Stress corrosion cracking and corrosion fatigue

Stress corrosion cracking is a brittle failure mode caused by the combined effect of tensile stress and a corrosive environment. Like pitting and crevice corrosion, stress corrosion of stainless steels is most frequently caused by solutions containing chloride or very alkaline solutions. However, most cases of stress corrosion cracking on stainless steels occur at high temperature. At elevated temperatures, solutions that are unlikely to cause pitting and crevice corrosion, due to their low concentrations of chlorides and oxidizing chemicals, may give rise to stress corrosion.

Depending on the environment, relatively low engineering loads may provoke stress corrosion. Residual stresses from different manufacturing operations, such as forming and welding, can be high enough to cause failure. In some practical situations annealing at an appropriate temperature can reduce this potential risk, but such an approach is often difficult for large constructions. Coarse grinding induces tensile stresses into the steel surface that may facilitate initiation of stress corrosion.

Most cases of stress corrosion occur at temperatures above 50℃ but failures at ambient temperature have occurred on standard grade austenitic steels, such as 304 and 316 in swimming pool atmospheres.

Stress corrosion attack on stainless steel typically takes the form of thin, branched cracks. Failure caused by stress corrosion cracking often happens abruptly and without warning, due to the high propagation rates of the cracks. A common cause of stress corrosion is the concentration of chlorides from evaporation on hot steel surfaces. The evaporating liquid might be freshwater or other very dilute solutions, which because of their low chloride concentrations are considered as harmless. When the water evaporates, the chloride concentrations of these liquids might, however, become high enough to cause stress corrosion.

Stress corrosion cracking and corrosion fatigue
6. Stress corrosion cracking in a stainless steel tube.

Steels with a ferritic or duplex (ferritic-austenitic) structure generally display a very high resistance to chloride-induced stress corrosion attack. Standard austenitic grades, such as 304 and 316, are rather sensitive to this kind of corrosion. High contents of nickel and molybdenum increase the resistance of austenitic grades to stress corrosion and for this reason the grades (EN) 1.4539, 1.4547, 1.4565, 1.4652 show excellent resistance to chloride-induced stress corrosion cracking.

It is well known that a material that is subjected to a cyclic load can fail at loads far below the ultimate tensile stress of the material. If the material is simultaneously exposed to a corrosive environment, failure may take place at even lower loads and after shorter times. This is caused by a form of corrosion known as corrosion fatigue, which has similarities to stress corrosion cracking. Both corrosion forms cause brittle failures. Corrosion fatigue often takes place at ambient temperature and in moderately concentrated solutions that could be considered harmless with regard to other forms of corrosion.

Residual stresses from manufacturing can adversely affect resistance to corrosion fatigue. These may be reduced by an appropriate heat treatment or by the introduction of compressive stresses in the surface, as achieved by shot peening. High mechanical strength increases the resistance of a stainless steel to corrosion fatigue. Duplex steels are thus often superior to conventional austenitic steels. One application where a change from austenitic to duplex steels has reduced the number of failures caused by corrosion fatigue is for suction roll shells in paper machines.

Stress corrosion cracking and corrosion fatigue
7. Stress corrosion cracking resistance of mill annealed austenitic and duplex stainless steels in the drop evaporation test with sodium chloride solutions at 120°C (stress that caused cracking shown as a percentage of yield strength).

* The present text has been registered at a notary office for the intellectual rights' protection.
Increase font size