DIN 9021 Fender Washers (Large OD) — A4 Marine Stainless Steel (SUS316)

FULLERKREG DIN 9021 A4 fender washers are manufactured from A4 stainless steel (SUS316) with a whitened and passivated surface finish. DIN 9021 specifies a large outer diameter washer — the outer diameter (d2) is approximately 3× the bolt nominal diameter — providing a significantly larger bearing area than standard DIN 125A washers to prevent pull-through and embedment in soft or thin materials such as timber, plastic, plywood, thin sheet metal, and composite panels.

A4 (SUS316) stainless steel contains 2–3% molybdenum (Mo), giving it a Pitting Resistance Equivalent Number (PREN) of ~24–26 — significantly higher than A2 SUS304 (~18–20). This makes A4 the definitive choice for marine, offshore, coastal, and chloride-containing environments where A2 may suffer pitting corrosion. Combined with the large bearing area of DIN 9021, these washers are the preferred choice for marine timber structures, coastal cladding, offshore platforms, and any outdoor application in high-humidity or salt-air environments.

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Product Specifications

Standard	DIN 9021 (Large OD flat washer, outer diameter ≈ 3× bolt diameter)
Type	Large outer diameter (fender / penny) washer — plain flat, no chamfer
Material	A4 Stainless Steel / SUS316 / 1.4401
Surface Finish	Whitened & Passivated
Size Range	M5 – M24
OD Ratio	Outer diameter ≈ 3× nominal bolt diameter (vs ~2× for DIN 125A)
Hardness	≤ 200 HV
PREN (Pitting Resistance)	~24–26 — significantly higher than A2 SUS304 (~18–20)
Corrosion Resistance	Excellent — superior to A2 in marine, chloride, and chemical environments
Magnetic	Slightly magnetic after cold working
Recommended Pairing	A4 stainless bolts (DIN 931 / DIN 933 A4) and nuts (DIN 934 A4) for full material compatibility
Related Standard	DIN 125A A4 (standard OD)  |  ISO 7093 (large washer series)

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Dimensions — DIN 9021

All dimensions in millimeters (mm). Data per DIN 9021 standard. Dimensions are identical for A2 and A4 grades.

Nominal Size (Bolt d)	d1 — Inner Ø (mm)	d2 — Outer Ø (mm)	h — Thickness (mm)	Weight (kg / 1000 pcs)
M5	5.5	15	1.0	0.73
M6	6.4	18	1.6	1.98
M8	8.4	24	2.0	4.40
M10	10.5	30	2.5	8.50
M12	13.0	37	3.0	15.2
M14	15.0	44	3.0	21.3
M16	17.0	50	3.0	27.3
M20	21.0	60	4.0	53.6
M24	25.0	72	4.0	76.9

* d1 = inner diameter (clearance hole); d2 = outer diameter; h = nominal thickness. All dimensions per DIN 9021. Weights are approximate based on A4 stainless steel density (7.98 g/cm³).

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A4 (SUS316) vs A2 (SUS304) — Why Choose A4 for Marine & Outdoor Applications?

Property	A4 — SUS316 (This Product)	A2 — SUS304
Molybdenum (Mo) content	2.0 – 3.0%	None
PREN (Pitting Resistance)	~24–26	~18–20
Chloride pitting resistance	Excellent	Good
Crevice corrosion resistance	Very good	Moderate
Marine / offshore suitability	Yes — preferred grade	Limited — may pit in high-chloride
Coastal / salt-air environments	Excellent	Good for low-salinity only
Timber treatment chemicals	Excellent resistance	May corrode with ACQ/CCA treated timber
Tensile strength (annealed)	≥ 515 MPa	≥ 500 MPa
Density	7.98 g/cm³	7.93 g/cm³

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DIN 9021 vs DIN 125A — Large OD vs Standard OD

Property	DIN 9021 (This Product)	DIN 125A (Standard)
Outer diameter ratio	~3× bolt diameter	~2× bolt diameter
M12 outer diameter	37 mm	24 mm
Bearing area (M12)	~940 mm²	~320 mm² (~3× smaller)
Pull-through resistance	High — for soft/thin materials	Standard — for hard materials
Best for	Timber, plastic, plywood, thin sheet, composite	Steel, concrete, hard structural materials
ISO equivalent	ISO 7093	ISO 7089

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Material & Surface Properties — A4 / SUS316

Property	Value
Material Designation	A4 / SUS316 / 1.4401 / 0Cr17Ni12Mo2
Chromium Content	16.0 – 18.0%
Nickel Content	10.0 – 14.0%
Molybdenum Content	2.0 – 3.0% — key differentiator vs A2
Tensile Strength	≥ 515 MPa (annealed)
Hardness	≤ 200 HV
PREN	~24–26 (vs ~18–20 for A2 SUS304)
Surface Finish	Whitened & Passivated
Corrosion Resistance	Excellent — superior chloride pitting and crevice corrosion resistance vs A2
Temperature Range	−50°C to +300°C (continuous service)
Magnetic	Slightly magnetic after cold working
Density	7.98 g/cm³

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Typical Applications

• 🚢 Marine timber structures — A4 is the mandatory grade for fastening in marine timber (jetties, pontoons, boat building) where seawater splash and salt air demand superior corrosion resistance; large OD prevents pull-through in timber
• 🌊 Coastal & offshore cladding — large OD distributes load across thin cladding panels in coastal buildings and offshore structures; A4 resists salt-air pitting
• 🏗️ Treated timber construction — ACQ and CCA timber preservatives are highly corrosive to A2 stainless; A4 provides the necessary resistance for treated timber fastening
• 🌳 Outdoor & garden structures — coastal — pergolas, decking, fencing, and garden structures in coastal or high-humidity environments where A2 may corrode
• 🧱 Marine composite & GRP panels — large OD prevents pull-through in fibreglass, GRP, and composite marine panels; A4 resists marine environment corrosion
• 🍼 Food & beverage processing — A4 SUS316 preferred grade for hygienic applications; large OD suitable for fastening insulation panels and equipment covers
• 🍁 Chemical plant — outdoor — large OD for thin panel and cladding fastening in outdoor chemical plant environments where chloride exposure demands A4

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Frequently Asked Questions

Q: Why choose A4 over A2 for DIN 9021 fender washers?
A: A4 (SUS316) contains 2–3% molybdenum, giving it a PREN of ~24–26 vs ~18–20 for A2 (SUS304). In practical terms, A4 resists chloride pitting and crevice corrosion significantly better than A2 — making it the preferred choice for marine, coastal, offshore, and treated timber applications. DIN 9021 fender washers are frequently used in timber and cladding applications that are exposed to outdoor environments, where A4 provides the necessary long-term corrosion resistance.

Q: Are A4 DIN 9021 washers required for treated timber?
A: Yes. Modern timber preservatives — particularly ACQ (Alkaline Copper Quaternary) and CCA (Copper Chrome Arsenic) — are highly corrosive to carbon steel and can also corrode A2 stainless steel over time. A4 (SUS316) stainless steel provides significantly better resistance to these preservative chemicals and is the recommended grade for fastening in treated timber per most timber construction standards.

Q: What is the difference between DIN 9021 and DIN 125A?
A: DIN 9021 has an outer diameter of approximately 3× the bolt nominal diameter, vs approximately 2× for DIN 125A. For M12: DIN 9021 d2 = 37 mm vs DIN 125A d2 = 24 mm — approximately 3× greater bearing area. DIN 9021 is used where the larger bearing area is needed to prevent pull-through in soft materials (timber, plastic, thin sheet). Both have the same inner diameter for the same bolt size.

Q: Are A4 DIN 9021 washers suitable for seawater immersion?
A: A4 (SUS316) provides significantly better seawater resistance than A2 due to its molybdenum content. For splash zone, tidal, and above-waterline marine applications, A4 is the correct choice. For continuous seawater immersion or subsea applications, even A4 can suffer crevice corrosion in stagnant conditions — specify Duplex 2205 or Super Duplex for fully immersed applications.

Q: What is the ISO equivalent of DIN 9021?
A: DIN 9021 is equivalent to ISO 7093 (large washer series). Both specify a large outer diameter washer with d2 ≈ 3× bolt diameter. DIN 9021 is the German national standard; ISO 7093 is the international equivalent.