Corrosion Fatique
Corrosion fatigue is the process of metal cracking prematurely in the presence of a corrosive environment, such as saltwater. The combined effects of cyclic stress and corrosion cause this type of failure. Corrosion fatigue occurs when metal breaks at stresses significantly lower than its tensile strength. Characterized by little or no branching, this corrosive process is accelerated when the fatigue causes the protective passive film to split. The environment in which the metal resides influences the beginning of the cracks and/or their growth rate.
Inspectors have difficulty detecting corrosion fatigue because the cracks can be very fine and unnoticeable; stressed coupons, which are metal samples, have been used for monitoring and testing purposes to detect failure. The end results of corrosion fatigue are catastrophic if not detected.

Types of stresses that contribute to fatigue failure include applied stresses, thermal expansion and contraction, poor assembly, welding, casting, soldering, mechanical cleaning, heat treatment, forming, shaping, cutting, and shearing.

Preventing corrosion fatigue requires designing and constructing the materials properly, reducing any stress, removing any environmental contributors such as oxygen and hydrochlorides, and eliminating stagnant areas and crevices (such as in heat exchangers) where chloride and hydroxide might be concentrated.

Corrosion fatigue and stress corrosion cracking share some of the same characteristics. Both forms can occur under similar conditions, coexist on the same fracture, initiate at the surface, and appear to be brittle.

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