304 and 316 stainless sheet:
the grade question,
answered without hedging

304 stainless steel sheet

Stainless steel sheet 304 is the grade most of the world means by stainless. It is 18 percent chromium and 8 percent nickel, which is where the old trade name 18-8 comes from, and it is austenitic — non-magnetic when annealed, and the reason a magnet is not a test for stainless.

Stainless steel 304 sheet comes in 0.5 to 3.0 mm from stock; above that it is plate. That boundary is not arbitrary: sheet is rolled, plate is rolled and then cut, and the mills quote them on different lines.

The same product travels under several search terms, and it is worth knowing they are all the same thing. SS 304 sheet and 304 stainless sheet are the same material under two trade names. Plat stainless steel 304 is a truncated version of the same query, and the product it means is 304 stainless steel plate, not sheet. If a supplier quotes you "plat" for a 2 mm requirement, one of you is using the word wrongly.

Stainless sheet 304 is ordered by finish as well as thickness; the finish is a separate line. 2B, No.4, hairline and mirror are all 304 at the same gauge and at four different prices, and a purchase order that names only the grade will be filled with whichever finish the mill has on the floor.

One property that bites in fabrication: 304 stainless steel sheet metal is not weldable without carbide precipitation at the seam, unless it is stabilised or the part is annealed afterwards. Chromium carbide forms at the grain boundaries in the heat-affected zone and takes the chromium out of solution — which is exactly where a weld starts to rust, six months later, on an otherwise perfect job. This is the mechanism behind almost every "the stainless is rusting at the weld" claim we have investigated.

316 and 316L

316 stainless steel sheet metal adds molybdenum, which is what resists chlorides. Two percent, usually. That is the entire difference from 304 in performance terms, and it is why 316 is specified near salt water and 304 is not.

316 stainless sheet costs about 35 percent more than 304 and is worth it near salt. Whether you are near salt is a site question, not a preference: a facade 400 m from the coast and a facade 4 km inland are two different specifications, and the buyer is usually the only person who knows which one applies.

316 against 316L, which is the real question

316l stainless steel vs 316 comes down to carbon: 0.03 percent against 0.08. That is the whole difference, and it is invisible on a finished part.

Stainless 316 vs 316l matters only after welding; unwelded, they behave the same. SS 316 vs 316l: if the part is welded and not annealed, specify the L grade. The low carbon is what stops the carbide precipitation described above, so the L grade exists purely to make welded 316 corrosion-resistant without a post-weld anneal.

Is 316l stainless steel good: yes outdoors, and yes after welding, for the same reason. It is the default grade for anything we make that will be welded and will also see weather, and it costs about the same as 316 in sheet form — which is why there is no argument for specifying 316 on welded exterior work.

What it is not: 316L is not a stronger grade, not a brighter grade, and not a grade that resists everything. It resists chlorides better than 304. That is a specific claim, and it is the only one worth making.

Thickness in the units buyers use

Gauge is a source of error that costs real money, because two people can specify the same gauge and mean two different thicknesses.

16 gauge stainless steel sheet metal is 1.59 mm, not 1.6; the difference shows at the joint. On a 3 m run that 0.01 mm is nothing. On a specified 1.6 mm reveal with a 0.5 mm tolerance it is still nothing. But the arithmetic matters when a drawing says "16 ga" and a purchase order says "1.6 mm" and the mill ships 1.59 — the paperwork no longer reconciles, and on an inspection-heavy job that is a non-conformance.

Sheet against plate

304 ss plate starts at 3 mm and is used for base plates, not for facings. SS plate 304 in 6 mm is a base plate; in a facade it would be a mistake — it would weigh three times what the fixing was designed for and would not fold.

The boundary is not cosmetic. Below 3 mm the material is supplied coiled or in flat sheet and can be folded on a press brake. Above it, it is plate, it is cut rather than sheared to shape, and it is welded rather than folded. A design that assumes a fold at 4 mm has assumed a process that does not exist.

Does 304 rust

Does 304 ss rust: yes, wherever chlorides sit and oxygen cannot reach. Stainless is not rust-proof; it is passive. A chromium oxide layer forms on the surface in the presence of oxygen and that layer is what stops corrosion — take the oxygen away, or put chlorides on it, and the layer cannot reform.

A 304 stainless sheet will show tea staining near a coast within two years. Tea staining is a brown surface discolouration, it is cosmetic rather than structural, and it is the single most common complaint on coastal stainless work. It is also entirely predictable, which is why we ask where the site is before we quote.

Where it actually starts

• In an aperture or a gap. A perforated panel or a shadow gap holds a film of moisture and excludes air. That is the geometry that fails first.
• At a weld that was not dressed. The heat-affected zone has lost its chromium to carbide; the roughened surface holds dirt.
• Under a deposit. Swarf from cutting, plaster dust, cement splash. Anything that sits on the surface and stays wet will start a pit underneath itself.

Will 316 stainless steel rust: in a warm chloride bath, yes, and no grade stops it. A pool hall ceiling, a coastal facade in a salt fog, a plant room with chlorine dosing — 316 buys you years, not immunity. Anyone who tells you a grade cannot rust is either selling or repeating something they were told.

What a mill certificate tells you, and what it does not

A mill certificate is a chemistry and mechanical test record for the heat the sheet came from. It tells you the actual percentages of chromium, nickel and molybdenum, the yield and tensile results, and the heat number that ties the sheet to the test.

What it does not tell you is anything about the surface. Surface condition, flatness, finish, and whether the coil was handled properly after it left the mill are all outside the certificate. A perfect certificate and a scratched sheet are entirely compatible.

The grades that are not building grades

316lvm stainless steel is a medical grade, vacuum melted; we do not use it on buildings. It appears in searches because the letters look like a variant of 316L. The V is vacuum melting and the M is a micro-cleanliness specification for implants — irrelevant to a wall, and priced accordingly.

Stainless steel grades renewable energy projects use are usually 316 for the wet side of a plant. That is not a different grade; it is the same 316 specified for a reason — condensate, cleaning chemicals, and the impossibility of inspecting a surface nobody can reach.

What we check, and how

Every batch we receive is read on the sheet with a handheld spectrometer, not taken from the delivery note. It takes a minute per sheet and it is the only way to know that the 316 you ordered is the 316 that arrived. The reading goes on the job file with the certificate.

Ask any supplier for the spectrometer reading as well as the certificate. A supplier who cannot produce both is passing on paperwork, not verifying metal.