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 degree to which they resist corrosion fatigue.

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.

In true corrosion fatigue, the fatigue-crack-growth rate is enhanced by corrosion; this effect is seen in all three regions of the fatigue-crack growth-rate diagram. The diagram on the left is a schematic of crack-growth rate under true corrosion fatigue; the curve shifts to a lower stress-intensity-factor range in the corrosive environment. The threshold is lower (and the crack-growth velocities higher) at all stress-intensity factors. Specimen fracture 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 environment, both corrosion type and fatigue load levels affect crack growth in varying degrees. Common types of corrosion include filiform, pitting, exfoliation, intergranular; each will affect crack growth in a particular material in a distinct way. For instance, pitting will often be the most damaging type of corrosion, degrading a material's performance (by increasing the crack-growth rate) more than any other kind of corrosion; even pits of the order of a material's grain size may substantially degrade a material. The degree to which corrosion affects crack-growth rates also depends on fatigue-load levels; for instance, corrosion can cause a greater increase in crack-growth rates at a low loads than it does at a high load.

Stress-corrosion fatigue

In materials where the maximum applied-stress-intensity factor exceeds the stress-corrosion cracking-threshold value, stress corrosion adds to crack-growth velocity. This is shown in the schematic on the right. In a corrosive environment, the crack grows due to cyclic loading at a lower stress-intensity range; above the threshold stress intensity for stress corrosion cracking, additional crack growth (the red line) occurs due to SCC. 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; crack-growth behavior is represented in the schematic on the left.