Stainless Steel A2 vs A4: Which Grade Should You Use?

The single most common question we get at Simfix: "What's the difference between A2 and A4 stainless steel, and which one do I actually need?"

If you only read one paragraph: A2 (also called 304) is fine for indoor, outdoor and workshop applications anywhere more than about 5km from the coast. A4 (316) is marine-grade and worth the small price premium if your fastener will be exposed to salt water, salt spray, or chlorinated water (swimming pools). If you're unsure, A4 is the safer choice - it does everything A2 does, plus salt.

That's it. You can stop here and pick with confidence. The rest of this article explains why, in case you want to know.

What "A2" and "A4" actually mean

Stainless steel comes in many alloy grades, but for fasteners there are basically two you'll encounter in the wild: A2 and A4. These are European standard codes (DIN 267 part 11). Their American equivalents are 304 and 316 - which is why you'll often see fasteners labelled "A2 (304)" or "A4 (316)". They're the same metal, just different naming conventions.

• A2 / 304 - ~18% chromium, ~8% nickel, balance iron. The everyday "stainless".
• A4 / 316 - ~16% chromium, ~10% nickel, ~2% molybdenum, balance iron. Marine grade.

That 2% molybdenum is the whole story. Everything else about A2 and A4 is essentially identical - same density, same machining behaviour, same magnetic properties (both are slightly magnetic when cold-formed), same temperature resistance up to ~500°C. Our stainless steel material guide goes deeper if you want the full metallurgy.

Why does molybdenum matter so much?

Both A2 and A4 form a thin, invisible "passive layer" of chromium oxide on the surface within seconds of being exposed to oxygen. That layer is what makes stainless steel "stainless" - as long as it's intact, the iron underneath can't oxidise (rust).

The problem comes when something breaks that layer. The most common culprit in the real world is chloride ions - Cl− - which can locally destabilise the passive layer and start pitting corrosion. Pitting starts as tiny pinhole defects you can barely see, but they're self-accelerating: once started, they grow deeper into the metal.

Where do chlorides come from?

• Sea water
• Salt spray (coastal air)
• De-icing road salt
• Swimming pool water (chlorinated)
• Some industrial chemicals

That 2% molybdenum in A4 dramatically increases the steel's resistance to chloride pitting. In practical terms, A4 will resist for decades in marine environments where A2 might show rust streaks within 12-18 months. For the full deep-dive on coastal use, see our marine-grade fasteners buying guide.

When A2 is the right choice

For about 80% of real-world UK projects, A2 is exactly what you want. It's cheaper, easier to source, and gives excellent corrosion resistance everywhere except salt-water-adjacent environments.

Good A2 applications:

• Indoor mechanical assembly, electrical enclosures, kitchen fittings - see our furniture and cabinetry range
• Outdoor decking, fencing, sheds, garden structures inland UK - see decking timber fixings and fencing and gate hardware
• Workshop and garage equipment - browse the workshop range
• Automotive interior trim and machinery - see the automotive range
• Food contact applications (A2 is food-safe, as is A4)

The classic A2 workhorses to buy first: M10 hex set screws (DIN 933), M10 hex nuts (DIN 934) and M8 socket cap screws (ISO 4762).

When you need A4

Stop and think about this list whenever you're spec'ing a fastener. If any of these apply, choose A4:

• Marine - boats, yachts, jetties, pontoons, dock fittings, sail rigging
• Coastal - anywhere within 5km of the sea (salt-laden air alone can pit A2)
• Swimming pools - water and surrounding fixtures - the bathroom and wet area range is A4-focused for this reason
• Roads and bridges treated with de-icing salt
• Industrial chemical exposure (sulphuric acid, chlorides, brines)
• Critical safety applications where you want the maximum safety margin

For A4 starters: M10 A4 hex set screws (DIN 933), M10 A4 hex nuts, M8 A4 socket cap screws and M10 A4 flat washers.

The cost difference

A4 typically costs about 20-30% more than the equivalent A2 fastener. On a single screw that's pennies; on a 200-pack it's £2-£4. For a coastal job it's the cheapest insurance you'll ever buy - having to redo the fastening 18 months later because the bolts are rust-streaked costs far more than the initial £4 saving.

How to tell them apart

You can't, by eye. Both look identical - silvery-grey, polished or brushed, slightly magnetic. The grade is usually stamped or laser-etched on the head of the fastener if it's big enough (typically M6 and above). Smaller fasteners are identified by their packaging only.

This is why buying from a supplier you trust matters - there's no way for the end user to verify A2 vs A4 in the bag. At Simfix every product page declares the grade clearly, every product label has an A2 or A4 badge, and we never mix stock between grades.

A common mistake: mixing grades in one assembly

One thing to avoid: don't mix A2 fasteners with A4 fasteners in the same connection. Different grades have slightly different electrochemical potentials, and the difference can drive galvanic corrosion at the contact point - accelerating rust on the less-noble of the two. Pick one grade per assembly. If you're not sure which standards your fasteners conform to, our DIN, ISO and BS standards guide decodes the markings.

Quick decision flowchart

1. Is the fastener going within 5km of the sea, or in a swimming pool, or exposed to road salt? → A4
2. Is the fastener going to be permanently underwater (fresh water)? → A4 to be safe
3. Otherwise: A2 is fine, and you'll save money.

Related Simfix guides

• DIN, ISO and BS Standards Explained
• Marine-Grade Fasteners Buying Guide
• DIY vs Trade Pack: When to Buy Bigger
• Fastener Head & Drive Types Glossary