As the name suggests, this type of failure results from a combination of two respective components. Initially, the action of a corrosif cell can result in the formation of a pit within the surface of the mother or father metal. The pit alone can often be obscured from look at due to the presence of a tubercle (cover). A pit within the the top of a pressure retaining aspect will induce a local tension concentration within the vicinity. The thickness of all pressure retaining boiler components is very carefully calculated to allow for the tensions and degradation mechanism that will affect pressure plant when in support. Cracks can initiate should the stress concentration exceed the critical value. Subsequent riding a bike of the boiler can result in the step-wise growth of the crack -- this is known as fatigue. Once the crack has propagated and there is no longer enough strength to retain the incident pressure, failure happens, sometimes catastrophically.

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Corrosion fatigue, since the above description details, can be initially characterised by unsightly pitting of the parent material that ultimately compromises its ethics. A pit within the surface of a parent material can be seen in figure 1. Following initiation, splits propagate with cycling, this leaves striations within the fracture surface as can be seen in figure 2.

The simplest way in which to avoid failures by means of deterioration fatigue is to prevent the oxidation in the first place. This is achieved by maintaining control over feed-water treatment during operation and also suitable lay-up during periods of loss of focus. This should be complimented by a regular system of inspections in ways that any potential issues can be dealt with during the early stages associated with development. Chemical cleaning, whenever executed both effectively and safely, offers the ability to stop ongoing corrosion in its tracks provided that subsequent feed-water treatment is accurately controlled.

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