When you're buying stainless steel screws, nails, or bolts for an outdoor build in Australia, you'll almost always be choosing between two grades: 304 (A2) and 316 (A4). They look identical. They're similarly priced. But they behave very differently in certain environments — and picking the wrong one can mean rust stains on a brand new deck, or structural fixings that corrode in five years instead of lasting fifty.
This guide explains the difference, tells you exactly which grade to use, and gives you a simple rule to remember on the job site.
The key difference: molybdenum
Both 304 and 316 are austenitic stainless steel alloys — the most common type used in construction fasteners. Both contain roughly 18% chromium and 8–10% nickel, which gives stainless steel its base corrosion resistance.
The critical difference is that 316 also contains 2–3% molybdenum. Molybdenum dramatically improves resistance to chloride attack — the corrosion caused by salt air, saltwater splash, and swimming pool chemicals. Without it, 304 can develop rust pitting when exposed to chlorides over time, even though it performs reliably in most other outdoor environments.
Which grade for which application?
| Application | Recommended grade | Why |
|---|---|---|
| Inland residential decking | 304 (A2) | No chloride exposure. 304 provides full protection for the life of the deck. |
| Suburban outdoor builds more than 5km from coast | 304 (A2) | Standard atmospheric exposure only. 304 performs reliably. |
| Coastal builds within 5km of ocean or estuary | 316 (A4) | Salt air causes chloride attack on 304. 316 molybdenum content resists this. |
| Swimming pool surrounds | 316 (A4) | Chlorinated water is a chloride-rich environment. 316 is essential. |
| Marine structures (jetties, pontoons) | 316 (A4) minimum | Direct saltwater exposure. 316 minimum; consider duplex grades for immersed fixings. |
| Roof batten fixing in coastal zones | 316 (A4) | Batten screws must be 316 to prevent early failure in salt air. |
| Cladding and fascia near the coast | 316 (A4) | Prevents rust streaks that 304 can develop in salt air over time. |
The 5km rule
The Australian Stainless Steel Development Association (ASSDA) recommends using 316 grade stainless within 5 kilometres of a coastline or tidal estuary. This is the practical rule most Australian builders and specifiers follow.
In practice, many builders default to 316 for any coastal or near-coastal job, even beyond the 5km threshold. The price premium over 304 is modest — typically 10–20% — and the cost of re-doing corroded fixings is far higher.
What do A2 and A4 mean?
You will sometimes see stainless fasteners labelled A2-70 or A4-80 rather than 304 or 316. These are ISO property class designations:
- A2 = 304 stainless steel
- A4 = 316 stainless steel
- The number (70, 80) refers to tensile strength in units of 100 MPa
For decking screws and nails, the grade (A2 or A4) is what matters for corrosion performance. SPAX labels their decking screws this way — their A2 range is 304 and their A4 range is 316.
Does 316 outperform 304 in strength?
No — 304 is actually slightly stronger in laboratory testing. But this is irrelevant for timber fasteners: the timber splits or the screw head strips long before either grade approaches its load limit. Choose based on corrosion environment, not tensile strength.
Quick-reference guide
- More than 5km from the coast, no pool? Use 304 (A2)
- Within 5km of coast or estuary? Specify 316 (A4)
- Pool surrounds or marine structure? 316 (A4) as a minimum
- Not sure? Step up to 316. The cost difference is small. The longevity difference is not.
Shop stainless fasteners at Toolup
Toolup stocks both 304 and 316 stainless steel fasteners from SPAX, INOX, AnchorMark, Macsim, and Paslode — including decking screws, coil nails, framing nails, brads, and batten screws. Free shipping on orders over $199 Australia-wide.
For trade quantities or volume pricing, call (03) 5241 9555.