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Corrosion fatigue
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 steel, 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. The effects of corrosive environments on the fatigue behavior of metals were studied as early as 1930.〔P. T. Gilbert, ''Metallurgical Reviews'' 1 (1956), 379〕 The phenomenon should not be confused with stress corrosion cracking, where corrosion (such as pitting) leads to the development of brittle cracks, growth and failure. The only requirement for corrosion fatigue is that the sample be under tensile stress.
== Effect of corrosion on S-N diagram ==
The effect of corrosion on a smooth-specimen S-N diagram is shown schematically on the right. Curve A shows the fatigue behavior of a material tested in air. A fatigue threshold (or limit) is seen in curve 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; this indicates a general lowering in fatigue strength, accelerated initiation at higher stresses and elimination of the fatigue limit. To meet the needs of advancing technology, higher-strength materials are developed through heat treatment or alloying. Such high-strength materials generally exhibit higher fatigue limits, and can be used at higher service stress levels even under fatigue loading. However, the presence of a corrosive environment during fatigue loading eliminates this stress advantage, since the fatigue limit becomes almost insensitive to the strength level for a particular group of alloys.〔H. Kitegava in ''Corrosion Fatigue, Chemistry, Mechanics and Microstructure'', O. Devereux et al. eds. NACE, Houston (1972), p. 521〕 This effect is schematically shown for several steels in the diagram on the left, which illustrates the debilitating effect of a corrosive environment on the functionality of high-strength materials under fatigue.
Corrosion fatigue in aqueous media is an electrochemical behavior. Fractures are initiated either by pitting or persistent slip bands.〔C. Laird and D. J. Duquette in ''Corrosion Fatigue, Chemistry, Mechanics and Microstructure'', p. 88〕 Corrosion fatigue may be reduced by alloy additions, inhibition and cathodic protection, all of which reduce pitting.〔J. Congleton and I. H. Craig in ''Corrosion Processes'', R. N. Parkins (ed.). Applied Science Publishers, London (1982), p. 209〕 Since corrosion-fatigue cracks initiate at a metal's surface, surface treatments like plating, cladding, nitriding and shot peening were found to improve the materials' resistance to this phenomenon.〔H. H. Lee and H. H. Uhlig, ''Metall. Trans.'' 3 (1972), 2949〕
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