Guide · Updated September 2026

Why Stainless Steel Washers Rust or Stick to a Magnet — and What to Check

These are the two complaints we hear most often from buyers, usually with a photo attached: "the magnet sticks, is this really 304?" and "the washers are rusting after three weeks on site." We make 304, 316 and 316L washers on coil-fed JH21 presses in Jiashan, so we see the same questions from the supply side. Below is what each symptom usually means and what to ask your supplier. That supplier might be us.

1. The magnet test: what it tells you and what it doesn't

304 and 316 are austenitic stainless steels. In the annealed state, which is how stainless coil arrives from the mill, the austenite structure is non-magnetic. Austenite in 304 is only just stable, though. Cold work converts some of it into martensite, and martensite is ferromagnetic. The Australian Stainless Steel Development Association states it plainly: 304 and 316 are non-magnetic when annealed, but once cold worked they will be attracted to a permanent magnet.

Stamping a washer is concentrated cold work. The punch shears the bore, the blanking die shears the outside diameter, and both edges take far more strain than the flat face. That is why, on a stamped 304 washer, a small magnet usually pulls hardest at the hole and the rim and only weakly on the face. Spring washers (DIN 127) and tooth lock washers take extra forming, so they often show a stronger response than a flat washer of the same grade.

316 carries 10–14% nickel against 8–10.5% in 304. Nickel stabilises austenite, so a stamped 316 washer normally responds less to a magnet than a stamped 304 one. "Less" is not "zero", and a slight pull on 316 is not by itself a reason to reject it.

Why the magnet can't catch the substitution buyers actually worry about

The real risk in cheap stainless fasteners is 201 sold as 304. 201 replaces much of the nickel with manganese and nitrogen: about 16–18% chromium, 3.5–5.5% nickel and 5.5–7.5% manganese (ASTM A240 ranges). It is also austenitic, so annealed 201 is non-magnetic too. After stamping, a 201 washer and a 304 washer can respond to a magnet in much the same way. 201 costs less because nickel is the expensive element, and it has noticeably weaker corrosion resistance outdoors and in humid air.

Where the magnet is useful: a strong, even pull across the whole face, the kind that lifts the washer cleanly, points to a ferritic grade such as 430 or to plated carbon steel. That deserves a proper check. A weak pull at the edges of a stamped 304 washer does not.

If your application genuinely needs low magnetic permeability, for example parts near sensors or instruments, say so in the RFQ with a numeric permeability limit. A standard stamped washer is not made to a permeability specification, and the limit decides whether 316, a different grade or a post-forming anneal is needed.

2. Why genuine stainless washers still rust

Stainless steel resists corrosion because of a thin chromium-oxide film on its surface. Anything that breaks that film locally, or stops it re-forming, lets corrosion start. In washers we see four mechanisms again and again.

Cause 1: free iron on the surface (spots that wipe or scrub off)

Small orange-brown spots, often in the first weeks, that come off with a non-scratch pad and leave clean metal underneath. This is carbon-steel particles sitting on the stainless surface and rusting, not the washer itself. Typical sources: grinding dust in a mixed workshop, carbon-steel tools, stainless packed in the same carton as zinc-plated or black steel parts, or steel strapping rubbing through the packing.

The standard remedy is chemical passivation, which dissolves the free iron and lets the oxide film re-form evenly. ASTM A967 specifies nitric and citric acid treatments for this and the tests to confirm the surface is clean. Our standard washer finishes are bright and polished; passivation is not one of them. If your drawing calls for ASTM A967, put it in the RFQ so the supplier quotes it explicitly instead of assuming it.

Cause 2: chlorides that 304 can't handle (pits)

Small, deep pits, often with a rust tail running down from them, on washers near the sea, around swimming pools, on roads treated with de-icing salt, or in wash-down areas. Chloride ions break through the passive film at weak points, and in 304 the pit keeps growing.

The usual cause is grade selection rather than a defect. Pitting resistance is ranked by PREN (Cr + 3.3 × Mo + 16 × N). 304 sits around 18–20 and 316 around 24–28, because of its 2–3% molybdenum. In chloride service the answer is 316 or 316L (A4), with the bolt and nut in the same grade. Our 304 vs 316 guide covers where each grade belongs.

Cause 3: the crevice under the washer (staining at the edge)

A washer creates a crevice by design: a narrow gap between its face and the part it sits on. If salty water gets into that gap and cannot drain or dry, the oxygen inside is used up, the passive film cannot repair itself, and corrosion starts under the washer. From outside it shows as a brown ring or streak at the washer's edge; when you undo the joint the damage is on the hidden face.

316 resists crevice corrosion better than 304 but is not immune in warm, stagnant chloride water. Joint design matters as much as grade: avoid details that hold water on horizontal surfaces, and in marine service a sealant or a sealing washer that keeps water out of the gap does more than an upgrade in grade alone.

Cause 4: contact with aluminium or zinc (the other metal corrodes)

White powder or pitting on an aluminium profile or a galvanised part around a stainless washer is galvanic corrosion. In wet conditions stainless is the more noble metal, so the aluminium or zinc next to it corrodes faster and the stainless washer stays clean. How bad it gets depends mostly on the area ratio. A small stainless washer on a large aluminium sheet is usually tolerable in the open air. A large stainless part against a small area of zinc or aluminium is not.

Two cases come up constantly. The first is a stainless washer under a zinc-plated bolt: the plating on the bolt goes first. Match the set, with stainless bolt, washer and nut in the same grade. The second is stainless fasteners on aluminium in wet or coastal service: isolate the metals with a nylon or EPDM washer and a sleeve so there is no direct contact.

And one that isn't corrosion at all: the wrong material

General, even rusting across the whole surface in ordinary outdoor air is not how 304 behaves. When we see photos like that, the material is usually not what the label says: 201, a ferritic grade, or plated carbon steel. The magnet will not settle it (see section 1). A PMI reading will, and so will an MTC whose heat number can be traced back to the coil.

3. Quick troubleshooting table

A diagnostic aid, not a failure analysis. Where a corroded fastener could cause injury or structural failure, have the joint examined by a qualified materials or corrosion engineer.

4. What to put in the RFQ so this doesn't happen

Most of these problems are settled before the order is placed, not after the goods arrive. Five lines on the enquiry cover it:

• Grade, written out: 304, 316 or 316L, or A2 / A4. "Stainless" on its own invites the cheapest grade that looks the same.
• Service environment: indoor, inland outdoor, coastal, pool, de-icing salt, wash-down, chemical. This decides 304 versus 316 more than anything else.
• Mating materials: what the washer sits on and what bolt it goes under. Aluminium, galvanised steel or plated bolts change the answer.
• Documents: an EN 10204 3.1 mill test certificate traced to the heat number, plus PMI if your goods-in team needs it.
• Surface and packing: bright or polished, whether passivation to ASTM A967 is required, and packing that keeps stainless away from carbon-steel parts.

The full checklist, including standards and tolerances, is in How to Specify Stainless Steel Washers for an RFQ.

5. How we handle it at HUIHUI, and what we don't claim

We stamp 304, 316 and 316L washers from full-thickness stainless coil on high-speed JH21 presses with automatic coil feeders, at our plant in Taozhuang, Jiashan (in production since 2008, ISO 9001:2015, certificate 74326Q00174R001). Each coil lot is received against its mill certificate and matched to its heat number. We confirm grade in-house with a handheld XRF (PMI) analyser and check hardness on a Rockwell tester, so a 316 order is 316 by chemistry, not by label. EN 10204 3.1 certificates and PMI results are available on request. See Quality & Inspection for the full control plan.

Where our statements stop:

• Stamped 304 and 316 washers can show a slight magnetic response at the edges. We do not sell standard washers as "non-magnetic".
• We do not publish a general salt-spray rating in hours. Neutral salt-spray testing is available on request for a specific order, and the result applies to that lot.
• Our standard finishes are bright and polished. We do not treat passivation or coatings as standard finishes.
• No stainless grade is corrosion-proof in every environment. Grade choice and joint design both count.

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