Corrosion fatigue crack film rupture

It has been proposed that corrosion fatigue proceeds by rupture of the surface film. There is considerable evidence to suggest that the rupture of a protective film leads to initiation of fatigue crack Figure 4.74 shows a schematic of a film rupture model. The film is ruptured at the crack tip by slip resulting in an increase in... 

State the difference between a SCC crack and a corrosion fatigue cracL What is the basis of film rupture mechanism Give one piece of evidence in support of the above theory. 

It has been said in the previous paragraph, that materials that were considered practically immune to SCC were found to be sensitive to it when affected by a crack. The existence of a crack is, therefore, fundamental for SCC to develop. Local chemistry that exists at the tip of a notch may be completely different from the general chemistry of the environment in which the material is operating and this can trigger a SCC that was never supposed to start. The effect of such a notch or crevice on corrosion has been studied in Sect. 13.7. It was shown how the passivizing film rupture at notch tip would lead to a slip dissolution process and, therefore, to corrosion. This initial notch or crack may not exist in the material, yet it can easily form as well. Fatigue may be the cause of crack initiation, but also a static stress can do it. In these cases we have what is referred to as pitting. 

In the first group, emerging persistent slip bands (PSBs) are preferentially attacked by dissolution. This preferential attack leads to mechanical instability of the free surface and the generation of new and larger PSBs, followed by localized corrosion attack, resulting in crack initiation. Under passive conditions, the relative rates of periodic rupture and reformation of the passive film control the extent to which corrosion reduces fatigue resistance. When bulk oxide films are present on a surface, rupture of the films by PSBs leads to preferential dissolution of the fresh metal that is produced.102... 

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