is eliminated. In curve C, the whole curve is shifted to the left indicating a general lowering in the fatigue strength, accelerated initiation at higher stresses and elimination of a fatigue limit. To meet the needs of advancing technology, higher strength materials are developed through heat treatments or alloying. Such higher strength materials generally exhibit higher fatigue limits and thus can be used at higher service stress levels even under fatigue loading. However, presence of corrosive environments during fatigue loading eliminates this stress advantage, as the fatigue limit becomes almost insensitive to the strength level for a particular group of alloys (2). This effect is schematically shown, for the case of several steels, in Fig.2.

. This figure brings out the debilitating effect of corrosive environments on the functionality of high strength materials under fatigue.

Corrosion fatigue in aqueous media is an electrochemical behavior. Cracks are initiated either by pitting or at persistent slip bands (3). Corrosion fatigue can hence be reduced by alloy additions, inhibition, and cathodic protection all of which reduce pitting (4) Since corrosion fatigue cracks initiate at the metal surface, surface treatments like plating, cladding, nitriding and shot-peening were found to improve the materials' resistance to this phenomenon (5).

[edit] Crack propagation studies in corrosion fatigue

In normal fatigue testing of smooth specimens, about 90% of the life is spent in crack nucleation and only the remaining 10 % in crack propagation. However,in corrosion fatigue,crack nucleation is facilitated by corrosion and typically about 10 % of life is sufficient for this stage. The rest, 90 % of life is spent in crack propagation. Thus it is much more informative to evaluate the crack propagation behavior during corrosion fatigue. Fracture mechanics uses pre-cracked specimens and effectively measures crack propagation behavior. Hence considerable emphasis is being given to crack propagation velocity measurements using fracture mechanics to study corrosion fatigue. Since fatigue crack grows in a stable fashion below the critical stress intensity factor for fracture (fracture toughness), the process is called sub-critical crack growth. Fig.3 shows a typical fatigue crack growth behavior.

.

In this log-log plot,the crack propagation velocity is plotted against the applied stress intensity range. Generally there is threshold stress intensity range below which the crack propagation velocity is insignificantly low. Three stages may be visualized in this plot. Near the threshold,crack propagation velocity increases considerably with increasing stress intensity range, a second region where the curve is nearly linear and follows the Paris' law(6), and the third region where crack propagation velocity increases rapidly with stress intensity range leading to fracture at the fracture toughness value.

Crack propagation under corrosion fatigue may be classified in to i) true corrosion fatigue, ii) stress corrosion fatigue and iii) a combination of true, and stress, corrosion fatigues.

[edit] True corrosion fatigue

In true corrosion fatigue, the fatigue crack growth rate is enhanced by corrosion and this effect is seen in all the three regions of the fatigue crack growth rate diagram. Fig.4 shows a schematic diagram of crack growth rate under true corrosion fatigue.

. It is seen from this figure that the whole curve is shifted to lower stress intensity factor range in the corrosive environment. The threshold is lower and the crack growth velocities are higher at all stress intensity factors. The fracture of the specimen occurs when the stress intensity factor range is equal to the applicable threshold stress intensity factor for stress corrosion cracking.

When attempting to analyze the effects of corrosion fatigue on crack growth in a particular, both corrosion type and fatigue load levels can each affect the crack growth to varying degrees. Common corrosion types include crevice, pitting, filiform, exfoliation, intergranular--each will affect crack growth in a particular material in a distinct way. For instance, pitting corrosion will often be the most damaging type of corrosion, degrading a material's performance (that it, speeds up crack growth rates) more than any of the other kinds of corrosion; even pits the order of a material's grain size can substantially degrade a material. The degree to which corrosion can affect crack growth rates can also be depending on fatigue load levels; for instance, corrosion can cause much greater increase in crack growth rates at low loads than it does at high loads (7).

[edit] Stress corrosion fatigue

In materials where the maximum applied stress intensity factor exceeds the stress corrosion cracking threshold value, stress corrosion will also add to the crack growth velocity. This is shown schematically in Fig.5.

. In the corrosive environment, the crack grows due to cyclic loading at lower stress intensity ranges,but above the threshold stress intensity for scc, additional crack growth (red line) occurs due to stress corrosion cracking. The lower stress intensity regions are not affected and the threshold stress intensity range for fatigue crack propagation is unchanged in the corrosive environment.

In the most general case, corrosion fatigue crack growth may exhibit both of the above effects and the crack growth behavior is represented schematically in Fig.6.

[edit] References

1. P. T. Gilbert,Metallurgical Reviews,1 (1956)379
2. H. Kitegava, in Corrosion fatigue, chemistry, mechanics and microstructure, O. Devereux et al. eds. NACE, Houston,(1972), p.521
3. C. Laird and D. J. Duquette, ibid,p.88
4. J. congleton and I. H. Craig, in Corrosion Processes, R. N. Parkins (ed), Applied Science Publishers London, (1982),p.209.
5. H. H. Lee and H. H. Uhlig, Metall. Trans., 3 (1972),2949.
6. P. C. Paris and F. Erdogan, J. Basic Engineering, ASME Trans., 85 (1963), 528
7. Craig L. Brooks, Scott A. Prost-Domasky, Kyle T. Honeycutt, and Thomas B. Mills. Predictive modeling of structure service life. In ASM Handbook Volume 13A, Corrosion: Fundamental, Testing and Protection, October 2003, 946-958.