Aircraft hardware (nuts n bolts) are cadmium plated, then conversion coated with iridite (a yellow chromate) surface conversion. This iridite conversion coating adds a good bit more protection. About 180 hours more in the salt spray tank.
Hardware can also be zinc plated, then iridite dipped.
Here is an example of a steel part that has been zinc plated, then dipped in iridite (chromate conversion coating)
Here is an example of aircraft hardware that is cadmium plated, then iridite'd.
Chromate conversion coatings can be applied to unplated steel, but it does very little to reduce corrosion. So, I suspect your hardware is zinc plated, then iridited.
Note: both cadmium and zinc plating produce a gray coating. Not yellow.
Here are some cadmium plated parts.