The bolts you are looking at are correctly called "Socket Cap Screws" They are a hard alloy steel that in most industries are used black or bare (no coatings other than oiled from the factory.) They can come in stainless steel and the alloy can be zinc plated which is what from the pictures they look like to me. You can coat any plain steel bolt in any plateing under the sun to fit a specific application. I can not speak to what coatings ST has coated or had coated the bolts. If regular zinc coating was used and chemical cleaners and regular wet envornment were used then I believe that it's very possible to get just such a result. There are some seriously corrosive wheel cleaners out there that specificly state to stay alway from aluminum surfaces in the disclaimer part of the bottle. The only other way to go would be to use a hardend strained stainless in the 300L family of bolts to achive the heat resistance and not cause embritlement on a standard commercial stainless bolt. They are a expensive option if you can find them.
2013 335is Vert Black on Black DCT
2008 - Z4MR - Sold
BMWCCA / ZSCCA Member