Why stainless chairs rust, and the passivation step that stops it
Stainless steel resists corrosion because of a chromium oxide film roughly 1 to 3 nanometres thick — thinner than the polish on the tube. Grinding, welding and carbon-steel tooling all damage or contaminate that film, and passivation is the cheap chemical step that puts it back.
Published by JPChairs · Chair manufacturer in Foshan, China · 2026-10-29
Why a stainless chair can still rust
Four causes account for almost every rust complaint we see on stainless frames, and only one of them is the steel grade:
- Free iron pickup: carbon steel wire brushes, shared grinding wheels and forming dies leave particles that rust on the surface within 24 to 72 hours in humid air.
- Weld heat tint: the straw and blue oxides that form somewhere between 300 and 600 °C are chromium-depleted, so the strip beside a good weld is the first to stain.
- Chlorides: bleach-based cleaners, indoor pool water and sea air. Grade 304 generally pits above about 200 ppm chloride at room temperature; 316 tolerates roughly five times that.
- Grade substitution: 201 carries 5.5 to 7.5 per cent manganese in place of nickel and typically lasts 96 to 200 hours in salt spray against 480 to 1,000 for 304.
Grade choice belongs on the drawing; this article is about what happens to the surface after the tube has been cut, bent and welded. Current frame options are listed with the stainless steel restaurant chairs.
Mill and polished finishes you can specify
- 2B: cold rolled, annealed, pickled and skin passed — smooth, slightly reflective, and the default for tube we bend.
- No.4 / hairline: directional polish with 180 to 320 grit to a roughness of Ra 0.2 to 0.8 µm. The most forgiving finish in service because scuffs blend into the grain.
- BA: bright annealed, near-mirror without polishing.
- No.8: full mirror polish. Every fingerprint and every handling mark shows, so we pack it in foam sleeves even for short domestic runs.
Hairline costs roughly 8 to 15 per cent more than 2B per frame and mirror 25 to 40 per cent, almost all of it hand finishing time. Surface roughness also drives corrosion: above about Ra 0.8 µm the microscopic valleys hold salt and the time to first visible staining drops sharply, which is why we do not recommend a coarse belt finish for coastal venues.
What passivation actually does
Passivation is a chemical treatment, not a coating. An acid bath dissolves free iron from the surface and leaves a chromium-rich oxide film behind. We run it to ASTM A967 after fabrication and before final packing:
- Nitric route: 20 to 25 per cent nitric acid by volume at 49 to 60 °C for 30 minutes, sometimes with sodium dichromate.
- Citric route: 4 to 10 per cent citric acid at 60 to 71 °C for 4 to 60 minutes. Cheaper to dispose of and safer on the line, so it is what we use for indoor frames.
- Metal removal: on the order of 0.002 mm, so threads and fit tolerances are unaffected.
- Sequence: alkaline soak 5 to 10 minutes, rinse, acid bath, rinse, deionised water rinse, forced-air dry.
Budget one extra working day in the schedule and roughly USD 0.60 to 1.40 per frame depending on size and how densely the bath can be loaded.
Heat tint has to be removed, not passivated over
Passivation brightens the steel but it does not dissolve the oxide scale inside and beside a weld. Those oxides are chromium-depleted, so they will stain regardless of what you do afterwards. Three ways to deal with them:
- Mechanical: flap wheel or Scotch-Brite belt to a uniform No.4 grain, then passivate. Fastest, and the only sensible option on 201.
- Pickling: hydrofluoric/nitric paste, 20 to 60 minutes dwell, neutralised and rinsed. Best for 316 and for welds a wheel cannot reach.
- Electropolishing: removes 10 to 30 µm, levels the bead and raises corrosion resistance; adds 2 to 3 days and roughly doubles the surface treatment cost.
Colour roughly tracks temperature: straw around 300 °C, brown at 350 to 400 °C, blue above 500 °C. A welder who runs hot and slow leaves the widest tinted band, which is why we set heat input limits on the stainless jigs rather than leaving it to the operator.
How we verify the result before packing
ASTM A967 gives five verification practices. We run two at production frequency and the rest when the grade or finish changes:
- Free iron: copper sulphate test, 6 minutes, no copper plating.
- Water immersion: 24 hours at 38 °C with no rust spots.
- High humidity: 24 to 48 hours at 38 °C and 97 per cent relative humidity.
- Neutral salt spray: ASTM B117 at 5 per cent NaCl and 35 °C. We hold passivated 304 frames to 480 hours and 201 to 96 hours before red rust.
- Pitting check for 316: ASTM G48 Method A in 6 per cent ferric chloride for 24 hours at 22 °C, mass loss below 1 g/m².
One coupon per bath per shift is enough for process control. Full panels are only pulled when the grade, the finish or the acid concentration changes.
Four lines to put on your purchase order
- Grade and certificate: 304 or 316 with an EN 10204 3.1 mill certificate.
- Finish and roughness: No.4 hairline at Ra 0.2 to 0.8 µm.
- Treatment: passivation to ASTM A967 citric route, heat tint removed before the bath.
- Verification: copper sulphate 6 minutes plus stated salt spray hours by grade.
Four sentences, and the surface becomes a measurable acceptance item rather than an opinion. The treatment line and the salt spray cabinet both sit inside our own plant; capacity, equipment and inspection flow are described on the factory stainless finishing lines.
