A2 vs A4 stainless: which grade belongs where
A2 and A4 are the two stainless fastener grades you will meet on almost every UK job, and the difference between them is molybdenum. A2 is roughly equivalent to type 304 and has none to speak of. A4 is roughly equivalent to type 316 and contains 2 to 3 per cent, which is what buys its resistance to chlorides. Almost everything else about choosing between them follows from that one line.
The two look identical, cost differently and fail differently. This guide covers where the boundary actually sits, why salt is the deciding factor rather than simply being outdoors, and the galvanic trap that catches people who have just upgraded to A4 for the first time.
- A2 is roughly type 304. A4 is roughly type 316, with 2 to 3 per cent molybdenum.
- Chlorides are the dividing line: coastal air, de-icing salt, pool halls and process water all mean A4.
- A2 in chlorides pits locally, and pitting is almost invisible until the fastener fails.
- A4 is not stronger than A2. Strength comes from the property class: 50, 70 or 80.
- A4 into aluminium or galvanised steel invites galvanic attack on the surrounding metal, not the fastener.
What actually separates A2 from A4
Both are austenitic stainless steels covered by BS EN ISO 3506, and both rely on a chromium oxide film that reforms itself when damaged. A2 sits at roughly 18 per cent chromium with 8 per cent nickel. A4 holds a similar chromium level, carries more nickel, and adds 2 to 3 per cent molybdenum.
Molybdenum does one job very well: it makes that passive film far harder for chloride ions to break down locally. That is the whole story. It does not make the fastener stronger, harder or more heat resistant.
- Strength is a separate decision. ISO 3506 property classes 50, 70 and 80 correspond to minimum tensile strengths of 500, 700 and 800 MPa. An A2-70 and an A4-70 are equally strong. Note the size limit: classes 70 and 80 apply up to M24, and above that class 50 applies unless something higher is specially agreed, so an A4-70 M30 holding-down bolt is not a shelf item.
- They look the same. There is no visual difference, and a magnet will not separate them, because cold working during heading and thread rolling leaves either grade slightly magnetic.
- Head markings are the only honest check. Marks read A2-70, A4-80 and so on. Unmarked loose stock cannot be graded by eye.
When molybdenum matters: chlorides, salt and pitting
A2 is a good general-purpose grade in ordinary inland atmospheric conditions: internal plant, dry structures, general engineering. It is not a marine grade. Once chlorides are in the picture it pits, and pitting is the dangerous failure mode because it is local, deep and hard to see from outside.
Treat all of the following as A4 territory:
- Coastal and estuary sites, quaysides and anything in salt-laden air.
- Highways structures exposed to winter de-icing salt spray.
- Pools, leisure centres and anywhere chlorinated water evaporates.
- Food, chemical and process plant with washdown, brine or chlorinated cleaning.
- Anything buried, boxed in or otherwise not inspectable once built.
A4 is better, not immune. Warm stagnant salt water, tight crevices under washers and permanent immersion will still attack 316 over time, and light brown surface staining on coastal handrail fixings is common and usually cosmetic. Where the duty is genuinely severe, the answer is a duplex or higher-alloy grade rather than more A4.
Galvanic corrosion: the trap after upgrading to A4
Stainless is noble. Zinc, aluminium and plain carbon steel are not. Put them in electrical contact with moisture between them and the less noble metal corrodes preferentially. That is why the classic site error is a stainless fastener through aluminium or galvanised steel: the fastener comes out perfect and the material around it is eaten away.
Two things govern how bad it gets. Moisture, because a dry indoor joint does very little. And area ratio, because a small stainless fastener in a large galvanised member spreads the attack thinly and is usually tolerable, while a small zinc or aluminium component clamped by a large stainless assembly is not.
- Isolate with a nylon or EPDM washer, an isolating sleeve, or a sealant film at the interface.
- Or match the materials, so galvanised fasteners go into galvanised steel.
- Do not mix A2 and A4 in the same joint in a chloride environment. The galvanic effect between two austenitic grades is negligible, but the A2 part becomes the corrosion weak link on its own account.
Buying, stocking and specifying
Because the grades are visually identical, mixed stock is a real risk, and it shows up months later as a run of pitted fasteners on a coastal handrail. Keep A2 and A4 physically separated and labelled, order them on separate lines, and never decant one into a bin that has held the other.
A few practical points at specification stage:
- Do not treat A2 or A4 as a like-for-like swap for a structural 8.8. Property classes are covered in our guide to choosing fasteners.
- Match the thread pitch exactly when replacing an existing fastener. The metric thread pitch guide has the table and shows how to measure one.
- Budget for anti-seize. Stainless threads gall, and larger diameters gall readily.
- Where a size or head form is not a standard stock item, a made-to-drawing route is often cheaper than redesigning the joint around what is available.
Common questions
Is A4 stainless the same as 316?
Effectively yes. A4 is the fastener designation in BS EN ISO 3506, and 316 is the material designation, and A4 fasteners are normally made from 316 or its low-carbon variant. The wording differs because the fastener standard sets composition ranges and mechanical properties rather than naming a single alloy, so A4 covers a small family of molybdenum-bearing austenitic steels.
Can I tell A2 from A4 with a magnet?
No. Both grades are austenitic and both are close to non-magnetic when annealed, but the cold working involved in heading and thread rolling leaves either one slightly magnetic. A weak magnetic response tells you the part was cold formed, not which grade it is. The reliable checks are the head marking, the supplier paperwork, a molybdenum spot test, or a handheld alloy analyser.
Is A4 stronger than A2?
No. Molybdenum buys corrosion resistance, not strength. Strength comes from the property class stamped after the grade: class 50 is annealed, class 70 is cold worked to a 700 MPa minimum tensile strength, and class 80 to 800 MPa. An A2-70 and an A4-70 of the same size carry the same mechanical rating, so pick the grade for the environment and the class for the load.
Is A2 ever acceptable outdoors?
Yes, inland and away from salt. A2 performs well in ordinary rural and urban atmospheres, and a great deal of external ironmongery on inland sites is A2 and lasts. The failure cases are all chlorides: coastal and estuary air, de-icing salt thrown off roads, and aggressive industrial atmospheres. If the fastener is also hidden or hard to inspect once built in, pay the difference and use A4.
Related
Back to the Fixings, fasteners & threads glossary · All glossary terms
Need this checked against a real specification?
Definitions only get you so far. Send us the actual requirement - a PPE schedule, a fixings list, a PQQ question or a marking you cannot make sense of - and the TES team will come back with what matches it. Trade pricing is on the B2B portal.