How do you specify outdoor stainless parts so tea staining does not appear?

What causes tea staining on coastal stainless parts?

Ranked by how often they decide the outcome: a grade too lean for the site, a surface that is too rough or polished across the rain path, sheltered or crevice-forming geometry that never gets rain-washed, contamination or missing passivation after the last machining or polishing step, and no washing in service.

1. Grade too lean for the exposure. ASSDA guidance is to use grade 316 or an equivalent as a minimum within five kilometers of surf and to step up to duplex 2205 or super duplex grades for splash zones, unwashed areas or rough surfaces, noting that 304 and 430 will probably become tea stained in marine environments (ASSDA). The International Molybdenum Association likewise reports that 316 works at typical coastal sites but can show light tea staining in severe locations without finish or design changes, and that 2205 is needed there when a pristine appearance is expected (IMOA, 2015).
2. Surface roughness and polishing direction. Per industry guidance from ASSDA (ASSDA), cut or abraded surfaces have a threshold near 0.5 µm Ra, and staining risk falls sharply below it, so a roughness under that value is the recommended target. A No. 4 callout on its own does not pin down roughness, so ASSDA treats it as an incomplete specification unless an Ra limit goes with it. Deep grooves hold salt, and horizontal abrasion lines on a vertical face act as tiny ledges.
3. Sheltered and crevice geometry. IMOA lists sheltered areas that cannot be rain-washed, and light salt-laden mist that dampens without cleaning, among the conditions that make a coastal site severe. Lips, overlaps, blind pockets, gaps under fasteners and upward-facing recesses all collect deposits.
4. Contamination and missing passivation. Embedded iron from carbon steel tooling, abrasive residues and exposed sulfide inclusions on machined bar surfaces weaken the passive film. ASSDA notes that bar product carries much more sulfur than sheet or plate and that an oxidizing passivation after final metal removal substantially improves corrosion resistance (ASSDA, seven ways to prevent tea staining).
5. No washing in service. Salt keeps working as long as it stays on the surface. Chloride salts absorb moisture from the air, so a salted surface stays damp well after the weather has dried, as the same ASSDA article explains.

Warm climates make every factor worse. ASSDA cites the general rule that corrosion rates roughly double for every 10°C rise in temperature, all else being equal (ASSDA), and the coastal-corrosion bulletin adds that higher temperatures can push tea staining 20 km or more inland. That is one reason the same grade and finish can stay bright on a cool coast and stain on a hot, humid one.

Which grade, finish and treatment prevent tea staining at your site?

Match the grade to the exposure first, then hold the exposed surfaces to a smooth finish with the polishing lines running in the direction rain drains, and finish with passivation or electropolishing after the last metal removal. Use the table below as the starting decision, and confirm the site with a corrosivity assessment when the stakes are high.

ASSDA expresses the grade step as a target Pitting Resistance Equivalent number, calculated as %Cr + 3.3 × %Mo + 16 × %N: about 18 away from marine influence, about 24 in marine atmospheres and about 34 in severe marine atmospheres (ASSDA). Those targets line up with 304, 316 and 2205 respectively. ISO 9223 gives the formal language for the site itself, ranking atmospheric corrosivity from C1 to CX, with coastal sites typically falling in the upper categories depending on chloride deposition (ISO 9223:2012).

Finish direction. Specify that brushed or abraded lines run vertically in the installed position, so water and salt drain along the grain instead of sitting in it. ASSDA lists horizontal abrasion lines on vertical surfaces among the features that raise corrosivity (ASSDA).

Electropolish or passivate? Both remove free iron and leave a chromium-rich passive film; electropolishing also levels micro-peaks, and ASSDA reports excellent tea staining resistance from electropolished surfaces even where Ra drops only slightly (ASSDA). For a machined part, the choice and its effect on dimensions are covered in the electropolishing vs passivation guide. Avoid free-machining 303 for coastal exterior parts; its added sulfur that makes it easy to machine also lowers its corrosion resistance, as the 303 vs 304 guide explains.

Cleaning existing stains. Mild stains usually come off with detergent, warm water and a soft cloth, followed by a clean-water rinse; ASSDA warns against abrasive cleaners, chloride cleaners, bleach and hydrochloric acid on stainless steel. Wiping the washed surface dry improves the result. If detergent does not lift it, ASSDA's view is that mechanical polishing alone is unlikely to remove the stain and stop it returning, and that chemical cleaning and passivation is usually the most effective treatment, with pastes and gels available for work on site (ASSDA). Pickling pastes contain nitric and hydrofluoric acid, so that step belongs with a trained contractor and a written safety and rinse procedure, not with maintenance staff. Cleaning removes the symptom; if the grade, finish or design is wrong for the site, the stain will return.

Who does what on a Saola order for outdoor stainless parts?

Machining and mechanical polishing are done at our lead CNC plant, T.K Viet Nam Precision Company Limited. Passivation and electropolishing are not on our published finishing menu today, so name them on the RFQ anyway and the quote states plainly whether that step is included, never dropping it silently, and gives the production site and country of origin for any partner-plant step.

The lead plant has run since 2017 with machining centers, CNC lathes, EDM, welding, and grinding and polishing, so exterior stainless fittings and brackets are cut on the CNC turning and CNC milling lines. Whether a stated roughness and lay direction on an exposed face can be reached by machining and polishing is confirmed on the quote, not assumed. The surface finishing page lists the finishes partner plants apply today. Partner plants are not named, they are not necessarily in Vietnam, and the production site and country of origin are stated on each quote.

The checklist in the previous section is what helps most at the RFQ stage: exposure, grade, roughness and lay, treatment specification, contamination and packing rules, and the records you need. Parts are inspected against those written requirements before they ship. ISO 9001 certification is in progress, not held. Send the drawing through the quote form.