This stainless steel curb bit has a 5" sweet iron medium port mouth and 6" cheeks.
The sweet iron material, when exposed to saliva/moisture and oxygen creates a surface layer of oxidization - rust - which is what gives the sweet feeling in the horses mouth, encouraging salivation.
Its totally harmless to your horse and there is no need to scrub it off, just a quick wipe with a clean cloth after a ride will keep your bit in tip top shape.
Sweet iron is a nice warm metal in the horses mouth that heats up to body temperature very quickly, encourages salivation and bit acceptance through the flavor. It can become rough with age, but a light sand with some superfine wet/dry sand paper can restore the smooth surface.