Stainless Steel A2-70 (304) Fasteners | 700 MPa Tensile · PREN 18–20 Corrosion Data
A2-70 is the most widely used stainless fastener class, made from 304 / 304L (18Cr-8Ni, no molybdenum) at 700 MPa nominal tensile strength. It relies on a passive oxide film for corrosion resistance and suits indoor dry, inland outdoor and food-equipment environments at a mid-range cost. Two points matter: its strength sits below class 8.8 and must not be treated as an equal-strength substitute, and because 304 carries no molybdenum it will pit — and can even stress-corrosion crack — in chloride service such as seawater, salt spray and de-icing salt. Coastal projects must move to 316. Stainless is not rust-proof in an absolute sense; the deciding question is whether chlorides are present.
Properties & Specifications
| Property Class | A2-70 per GB/T 3098.6 / ISO 3506 |
| Tensile Strength | 700 MPa (nominal) |
| Yield Guarantee | No yield guarantee equivalent to class 8.8 — do not substitute for 8.8 |
| Base Material | Austenitic stainless 304 / 304L, 18Cr-8Ni, no molybdenum |
| Pitting Resistance Equivalent (PREN) | ≈ 18 – 20 |
| Finish | Solution treated and passivated (clean passivation as standard) |
| Corrosion Envelope | Good indoors and inland outdoors; pitting in chloride environments |
| Chloride SCC Threshold | Risk in chlorinated water above 60°C (austenitic-specific failure mode) |
| Relative Cost | Medium — above carbon steel + zinc, below 316 |
Material Advantages
A self-healing passive film: once scratched, the oxide layer re-forms in the presence of oxygen — something a zinc coating on carbon steel cannot do
Coating-free by design: no coating wear, no salt-spray expiry, no hydrogen embrittlement, so it is safe for food and pharmaceutical contact
Balanced corrosion cost: decades of stable service indoors and inland outdoors, saving the molybdenum premium that 316 carries
Good low-temperature toughness: none of the brittleness or delayed-fracture sensitivity of class 12.9, so it works in cold service
Matched A2 class 1 / Type 2 nuts and DIN985 lock nuts in the same material, avoiding galvanic couples between dissimilar metals
Applications
Food and beverage plant — conveyors, fillers, agitator vessels
Kitchen equipment and commercial refrigeration
Indoor and inland outdoor fittings, guard rails and facade hardware
Pharmaceutical and cosmetics equipment
Water treatment and pump stations (non-seawater)
Not recommended: coastal salt spray, de-icing salt, chlorinated chemical media, chlorinated water above 60°C
Grade Selection
| Class / Grade | Service temperature | Notes |
|---|---|---|
| A2-70 | 含氯水 > 60°C 有应力腐蚀风险 | The workhorse 304 class, 700 MPa tensile, first choice indoors and inland outdoors |
| A2-80 | 同 A2-70 | A colder-worked, tighter class listed in the legacy table; confirm against ISO 3506 |
| A4-80 | 耐氯性明显更好 | 316 with 2–3% Mo — the correct answer at the coast and in chemical service |
| 2205 Duplex | 高温含氯工况 | PREN 33–35, special order; the upgrade where stress-corrosion risk is unacceptable |
Frequently Asked Questions
Yes. 'Stainless' means resistant to oxidation-type media such as nitric acid, not immune to rust in every environment. 304 pits — and can stress-corrosion crack — in chloride environments such as seawater, salt spray and de-icing salt. The deciding question is not 'is it stainless' but 'are there chlorides in the environment'.
No. A2-70 has 700 MPa tensile and no equivalent yield guarantee, and treating stainless as a high-strength option is one of the selection errors called out in our guide. Under load-bearing or vibrating conditions it is a safety issue. Where strength and corrosion resistance are both required, specify a Delta-Tone coated class 10.9 carbon steel bolt instead.
Two usual causes. First, carbon-steel contamination: steel swarf from machining or handling transfers onto the stainless surface and rusts — keep assembly tools and dunnage clean. Second, crevice corrosion: the oxygen-starved gap between nut and washer cannot re-passivate, so apply an anti-seize compound on critical joints. Tea-coloured staining does not affect function; where appearance matters, specify electro-polishing or passivation.
Quantify it with PREN: 304 sits around 18–20 while 316 reaches 24–26, roughly 35% higher. In engineering terms, 316 moves 304's pitting safety margin by a whole environment class — which is exactly why coastal projects must use 316.
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