Galling on stainless steel threads usually occurs when you do not want it to. I have seen it happen with new pipe fittings and with new stainless bolts, just running them together by hand with dry threads. Usually, the fittings or fasteners gall so hard together, we throw them out and get new ones. The few we tried to get apart had the threads messed up badly and we had to then spend time with needle files, dressing galls off the male threads.
I would think galling is a function of how close a fit the threads are, and making sure the same alloy of stainless steel is used for both parts.
Also make sure the parts are absolutely free of any oil or other lubricating substance when making them up. If the threads are a close fit, you will be lucky to get the parts fully made up before the galling occurs.
"Permanent" is a matter of interpretation. Galled threads on stainless parts can sometimes be gotten apart if you use a big wrench and sometimes put a cheater on it. IOW, "Permanent" depends on how much effort is considered "yeah, it's a PITA, it took a big cheater, but I got it apart" vs. "the heck with it. Sucker won't come apart...." face it: ANYTHING made by one guy can be taken apart by another. It is just a question of what "taking apart" consists of. To some people, if they can't get stuff apart with normal hand tools, it's "permanent". To others, such as those of us who work in the powerplants, if something won;t come apart, we put heat on it, use bigger wrenches or take a torch or grinder & cutoff wheel to whatever has to come apart. As I tell my own son: "What one man designed and put together, another man can figure out and get apart."
Like I said, "Permanent" is relative. How "permanent" do you want those stainless parts put together ? Galled parts may give way to a big enough wrench. You ask about "chemical welding". I do not know of anything which would cause a galvanic action between two stainless parts to "chemically weld them". The heaviest duty grades of Loctite will do a great job of locking things together so that wrenches and cheaters probably won;t work. We use enough of the green, permanent Loctite on turbine and locomotive parts and I can say the stuff produces a very strong bond. However, with the application of heat, the Loctite will lose it's bond. A plus to Loctite is it acts as a lubricant when the stainless parts are being assembled, then cures to lock them.
You want "permanent" ? IMO, for most people, "permanent" can be achieved by welding.
If you put the stainless parts together and want to be sure they do not come apart, consider putting some TIG stitch or tack welds on things to lock them for keeps. Nothing short of a die grinder or plasma cutter will get things apart once that is done.
Joe Michaels