They are all stainless, but not all of them resist the same environment. The wrong choice costs you either a lot (the most expensive material when it isn't needed) or more (rust and failure within months). Here are the simple rules.
When to Choose Each Material
EN 1.4301 · A2 series
A2 (AISI 304)
The standard choice for general use when the environment has no chlorides.
Outdoors in ordinary atmosphere, away from the sea, with no road salt.
Food industry where there are no chlorinated detergents or high salt.
When cost is the priority and the environment is mild.
Don't choose it when
Near the sea or in salty air (rust stains appear within months).
Swimming pools, road salt, chloride chemicals or acids.
When the part is safety-critical and not easy to replace.
EN 1.4401 / 1.4404 · A4 series
A4 (AISI 316)
With molybdenum: far better resistance to salt and chemicals.
Choose it when
Coastal and marine environments with salt spray (not submerged).
Chemical, pharmaceutical and food industry with harsh cleaning (CIP).
Polluted industrial or urban environments, and outdoor pools.
When A2 has already failed in the same place, or replacing the bolts is hard.
Don't choose it when
Continuous immersion in warm seawater or very high chloride.
Chlorides combined with mechanical load above about 60 °C (stress corrosion cracking risk).
When very high strength is needed; consider duplex instead.
EN 1.4462 · duplex family
Duplex (2205)
Around twice the strength and high resistance to chlorides and cracking.
Choose it when
Marine structures, ports, submerged service and desalination plants.
Oil, gas and chloride chemicals where A4 isn't enough.
Critical points above indoor pools and in tunnels with road salt.
When you need high strength and corrosion resistance together without moving to a pricier steel.
Don't choose it when
Continuous temperatures above about 300 °C (embrittlement).
When A4 is enough: it costs more, so choosing it needlessly wastes budget.
When you need delivery at once: it isn't always in stock, often made to order.
Side by Side
Property
A2 (AISI 304)
A4 (AISI 316)
Duplex (2205)
Steel type
Austenitic, chromium-nickel
Austenitic, chromium-nickel-molybdenum
Duplex (austenite + ferrite)
fastenerGuide.cmp.composition.label
Cr 17.5–19.5 · Ni 8–10.5
Cr 16.5–18.5 · Ni 10–13 · Mo 2–2.5
Cr 21–23 · Ni 4.5–6.5 · Mo 2.5–3.5 · N
PREN
≈ 18
≈ 24
≈ 35
Strength classes (ISO 3506)
A2-50, A2-70, A2-80
A4-50, A4-70, A4-80
On request and by standard
Strength
Class 70: Rm min. 700 MPa (up to M24)
Class 80: Rm min. 800 MPa (up to M24)
Material yield strength about 450 MPa
Chloride resistance
Low
Good
High
Stress corrosion cracking
Risk above about 60 °C with chlorides
Risk above about 60 °C with chlorides
Very good resistance
Magnetic
Slightly (after cold forming)
Slightly (after cold forming)
Yes
Temperature
From cryogenic up to about 400 °C
From cryogenic up to about 400 °C
Up to about 300 °C
Cost
€
€€
€€€
Availability
Very good
Good
Often made to order
Which One by Environment
A quick guide: find the row that looks most like where it will be fitted.
Environment
A2
A4
Duplex
Indoors, dry
Yes
Yes
Yes
Outdoors, away from the sea
Yes
Yes
Yes
Food industry (with cleaning)
Conditional
Yes
Yes
Coastal, salt spray
Conditional
Yes
Yes
Road salt, tunnels
No
Conditional
Yes
Indoor pool (chlorine vapour)
No
Conditional
Yes
Chemicals and chlorides
No
Conditional
Yes
Submerged in seawater
No
No
Conditional
Very high mechanical load
Conditional
Conditional
Yes
Above 300 °C continuously
Conditional
Conditional
No
This guide is indicative and does not replace an engineer's calculation for critical projects. The real environment (temperature, chloride concentration, cleaning) can change the choice.
Not sure about your environment? Tell us where it will be installed and we'll recommend the right material.