304 vs 316 Stainless Bolts: The Corrosion Gap Measured by PREN
The pitting equivalence number PREN (%Cr + 3.3x%Mo + 16x%N) quantifies the gap between 304/A2 and 316/A4: 18-20 vs 24-26, about 35% higher. Conclusions for indoor, inland-outdoor, coastal and chemical environments, with strength classes (A2-70/A4-80) and cost compared.
Overview
Stainless means resistant to oxidizing media such as nitric acid - not immune everywhere. The deciding question is whether the environment contains chlorides. The difference between 304 and 316 concentrates in chloride pitting resistance; this comparison quantifies it with PREN and gives conclusions for four service environments.
Parameter Comparison Table
| Property | 304 / A2 | 316 / A4 |
|---|---|---|
| Typical composition | 18Cr-8Ni·无钼 | 16Cr-10Ni-2~3Mo |
| PREN (approx.) | 18-20 | 24-26 |
| Fastener classes | A2-70 / A2-80 | A4-70 / A4-80 |
| Tensile / yield (common class) | 700 / 450 MPa | 800 / 600 MPa |
| Coastal / salt spray | ✗ pits through | ✓ recommended |
| Chemical / pool (chloride + stress) | ✗ | ✓ (duplex/Ni if >60C) |
| Relative cost | Medium | High |
| Typical scenario | Food equip. / indoor / inland outdoor | Seaside / chemical plant / marine |
Selection Conclusion
PREN rising from 18 to 26 is about +35%, which in engineering language means 316 pushes the pitting safety boundary one full environment class beyond 304 - the quantified reason seaside projects must use 316. Indoor dry: 304 suffices. Inland outdoor: 304, accepting occasional tea-colored flash rust that does not affect function. Coastal/salt spray: 316 mandatory. Chemical/pool: 316 or duplex; above 60°C in chlorided water under stress there is stress-corrosion-cracking risk, so switch to duplex 2205 (PREN about 33-35) or nickel alloys. Also mind two overlooked rust causes: carbon-steel contamination and crevice corrosion.
FAQ
Yes. In chloride environments (seawater, salt spray, de-icing salt) 304 pits and can even suffer stress-corrosion cracking; inland outdoor use may show tea-colored flash rust. The test is chlorides in the environment, not the word stainless.
Corrosion-wise yes, cost-wise not always: 316 is clearly pricier and dry indoor service runs stable on 304 for decades. Also the strength systems differ (A2-70/A2-80 vs A4-70/A4-80) - re-check load capacity by class when substituting.
Above 60°C in chlorided water under stress (chemical plant, pool plant room), austenitic stainless faces stress-corrosion cracking. Switch to duplex 2205 (PREN about 33-35, fasteners on special order) or nickel alloys, and add coating isolation where needed.
Usually carbon-steel contamination: iron fines picked up in machining or transport rust on the surface. Keep tools and dunnage segregated between carbon steel and stainless; on critical joints also guard against crevice corrosion under nuts and washers with an anti-seize compound.
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