Corrosion fatigue occurs when the simultaneous action of cyclic stresses and chemical environment is in play. Corrosive environment generally shorten the fatigue life and reduce the endurance limit, as shown in Figure 5.66. There are various ways in which corrosive environment can affect the fatigue behavior. One could be generating pits (pitting corrosion) that can lead to crack nucleation sites. Also, the crack growth rate is increased as a result of the chemical environment. The mode of load application also affects the corrosion fatigue behavior, such as reduction in loading frequency during fatigue testing may result in longer period of time through which the crack stays in opening mode in contact with the corro­sive medium, leading to a reduction in fatigue life.

Corrosion Prevention Methods There are a host of corrosion prevention methods that can be introduced. The first choice of prevention would be to develop an intrin­sically corrosion-resistant alloy if it is economical. One way is to create passive films naturally on the material. There are yet several other ways that can prevent or mini­mize corrosion. For example, even though aluminum appears as very active on the electrochemical series, it forms a thin, impervious layer of alumina, thus protecting it from a variety of corrosive environments. Anodizing is a commercial process by which a much thicker alumina film is developed on aluminum to have usefulness in marine environment. Generally, the thin passive film of aluminum oxide gets destroyed under the exposure of chloride ions in seawater. As already noted, stain­less steel is mainly stainless because of the formation of a thin, impervious film of chromic oxide that occurs due to higher chromium content.

Inhibitors are sometimes used to provide passive surface film over surfaces. Inhibitors are highly oxidized chromate and the like solutions that are adsorbed on the metal surface to be protected. The passive layer formed by inhibitors works like

passive film, but cannot be regenerated. Hence, new inhibitors need to be applied if the layer gets washed out.

Galvanic protection is another way of preventing corrosion. As already men­tioned, one of the ways of achieving galvanic protection is to use a more active metal as anode (sacrificial anode). The sacrificial anode needs to be replaced after it gets exhausted (Figure 5.67). Another way of achieving galvanic protection is to apply a direct current (DC) that feeds electrons into the metal part that needs to be protected. An example would include the use of impressed current minimizing or even stopping corrosion occurring at the underground pipeline (Figure 5.68).

Various types of paints, enamel coating, and so on are also used in various appli­cations for prevention of corrosion. However, they need to be reapplied in regular intervals as they may exfoliate under service condition and restart corrosion.

5.3.3