Suspension bolts are mostly phosphated, aka Parkerized, same as the gun metal finish on military weapons.
You can DIY phosphating at home in your spare time with gunsmithing chemicals and a garage sale stainless electric skillet.
You can DIY zinc plating with a battery charger or power source, and you can do home chromating also. Chromate is a dip for newly electro-zinc plated parts that increases corrosion resistance and changes the color to yellow-rainbow, black, etc. Google is your friend.
In the past I have offered zinc anodes for plating here on Moparts parts for sale. Some day maybe I'll post how to get started with electro-zinc plating and chromating. Meantime I'm busy with restoring. Sorry.