A couple of years back a company learned that the hard way, carbon steel studs on any stainless , let alone Duplex , is a loose-loose situation, spending a few extra bucks for 309L or 316L studs and investing in the option of shielding gas ( Argon ) to the weld zone turned out to be the winning ticket. As far as the sticking ferrules, they were kept in a dryer up to the time of use I hope, ferrule ceramics, depending on their age and style will be a great " sponge " for atmospheric humidity, morning dew and sweating-dirty hands, etc. Duplex Stainless is very sensitive to ALL types of contaminations. Because the stud welding operation is a quick shot of intense heat in a very localized area, you will need to do planning and experimenting with the settings to develop a workable procedure, we even went so far as to heat the area up with a small LP (plumbers torch) to only about 150-160 deg. , the base metal was only 3/16 thick and the reward was satisfying. Duplex can be easy , IF you follow the rules, A little bit of preparation can save a lot of unexpected cost later.