A2 vs A4 Stainless Steel: Which Grade?

A2 vs A4 Stainless Steel: Which Grade Do You Need?

The difference between A2 (grade 304) and A4 (grade 316) stainless steel is one of the most common questions we receive. Both are excellent choices for most applications — but in certain environments, using the wrong grade can lead to premature failure.

PTS By the PTS technical team· Updated June 2026· 5 min read

Key takeaways

A2 contains 18% chromium + 8% nickel (304 grade) — strong all-round corrosion resistance.
A4 adds 2–3% molybdenum which dramatically improves resistance to chlorides.
A4 is required for marine environments, swimming pools and coastal structures.
A2 is fine for most indoor, sheltered outdoor and general engineering applications.
PRE values: A2 = ~18–19, A4 = ~24–26 — a measurable difference in pitting resistance.

Chemical composition

Both grades share the same stainless-steel foundation — high chromium content that forms a passive oxide layer on the surface, preventing rust. The key difference is molybdenum:

Property	A2 (304)	A4 (316)
Chromium	17–20%	16–18%
Nickel	8–10.5%	10–14%
Molybdenum	None	2–3%
PRE value	~18–19	~24–26

The PRE (Pitting Resistance Equivalent) is a calculated measure of resistance to chloride-induced pitting. The higher the PRE, the better the material handles salt and chemicals. A4’s PRE of 24–26 is meaningfully better than A2’s 18–19.

When to choose A4

Specify A4 (316) if any of the following apply:

Within 1 mile of the sea — salt-laden air attacks A2 over time; A4 withstands it reliably.
Exposed to chlorinated water — swimming pools, water treatment, irrigation.
Food processing environments — where cleaning chemicals (often chlorine-based) are used regularly.
Chemical plant — where acids, solvents or harsh cleaning agents are present.

When A2 is the right choice

A2 (304) is the workhorse grade — strong, reliable and cost-effective for the vast majority of applications:

General indoor engineering and manufacturing.
Sheltered outdoor use where salt exposure is low.
Architectural and construction fasteners in inland locations.
Any application where A4 would be over-specification.

Frequently asked questions

Is A4 always better than A2?

Better in chloride environments, yes — but A2 is entirely adequate for the majority of applications. Using A4 where A2 would work fine adds unnecessary cost. Match the grade to the environment.

Can I mix A2 and A4 fasteners?

Technically yes, but it’s better practice to use the same grade throughout an assembly. Mixed grades in aggressive environments can create galvanic micro-cells at contact points.

Are A2 and A4 the same strength?

For standard grades (A2-70 and A4-70), yes — the “70” refers to the minimum tensile strength class (700 MPa). Both are available in the higher A2-80 / A4-80 grades for extra load capacity.

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