Do Bi-Metal Screws Really Beat Stainless Steel on a Salt-Soaked Deck?

Last June, around five in the morning, I stood on a fishing dock south of Qingdao while the crew pulled a six-millimetre self-drilling screw out of a pressure-treated stringer. The thread came out grey and powdery. The foreman held a 304 stainless screw up to the fog — fourteen months in, he said, and look at them. Chewed. On the bench sat a box of bi-metal screws somebody had brought back from a job in Hokkaido, and none of the crew could tell me why those should behave any differently from the stainless ones. Honestly, neither could I. Not then.

First, what a bi-metal screw isn't: it isn't plated. You don't get one by dipping carbon steel in zinc or spraying on a coating. During cold heading, a hardened medium-carbon core gets wrapped in an austenitic stainless shell, and the two metals bond at the interface — they don't just sit on top of each other. That distinction matters more than it sounds. The drill point keeps enough hardness to chew through structural steel, while the exterior keeps its face against salt, chlorides, and the wet chemistry of treated lumber. Mechanically, the core is assessed the way any carbon drilling fastener is — ISO 10666 sets the surface and core hardness, drilling torque, and torsional strength. Out in the field, the shell behaves like stainless. Same geometry, two jobs.

Look at the table long enough and one row does all the explaining. Bi-metal takes the strength column from carbon steel and the corrosion column from stainless. That's the whole pitch.

People keep asking three things: do bi-metal screws rust, are they better than stainless, and where would you actually use them. The honest answers are shorter than the marketing. Yes, the visible thread resists rust — but only if the shell came through heading without cracking, which is why a clean cold-heading line counts for more than any coating certificate. Better than stainless? Not universally. Indoors, on a purely decorative job, 304 is already overkill and bi-metal is overkill twice over. But where shear load meets salt air, bi-metal earns its keep: corrosion is a surface fight, strength is a core fight, and one screw body takes both. Use them for coastal decking, metal roofing in chloride-heavy zones, and steel-to-steel fastening where a zinc screw dies young. One caution engineers keep raising: the two metals expand at different rates under heat, so the bond has to survive thermal cycling, not just the salt-spray cabinet. And cheap production gets caught at batch level — an uneven shell shows up as rust islands at the second-year inspection.

In conclusion

Fasto makes bi-metal screws the way the process actually has to run: one continuous cold-heading flow, carbon core and stainless shell bonded under controlled deformation, then an ASTM B117 salt-spray audit on every production lot before it leaves the workshop. The core is quoted against ISO 10666, and shell integrity is verified by cross-section sampling — not by a catalogue photo. For a procurement manager weighing marine-grade bids, that traceability is the difference between a fastener you trust for a decade and one you replace after two winters.