Did you try it in SEM once? Because SEM is the better option for analysis the fracture surface morphology after SCC or ant other failure. From your optical micrograph cracks are not so much cleared. So, my advice it try SEM once and send the images.

May be I am wrong in my first answer. I missed the point degradation. Can you please tell me in which condition you took these images and also by which microscope. details are required. 

I took these pictures in optical metallography. 200x . It is a replica taken at site. No option to do sem

which steel it is? exact grade?

Astm a516 gr 70

See from these figures, no such cracks are observed. From your micrographs, grains are looking like austenitic. And the dark spots are formed due to the surface degradation or dissolution or effect in presence of corrosive medium. I don't know about the types for this. But one thing you must notice that the environmental attack is not uniform. That is why you can say it is a localized deformation under specific condition (you know more specif about the conditions).

Last figure , the big dark spot is may be the etch pit or sulfur inclusion.

From the images it seems the material has undergone HIC. But you need SEM Analysis to be sure and actually clearly see the cracks in the grain boundary.  Without that it is difficult to comment on the corrosion type, whether it is stress induced or high temperature!!

Are all microstructures from same operating conditions. If yes, high temperature hydrogen attack is most likely