Almost every stainless fixing we sell comes in one of two grades: A2 or A4. They look identical in the hand. They are not interchangeable, and choosing wrong is the single most common reason a stainless fixing fails.
What the grades actually mean
A2 is the common austenitic stainless, sometimes written 18/8 — roughly 18% chromium, 8% nickel. It corresponds to AISI 304. The chromium forms a passive oxide layer that repairs itself when scratched, which is what makes stainless stainless.
A4 is the same family with one crucial addition: 2–3% molybdenum. It corresponds to AISI 316. That molybdenum is the whole story. It makes the passive layer far more resistant to chlorides, which are the one thing that reliably breaks stainless down.
The failure A4 prevents
Chlorides attack the passive layer at a single point rather than across the surface. The result is pitting corrosion: a fixing that looks fine from outside while a pit eats down into the shank. It fails suddenly, under load, with no rust streak to warn you.
Chlorides come from more places than people expect:
- Salt water and salt-laden air — anywhere within a mile or so of the coast
- Road salt, including spray thrown onto trailers, gates and railings
- Swimming pools and hot tubs, where chlorine sits in the air as well as the water
- Some cleaning products, and unwashed hands on food-industry equipment
When A2 is the right answer
A2 is not a lesser product. It is the correct specification for most work, and paying for A4 where A2 will do is simply spending more:
- Anything indoors — furniture, cabinetry, enclosures, electronics
- General outdoor use inland: fencing, gates, garden furniture, signage
- Machinery and general fabrication away from salt
When to insist on A4
- Boats, yachts and any chandlery — standing rigging, deck fittings, cleats, shackles
- Coastal property: balustrades, handrails, external ironmongery
- Poolside and spa hardware
- Trailers, horseboxes and anything that sees gritted roads through winter
- Lifting and load-bearing work outdoors, where a hidden pit is a safety problem rather than a cosmetic one
The mistake that ruins good fixings
Do not mix grades in a wet joint. Putting an A2 nut on an A4 bolt, or a stainless bolt through a galvanised bracket, sets up a galvanic couple — two dissimilar metals plus moisture make a small battery, and the less noble metal corrodes to protect the other. In salt water this happens fast.
Buy the bolt, the nut and the washer in the same grade. It costs pennies more and removes the problem entirely.
What about “marine grade”?
“Marine grade” has no legal definition. When we use it, we mean A4 316 and we say so in the product title and description. If a supplier says marine grade without naming a grade or standard, ask which one — plenty of A2 gets sold that way.
A quick note on magnetism
A fixing sticking to a magnet does not mean it is not stainless. Cold working — thread rolling, heading — makes austenitic stainless slightly magnetic, especially on small sizes. It is a poor test for grade or quality.
Where to start
Our Marine Hardware range is A4 316 throughout, including shackles, eye bolts and eye nuts and wire rope fittings. In Fasteners we stock both grades side by side — the grade is stated in every product title, so you always know what you are buying.