The brackets were 316. The washers holding them on were zinc-plated mild steel.

The situation

A coastal resort operator ordered 300 aluminium-framed terrace armchairs for a poolside deck on an exposed coastline. The specification was written around the environment rather than around a price: *AISI 316 stainless steel to all exposed brackets, bolts, washers and fixings, suitable for ISO 9223 category C5 exposure.*

The chairs arrived with the armrest brackets and seat brackets conforming. What arrived in the fixings bags was a mixture. The brackets were stamped 316 and verified as 316. The washers in 18 of the 24 sampled bags were zinc-plated mild steel — the kind bought by the bag as a general stores item and used across a factory's whole outdoor range.

Nothing about the chairs looked wrong, and that is the reason this kind of substitution reaches the deck. It sits in the smallest component, concealed under an armrest, in a part nobody looks at after the carton is opened.

What the magnet test proves, and what it never will

So the magnet usefully separates austenitic from everything else. It is a screening test and nothing more. **It cannot distinguish 304 from 316, and no version of it ever will** — the gap between those two grades is 2–3% molybdenum, which is not a magnetic property. A report that records *magnet test: pass* as evidence of 316 has recorded a test of the wrong property and should be read back as a statement about the wrong thing.

• **304 and 316 are both austenitic and nominally non-magnetic.** In the annealed condition a magnet picks up neither. This is exactly why the magnet is the wrong instrument for the question being asked — the question is 304 *or* 316, and the magnet is blind to the difference.
• **Cold work makes austenitic stainless magnetic.** Brackets are cut, bent and brushed. The deformation converts part of the austenite into martensite, and the component takes on a weak attraction that indicates nothing about the alloy. A weak positive on a 316 bracket is normal, expected, and not a defect.
• **A strong positive does mean something.** The washers came from the general fixings bag — zinc-plated mild steel, which is ferritic, strongly magnetic, and not stainless of any grade. Plating does not change the substrate’s magnetic response, so a magnet that pulls hard is telling you the material underneath is not stainless, whatever the plating looks like.

Why mixed fixings fail at the coast

The first failure is galvanic, and it is the one that gets found. Zinc is anodic to stainless steel, so a zinc-plated washer clamped against a 316 bracket corrodes first — deliberately, because that is how zinc coating is meant to protect the steel underneath. In a mild atmosphere that trade is harmless. In category C5 salt spray the zinc is consumed within months, and once it is gone the washer has become a thicker, crusted gap between two plates.

A washer’s job is spring tension. It takes up the tolerance in the joint and keeps the bracket clamped as an aluminium frame moves with daily temperature swing and wind. A washer that has lost its zinc and rusted into a thicker disc does not restore the clamp. The joint works loose, the bracket starts to move, and by the time anyone can see it from the terrace floor the fixing has already spent two seasons in a wet chloride atmosphere.

The second failure was the one that could not be seen at all. **A 304 washer mixed into a 316 assembly passes the magnet test.** Same austenitic family, same non-magnetic response, same appearance, and close enough in price that nobody is looking. 304 carries no molybdenum; in chloride-laden spray it pits where 316 does not. That substitution is not detectable at the loading dock with hand tools under any circumstances — it needs XRF on the batch and mill certificates to match against.

What the report recorded

That last line is the one that carries commercial weight. 300 chairs is 300 bags; 24 were sampled and 18 failed. The defensible statement is not *the order fails* and not *the order passes*. It is *a failure rate of 75% within the sampled bags, the remaining 276 units unverified, and a remedy that is cheap and fast* — which is a decision the buyer can make, rather than an argument they have to have.

• **What was specified**, quoted from the purchase order, with the alloy designation, the standard and the environment category.
• **What was verified and by what method** — XRF readings including molybdenum values for the brackets, so a reader can see the alloy rather than take the report’s word for it. The magnet appears separately, labelled as a screening result.
• **What was found, per component and per sample size** — 18 of 24 bags, 24 washers per bag, rather than *some fixings appeared to be zinc plated*.
• **What could not be determined on site** — the 276 bags never opened are recorded as unverified, not as passed.

What the specification should have said

Written this way, the zinc washers are not a finding. They are an impossibility. Most of the cost of an inspection is avoided by the cost of a clause.

• **Itemise every stainless component separately** — bracket, bolt, washer, spring, clip, bushing, anchor. Never write *stainless steel fixings* as a single line. The substitution happened in a part too small to be worth naming, so name it.
• **Give the alloy designation and the standard** — *AISI 316 (1.4401) to ASTM A240 for sheet components and ASTM F593 for fasteners.* A designation can be checked; *stainless* cannot.
• **Tie the requirement to the environment** — *suitable for ISO 9223 category C5 exposure.* This is what makes a later refusal to substitute defensible, because the requirement is anchored to a service condition rather than to a preference.
• **State the evidence up front** — *mill certificates to EN 10204 3.1 for all stainless components, plus PMI by XRF on a minimum of five components per lot per type.* This converts trust into verification: one certificate and five readings.
• **Forbid substitution without written approval, including small parts** — *no substitution of grade on any component, including fixings below M6, without prior written approval.* The words *including small components* are what close this case.
• **Write a rule for the fixings bag** — *bags shall be assembled against the bill of materials for the model and signed off by name.* A bag pulled together by a stores picker is where a consistent specification quietly stops being applied.