Brown or tan discolouration on stainless steel balustrade, handrails, pool fencing, letterboxes and door furniture is called tea staining, and it is a standard feature of coastal Sydney.
It is surface corrosion. Which means cleaning it is only half the answer.
Why stainless steel stains at all
Stainless is not rust-proof. It resists corrosion because chromium in the alloy forms an invisible passive oxide layer on the surface. Scratch it and it re-forms, as long as there is oxygen available.
Salt breaks that. Chloride ions attack the passive layer locally, and where they sit undisturbed the layer cannot reform. The result is shallow surface corrosion — the brown bloom.
It is cosmetic at first. Left long enough it progresses to pitting, which is structural, and on a balustrade fixing that matters.
304 versus 316
The single most important thing to know.
Grade 304 is the common, cheaper stainless. Fine inland. Near the coast it tea stains, often within a year.
Grade 316 contains molybdenum, which substantially improves chloride resistance. It is the marine grade and it is what should be specified anywhere near surf.
A great deal of Sydney coastal balustrade and pool fencing was installed in 304 to save money, and tea staining is the consequence. No cleaning regime fully compensates for the wrong grade.
Even 316 tea stains in a severe marine environment if it is never washed.
Where it appears
Same rule as all salt corrosion: worst where salt lands and rain does not wash it off.
The underside of a handrail. The back of a post. Inside a fixing recess. Under a glass clamp. The sheltered elevation. Those areas go first while the exposed faces stay clean, which is counterintuitive and catches people out.
Same pattern as powder-coated aluminium and coastal exposure generally.
Removing it
Light staining: warm water with a mild detergent and a soft cloth, then rinse and dry. Often enough if caught early.
Established staining: a proprietary stainless steel cleaner, applied with a soft cloth, working with the grain of the metal. Stainless has a directional finish and cleaning across it shows.
Heavier staining: an oxalic-acid-based stainless cleaner or a passivating product that removes the corrosion and helps restore the oxide layer.
Severe: mechanical polishing with a fine non-metallic pad, then re-passivation. Specialist work.
What makes it permanent
Never use chlorine bleach on stainless. This is the critical one. Hypochlorite is a chloride source, and chloride is what causes the corrosion in the first place. Bleaching tea stain accelerates it and causes pitting, which does not come out.
This matters because hypochlorite is exactly what gets used on the render and the fencing around it. Stainless balustrade near a wall being soft washed must be protected and rinsed.
Also avoid: steel wool and wire brushes, which embed carbon steel particles that then rust and make it worse; abrasive scouring pads, which scratch through the passive layer; and hydrochloric acid of any kind.
Preventing it
Washing frequency does more than anything else.
Fresh water rinse, including the undersides and the sheltered faces, every few weeks on a front-line property and monthly elsewhere. It costs nothing and it removes chloride before it can do damage.
Dry after washing where you can — standing droplets concentrate salt as they evaporate.
Products that leave a protective film are available and genuinely extend the interval, though they need reapplying.
And if you are specifying new work near the coast, pay for 316. The cost difference is small against the cost of replacing stained 304 balustrade.
Where it matters most
Pool fencing, because it is safety equipment and because it is wet and salty constantly — see glass pool fencing and balustrades.
Balcony balustrade on coastal apartment buildings, where the fixings are structural and corrosion is a committee-level maintenance issue — here.
And security screens, which are usually 304 mesh and show it first — here.
