I've never heard of a metal sippy cup...sounds intriguing...I'm guessing it's like a metal thermal travel coffee mug?
I just found out recently that you're not "supposed" to put ANY of those through the wash (i live under a rock, apparently, I've been putting them in the dishwasher in pieces for YEARS, without any noticeable difference unless a lid flipped out of it's cage and onto the heating element counts). The change in heat from the air chamber to the outside ends up breaking the seal between the metal and plastic for many people & they end up getting water or worse in that chamber. Same for that matter, the insulated plastic ones. Since we could never manage to hang onto the insulated plastic ones for more then a month or two...we never actually noticed in those either. Makes sense though. In fact my husband just bought one where the plastic liner comes out and the bottom unscrews so the whole thing can go in and not have that problem. Huh. That's me. Living under a rock.
If I'm right about the type...then I wouldn't boil them either...same problem. My sons hypoglycemic, so we always had a milk sippy or 3 around...and I invariably would find them under the seat in the car/behind the crib/wedged into a shoe/etc. I'm not talking about fresh ones. Eeeew eeeew eeeeeeeeeew. We had quite a bit of success of washing them, filling them with soap & water and letting them sit for a few days, and washing again. Hadn't thought of the bleach. Here's what I would do with the addition of the no brainier of "duh! Bleach!", obviously if it passed the sniff test, stop:
- soak in 10% bleach
- wash until the bleach smell is gone
- fill with soapy water
- let sit a day or two
- dishwasher (at this point what do you have to lose?)