This is more than likely one your probable culprits the root pass deposit of the ER 2594 (100X) using GTAW...Now I don't know the exact chemical composition of the root pass filler but if there was an insufficient amount of nickel necessary in order to hold the all weld metal ferrite levels low enough for good ductility the conditions as such would certainly be enough to warrant enough IGSCC (Inter-Granular Stress Corrosion Cracking) throughout the thickness of almost the entire weld metal deposit passes to cause enough corrosion and show the pitting marks as well, because if the base/parent metal has a 5 to 6% Nickel content and the root filler doesn't have at least an 8 to 10% of Nickel in order to keep the ferrite number low enough to avoid brittle fracture due to IGSCC that is, so long as excessive dilution is avoided, then IGSCC can originate where the first pass of the flux cored deposit of Supercore (E2209TO/1-4) diluted, and more than likely excessively with the root pass made up of an alloy mix with insufficient Nickel. this is one of many probabilities however slim it may be, I'm leaning more towards excessive dilution due to the higher heat input from the FCAW process... So the next questions would be: What were the parameters used in welding with the FCAW process, and what were the parameters for the GTAW process and finally, what was the exact composition of purging gas used - NOT the shielding gas!!!
I ask this because if there wasn't a slight amount of Nitrogen added to the purging gas this could also have led to setting up the conditions for IGSCC as well as having too many ppm's of oxygen in the purging gas, not the shielding gas!!! In summary, if there isn't sufficient data bought forth for us to analyze completely to cover all possible scenarios by having all of the pertinent information available to review, then we could come up with all types of possible root causes that may or may not bear fruit with or without complete confidence! ;)
Nanjing you need to reread the original post where Welder 5354 does mention the base or as you might understand it as the parent material being:
4", sch. 120 (SA-790 (S31803) which is 2205, or 22% Chromium version.
Respectfully,
Henry