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Corrosion Fatigue

   
 
 
                                                                                

Corrosion Fatigue.

Many materials will exhibit a substantial reduction in fatigue life when exposed to a corrosive environment. In some cases, the reduction is severe, in other cases it is less dramatic, but only a very few materials show a fatigue resistance in a corrosive environments as great as that in dry air.

Definition.

Corrosion fatigue is the reduced ability of a metal to withstand repeated stress when exposed to the combined action of stress and a corrosive environment as compared to the effects of stress alone.


Mechanism of Corrosion Fatigue.

Fatigue resistance can be reduced by corrosion activity in many ways. In materials that are susceptible to stress corrosion, fatigue resistance is probably lowered by the rapid propagation of fatigue cracks after they reach the size required for stress corrosion cracking. In materials not susceptible to stress corrosion cracking, corrosion probably enhances crack propagation through direct attack at the crack tips, or by the formation of stress risers such as pits. Corrosion fatigue is usually 4-20 more severe at low cycling frequency where the longer time to failure allows more corrosion activity to occur.


Examples of Corrosion Fatigue.

High strength steels are susceptible to substantial reduction in fatigue resistance in many environments. The endurance limit (stress below which fatigue failure will not occur) is often reduced by a factor of ten from that measured in air. Cathodic protection can increase the resistance of steels to corrosion fatigue, but care must be taken not to overprotect them as hydrogen embrittlement would then occur. Titanium alloys, which are not subject to stress corrosion cracking, are particularly resistant to corrosion fatigue as are some of the more corrosion resistance nickel alloys, such as Inconel 625 and Inconel 718. Copper alloys and stainless steels are also susceptible to corrosion fatigue with a reduction of one-half in their endurance limit being common.


Appearance of Corrosion Fatigue.

Corrosion fatigue gives a fracture surface similar to ordinary fatigue except that in some cases, corrosion products are present in the outer sections of the cracks.


Significant Measurements.

In the simplest corrosion fatigue test, the electrolyte is simply dripped over the surface of a rotating beam fatigue test specimen. In more sophisticated tests, flat specimens are stressed as cantilever beams and only tensile stresses are induced on the surface exposed to the corrosive environment. When cyclic loading is a factor in design, fatigue data from tests that include the corrosive environment must be used.

Source : "Corrosion Control" NAVFAC MO-307 September 1992

see also:

Corrosion Types of Corrosion Forms

What is Corrosion?