I'd like to know if a Fe-Si magnetic alloy can be considered as NACE compliant, how is it possible to justify (I've checked, it is not sayed into the MR0175 Nace standard).
I'm not familiar with NACE MR0175 so have no idea what properties you are looking for but silicon irons in general have virtually no corrosion resistance. I have had good success with electroless nickel plating on silicon iron for moderately corrosive environments (5% salt spray for instance).