I am lookng into a problem that require seperation of this two mechanisms through factographic features and/or microstructural characterization.
To me, this is a case of semantics and it sounds like your issue is whether the fatigue fracture initiated due to corrosion or whether corrosion accelerated the fatigue fracture.
In the case of the fatigue being intiated by corrosion, you should check the initiation point for signs of pitting or severe localized attack. Remember that the pit either has to be large enough or sharp enough to serve as a stress intensifier to initiate the crack. If you cannot find any signs of accelerated corrosion at the initiation site, then it points toward corrosion accelerated fatigue.
As for corrosion accelerated fatigue, you best bet there would be to create a fatigue crack in the same material with the same load but without the corrosives and then compare the length of the striations (and N). If corrosion is accelerating the fracture, the length of the striations should be longer. If the striations are the same size and N for both samples, this points back to a initiation at a pit.
Chemical analysis of fracture surface in the initiation and propagation regions using EDS or XPS may reveal the role of corrosive environment on fatigue cracking process. Sometimes the fatigue features on the fracture surface such as striations, river marks could be dissolved by the corrosive medium or rubbing action between the crack walls. Intergranular cracking mode is a sign of corrosion assisted fatigue and this can be understood by either SEM or microstructural characterization.
It should be noted that corrosion assists initiation as well as propagation rates of a fatigue crack. Thus the effect of corrosion depends on the availability of corrosive medium at the initiation and propagation sites (crack tip) and also the duration of exposure of corrosive medium at those sites (as corrosion is a time-dependent phenomenon).
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