Not sure kev. But I think it's the wrong potential to cause SCC, i.e. hydrazine is strongly reducing - it's used as a boiler water additive as it reduces boiler scale (iron III oxides) back to iron II oxides. So, even with chromium depletion, hydrazines shouldn't have much corrosivity to those, or any other steel.
Could be totally wrong, as there is such a thing as SCC in steels exposed to strong alkalis - but I think those occur as the temperature is increased (200 F or higher for NaOH solutions?) - and at higher temperatures like that, with neat hydrazine vs. the low concentrations used in boiler water, you are going to have problems with auto decomposition of the hydrazine, i.e. fast bangs, not slow decay like SCC.
If you do have higher temperatures (thruster soakback to a valve or something), any reason not to use a stabilized stainless (321 or 347) or titanium?
Actually, I did make a suggestion for a stabilized grade, 347 and stabilize heat treat 1650+/-25F for 4 hrs min. after welding. The dilemma is that the 430 needs a pwht anneal to restore ductility so I couldn't heat treat below the sensitization temperature. At the same time I don't want to cause excessive grain growth >1000C (1832F). I found an article that was helpful too. Apparently, I can use this heat treatment to minimize sensitization with even the 304L combination provided that I perform a rapid quench. See the attached.
