Stress corrosion cracking crack external surface

Pitting corrosion may occur generaHy over an entire aHoy surface or be localized in a specific area. The latter is the more serious circumstance. Such attack occurs usuaHy at surfaces on which incomplete protective films exist or at external surface contaminants such as dirt. PotentiaHy serious types of corrosion that have clearly defined causes include stress—corrosion cracking, deaHoying, and corrosion fatigue (27—34). 

Microstructural examinations revealed that the cracks originated on the external surface (Fig. 9.15). The cracks were highly branched and transgranular. The branched, transgranular character of these cracks is typical of stress-corrosion cracking of austenitic stainless steels. The thick-walled fracture faces are also typical of cracking by this mode. 

Stress corrosion cracking also involves localized breakdown of the protective film. The corrosion is narrowly confined within the metal due to stress factors which may arise from either residual Internal stress or applied external stress. In some cases the stress failure can be accelerated by chemical factors, such as surface adsorption or hydrogen dissolution from cathodic hydrogen leading to embrittlement. 

Stress Corrosion Cracking One of the serious forms of pipeline corrosion is see. This form of corrosion consists of brittle fracture of a normally ductile metal by the conjoint action of a specific corrosive environment and a tensile stress. In the case of underground pipelines, see affects the external surface of the pipe, which is exposed to soil/ground water at locations where the coating is disbonded. 

Because stress corrosion cracking is a localized corrosion process, involving the spatial separation of the local anode (in the crack) and the local cathode (on the external surfaces), the phenomenon of IGSCC is expected to fall under the theoretical umbrella of the Differential Aeration Hypothesis (DAH), as depicted in Fig. 82. The DAH, which was first postulated by Evans in the 1920s and which has since been recognized as the theoretical basis for essentially all localized corrosion phenomena requires that, in order to maintain the spatial separation between the local anode and local cathode, a positive ionic coupling current flows through... 

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