Stainless Fastener Grades Explained: A2 vs A4 (304 vs 316)
Every stainless bolt you buy is stamped with a class like A2-70 or A4-80, and most procurement mistakes in stainless fasteners come from reading only the “A”. The digit family decides corrosion resistance; the strength class decides whether the joint holds. Both matter, and they are priced differently.
What the stamp actually says
Under ISO 3506, “A” means austenitic stainless steel. The second digit is the chemistry family: A2 corresponds to 304 (18/8 chromium-nickel), A4 to 316 (18/10 with 2–3% molybdenum). The dash number is the property class — 70 means 700 MPa tensile strength, 80 means 800 MPa, usually achieved by cold working.
So A4-80 is a molybdenum-alloyed stainless bolt at 800 MPa. Same stamp format, two independent decisions baked into one marking.
When A2 (304) is the right call
A2 handles the overwhelming majority of indoor and general outdoor duty: furniture, food equipment, indoor railings, machinery covers, freshwater environments. It machines well, is widely stocked, and costs roughly 30–40% less than A4 at comparable sizes. If the joint will not see salt, chlorides or sustained chemical exposure, A2-70 is the default and paying for A4 buys nothing but margin.
When A4 (316) earns its premium
The molybdenum in A4 exists for one job: resisting pitting and crevice corrosion from chlorides. Specify A4 when any of these apply:
- Coastal or marine exposure — salt spray within roughly 5 km of open coastline
- De-icing salt contact — bridge fittings, parking structures, roadside hardware
- Swimming pools, spas and water treatment — chlorinated water attacks 304 aggressively
- Chemical, pharmaceutical and food-processing plants with cleaning acids
In these environments, an A2 bolt does not fail politely. It pits, seizes and snaps, usually after the warranty window. The cost of one failed coastal railing bracket dwarfs a pallet of A4 bolts.
Two traps to avoid
First: stainless does not mean non-magnetic or non-galling. A4 bolts gall under dry installation; use an anti-seize compound or coated nuts on any critical stainless joint.
Second: matching grades matters. An A4 bolt through an A2 plate still leaves a weaker link — the plate pits first. Specify the grade of the whole assembly, not just the fastener.
The one-line rule
Indoor and freshwater: A2-70. Salt, chlorides or chemicals: A4-80, installed with anti-seize. Everything else is a sourcing conversation, not an engineering one.