Ask for "stainless steel bolts" and you'll usually be asked one question back: A2 or A4? These two grades cover the vast majority of stainless fastener demand, yet the differences between them — and the reason a third grade, 303Cu, quietly dominates the turned-parts world — are often misunderstood. All three belong to the austenitic family, which means they share the same basic crystal structure, are non-magnetic in the annealed condition, and can't be hardened by heat treatment. Where they part ways is chemistry, and that chemistry decides whether your fastener survives ten years on a coastal handrail or starts weeping rust stains within the first season.
A2 (304): The Workhorse
A2 is the fastener-industry designation, under ISO 3506, for what metallurgists call AISI 304 — the classic "18/8" stainless, named for its roughly 18% chromium and 8% nickel content. Chromium is what makes stainless steel stainless: it forms a thin, self-healing passive oxide layer on the surface that shields the underlying steel from attack. Nickel stabilises the austenitic structure and gives the material its excellent ductility.
For general-purpose work, A2 is hard to beat. It resists atmospheric corrosion, fresh water, and a wide range of mild chemicals. It cold-forms beautifully — which matters for fastener manufacturing, where heads are formed rather than machined — and it welds well by all common fusion methods. It's also the most economical of the three grades, which is why standard stainless screws, bolts and nuts default to A2 unless the application says otherwise.
Its weakness is chlorides. In saltwater, coastal atmospheres, or around pool chemicals, the passive layer breaks down locally and A2 becomes vulnerable to pitting and crevice corrosion. A crevice is exactly what you create under every bolt head, nut face and washer — so fasteners are, if anything, more exposed to this failure mode than the surrounding structure. If the job is anywhere near the sea, keep reading.
A4 (316): The Marine Grade
A4 corresponds to AISI 316 and starts from the same recipe as A2 with one decisive addition: 2–3% molybdenum, usually accompanied by a bump in nickel to 10–14%. Molybdenum dramatically improves resistance to pitting and crevice corrosion in chloride environments, which is why 316 has earned the nickname "marine grade."
In practical fastener terms, A4 is the specification for coastal construction, marine hardware, chemical and petrochemical plant, pharmaceutical and food equipment in aggressive wash-down environments, and anywhere de-icing salts or pool water are in play. Mechanically it behaves much like A2 — similar strength, good weldability, slightly less ductile owing to the higher alloy content — and it holds its strength a little better at elevated temperature.
The trade-off is cost. Molybdenum and the extra nickel make A4 meaningfully more expensive than A2, and prices swing with the alloy surcharge market. But specifying A2 to save money in a chloride environment is a false economy: the cost of replacing seized, pitted fasteners (and repairing tea-staining on the surrounding stainless work) dwarfs the initial saving.
A note on markings: stainless fasteners are marked with both grade and property class — A2-70, A4-80 and so on. The number after the dash is the minimum tensile strength in MPa divided by ten, achieved through cold working. So an A4-80 bolt is 316 stainless work-hardened to 800 MPa tensile. Grade tells you corrosion resistance; property class tells you strength. You need both to specify a stainless fastener properly.
303Cu: Built for the Screw Machine
The third grade in this comparison serves a different master entirely. 303 is a 304 derivative deliberately doped with sulfur (and sometimes selenium) to create manganese sulfide inclusions throughout the material. Those inclusions act as built-in chip breakers: instead of the long, stringy, tool-wrecking swarf that 304 produces, 303 machines with short chips at high speeds with far less tool wear. The "Cu" variant adds around 0.5–1% copper, which further improves machinability and cold formability, and is particularly common in material from Asian mills.
That machinability comes at a real price. The same sulfide inclusions that break chips also act as initiation sites for pitting corrosion, so 303Cu resists chlorides noticeably worse than even A2 — the copper helps marginally, but doesn't change the picture. Weldability is poor, too: the high sulfur content makes 303 prone to hot cracking, and it's generally excluded from welded applications altogether. Formability suffers similarly, ruling out deep drawing.
So where does it belong? Anywhere parts are machined in volume and the environment is benign: turned specials, precision shafts, fittings, valve bodies, electrical connectors, and the endless output of CNC lathes and automatic screw machines. It's worth noting that although the raw material can cost slightly more than 304, the finished part often costs less, because machine time and tooling dominate the economics of turned components.
One caution for buyers: because 303 and 304 look identical and share most mechanical properties, 303 occasionally turns up where 304 was specified — especially in machined specials. If the part is destined for a corrosive environment or will be welded, insist on material certification (EN 10204 3.1) and verify the grade, not just the appearance.
Side by Side
| **A2 (304)** | **A4 (316)** | **303Cu** | |
|---|---|---|---|
| Distinguishing chemistry | ~18% Cr, ~8% Ni | + 2–3% Mo, higher Ni | + S (chip-breaking), + Cu |
| General corrosion resistance | Good | Excellent | Reduced |
| Chloride / marine resistance | Poor — pits | Excellent | Poor — pits readily |
| Machinability | Fair, work-hardens | Fair, work-hardens | Excellent — free-machining |
| Weldability | Excellent | Good | Poor — hot cracking risk |
| Cold formability | Excellent | Good | Poor |
| Typical fastener use | Standard screws, bolts, nuts | Marine, coastal, chemical | Turned specials, fittings, shafts |
| Relative material cost | Base | Higher | Slightly above 304; part cost often lower |
The Bottom Line
Choosing between these grades comes down to two questions: what will the fastener face, and how will the part be made?
If the environment is dry or mildly corrosive and the part is a standard formed fastener, A2 is the sensible default. The moment chlorides enter the picture — coast, marine, pools, road salt, aggressive chemicals — step up to A4 and don't look back. And if the part is a machined component running through a lathe in quantity, in a forgiving environment, 303Cu will save real money on the shop floor — just never let it near a weld or a jetty.
As always, the cheapest fastener is the one you only install once.