Don’t judge a bit by the way it looks! Sweet iron is designed to rust slightly when exposed to your horses saliva. The saliva/moisture and oxygen creates a surface layer of oxidisation (rust) which appeals to your horse by giving a sweet tingly feeling in their mouth and encourages further salivation.
The rust on a sweet iron bit is not dangerous or harmful to the horse, as it will not chip off/flake like rusty steel would. Most horses love these bits and they will last a long time even if the bit looks a little rusty and brown. The cheeks on Sweet Iron bits usually are made of steel and therefore don’t change cosmetically like the Sweet Iron material.