Are you stuck trying to decide between A2 or A4 stainless steel fasteners? You're not alone, and it's worth taking a few minutes to read through this post. Is A4 worth the extra money? Is A2 sufficient for you? Keep reading, and I'll explain the key differences for you.
Before i properly get into the A2 vs A4 stainless steel question, it is important to clarify this first. A2 stainless is 304 and A4 stainless is 316, i will be referring to both name types throughout the post.
The short answer: A2 for most jobs, A4 near salt water
If you are stuck choosing, here it is. A2 handles most fixings, indoors and out, and costs less. Choose A4 when the fixing lives near the sea, a pool, road salt or harsh chemicals, because A4 resists the salt that would slowly stain A2.
Just to clarify, because it saves money. A4 is not stronger than A2, not at the same grade number. The letter is about differentiating between grades; more on that below.
A2 vs A4 is the same question as 304 vs 316
Half the confusion here is just names. A2 is the fastener trade name for A2 stainless steel, which is 304, the classic 18/8. A4 is the fastener name for A4 stainless steel, which is 316, the one people call marine grade.
So "a2 vs a4" and "304 vs 316" are the same question asked two ways, and this page answers both. You will see 304 on sheet, bar and tube, and A2 on the box of bolts. Same steel, so if a spec says 304 and the fixings say A2, you have the right part.
A2 vs A4 stainless steel at a glance
Here is the whole comparison in one place. Everything below it is just the detail.
| A2 (304) | A4 (316) | |
| Also known as | 304, 18/8 | 316, marine grade |
| Key ingredient | About 18% chromium, 8% nickel | About 17% chromium, 11% nickel, 2 to 3% molybdenum |
| Salt resistance (PREN) | Around 18 to 20 | Around 23 to 28 |
| Best for | General, indoor, garden, normal outdoor | Coast, marine, pools, road salt, washdown |
| Not ideal for | Salt spray and chlorides | Nothing A2 does, it just costs more |
| Strength at the same class | A2-70 = 700 MPa minimum | A4-70 = 700 MPa, identical |
| Cost | Lower | Higher |
| Magnetic? | No (slightly, if cold worked) | No (a touch less prone) |
The only real difference is molybdenum
Both grades are austenitic stainless steel, which makes them tough, easy to weld, and "almost" non-magnetic. The difference between them comes down to one added ingredient.
A2 (304) is roughly 18% chromium and 8% nickel, which is where the 18/8 nickname comes from. A4 (316) has a bit more nickel and adds 2 to 3% molybdenum.
That molybdenum is the critical difference. When added, it fights the pitting and staining that chlorides in sea and road salt, and also harsh chemicals, cause in plain stainless.
There is a rough score for this called PREN, the pitting resistance number. Higher is better. A2 sits around 18 to 20 and A4 around 23 to 28, according to the British Stainless Steel Association.
That gap is exactly why A4 holds up in harsher conditions where A2 would fail.
A4 is not stronger than A2, at the same class
This is the one that costs people money, so let me be blunt. A4 is not stronger than A2. People assume the dearer grade must be tougher, but not when you compare like for like.
Stainless fasteners carry two markings. A grade letter, A2 or A4, which tells you the corrosion resistance. And a number after it, like A2-70, which tells you the strength.
The strength number is set by the standard ISO 3506, and it is the same whichever letter sits in front of it.
So A2-70 and A4-70 both meet the same 700 MPa minimum pull strength. Match the numbers and they are equally strong. The letter only changes how well they cope with rust.
What the dash number means: A2-70, A4-80
That number is a strength class. Here is what each one stands for.
| Class | Min tensile strength | Min proof stress | Typical use |
| 50 | 500 MPa | 210 MPa | Soft, annealed |
| 70 | 700 MPa | 450 MPa | Cold worked, the usual one |
| 80 | 800 MPa | 600 MPa | Harder, higher strength |
Class 70 is the common one, cold worked and strong enough for most jobs. Class 80 is harder again. Class 50 is soft and rarely what you want for a working bolt.
One honest caveat. Stainless is not high tensile steel. If you are swapping out a high strength bolt on a car or machine, check the original grade first, because a stainless bolt can be weaker than the one it replaces.
When A2 is the right choice
A2 is the default, and for good reason. It handles the vast majority of fixing jobs without any issues, and it costs less than A4.
So, opt for A2 for general engineering, indoor work, furniture, brackets, and normal outdoor use like a garden, fence or shed that just sees rain and damp. Rain alone does not affect stainless. Salt does the damage.
When you need A4
A4 is your choice the moment chlorides come into it. That means the coast, boats and marine work, swimming pools, and anywhere road salt flies around in winter. It also suits food, dairy and chemical washdown areas cleaned with harsh chemicals.
If your application is in or around a harsh environment, go for A4 every time as it will far outlast A2 and provide the peace of mind that it can cope a lot better than the A2.
Where even A4 struggles
It is important to note that A4 is not 100% tolerant and can still corrode if fully submerged in seawater for any length of time, and can still be prone to pit over time. In these applications or where severe chlorides exist, the best move is to step up again, to a duplex or super austenitic grade, rather than expect 316 to last forever. Otherwise, you should be aware of the potential need to inspect and replace if necessary.
For everyday coastal and marine work though, A4 is the one.
Is A4 worth the extra money?
A4 costs more than A2, and that is down to the alloy. Molybdenum and the extra nickel are pricier, so 316 naturally costs more to produce than 304.
Because of that, do not opt for A4 out of habit. On a job that never meets these types of conditions, you are paying extra for protection you will never need, and A2 does the same work for less.
A2 vs A4 FAQ
Is A2 or A4 stainless steel magnetic?
Neither, mostly. Both are austenitic, so they are non magnetic when soft, and only go slightly magnetic when worked cold, like a rolled thread. A4 is a touch less prone, and a magnet is not a reliable way to tell them apart.
Is 316 stainless A2 or A4?
316 is A4. They are the same steel; 316 is the material name and A4 is the fastener name. In the same way, 304 is A2.
Can you use A2 outdoors?
Yes, for most outdoor jobs. A2 copes fine with rain, damp and general weather. Where it struggles is salt, so near the coast or on gritted winter roads, switch to A4.
Is 304 more expensive than 316?
No, it is the other way round. 316 (A4) costs more than 304 (A2) because of the molybdenum and extra nickel in it. That is the main reason not to buy A4 unless the job actually needs it.
Can you mix A2 and A4 fasteners on one job?
Yes, and often it is the smart thing to do. Put A4 where the salt is and A2 everywhere else. Just be careful that the A2 are far enough away or sealed away from the harsher conditions that A4 are meant for.
Can you weld 304 and 316 together?
Yes. They weld together fine, and it is common to use a 316 or low carbon 316L filler. Just remember the joint is only as corrosion resistant as the lower grade, so a 304 part welded in still rusts like 304.
So, A2 or A4?
For most jobs, A2 is the sensible pick, strong enough and cheaper. Save A4 for salt, coast, pools and chemicals, where the molybdenum actually earns its keep. And whatever you land on, match the strength number rather than the letter, because A2-70 and A4-70 pull just as hard as each other.
Not sure which way to go on your own job? That is exactly what we are happy to help with, so get in touch, and we'll point you in the right direction.
