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Stress Corrosion Cracking Testing

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The mechanism of stress corrosion cracking (SCC) involves a very complicated sequence of corrosion and fracture. It has been shown that, for some materials, fine pitting is needed to start the activity. Further progress is then a process alternating between corrosion and mechanical cracking until the material fails completely.

Transformable ferritic steels are susceptible to hydrogen-induced SCC in a wide range of aqueous media. Hydrogen formed on the surface of the steel by a cathodic corrosion reaction can diffuse into the material, causing embrittlement, and, if the region is subject to tensile stress, cracking can result.

If conditions are right, almost any metal can be made to fail by stress corrosion cracking. One characteristic of SCC that is very useful for diagnosis is the direction of the crack. It always follows the plane of maximum stress and, therefore, has branches in its form. These branched cracks are often visible without any other assistance. When viewed under a microscope, the branched direction of the cracks can be seen and they are practically always transgranular. Exceptions are caustic solution and improperly heat-treated stainless steels. In these cases, the path of the crack is intergranular.

There are several species that can lead to SCC, of which the following are relevant to refinery environments:  

 Amine SCC Chloride SCC
 Anhydrous ammonia SCC  Deaerator cracking
 Carbonate SCC  Polythionic acid SCC
 Caustic SCC  Sulphide SCC

It is TWI's wealth of knowledge in corrosion and the effects of hydrogen on the properties of materials, combined with our testing facilities and broad experience in the field of metallurgy and corrosion which sets us apart from our competitors in Stress Corrosion Cracking (SCC) testing.

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