If you were to fit ordinary stainless near the sea, or for use in the sea, it'll not be long until it lets you down. Not straight away, but a few months later when brown staining creeps in around the heads. That is where A4 comes into its own with its higher tolerance of the conditions and higher resistance to rusting.
Here it is without the jargon. What A4 stainless steel is, why it is the same thing as 316, what the molybdenum in it does, and how to read the A4-70 and A4-80 codes before you order.
What A4 stainless steel is, and why it is the same as 316
A4 is a grade of austenitic stainless steel made for fasteners. It is the fixings trade name for marine grade stainless, commonly on boats, docks, coastal railings and anything else that is exposed to a marine environment.
Austenitic just means the family it belongs to, it is tough, easy to weld and does not go brittle in the cold. A4 sits under a standard called ISO 3506, the rulebook that sets what fastener grades are made of and how strong they are.
Here's the bit that trips people up. A4 and 316 are the same steel. A4 is the fastener name and 316 is the material name you see on sheet, bar and tube. When a job spec says 316 and your fasteners say A4, you can be sure they are the same thing and are compatible.
A4 and its equivalents
| Name | What it refers to |
| A4 | The fastener grade, set by ISO 3506 |
| 316 | The material grade for sheet, bar and tube, often called marine grade |
| 316L | The same steel with less carbon, for better welding |
| EN 1.4401 / 1.4404 | The European material numbers for 316 and 316L |
| UNS S31600 / S31603 | The US material numbers for the same two |
What the molybdenum actually does
The difference between A4 and everyday A2 comes down to one added ingredient. A4 has 2 to 3% molybdenum in the mix, alongside its chromium and nickel. A2 has none.
Chromium is what makes any steel stainless in the first place, by forming a thin invisible film on the surface that keeps rust at bay. Molybdenum toughens that film against chlorides, which is the salt in sea air, sea water, pool water and road salt.
Pitting and crevice corrosion in plain English
Chlorides attack stainless in two ways. Instead of rusting all over, they punch tiny pinholes down into the surface, which is called pitting. And in the tight gaps where water sits and cannot dry out, under washers, inside threads, between clamped parts, they eat away hidden from view, which is called crevice corrosion.
The molybdenum in A4 makes both much harder to start. In numbers, the pitting resistance of 316 works out around 24 on the industry pitting scale called PREN, against roughly 18 for ordinary 304. That gap is why A4 lasts by the sea where A2 stains.
A4-50, A4-70 and A4-80 explained
That dash number catches people out. It says nothing about salt resistance. A4-50, A4-70 and A4-80 all fight chlorides identically, because the material makeup is the same, so the code is purely about how hard the steel was worked and how much load it will carry.
Nearly every A4 bolt and screw you pick up is A4-70. It is the default supply condition, plenty for general work. A4-50 is the soft version, A4-80 is worked harder for more strength and tends to cost a bit more. Here are the minimum figures the standard sets.
A4 property classes (ISO 3506-1)
| Class | Min tensile strength | Min proof stress | Supply condition |
| A4-50 | 500 MPa | 210 MPa | Soft, annealed |
| A4-70 | 700 MPa | 450 MPa | Cold worked, the usual one |
| A4-80 | 800 MPa | 600 MPa | Harder cold worked |
In practice you rarely have to choose a class. A listing that only says A4 is almost certainly A4-70, so unless a job specifically calls out A4-80 for strength, the standard grade is the one you want.
When to step up from A2 to A4
Most fixings do not need A4. For general indoor and outdoor use, ordinary A2 stainless steel is cheaper and copes fine with rain and damp. The step up is worth it when chlorides are present.
The simple rule is salt. If the job lives near the coast, gets splashed with sea water, sits around a swimming pool, or takes a hammering from winter road salt, go A4. The same goes for food, dairy and chemical work where things get washed down hard.
Anywhere chlorides turn up regularly, the extra spend on A4 pays for itself in fixings that are still sound years later.
Where A4 is not enough
A4 is far better than A2 in salt, but it is not rust proof, and it is fairer to say so. In warm, stagnant, very salty spots, especially tight crevices that never dry out, even A4 can eventually pit.
For genuinely brutal chloride jobs, the grades above A4 are duplex and super-austenitic stainless, which cost far more and are usually specified by an engineer. For normal coastal and marine fixing work, A4 is the sensible, honest choice. If your job is structural or safety-critical, check the spec for your application rather than going off a web page.
Sizes, magnetism and what to expect
Those class figures hold for everyday diameters, roughly up to and including M24 for A4-70 and A4-80. Go bigger and the numbers become a matter of agreement between buyer and supplier, with large diameters often supplied a touch softer, because 316 gets harder to cold work the thicker it is. So expect A4-70 on normal sizes, and check the spec if you are buying something chunky.
Magnetism throws people too. A4 is non-magnetic by nature, yet the cold working that forms a bolt can leave a faint pull on a magnet. That faint pull is normal and does not mean the grade is wrong.
Where to buy A4 fixings
You will find A4 here in many sizes and head types. For socket screws, start with the A4 socket cap screws collection, or browse the wider socket cap screws range to compare head styles.
The handy part for your projects, there is no minimum order. Buy three A4 bolts or three hundred, whatever the work actually needs, rather than a bag of a hundred just to get the handful you are short.
A4 stainless steel FAQ
Is A4 stainless steel the same as 316?
Yes. A4 is the fastener grade name and 316 is the material grade name, but they describe the same marine grade steel. If a spec calls for 316 and your fixings are marked A4, you have the correct grade.
Is A4 316 or 316L?
It can be either. A4 covers the 316 family, and both 316 and the lower-carbon 316L meet it. 316L is often chosen for welded parts, because the reduced carbon helps the weld resist corrosion. For most bolts and screws it makes no difference to how you use them.
Which is stronger, A4-70 or A4-80?
A4-80. It meets a minimum tensile strength of 800 MPa against 700 for A4-70, because it has been worked harder. Both resist salt exactly the same, so pick A4-80 only when you actually need the extra strength, not for better corrosion resistance.
Is A4 stainless steel magnetic?
No, not really. As supplied it is essentially non-magnetic. The cold working that shapes a bolt can add a faint magnetic pull, and that is normal rather than a sign of the wrong grade.
Is A4 better than A2?
Only where it counts. A4 is much better in salt and chlorides thanks to its molybdenum, so near the sea it wins easily. Away from salt, A2 does the same job for less, so better really depends on where the fixing is going.
What is A4 stainless steel equivalent to?
316 and 316L, written as EN 1.4401 and 1.4404 in Europe, or UNS S31600 and S31603 in the US. In everyday terms, marine grade stainless.
The short version
A4 is marine grade stainless, the same steel as 316, and the molybdenum is what fights off salt. A4-70 is the everyday strength class, and stepping up from A2 is worth it the moment chlorides are in play. It holds up very well by the sea, just not forever, and that honest limit is the useful thing to carry away.
Still weighing up whether A4 is overkill or exactly right for your job? Drop us a line before you order. Getting you the grade that suits the work matters more to us than shifting a box of the wrong ones.