Stainless Steel Fasteners Buying Guide: 304 vs 316 Bolts, Nuts & Washers — How to Specify Marine-Grade Hardware
Stainless fasteners — bolts, nuts, washers, and anchors — are the smallest line on the PO and the most common cause of rust-streak callbacks. A 316 structure with 304 bolts rusts at every connection; carbon-steel bolts in stainless flanges corrode galvanically and seize. Match the fastener alloy to the environment and the parent metal, and the hardware outlasts the structure.
This guide compares 304 vs 316 vs 2205 fasteners, strength classes, anti-galling practice, and a complete RFQ checklist.
304 vs 316 vs 2205 Fasteners
| Grade | Key trait | Use |
|---|---|---|
| 304 (A2) | General stainless, economical | Indoor, dry structures, food equipment |
| 316/316L (A4) | Mo-bearing, chloride-resistant | Marine, coastal, chemical — the outdoor default |
| 2205 duplex | Double strength, PREN ~35 | Offshore, high-load seawater bolting |
Strength Classes (What A2-70 / A4-80 Mean)
| Marking | Meaning | Advice |
|---|---|---|
| A2-70 / A4-70 | 700 MPa tensile | Standard stainless bolting |
| A2-80 / A4-80 | 800 MPa tensile | Higher preload, structural joints |
| A4L-80 | Low-carbon 316L | Welded assemblies, intergranular-corrosion risk |
Anti-Galling Rules (Read Before Assembly)
- Always lubricate stainless threads — moly paste, PTFE, or nickel anti-seize; dry assembly galls and snaps.
- Use different alloys or hardnesses for nut and bolt (e.g., 316 bolt + 316L nut) to cut galling.
- Slow down — high-speed impact drivers weld stainless threads; run nuts by hand, then torque.
- Isolate stainless bolts from carbon steel with washers/sleeves to stop galvanic corrosion.
Standards Buyers Quote
- ISO 3506: stainless fastener grades and property classes — quote A2/A4 + class (70/80).
- ASTM F593/F594/F738M: US stainless bolts, nuts, and metric bolting references.
- ISO 4762 / 4014 / 4032: socket-head, hex bolt, and nut dimensions.
- Salt-spray alone proves nothing — demand alloy certs (316 needs Mo ≥ 2.0%), not just hours in fog.
Common Applications
- Curtain walls, roofing, and coastal structural connections.
- Marine cleats, railings, and offshore secondary steel.
- Food, beverage, and pharmaceutical equipment.
- Chemical pipe flanges and tank attachments.
RFQ Checklist
- Grade: A2 (304) / A4 (316) / duplex — confirm Mo content for A4
- Product + class: bolt/nut/washer + 70/80, thread (coarse/fine), finish (plain/passivated)
- Size x length, quantity per size, test certs (EN 10204 3.1 for structural lots)
- Destination / Incoterm
FAQ
304 or 316 bolts outdoors? 316/A4 within ~10 km of the coast or any chemical exposure; 304/A2 only for dry inland service.
Why did my stainless bolts seize? Dry threads run at speed — lubricate, slow down, and use rolled (not cut) threads.
Can I use carbon bolts on stainless flanges? Only with isolation kits — otherwise galvanic corrosion eats the bolt and stains the flange.
Why Source Stainless Fasteners from Yihang Metal
Yihang Metal supplies stainless fasteners in A2-70/80 and A4-70/80/316L, plus 2205 duplex bolting, with verified Mo content, thread gauges, and EN 10204 3.1 certs on structural lots, packed by size in export cartons and pallets. Standard sizes typically ship in 20–30 days.
Ready to specify your stainless fasteners? Send us the grade, product and class, sizes, quantities, and destination, and we will confirm availability and lead time with a fixed-price quote. Email [email protected] or use the quote form on this page — our team replies within one business day.