Why has my sweet iron bit gone rusty?

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.