Non corrosive coating for 420 SS decking fasteners

A. Why was Type 420 stainless chosen as the alloy? If a coating needs to be applied anyway, why not just use a lower cost high strength alloy steel like SAE 4037 and coat it with a duplex fastener coating like Magni 565 or Dörken Delta-Protekt KL1000? Either way, a thin layer of a black organic coating (with a friction modifier) is often used for carbon & alloy steel fasteners. You should discuss these options with your stainless steel vendor and the coating supplier, as the passivation process normally used for maximizing corrosion resistance of the stainless steel may not be result in good adhesion of the organic coating.

A. I noticed after I wrote my first response that the hardness level you have specified is 47-50 HRC. This is considerably in excess of what a low alloy steel like SAE 4037 is capable of producing without encountering problems of brittle fracture, etc. At these types of strength/hardness levels, it is difficult to achieve adequate corrosion resistance in the base metal, unless you use very highly alloyed stainless steels, titanium alloys, or multiphase alloys (Ni-Cr-Co-Mo).

A. 420 ss will not, of course, stand up to any degree of salt spray near the seashore. If you MUST use this grade of steel you will need to coat it or plate it. Any coating will surely have some pinholes sporadically which will rust and cause failure of the fastener eventually. Plating the fastener or electropolishing it may give you sufficient resistance, but this would be expensive compared to making the fastener out of a more resistant alloy.