Pickling, passivation and electropolishing stainless

At a glance

Degrease first: acid cannot be relied on to remove oil or grease (Euro Inox). Sources: Euro Inox Vols. 4 and 11, IMOA, HSE, NAAMM. They are trade-body and government guidance, not the standards, so ask the finisher for the method, standard and test in writing.

Why welded stainless goes brown, and why it looks rusty

Stainless steel resists corrosion because of a thin, clear, chromium-rich film on its surface, called the passive layer. Euro Inox says it forms on its own whenever a clean surface meets enough oxygen, and re-forms quickly if the surface is scratched. Two things on a fabricated piece can stop that working.

• Heat tint. Welding thickens the film into colours from pale straw to dark blue. Chromium is drawn up to the surface, which leaves a layer just below it with less chromium and so less corrosion resistance (Euro Inox). Euro Inox calls removing all visible heat tint good practice, and says it should also come off the hidden faces of welds that will meet the service environment. The HSE guidance says the same thing in its own words: the damaged areas have lower corrosion resistance and are typically treated by pickling.
• Iron contamination. Particles of carbon steel or iron on the surface rust. Euro Inox says this is often mistaken for corrosion of the stainless steel itself, anything from a slight brown bloom or rusty scour marks to pitting on handrails, and that it is a common cause of reported problems after installation and at handover. The NAAMM finishes manual makes the same point: the rust gives the wrong impression that the stainless steel is rusting.

The two have different cures. Heat tint needs the tinted layer and the chromium-poor metal under it taken off. Iron needs to be taken off completely, not spread. The drawing shows, in section, what each treatment does to the surface.

What each treatment does, and what it does to the look

• Pickling takes off metal. Euro Inox describes it as removal of a thin layer of metal, usually with mixtures of nitric and hydrofluoric acids, and as the process used for weld heat tint. Tank immersion is normally done off site at the fabricator or a specialist's plant. Euro Inox calls it the best option on health and safety grounds, and the one that treats every surface uniformly. Spray pickling can be done on site but only by specialists with the right safety and acid disposal. Brush-on pastes or gels and electrochemical cleaning suit small areas such as welds, with adequate expertise and supervision. Contact times matter: Euro Inox warns that excessive contact causes pitting, and that all pickling residues must be rinsed off.
• Passivation adds no metal and takes none off. An oxidising acid, nitric or the weaker citric, speeds up the formation of the passive film. Euro Inox says nitric acid alone passivates but does not pickle, that acid passivation should be seen as the exception, because steel from the mills and reputable stockholders is fully passive, and that it may be needed on intricately shaped machined parts where oxygen cannot easily reach every surface. IMOA adds that if the steel's sulphur content is above 0.005, the finished component should be passivated with phosphoric or nitric acid after fabrication, and that this is not necessary at 0.005 or below.
• Iron removal is its own job. Euro Inox says any process that removes all the embedded iron will do, but it must be removed and not just spread. A process with an acid stage is preferable to wire brushing or an abrasive nylon pad alone, and strong nitric and hydrofluoric pickles should be avoided where decontamination is all that is needed, because they can etch the surface. NAAMM names phosphoric acid, nitric acid or a citric acid and sodium nitrate solution, degreased first and rinsed after, and says deeply embedded iron may need grinding and refinishing.
• Electropolishing removes metal evenly. The part is made the anode in a bath of mostly sulphuric and phosphoric acid and metal is removed ion by ion. Iron and nickel come off more readily than chromium, so the surface is left chromium-rich, which speeds passivation (Euro Inox, 2010). The aim is to cut micro-roughness so that dirt sticks less and cleaning is easier. Typically 5 to 10 µm of metal comes off, up to 50 µm for extra smoothing, and more from corners and edges unless they are shielded.
• Mechanical cleaning is an alternative. HSE lists blasting with clean glass beads or walnut shells, a stainless-only brush, grinding with an abrasive disc and buffing, and warns that buffing can smear and later cause corrosion.

Appearance. Each of these changes the surface, so none is a neutral extra. Euro Inox shows a matt grey on annealed mill products after pickling, and warns of patchy, uneven pickled surfaces if the metal is not clean before the acid. NAAMM says refinishing will be needed after a pickling paste has removed deeply embedded iron, and that a visual standard should be considered for weld clean-up. On a brushed or polished face, then, approve a sample after the chemical treatment, not before it. Euro Inox adds that a roughness, reflectivity or gloss figure can be agreed, but should always be confirmed by representative samples made by the finishing contractor. Our finish codes sheet shows how to name the finish itself.

Electropolishing, Euro Inox says, can look the same as a buffed finish: the two can give the same roughness reading and look alike to the eye, yet differ under magnification. Unless the specification says otherwise, a bright lustre is usually taken as acceptable, so say bright or dull, or give a sample. Roughness figures, it adds, have no real bearing on how easily an electropolished surface cleans. Faults in the base metal, such as scratches, porosity or inclusions, show through, and the base can be ground and buffed first. Stabilised grades such as 1.4541 and 1.4571 are not used for electropolishing. For the corrosion score, IMOA gives the same −2 points to a pickled or electropolished surface as to a smooth finish of Ra 0.3 µm or less; see the stainless scoring worksheet.

Where iron comes from, and how to check

Euro Inox says stainless from reputable mills, stockholders and fabricators is normally clean, and that a part properly made from a suitable grade and finish will not rust-stain unless iron has been introduced. It is easily avoided with good handling and fabrication controls, and costly to put right after handover. The table gathers the sources the guidance names.

Checking. Euro Inox says ASTM A380 and A967 outline tests for iron contamination. Some only look for rust stains after a set time in water or a humid atmosphere. To find the free iron that causes later staining, it says the ferroxyl test should be used: a test solution of nitric acid, distilled water and potassium ferricyanide, sensitive to free iron or iron oxide, with the procedure in ASTM A380 (section 7.3.4 in the edition it cites). If iron is found, every trace has to be removed. Ask for the test, where it was taken and the result, in writing.

What to write in the specification

Write the outcome and the test, and let the finisher choose the method. There is a safety reason as well as a practical one. HSE says pickling pastes contain a combination of hydrofluoric and nitric acids, which can cause serious burns, and that where a contract specifies pickling paste, the employer should challenge the specification and consider a less hazardous alternative. HSE lists mechanical cleaning and electrochemical cleaning as alternatives, and says a COSHH assessment is still needed for the milder electrochemical acids. It also says pickling surface cleaners and passivating solutions do not generally contain hydrofluoric acid. The usual order of work is shown below.

• The outcome. Heat tint and weld scale removed, on visible faces and on hidden faces that will be exposed. No embedded iron. A surface matched to an approved sample.
• The method, in writing, before work. The finisher states it, and carries it out under their own COSHH assessment. On-site chemical work only by specialists (Euro Inox).
• The standard and the test. Name the standard with the edition to be confirmed, ask for a free iron test, and for electropolishing a passivation test and a test report where wanted.
• The sample. Approved after the chemical treatment, in the light the work will be seen in. See the sample approval sheet.
• The sequence with other finishes. Weld, grind and polish first, as our weld finish sheet says, and coat or colour last. Whether a coater needs a pickled or electropolished surface is for the coater to say.
• Protection until handover. NAAMM says strippable film is common practice and should come off, with adhesive residue, as soon as possible after installation. It says temporary coatings of wax, oil or lacquer on stainless are unnecessary and can accelerate corrosion as they fail.

Which standards turn up, and what to make of them

None of these is free to read, so the table gives only what their listings and the guidance say. No treatment time, concentration or temperature is quoted. Name the standard and the edition to be confirmed, and let the finisher show how their process meets it.

Remove all weld heat tint and scale on [stainless steel, grade 1.4404] from [visible faces, and from hidden faces of welds exposed to the service environment], leaving a clean surface uniform with sample [ref. __]. The finisher states the method in writing before work starts and works under its own COSHH assessment.

Surfaces to be free of embedded iron and carbon steel contamination at handover. Tools, abrasives, brushes and handling equipment used on stainless steel are for stainless steel only, and work is done away from, or separated from, carbon steel work. Free iron test (the ferroxyl test in [ASTM A380/A380M, edition to be confirmed]) on [__ surfaces]; result recorded.

Where [sulphur on the certificate exceeds [0.005]%, or the part is an intricate machined part], passivate the finished component with [nitric or citric acid] to [ASTM A967/A967M or BS EN 2516, edition to be confirmed by the finisher], rinse with [deionised] water and dry. Method and test stated in writing.

Electropolish [item ref. __] to [ISO 15730, edition to be confirmed]. Alloy [ ]. Appearance [bright / sample ref. __]. Electrical contact marks acceptable at [ ]. Dimensional tolerance after treatment [ ]. Passivation test [ ]; test report [required]. Base surface [ground and buffed to __].

Protect finished surfaces with strippable film until [practical completion]. No wax, oil or lacquer as temporary protection. Remove film and adhesive residue by [ ] on completion. No carbon steel tools on or near finished stainless steel.

Standards and sources

• Euro Inox, Pickling and Passivating Stainless Steel (Vol. 4, 2007) and Electropolishing Stainless Steels (Vol. 11, 2010)
• IMOA, Which Stainless Steel Should Be Specified for Exterior Applications? (brochure): sulphur and passivation.
• HSE, Post weld cleaning using pickling pastes: web guidance; check the current page.
• NAAMM/NOMMA AMP 500-06, Metal Finishes Manual (2006), Chapter 3: US trade guidance.
• ASTM A967/A967M-25, ASTM A380/A380M, BS EN 2516:2023, ISO 15730:2023: listings only; text not read; confirm editions.
• ISO 16048:2003 (fasteners): listing only; fasteners, not general passivation.

Questions we are asked

Why has my welded stainless steel gone brown or straw coloured?

That is heat tint: welding heat thickens the surface film, and the colours run from pale straw to dark blue. Euro Inox says chromium is drawn to the surface, which leaves a layer beneath it with less chromium and less corrosion resistance. It calls removing all visible heat tint good practice, and says it should come off the hidden faces of welds that will meet the service environment too.

Is passivation always needed?

No. Euro Inox says passivation usually happens naturally, that steel from the mills and reputable stockholders is fully passive, and that acid passivation is the exception, for example on intricate machined parts. IMOA says to passivate the finished component with phosphoric or nitric acid if the steel's sulphur content is above 0.005, and that it is not necessary at 0.005 or below. Heat tint and iron need removing first, whichever route is taken.

What is the difference between pickling and passivation?

Pickling removes a thin layer of metal with acid, which takes weld heat tint and the chromium-poor layer with it. Passivation removes no metal: an oxidising acid helps the clear film form quickly on a clean surface. Euro Inox says nitric acid alone will passivate but is not an effective pickle. Both come after degreasing, because acid cannot be relied on to remove oil or grease.

Why are there rust spots on new stainless steel?

Usually iron contamination, not the stainless itself. Euro Inox lists carbon steel tools and handling equipment, and mixed-metal workshops without separation. IMOA says it can happen at the job site as well as in fabrication, and adds steel wool, brushes, airborne steel particles and poor storage. A ferroxyl test finds free iron; the iron then has to be removed completely, not spread.

Is electropolishing the same as polishing?

No. Mechanical polishing leaves microscopic scratches and can embed abrasive and cold-work the surface. Electropolishing removes metal ion by ion and leaves an undisturbed surface, but Euro Inox says the two can look alike to the eye at the same roughness. Ask for it where cleaning and hygiene matter, say bright or dull or give a sample, and expect 5 to 10 µm to come off, more at edges.

Can the work be done on site?

Some of it. Euro Inox says tank pickling is done off site, and is the best option on health and safety grounds, while spray pickling can be done on site only by specialists. Brush-on pastes and electrochemical cleaning suit small areas, with adequate expertise and supervision. HSE asks employers to challenge a specification that insists on pickling paste, so write the outcome and let the finisher choose.

Which standard should I name?

Name the outcome and the test first. For passivation the sources point to ASTM A967 and ASTM A380, and Euro Inox cites EN 2516, whose current edition BSI lists in the aerospace series. For electropolishing, ISO 15730. ISO 16048 is for fasteners. We could not read any of them, so confirm editions with the finisher.