I posted this question on homebrewtalk.com, but I've since realized that Beeradvocate is a much better forum. So I built a stir plate, and I tested it with a small chain of nickel-coated neodymium magnets as my stir bar. Everything seemed to work just fine, so I ordered a teflon coated stir bar, but it turns out the one I got is a bit too large and gets thrown pretty quickly. The chain of magnets worked fine, so I'm wondering, will a nickel-coated stir bar (vs one of the teflon coated ones) have any adverse effects on a yeast starter?
My gut says you really don't know what those magnets are plated with. Plating baths are filled very with nasty chemicals and who knows what their processing method is. They may be plating other things in there as well. Odds are its fine, but you'll never really know. Personally, I'd try another teflon bar.
Experience is the best teacher. I stirred 2 cups of wort to try to revive some yeast this weekend using the magnet stir bar, and the wort turned kind of grey. The stir bar looked a little bit tarnished. So, case closed.
Hopefully you only lost the 2 cups of wort but I want to chime in on how nasty using the nickel plated stir bar could be. I used to work testing plating solutions for the plating industry fro 12 years so I have a little bit of knowledge. Nickel Plating baths typically have hydrochloric acid in them to make the solution acidic to allow the nickel metal to go into solution. Since wort is acidic I'm sure you figured out what the gray color is. If the nickel plating was bright and shiny (decorative nickel) it gets even nastier. The bright shiny chrome color is actually from chrome plating underneath the thin nickel plating. Chrome besides being a carcinogen has a list of health impacts. The form of chrome typically used in plating is hexavalent chrome (the Erin Brokovich type) Sorry I couldn't post this before you ruined 2 cups of wort but i want to make sure no one considers using anything but a teflon coated stir bar.
Your comments about chromium are concerning, as I use a chromium-plated pipe as part of my immersion heat stick setup. Have any data on the dissolution of Cr at a pH of 4-6 from 140-212F? Links to other discussions, papers, etc are ok too. Edit: I've used this Cr-coated brass pipe (as part of the heat stick) to boil ~20gal and have noticed no distinct change in the Cr surface finish
I know the chrome plating baths require a lower pH bath than the nickel plating baths. Even the spent chrome baths that weren't being maintained and wouldn't plate had pH values of less than 2 and a properly functioning bath has a pH of roughly 0, far below a nickel plating bath which usually has a pH around 4. The pH range you listed is far higher than what a chrome bath requires but I can't confirm that absolutely no dissolution occurs at the range you listed. if you want BM me tomorrow and I'll look at some of my industry references to see if I can come up with more information for you.
