Stainless Steel Fasteners: 304 vs 316 vs Duplex
> In brief > - 304, 316 and duplex are not interchangeable grades. > - The environment decides the grade, not the price list. > - Over-specifying stainless costs more and solves a problem you do not have.
Stainless is chosen for corrosion resistance, but the grade must match the chloride exposure or the part corrodes anyway — sometimes faster than a galvanized carbon steel part would have.
Our own production and test data are available on request — see how our factory works and contact the engineering team for the documents behind any figure quoted here.
A2 (304) is the general-purpose austenitic grade suitable for rural, urban, and most indoor environments, and in fresh water, but it pits in chloride-rich conditions. A4 (316) refers to an austenitic grade with added molybdenum, improving pitting and crevice corrosion resistance in chloride environments, making it suitable for coastal and marine exposure. Duplex means higher strength and better chloride resistance, used for high-load marine applications and where stress corrosion cracking is a risk, though it is more costly and less available in small fasteners. Galling is the cold-welding of threads under pressure, a common issue with austenitic stainless threads, especially on new, clean threads and at high tightening speeds.
The grades in construction.
A2 (304): the general-purpose austenitic grade. Good in rural, urban and most indoor environments, and in fresh water. It pits in chloride-rich conditions — coastal atmospheres, de-icing salts, marine spray.
A4 (316): adds molybdenum, which substantially improves pitting and crevice corrosion resistance in chloride environments. The standard choice for coastal and marine exposure and for chemical or water treatment plants.
DUPLEX AND SUPER DUPLEX: higher strength and better chloride resistance again, used for high-load marine applications and where stress corrosion cracking is a risk. Higher cost and less available in small fasteners.
Standard basis: EN 10204:2004 — Types of inspection documents
Why stainless still corrodes.
THREE conditions defeat any stainless grade: chloride concentration above what the grade tolerates, oxygen starvation in crevices (under a washer or in a thread root), and elevated temperature combined with chlorides.
CONTAMINATION is the fourth and most common: carbon steel particles embedded in a stainless surface — from a wire brush, a grinding wheel, or a carbon steel workbench — create local galvanic cells. The stainless stays intact and the embedded particle rusts, staining the surface and sometimes initiating pitting. Site rule: dedicated stainless tools, and passivation after any mechanical work.
The galling problem.
Austenitic stainless threads gall: under pressure, the oxide layer breaks and the two surfaces cold-weld, seizing the joint. It happens most on new, clean threads and at high tightening speed. It is the single most common complaint about stainless fasteners.
Practical mitigations: use a lubricant or anti-seize formulated for stainless, slow the tightening, avoid power tools on stainless threads, and consider using a stainless nut on a stainless bolt only where necessary — a coated carbon steel nut is sometimes acceptable if the nut is not exposed.
Selection guidance.
Indoor, dry, non-chloride: A2 (304) is sufficient and cheaper.
Coastal, marine, de-icing salt, swimming pool atmosphere: A4 (316) as a minimum.
Permanent embedment in chloride environment, or high load with chlorides: duplex or a non-metallic alternative, and specify the surface condition (pickled and passivated).
Galvanized carbon steel remains the economic choice for large volumes of permanent embedment in non-chloride environments.
Standard basis: ISO 965-1
Specification points.
State the grade by standard designation (A2-70, A4-80 and the equivalent ISO class), the surface condition, and whether passivation is required. For safety-related applications, state the strength class explicitly: stainless fasteners are not automatically stronger than carbon steel, and a common error is replacing an 8.8 carbon steel part with an A2-70 part of lower strength.
| Grade | Corrosion Resistance | Consequence for Application |
|---|---|---|
| A2 (304) | Good in rural, urban, indoor, fresh water | Pits in chloride-rich environments |
| A4 (316) | Improved in chloride, marine, chemical plants | Standard for coastal and marine exposure |
| Duplex | High in chloride, high load, stress corrosion | Higher cost, less available in small sizes |
| Galvanized Carbon Steel | Adequate in non-chloride environments | Economic for large volumes, non-chloride use |
| Non-Metallic Alternatives | Resistant in chloride, high load | Suitable for permanent embedment, higher cost |
References and standards.
- EN 10204:2004 — Types of inspection documents
- ISO 898-1 — Mechanical properties of fasteners made of carbon steel
- ISO 3501 — Corrosion resistance of stainless steel fasteners
- NIST material measurement standards
- ISO 10683 — Hot dip galvanized coatings on fabricated iron and steel articles
- ASTM A36/A36M — Standard specification for carbon structural steel
About us: This article covers the differences between 304, 316, and duplex stainless steel fasteners and their appropriate use in various environments. Our production and test data are available on request. The contact page provides a way to reach our engineering team for detailed information, and the about page documents how our factory operates. This article was reviewed by the Qianheng Formwork Engineering team.