i believe you are refering to a form of interganular corrosion.
magnetite might be forming in the intergranular boundaries , do you have a metalurgical microscope picture or a SEM ? to take a look, i am no metalurgist but have several friends that are and rate between the best.
we have worked in superheaters for hidrogen co mixtures for 25 years and they have developed severall treatments to disolve carbides and reduce or eliminate metal dusting , so they might be usefull.
Sponge Iron (DRI) handling
Sponge Iron DRI Automated Storage Firefighting and Root Cause Analysis
Pneumatic Conveying Consultants
Phone 5281 8300 4456.