October 18, 2026 · Technical Guide

Stainless Steel Pickling and Passivation After Welding: What to Remove and What to Skip

Stainless steel pickling and passivation are two different jobs, and most rust claims come from treating them as one. Pickling dissolves the weld heat tint and the chromium-depleted layer under it. Passivation is what happens next: the cleaned surface is treated so the chromium oxide film rebuilds. Any part that sees water, food, chemicals or salt needs both. A properly pickled weld looks dull grey, not polished. That is normal, and it is the point.

I quote stainless sheet, coil and laser-cut blanks to fabricators every week. The question that comes back most often is whether the weld has to be pickled at all. Short answer: if the part sits in a wet environment, yes. Here is what the standards and shop practice actually say, with the numbers.

Why does a wire brush not bring the corrosion resistance back?

Welding heats stainless past roughly 300C. From that point the surface oxidises into the straw, brown and blue tints along the seam. The colour is only half the problem. Forming that oxide pulls chromium out of the metal just below the surface, so the layer under the weld is leaner in chromium than the parent sheet.

That lean zone is where pitting starts. A wire brush polishes the colour off and leaves the depleted layer in place. Grinding removes more, but it also embeds iron particles if the wheel has touched carbon steel. TWI puts it plainly: chemical pickling preferentially dissolves the depleted zones and leaves sound metal behind, while brushing largely polishes over them.

The UK Drinking Water Inspectorate code of practice takes a practical line. Any tint deeper than pale yellow has to come off, by mechanical dressing followed by acid pickling. I use that as the working rule for any wetted part, not just drinking water. If you can see the colour, remove it.

When do you actually need to pickle?

Not every weld needs acid. Dry indoor structure is a different job from a tank that holds chloride. This is the split I use when a customer asks.

Service	Tint removal	Passivation	Why
Drinking water, food, pharma contact	All tint, both sides if wetted	Yes, per ASTM A967	Chlorides and bacteria settle in the depleted zone
Marine, coastal, pool, de-icing salt	All tint	Yes	Chloride pitting; 316L minimum, 2205 for splash zones
Chemical process, tanks, pipework	All tint	Yes	Acid and chloride duty; rinse with low-chloride water
Outdoor architectural, wet climate	Anything darker than straw	Recommended	Rain keeps the surface wet for hours
Indoor structure, dry, painted	Optional	No	Condensation is the only real driver
Decorative, dry interior	Cosmetic match only	No	Appearance, not corrosion

Two rows cause most of the arguments. The first is drinking water and food contact, where a visible tint is a defect, full stop. The second is indoor dry structure, where a fabricator can reasonably skip the acid bath and spend the money on a better weld schedule instead.

Paste, immersion bath, electrolytic or laser cleaning?

Four methods, four cost profiles. What matters is that the chromium-depleted layer goes, not which tool removes it. The numbers below are the ones I see in supplier data sheets and in HSE guidance.

Method	Typical parameters	Best for	Watch out
Pickling paste or gel	About 20% nitric acid plus 5% hydrofluoric acid by weight; 35-70 min at 20C, duplex 90-180 min	Site welds, vertical and overhead joints, repairs	Hydrofluoric burns are delayed and serious; coverage goes uneven on rough welds
Immersion bath	10-15% nitric acid plus 0.5-3% hydrofluoric acid; roughly 20-40 min at 20-30C	Shops with volume, small and medium parts	Over-pickling on thin sheet; drag-out contaminates the rinse
Electrolytic brush	Seconds per spot instead of tens of minutes of dwell	Field welds where speed matters	Residue left behind; results depend on the operator
Laser cleaning	Pulsed fiber laser, no acid and no rinse water	Tint and free iron where acid is impractical	Cannot repair sensitised grain boundaries; capital cost

Paste is what most workshops use, because it stays where you put it. Buy the lower-hydrofluoric grade if you only clean thin sheet, and keep the stronger one for duplex. On a vertical or overhead joint, a gel beats a thin liquid every time.

One safety note that is not negotiable. Hydrofluoric acid burns are delayed, sometimes by hours, and the damage keeps going after the skin is washed. UK workplace exposure limits sit at 1.8 ppm over eight hours and 3 ppm over 15 minutes. Gloves, face shield, extraction and a calcium gluconate gel on site are the minimum, not the ideal.

What does ASTM A967 passivation actually specify?

Two standards get mixed up. ASTM A380 covers cleaning, descaling and passivation practice, in other words how to get the surface clean. ASTM A967 lists the chemical treatments and the tests that prove they worked. When a drawing says passivate per A967, this is the table it means.

Treatment	Solution	Minimum time	Temperature
Nitric 1	20-25 vol% nitric acid plus 2.5 wt% sodium dichromate dihydrate	20 min	50-55C
Nitric 2	20-45 vol% nitric acid	30 min	20-30C
Nitric 3	20-25 vol% nitric acid	20 min	50-60C
Nitric 4	45-55 vol% nitric acid	30 min	50-55C
Citric 1	4-10 wt% citric acid	4 min	60-70C
Citric 2	4-10 wt% citric acid	10 min	50-60C
Citric 3	4-10 wt% citric acid	20 min	20-50C

Citric is the one buyers get wrong. It passivates well, it is far safer to handle, and Citric 1 finishes in 4 minutes at 60 to 70C. But it does not remove heat tint or the depleted layer. Citric on an unpickled weld is a wasted shift. Pickle first, then passivate.

How do you check the job is done?

Visual inspection proves nothing. A bright weld can still hide a depleted layer, so the acceptance test has to be agreed before the work starts. ASTM A380 lists the ones that count.

Water break test — clean water should sheet evenly across the surface. If it beads into droplets, free iron or oil is still there.
Copper sulfate test — a simple check for free iron. Any copper deposit means contamination, and the part goes back for re-pickling.
Ferroxyl test — the precision version, used where the customer wants documented evidence rather than a visual pass.
High humidity test — warm humid air for a set period. Rust spots show exactly where the passive film did not form.

On our own shipments we check mill finish, thickness and flatness before loading, but the pickled weld belongs to the fabricator. Write the test method into the order and you avoid the argument three months later, when the part is already installed.

Five mistakes that come back as rust claims

Grinding and calling it pickling. The colour goes, the depleted layer stays. Critical pitting temperature does not recover.
Carbon steel brushes, wool or a shared blasting cabinet. Free iron gets pushed into the surface and shows as rust spots within weeks. Keep stainless tooling separate and label it.
Chloride in the rinse water. Rinsing a freshly pickled tank with chlorinated or dirty water starts pitting straight away. Use clean, low-chloride water and rinse until the run-off is neutral.
Over-pickling thin sheet. On 0.5 to 1.0mm material, long contact times with a strong paste thin the section and open pits. Shorten the dwell rather than strengthening the acid.
Passivating over an unremoved tint. The oxide blocks the chemistry, the test fails, and the second attempt costs more than doing it right the first time.

What should go on the purchase order?

Most quality disputes I see are specification disputes. Five lines on the order prevent them.

Base material — ASTM A240 Type 304 or 316L, or EN 10088-2 1.4301 or 1.4404. Name the standard, not just the grade number.
Weld cleaning — pickled per ASTM A380, all heat tint removed, including the weld toe.
Passivation — ASTM A967, and name the treatment, for example Nitric 2 or Citric 2.
Acceptance — copper sulfate or ferroxyl test per ASTM A380, with no tint darker than pale yellow.
Rinse and handling — low-chloride water, no carbon steel contact after cleaning, parts kept off the floor.

If you buy cut blanks rather than welded assemblies, the edge is less of an issue. Nitrogen-cut fiber laser cutting leaves an oxide-free edge that does not need pickling, though the sheet still has to be the grade you ordered. Most of our stainless work goes out as 2B or No.4 sheet, and the stainless steel sheet and coil page lists what we keep in stock.

If the parts are welded, the weld schedule matters as much as the acid. A laser welding machine with good gas coverage leaves a narrow tint band and a shallow depleted zone, which makes the cleaning step shorter and more predictable. Clean welds are cheaper to pickle.

Frequently Asked Questions

Do I still need to pickle a weld that looks clean?

If the part sees water, food, chemicals or salt, yes. A straw or light blue tint is still an oxide layer, and the metal under it is lean in chromium. Laser welding leaves a narrower tint band, which is one reason shops move to it, but the depleted layer is still there. Passivation on its own will not remove either.

Is stainless steel pickling and passivation the same thing?

No. Pickling is a removal step: acid dissolves the heat tint, the scale and the chromium-depleted layer. Passivation is what follows, letting the chromium oxide film rebuild on clean metal. ASTM A380 covers the cleaning practice and ASTM A967 covers the passivation treatments and the tests that accept them.

Can I use citric acid instead of nitric and hydrofluoric acid?

For passivation, yes. A citric bath at 4 to 10 weight percent works and is much safer to handle. For tint removal, no. Citric acid does not take off heat tint or the depleted layer, so a citric-only job on an unpickled weld fails the corrosion test sooner or later.

How long does pickling paste stay on the weld?

At 20C, plan on 15 to 70 minutes for 304 or 316L, and 90 to 180 minutes for duplex. Cold slows the reaction down and above roughly 40C the paste dries out and stops working. Keep the layer thick and re-wet it rather than leaving a thin film to bake.

Does laser cleaning replace pickling?

It removes heat tint and free iron without acid or rinse water, and that is genuinely useful on site. What it cannot do is repair grain boundaries that were sensitised by too much weld heat. If the joint was welded with a bad schedule, clean it and re-test it, or cut it out.

Which stainless grades need post-weld cleaning most?

304 and 316L in wet service, and 2205 duplex in marine or chloride duty. Higher molybdenum helps, but it does not save an unpickled weld. The grade choices and finishes we stock are listed on our stainless steel sheet and coil page.

Sources: British Stainless Steel Association, post-weld cleaning and finishing of stainless steels (nitric 10-15% with 0.5-3% hydrofluoric acid bath; grinding at 360 grit or finer; heat tint forms above about 300C in the heat affected zone); TWI technical FAQ on replacing weld pickling (pickling preferentially dissolves chromium-depleted zones; brushing and blasting can embed contamination); UK Health and Safety Executive, post-weld cleaning with pickling pastes (typical paste about 20% nitric acid and 5% hydrofluoric acid by weight; 15-60 minute dwell; workplace exposure limit 1.8 ppm eight-hour TWA and 3 ppm 15-minute reference); Buehler Welding pickling handbook and TACAB pickling paste data sheet (nitric 15-20%, hydrofluoric 3-5%, sulphuric 7-10%; 35-70 minutes at 20C for stainless, 90-180 minutes for duplex); ASTM A967/A967M-25 chemical passivation treatments (Nitric 1 to 4 and Citric 1 to 3 concentrations, minimum times and temperatures as tabulated); ASTM A380 cleaning, descaling and passivation practice (water break, copper sulfate, ferroxyl and high humidity tests); Journal of Laser Applications, 2021, laser-assisted removal of weld heat tints from stainless steel. Treat the timings as shop starting points and confirm against your own acid supplier data sheet and contract specification.

Written by David Chen, Sales Engineer at FANY LASER. Connect on LinkedIn for stainless steel grades, weld finishing requirements and price data.

Buying fabricated stainless parts from China? Send the drawing, the service environment and the finish you need. We will tell you which grade to specify, how the welds should be cleaned, and what to put in the inspection clause. Talk to our engineers here.