Crack initiation electrochemical tests

The mechanistic differences between SSC and SCC (anodic type) have a significant influence on the test methods used. Stated simply, the critical vciriable in SSC of carbon and low-alloy steels is the stress (strain) required to initiate and sustain cracking in an arbitrarily selected severe environment. In SCC, the critical variables are the electrochemical polarization characteristics of the metal/environ-ment system that control the anodic processes involved in crack initiation and growth. Therefore, alloy SCC susceptibility comparisons are done by determining the critical environmental factors (e.g., temperature, H2S partial pressure, chloride concentration) required to cause failure of a severely stressed specimen. 

The stress-strain curves corresponding to pre-exposure and solution tests revealed similar loss of ductility. Consequently, there are no synergetic effects between the solution and the stress. Pitting developed on the specimen surface behaves as a notch in tests performed on pre-cracked specimens, increasing the stress level. This increases the probability of cracks, reducing the time to failure but once initiated the crack grows because of mechanical factors rather than electrochemical. 

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