1. The short answer, and where it stops being true
For wine storage, specify 304 unless there is a documented reason not to. Wine cellars and cabinets are mildly corrosive environments by design: they run cold, so surfaces sit below the dew point of the room air and condense; they hold liquid that is acidic; and they are cleaned. Add a coastal site and the chloride load rises again. 201 has less nickel and relies on manganese to stabilise its austenitic structure, which makes it more economical and correspondingly less tolerant of exactly those conditions.
The statement stops being true in one direction: an indoor, climate-controlled, non-coastal interior where the metal is dry most of the time and cleaned with neutral products. There, 201 can give years of service — particularly for racking and trim that never sees condensate. The problem is not that 201 exists. It is that 201 is frequently sold as "stainless steel" without the grade being named, at a price that implies 304.
2. What the alloy numbers actually mean
Both grades are austenitic. The practical difference is the alloying balance:
| Element (nominal) | 304 / SUS304 | 201 | Practical effect |
|---|---|---|---|
| Chromium | ~18% | ~16–18% | Forms the passive layer; the primary corrosion defence |
| Nickel | ~8% | ~3.5–5.5% | Stabilises austenite, improves toughness and chloride tolerance |
| Manganese | ~2% max | ~5.5–7.5% | Substitutes for part of the nickel; cheaper, less corrosion margin |
| Carbon | ≤0.08% | ≤0.15% (grade dependent) | Higher carbon raises sensitisation risk at welds |
A useful shorthand used in corrosion engineering is the pitting resistance equivalent number, which weights chromium, molybdenum and nitrogen. Neither grade contains molybdenum, so both sit at the low end of the austenitic family — but 304's higher chromium and nickel put it meaningfully ahead of 201. That gap is small in a dry room and decisive in a wet one.
3. Corrosion: pitting, crevices and chloride
Stainless steel does not resist corrosion by being inert; it resists by maintaining a thin chromium oxide passive layer that re-forms when oxygen is available. Anything that breaks that layer locally — chloride ions, stagnant water, a crevice where oxygen cannot reach — starts a pit.
In a wine cellar there are four routine sources of chloride:
- Condensate. Cold frames and glass edges run wet for hours; standing water concentrates salts.
- Cleaning chemicals. Many "stainless steel cleaners" and general sanitizers contain hypochlorite or hydrochloric acid. Both attack the passive layer. This is the most common self-inflicted failure.
- Coastal air. Salt aerosol settles on surfaces and is re-wetted by condensation.
- Wine and food spill. Acidic, and in a bar counter environment, frequently left in crevices.
Pitting is not uniform. It appears first at welds, at cut edges, under scratches, and in crevices where a panel meets a frame. This is why a cellar can look perfect for eighteen months and then show a line of rust spots along one welded seam. For a broader treatment of how the enclosure itself drives condensation, see the temperature and humidity specification guide.
4. Welding, heat tint and passivation
Welding destroys the passive layer in the heat-affected zone and leaves a coloured oxide — heat tint — whose composition is less protective than the base metal. On 201, with its higher carbon and lower nickel, the same weld is more likely to sensitise: chromium carbides form at grain boundaries and deplete chromium locally, leaving a narrow band that is markedly less corrosion resistant.
The correct response is mechanical and chemical: remove the tint, then passivate. In practice, a supplier's quality system can be judged by whether they can describe this sequence for the specific parts they make — pickling paste or electrolytic cleaning, then nitric or citric passivation, then a rinse that does not leave chloride behind. "We polish it" is not passivation.
5. Forming: springback, flatness and the bend radius
201 work-hardens faster than 304. For a buyer, that shows up in three places:
- Springback. A bent panel does not hold the angle as accurately, so mitres and door gaps need more correction — or are shipped with visible gaps.
- Bend radius. Tight radii on 201 are more prone to cracking on the outside of the bend, particularly in thicker gauges.
- Flatness. Large laser-cut panels can distort more after folding, which is visible as a wavy reflection on a brushed surface — the exact defect a luxury cellar cannot hide.
None of this means 201 cannot be formed well; it means the tooling and the tolerances have to be set for it. When a supplier quotes a tight door gap and a mirror-flat panel at a 201 price, one of the three is probably not true.
6. Finish: brushed, mirror and PVD colour
Finish interacts with corrosion more than most buyers expect:
- Brushed / hairline. Hides fingerprints and fine scratches, and the directional grain can mask early staining. It does not prevent pitting in a crevice.
- Mirror. Shows every defect, including corrosion initiation. It is the least forgiving finish and the easiest to ruin with an abrasive cleaner.
- PVD colour coatings (titanium gold, black). Hard and decorative, but a coating is only as good as its adhesion. At cut edges and pinholes, corrosion can start underneath and lift the coating — which reads to the client as "the gold is peeling", not as a metallurgy problem.
For a cellar interior, a consistent brushed grain with a documented passivation step is usually the most durable specification. A worked example of the finish question on a real configuration is the dual-zone stainless cabinet, where the interior and exterior finishes are specified separately.
7. The magnet myth
A magnet is not a grade test. Annealed 304 is essentially non-magnetic, but bending, rolling and drawing transform some austenite to martensite, and the deformed areas become slightly magnetic. 201 is generally more magnetic after forming. So a magnet tells you something about how a part was made, and nothing reliable about which alloy it is.
We have seen buyers reject correct 304 because a bend attracted a magnet, and accept 201 because a flat panel did not. If grade matters to the contract — and it should — verify it with documentation or instrumental analysis, not a fridge magnet.
8. Where 201 is genuinely acceptable
Being precise about this is more useful than a blanket prohibition:
- Interior racking in a dry, stable room — no condensate, no coastal exposure, neutral cleaning.
- Decorative trim and concealed brackets that are not in the condensation path.
- Budget-sensitive projects where the grade is disclosed in writing and the client accepts a shorter cosmetic life.
It is not appropriate for exterior panels, cold frames, bar counters in commercial service, coastal projects, or any surface cleaned daily with sanitizer. In those positions the cost difference between the grades is small relative to the cost of replacing a finished panel.
9. Spec language that protects the buyer
Vague words are where grade substitution lives. Replace them with clauses a supplier can be held to:
| Weak wording | Replace with |
|---|---|
| "High-quality stainless steel" | "Austenitic stainless steel grade 304 (UNS S30400 / EN 1.4301), mill certificate EN 10204 3.1 to be supplied" |
| "Rust-proof" | "Suitable for the stated environment; passivated after welding per the supplier's documented procedure" |
| "Mirror finish" | Ra value or equivalent surface roughness, plus a agreed sample approved before production |
| "Gold finish" | PVD coating type and thickness, adhesion test method, and a defined edge-treatment |
| "Salt spray tested" | Test standard, duration, specimen preparation and acceptance criteria — an unqualified hour figure means little |
The single most effective clause is the mill certificate: it ties the delivered metal to a heat number and a chemistry. Everything else is secondary. For the structural side of the same purchase — how racking carries load and what the fixings assume — the architectural racking range and the cellar systems index show how the material choice plays out in finished assemblies.
10. Receiving inspection: what actually verifies grade
- Documentation first. Mill certificate with heat number, chemistry and standard. Check that the heat number appears on the packing list.
- Positive material identification (PMI) on a sample of parts, using X-ray fluorescence or optical emission. This is the practical field method; it reads alloying elements directly.
- Chemical spot-test kits can indicate molybdenum-bearing grades but are poor at distinguishing 201 from 304 and can leave staining. Do not rely on them for acceptance.
- Visual weld inspection for heat tint, undercut and unresolved crevices, before the protective film comes off.
- Record photographs of welds and passivation on concealed assemblies, since these are what a warranty claim will turn on years later.
Need a material clause written into your specification?
Send the drawings and the environment — coastal or inland, humidity, cleaning regime — and our engineering desk will return the grade recommendation and the wording to put in the purchase order. No obligation, and no sales sequence.