Sweet iron is a covering on top of a bit made of stainless steel. We have both grey and blue sweet iron coatings.
The purpose is to disguise the metal taste from stainless steel and create a pleasant, sweet taste, encouraging the horse to seek more contact and hold on to the bit. Following use, sweet iron will start to oxidize and turn brown/grey, this is to be expected. The more the sweet iron oxidizes, more ‘sweet’ taste will be released. When this happens the ‘blue’ will almost disappear.
The sweet iron bit should not be left in water, the only thing you have to do after riding is just wipe it off with a towel and keep it in a dry space when you're not using it.
The bit will still rost, as that's the point of the sweet iron material, but if you just give it a quick wipe after riding it will keep a nice, smooth surface.