Which screw material to use, and what rusts
Material matters most in two situations: anywhere that gets wet, and anywhere near a magnet or a sensor. The rest of the time, plain plated steel is fine.
The everyday options
- Zinc plated steel — cheap, strong, indoor use only. The plating is a thin sacrificial layer, so a scratched screw rusts.
- Black oxide steel — the dark screws inside most electronics. Almost no corrosion protection; it is there for looks and glare.
- Stainless A2 (304) — the default for anything that might get damp. Not fully rustproof in salt.
- Stainless A4 (316) — marine grade, resists salt water. Costs more, worth it near the coast.
- Brass — soft, non-magnetic, corrosion resistant, used in instruments and electrical work.
- Titanium — light and strong, common on bikes and drones, expensive and easy to gall.
- Nylon — non-conductive, non-magnetic, for insulating standoffs and antenna mounts.
Two metals touching: galvanic corrosion
Put a stainless screw into bare aluminium and add water, and the aluminium corrodes around the screw. Outdoors this can seize a fitting permanently in a couple of seasons.
Fixes: an anti-seize compound, a nylon washer, or matching the materials where you can.
Stainless galling
Stainless screwed into stainless can cold-weld and lock solid halfway in. Go slowly, use a lubricant on anything M4 and up, and never run a stainless screw in fast with a power driver.
Magnetic or not
Near a compass, a speaker, a sensor or an MRI, magnetism matters. A2 and A4 stainless are only very slightly magnetic; brass, titanium and nylon are not magnetic at all. Plated steel very much is.
Common questions
Is stainless steel stronger than plated steel?
Usually not. A standard A2 stainless screw is roughly grade 8.8 equivalent at best, while hardened alloy steel screws go much higher. Stainless is chosen for corrosion resistance, not strength.
Which screws for a boat or the coast?
A4 / 316 stainless, and keep it away from bare aluminium or use an isolating washer.
Are stainless screws magnetic?
A2 and A4 are austenitic and only weakly magnetic, and cold working during manufacture can make them a bit more so. If you need truly non-magnetic, use brass, titanium or nylon.
Keep reading
Tools and screws for this job
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Metric screw assortment kit
One box covering the sizes in this guide
Precision driver and bit set
The right bit is what stops rounded heads
Digital caliper
Measure before you buy anything