Pre-existing active path

Fig. 8.1 Pre-existing active path mechanisms, in which H represents cathodic hydrogen...

On the basis of the progress made in the understanding of the see phenomenon in recent years, the existing mechanisms of See can be divided into three categories (1) pre-existing active path mechanisms, (2) strain assisted active path mechanism and (3) adsorption related mechanisms [64-66]. These subdivisions are only from a point of view of understanding of one continuous process with only shift of emphasis from one... 

To conclude, it is observed that in mechanisms based on a pre-existing active path, the susceptibility to SCC at one end is predominantly controlled by localized corrosion where stress is not necessary, such as carbon, steel in NO, and Al-Zn-Mg in Cl , brass in NH3 solution, to the other limit where the stress influences the propagation of a crack from an intergranular notch, such as in Mg-Al alloys in CrOJ, C1 and high strength steel in water. 

Explain the main difference between the pre-existing active path, strained generated active paths and specific adsorption theories. 

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