304 vs 316 vs Duplex 2205/2507: Choosing Marine Stainless Fasteners That Actually Last

Salt water is the fastest way to kill an ordinary stainless bolt. Chloride ions attack the passive oxide film, and what looks fine in a warehouse can be pitted, crevice-corroded or stress-cracked within a season on an open deck. This guide compares the three families of marine stainless fasteners — 304, 316 and duplex 2205/2507 — on corrosion resistance, mechanical strength and cost, using the internationally recognised corrosion-classification system in ISO 9223 and the fastener property grades of EN ISO 3506-1.

Why Ordinary Stainless Fails in Marine Service

In open, chloride-rich conditions, stainless steel corrosion follows three main mechanisms:

• Pitting corrosion — local breakdown of the passive film by chloride ions, creating deep cavities.
• Crevice corrosion — accelerated attack inside clamped joints, under washers, nuts and poorly sealed threads where stagnant salt water is trapped.
• Stress corrosion cracking (SCC) — a combination of tensile stress and a corrosive medium that causes brittle cracking far below the material's normal strength.

All three are measured and managed by the alloy's PREN (Pitting Resistance Equivalent Number), calculated as PREN = Cr% + 3.3 × Mo% + 16 × N%. The higher the PREN, the better the resistance to chloride pitting and crevice attack.

The Three Material Families

Note: compositions per ASTM A240; yield values per ASTM specification; PREN is a comparison basis, not a standard guarantee. Selection must be confirmed by applied material standard and service conditions.

Property Grades: A2 vs A4 per EN ISO 3506-1

EN ISO 3506-1 classifies corrosion-resistant stainless fastener groups and property grades. The letter denotes the alloy group — A2 (304-type) and A4 (316-type) are the two most common — while the following two digits state minimum tensile strength in MPa ×10. For example A4-80 means 316-type material with a minimum tensile strength of 800 MPa; A2-70 is 304-type at 700 MPa.

Matching Material to Corrosion Class (ISO 9223)

ISO 9223 defines atmospheric corrosivity categories from C1 (very low) to C5 and CX (very high / extreme). On board and offshore, the relevant bands are C5-M (high salt, marine/industrial exposure) and CX (extreme offshore, prolonged salt-water wetting). A practical rule:

• C3 / sheltered interior — A2 (304) grades are usually sufficient.
• C4 / sheltered deck, coastal air — A4 (316) is the standard choice.
• C5-M / open deck, splash zone — A4-80 or higher; consider duplex 2205 where cyclic load is combined with corrosion.
• CX / offshore, sub-sea, permanent wetting — duplex 2205 or super duplex 2507.

Why Designers Shift to Duplex

Duplex 2205/2507 combine roughly double the yield strength of 304/316 with markedly higher PREN. In demanding fasteners — pipe flange studs, structural and mooring attachments — duplex delivers both corrosion resistance and strength in one grade, reducing section size and weight. The trade-off is higher base-material cost, so the choice is a balance of service life, load and budget.

Certificates and Verification You Should Ask For

• Material certificates — confirm grade and heat traceability.
• Spectrochemical analysis report — validates Cr, Mo and N for the claimed PREN.
• Neutral salt-spray test per ISO 9227 / GB/T 10125 where required.
• Third-party inspection — can be arranged to the specified classification or project standard.

sinofastener supplies marine fasteners with full material traceability, and can arrange third-party inspection on request. For assistance in matching material to your deck, pipe or structural application, contact us below.