Descaling and passivating SS

Couldn't seem to find anything that directly addressed this issue on
Google. So maybe somebody here can advise me.
I'm making small items of stainless steel for costume accessories. I
have to heat the square steel and twist it tightly then form it into
circles. The heating of course causes scaling even though the forge is
adjusted to a reducing atmosphere.
This scale is tough! It doesn't come off easily. Tumbling the items
didn't work well idea because my tumbler has been used for years to
descale ordinary carbon steel and is throughly contaminated with steel
dust and crud. The last time I tried tumbling SS in it, the stainless
rusted worse than the mild steel afterward. Not sure whether a
post-tumbling acid bath would have prevented that or not.
Oh, the medium in that tumbler is steel scrap, which certainly isn't
making the situation better.
So I'm asking: What's the best way to remove forge scale and passivate
to make sure the SS items don't develop rust blemishes afterward from
iron contamination?
Help me, Obi Wans, you're my only hope! :)