- Cutting, grinding and welding all leave free iron on the surface.
- Pickling removes it; passivation regrows the chromium oxide film.
- Finish is a specification: No.4 brushed with a stated grain direction.
Stainless steel is not corrosion resistant because of what it is made of. It is corrosion resistant because of a chromium-oxide film a few atoms thick that forms on its surface in air. Everything a workshop does to a sheet (shearing, folding, grinding, welding, dragging it across a bench) damages that film or contaminates the surface with iron that will rust on top of it.
The rust that is not the steel's fault
The most common call we get about a new stainless installation in Sydney or on the NSW coast is orange staining within a few weeks. Almost invariably it is not the stainless corroding. It is free iron sitting on the surface: particles from a grinding disc that had been used on mild steel, from a wire brush, from swarf left on a bench, or from carbon steel packaging straps. The iron rusts, the rust stains the stainless, and the stainless underneath is untouched.
- Separate abrasives, brushes and benches for stainless only.
- No carbon steel clamps or straps in contact with finished surfaces.
- Swarf cleaned off before a part is stacked.
- Protective film left on until the install is complete.
Pickling versus passivation
They are different steps and they are often conflated. Pickling is aggressive: an acid treatment that removes a thin layer of the surface itself, taking heat tint, scale and embedded contamination with it. Passivation is gentler and additive in effect: it dissolves free iron and encourages the chromium-oxide layer to reform fully and evenly. Weld zones generally need both. A clean, unwelded, machined surface usually needs only passivation.
Finish is a specification, not a preference
No.4 brushed is the default for commercial work: a directional satin finish, typically 150 to 180 grit, that hides fingerprints and everyday scratches and can be repaired on site. Beyond the grit there are two things worth stating on a drawing.
| Specify | Why it matters |
|---|---|
| Grit and finish name | So a repair in two years matches |
| Grain direction per surface | Adjacent panels running different ways look like an error |
| Weld dressing standard | Flush and grain-matched, or ground and left |
| Where the grain runs on horizontals | Front-to-back cleans better than left-to-right |
Handing it over
We film-wrap fabricated work before it leaves Moorebank and leave the film on through install, which annoys everyone until the day a trowel is dropped on a bench in Sydney or Brisbane. Once it comes off, the only maintenance a No.4 surface needs is warm water, a mild detergent and a cloth worked with the grain. Chloride-based cleaners, steel wool and abrasive creams will each do more damage in one clean than a year of Australian trading.
Nobody has ever asked us about passivation before a job. Plenty have asked about it afterwards.