Ask any fastener supplier which material question comes up most often, and it is usually this: “Should I use 304 or 316?” — or, in the language of international standards, A2 or A4.
It is a fair question. Stainless steel fasteners are used everywhere — from building facades and food processing lines to marine deck fittings and chemical plants — and the two grades differ in ways that directly affect service life and cost. Choose 316 where 304 would corrode and you avoid a failed joint; choose 304 where 316 is overkill and you save money on every unit.
This guide explains the real differences between A2 (304) and A4 (316) stainless fasteners, how the strength classes work, and how to specify them correctly in your next order.What Do A2 and A4 Actually Mean?
Stainless steel fasteners are classified under ISO 3506, which groups them by material family:
- A2 refers to austenitic stainless steel in the 300-series family — most commonly 304 (with ~18% chromium and 8% nickel). This is the general-purpose stainless fastener grade.
- A4 refers to austenitic stainless steel with added molybdenum — most commonly 316 (with ~16-18% chromium, 10-14% nickel and 2-3% molybdenum). The molybdenum addition is what gives A4 its superior resistance to chlorides.
The letter and the number together describe the material (A2/A4) and the strength class (50/70/80) — for example, A2-70 means 304 stainless, 700 MPa minimum tensile strength.304 vs 316: Composition Compared
| Element | A2 (304) | A4 (316) | Why it matters |
|---|---|---|---|
| Chromium | 17.5–19.5% | 16–18.5% | Forms the protective oxide layer that gives stainless steel its corrosion resistance |
| Nickel | 8–10.5% | 10–14% | Stabilizes the austenitic structure; improves toughness and formability |
| Molybdenum | — | 2–3% | The key addition — dramatically improves resistance to chloride pitting and crevice corrosion |
| Typical designation | 304 / A2 | 316 / A4 | Commonly ordered names in the fastener market |
The practical takeaway: the corrosion difference between 304 and 316 is mostly about chlorides. In dry indoor environments, both perform well. In coastal air, de-icing salt, seawater or chemical exposure, A4’s molybdenum makes a real, measurable difference.Corrosion Resistance: Where the Difference Really Shows
Both grades resist general rusting far better than carbon steel, but they fail differently in aggressive environments:
- A2 (304): excellent in atmospheric, indoor and mildly humid conditions. The most widely used stainless fastener grade for general applications — but it can pit in chloride-rich environments, such as coastal zones and swimming pool atmospheres, especially where water can pool in crevices.
- A4 (316): the molybdenum content resists chloride pitting and crevice corrosion much better, making it the standard choice for marine, offshore, chemical and food-processing environments.
Rule of thumb: in coastal zones, in contact with salt water, or in chemical/chloride atmospheres, specify A4. For general indoor, architectural and mild outdoor use, A2 is usually sufficient.Mechanical Property Classes: A2-70 vs A4-70 vs A4-80
Stainless fastener strength is expressed by the number after the letter, per ISO 3506:
| Class | Typical steel | Min. tensile strength (MPa) | Typical application |
|---|---|---|---|
| A2-70 | 304, cold worked | 700 | The standard stainless bolt/screw grade for general use |
| A4-70 | 316, cold worked | 700 | Corrosion-critical environments at standard strength |
| A4-80 | 316, heavily cold worked | 800 | Corrosion-critical environments with higher strength |
In practice, A2-70 covers the vast majority of stainless fastener orders. A4-80 is used where higher strength and corrosion resistance are both required. Note that high-strength stainless classes are achieved through cold working, not heat treatment, so the trade-off between strength and corrosion resistance needs to be handled by an experienced supplier.Are Stainless Fasteners Magnetic?
A common misunderstanding: “stainless must be non-magnetic.” Austenitic stainless steel (A2/A4) is generally non-magnetic in the annealed condition, but cold forming during fastener production can make it slightly magnetic — a bolt that attracts a magnet is not proof it is low grade. Conversely, some stainless grades (martensitic, ferritic) are magnetic by design.
The reliable way to verify a stainless fastener is by its markings and documentation (ISO 3506 class marking, material certificate), not by a magnet.How to Choose: Application Guide
| Environment / application | Recommended grade |
|---|---|
| Indoor general assembly, furniture, light machinery | A2-70 (304) |
| Building facades, handrails, general outdoor structures | A2-70 or A4-70 (coastal) |
| Coastal, marine, docks and offshore | A4-70 / A4-80 (316) |
| Chemical plants, chloride exposure, pools | A4 (316) |
| Food processing and washdown areas | A4 (316) |
| High-strength requirement + corrosive environment | A4-80How to Specify Stainless Fasteners in Your RFQ To avoid ambiguity, specify all four dimensions of the material choice: 1. Material family: A2 (304) or A4 (316), per ISO 3506. 2. Strength class: e.g., A2-70, A4-70 or A4-80. 3. Standard and dimensions: e.g., DIN 933 hex bolt, M10 × 40 mm. 4. Additional requirements: passivation, lubricant/coating, marking, test certificates. A complete specification protects both sides — the supplier quotes the right material, and you receive exactly what your application needs.Frequently Asked Questions Can I use 304 fasteners outdoors? Yes, in most atmospheric conditions. For coastal or chloride-exposed locations, choose 316 (A4). How can I tell if my stainless fasteners are really 316? Check the head marking (A4-70, A4-80) and request the material certificate. A magnet test cannot reliably distinguish 304 from 316. Is 316 always better than 304? For corrosion resistance, yes — especially in chloride environments. For cost and availability, 304 remains the sensible default for general applications. Do stainless fasteners suffer from galling? Stainless threads can gall (seize) during tightening. Anti-seize lubricants and controlled tightening torque prevent most cases.Get the right stainless grade, from the right source Whether your project calls for A2-70 or A4-80, the right specification is only as good as the supplier behind it. FASTENERHALL connects you with certified fastener manufacturers directly connected to source factories — covering bolts, nuts, screws, washers, retaining rings, pins and custom parts across construction, machinery, solar, railway, automotive and industrial applications. Send your specification to FASTENERHALL and get matched with verified suppliers — no middlemen, transparent pricing, reliable quality. Email: info@fastenerhall.com WhatsApp: +86 185 3100 3390 / +86 136 0330 7393 Contact: Mr. Wu FASTENERHALL — Global Digital Exhibition Hall for the Fastener Industry. Source high-quality bolts, nuts, screws, washers, anchors and custom fastening solutions from China’s leading fastener supply chain. |
