Q. Which is the better coating for a fastener made from C40-UNI-7845, cadmium or black oxide?
A. Either one of these coatings will work on this type of steel, but before you choose one over the other, consider where the fastener itself will be used. Cadmium coatings offer good corrosion protection and lubricity, but they are toxic. Black oxide offers some corrosion protection but no lubricity. And while attractive, in my experience black oxide does not provide the type of corrosion protection required in difficult environmental conditions.
If you are a plater, consult with your customer about where the fastener will be used and which coating is more suitable. If you are an engineer, look at how your similar parts are coated and consider finishing these fasteners in the same way.