The short answer
304 is fine for dry and intermittently wet interiors — hotel rooms, offices, residential bathrooms. 316 is required where chlorides are present: shower interiors, pools, coastal sites, and anywhere harsh disinfectants are used. The cost difference is roughly 20–35% on the bar; the difference in service life in a coastal bathroom is measured in years versus months.
The chemistry in one table
| Element | 304 | 316 | What it does |
|---|---|---|---|
| Chromium (Cr) | 18–20% | 16–18% | Forms the passive oxide layer — the thing that makes it "stainless" |
| Nickel (Ni) | 8–10.5% | 10–14% | Stabilises austenite, improves toughness and formability |
| Molybdenum (Mo) | — | 2–3% | The whole difference. Resists chloride pitting |
| Carbon (C) | ≤0.08% | ≤0.08% | Strength; lower is better for corrosion |
| PREN (pitting resistance) | ~18 | ~25 | Higher = more resistant to pitting |
Our 316 stock is verified at Cr 24–26%, Ni 19–22%, Mo 2–3% — above the ASTM minimum, which is what you want for a bathroom product. Mill certificates ship with the order.
Why 304 rusts and 316 does not
Stainless steel protects itself with a chromium oxide layer a few nanometres thick. That layer reforms instantly when scratched — which is why a scratched 304 sink still looks fine.
Chlorides break the layer. Chloride ions (in salt, and in many cleaning products) attack the oxide film at weak points and start a pit. Once a pit starts, it self-propagates: the inside becomes acidic and oxygen-starved, and the pit grows from the inside out. This is why you see small brown dots rather than an even film of rust.
Molybdenum in 316 slows this process dramatically. It is not that 316 is immune — leave 316 in standing seawater and it will pit too. It is that 316 tolerates the chloride levels found in a bathroom, a pool deck, or a coastal building, and 304 does not.
Matching alloy to environment
| Environment | Alloy | Reasoning | Risk if you use 304 |
|---|---|---|---|
| Residential bathroom, interior | 304 | Intermittent moisture, low chloride | Low |
| Hotel guest room | 304 | Same, cleaned with neutral products | Low |
| Office / commercial washroom | 304 | High traffic but low chloride | Low–moderate |
| Shower interior | 316 | Constant wet, soap and body salts | Tea-staining within 12–24 months |
| Pool deck / spa | 316 | Chlorinated water, splash zone | Visible pitting within months |
| Coastal building (<5 km) | 316 | Airborne chloride | Pitting, often within a year |
| Healthcare, harsh disinfection | 316 | Repeated chloride disinfectants | Rapid staining |
| Industrial / marine direct exposure | 316L or 2205 | Beyond standard 316 | Fail |
"Marine grade" in a specification means 316. If a tender says "marine grade stainless" without naming an alloy, write 316 into the submittal before you price it.
Salt spray: what the numbers mean
ASTM B117 exposes samples to a 5% NaCl fog at 35 °C and records hours until red rust appears. It is a comparative test, not a life prediction — but the ratio between alloys is informative.
| Alloy / finish | Typical B117 hours to first corrosion | Interpretation |
|---|---|---|
| 304, satin | 72–200 h | Adequate indoors, fails coastal |
| 304, polished | 100–250 h | Smooth surface helps; alloy still limits |
| 316, satin | 500–1000 h | Handles bathroom and pool environments |
| 316, polished | 700–1500 h | Best standard option |
| 316 + PVD coating | 1000 h+ | Coating adds barrier; quality varies by supplier |
Two caveats. First, B117 results are heavily influenced by surface finish and passivation — a well-passivated satin bar outperforms a poorly finished polished one. Second, real bathrooms are not salt fog; intermittent wetting and drying is usually less aggressive than continuous fog, so treat these numbers as a ranking, not a countdown.
The magnetism myth
People test stainless with a magnet and conclude "it's fake" if it sticks. This is wrong in both directions:
- Annealed 304 and 316 are non-magnetic. A magnet should not stick to raw tube.
- Cold-worked 304 becomes slightly magnetic. Bending, forming and drawing transform some austenite to martensite, which is magnetic. A formed grab bar bracket in 304 will often attract a magnet. This is normal and not a defect.
- 316 stays less magnetic after cold work because of its higher nickel, but it is not a reliable test either.
- Cheap 201 or 430 will attract a magnet strongly — but so will perfectly good 304 after forming, so a magnet cannot distinguish them reliably.
The only reliable verification is a mill certificate (chemical composition) or a spectrometer reading. Ask for the mill cert; any real factory can produce one in a day.
The real cost difference
| Cost element | 304 | 316 | Difference |
|---|---|---|---|
| Raw material (per kg) | Baseline | +30–45% | Driven by nickel and molybdenum |
| Finished bar (typical 36″) | Baseline | +20–35% | Material share of total cost |
| Machining / polishing | Same | Same | 316 is slightly tougher to polish |
| Replacement cost if it fails | Labor and disruption typically 3–10× the hardware price | — | |
This is the argument to make to a client who wants to save 25% on hardware in a coastal project: the bars will be the cheapest line item in the replacement cost, and the labor to swap them out in an occupied building is not.
How to verify what you bought
- Ask for the mill certificate before the PO. It states the heat number, chemical composition and standard. A factory that cannot produce one within a day is not buying from a mill.
- Check the Cr, Ni and Mo figures against ASTM A240. Mo absent means it is not 316, whatever the label says.
- Match the heat number on the certificate to the packing list. Certificates not tied to a specific heat are decoration.
- Test one sample with a spectrometer if the project value justifies it. A handheld XRF reading takes ten seconds and settles the question.
- Check the passivation — a properly passivated part has a uniform, bright surface. Acid residues or welding discolouration left on the surface will corrode first regardless of alloy.