Sweet iron bits are designed to rust slightly when exposed to your horses saliva. When the moisture from the salvia makes contact with the sweet iron, along with oxygen a surface layer of oxidisation (rust) is created. This 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 safe for your 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.