A2 and A4 are fastener-specific stainless steel designations from ISO 3506, not separate alloys in their own right — A2 roughly corresponds to grade 304 stainless steel, and A4 roughly corresponds to grade 316. If you’re buying bolts, screws, or nuts and see “A2” or “A4” stamped on the head, you’re looking at the same chromium-nickel-molybdenum trade-off that defines 304 vs 316, just under fastener-industry naming.
The Short Answer
- A2 ≈ 304 — general-purpose stainless, good corrosion resistance, lower cost.
- A4 ≈ 316 — adds molybdenum for much better resistance to saltwater and chlorides, higher cost.
- The number after the dash (A2-70, A4-80) is a separate tensile-strength class, not a corrosion-resistance rating — don’t confuse the two.
Comparison
| Feature | A2 Stainless | A4 Stainless |
|---|---|---|
| Equivalent grade | ~304 (18/8) | ~316 |
| Key addition | None (base 18% Cr / 8% Ni) | Molybdenum (~2–3%) |
| Corrosion resistance | Good, general atmospheric conditions | Superior — resists pitting/crevice corrosion from salt and chlorides |
| Common designation | “Commercial grade” | “Marine grade” |
| Typical uses | Indoor hardware, general outdoor, food-processing fasteners | Marine hardware, coastal structures, pools, chemical plants |
| Relative cost | Lower | Higher |
Why the Naming Splits from “304” and “316”
A2 and A4 come from ISO 3506, the standard covering corrosion-resistant stainless fasteners — it’s mostly a European/international fastener convention, running alongside (not replacing) the ASTM grade numbers used for raw stock. The reason it exists separately is that fastener specifications need to call out both the material and a guaranteed mechanical property (tensile strength) in one compact marking, which raw-material grade numbers don’t do on their own.
Reading the Full Marking: A2-70, A4-80
The number after the dash is a tensile-strength property class, not a corrosion rating. A2-70 and A4-80 both use fasteners with a nominal tensile strength tied to that number (roughly 700 MPa and 800 MPa respectively, per ISO 3506’s property classes) — but the “-70” or “-80” says nothing about how corrosion-resistant the fastener is. That’s entirely down to whether it’s stamped A2 or A4. A full property-class breakdown across every A2/A4 strength grade wasn’t available in the sources for this guide, so if a project has a specific strength requirement, check the fastener against the current ISO 3506-1 property tables directly rather than assuming from the marking alone.
Which Should You Choose?
- A2: general construction, interior applications, or standard outdoor projects with no meaningful salt exposure.
- A4: anything that will be submerged in or regularly splashed by saltwater, exposed to de-icing salt, used in a pool (chlorine), or located within a few miles of the coast.
A4 isn’t automatically stronger than A2 — the strength class after the dash determines that, independent of the A2/A4 letter. The reason to pay more for A4 is the same reason you’d choose 316 over 304: corrosion resistance in a chloride environment, not raw strength.
If You’re Working with Raw Stock, Not Fasteners
If you’re sourcing sheet, bar, or tube rather than bolts and screws, you’ll see the underlying grade numbers (304, 316) directly instead of the A2/A4 fastener marking. See our full 303 vs 304 vs 316 stainless steel comparison for the metallurgical detail behind the corrosion-resistance difference that A2/A4 is built on.
Related reading: 303 vs 304 vs 316 Stainless Steel · Where to Buy Stainless Steel
