Pickling vs passivation of stainless steel: what is different?

This comparison covers stainless steel parts and fabrications after cutting, machining, forming or welding. It does not cover descaling at the mill or cleaning of equipment that has been in service, which ASTM A380/A380M-25 also leaves outside its scope. Electropolishing, which also removes heat tint, is compared in passivation vs electropolishing for stainless steel.

How do pickling and passivation differ on stainless steel?

Pickling and passivation differ on stainless steel in what they remove: pickling dissolves a thin layer of metal together with heat tint and scale, while passivation dissolves only free iron and other surface contamination and leaves the metal itself in place (Euro Inox, 2007; Euro Inox, 2015). The acids follow from that. Nitric acid is oxidizing and will not dissolve oxide scale from stainless steel when used alone, so pickling solutions combine it with a reducing acid, most often hydrofluoric acid (Nickel Institute, 2020). The Nickel Institute handbook cited in this post was first published by AISI in 1982 and republished in 2020 (Nickel Institute, 2020). Passivation uses nitric acid alone or a milder citric acid treatment (Euro Inox, 2015). ASTM A380/A380M-25 is the practice that gives recommendations for cleaning, descaling and passivating stainless steel parts, assemblies, equipment and installed systems (ASTM International, 2025).

The property table below decides which treatment a stainless surface needs; each value names its condition and its source.

The deciding row is heat tint and scale: pickling removes them with the chromium-depleted layer (Euro Inox, 2007), and passivation does not (Avery, Nickel Institute, 2011).

Which surface conditions call for pickling, passivation or both?

The surface condition of the stainless steel decides between pickling, passivation or both: oxides from heat call for pickling, contamination on clean metal calls for passivation, and a pickle in a non-oxidizing acid calls for both. The decision table below decides the treatment for each common surface condition on a stainless fabrication.

The key row is weld heat tint: it needs pickling, and a nitric-hydrofluoric pickle generally makes a separate nitric acid passivation unnecessary (Nickel Institute, 2020). Oil and grease on any surface sit outside the table, because acid treatments alone cannot be relied on to remove them, so every route starts with degreasing (Euro Inox, 2007). Most stainless products from the producing mill have already been pickled, so descaling is needed only after the material has been exposed to high temperatures in fabrication (Euro Inox, 2015). The grade notes in the next section adjust these rows for duplex, ferritic and martensitic grades.

When does stainless steel need pickling before passivation?

Stainless steel needs pickling before passivation when the surface carries oxide scale or heat tint, because before acid passivation the surface should be descaled and any chromium-depleted heat tint layer removed by pickling (Euro Inox, 2007).

The sequence has four steps. First, degrease, because contamination can make pickling ineffective (Euro Inox, 2015). Second, pickle by immersion, spray or paste (Euro Inox, 2007); read the acid supplier's SDS before use, wear face shields, rubber gloves and rubber protective clothing, and keep the space ventilated (Euro Inox, 2015), and train personnel on what to do in case of an accident (Euro Inox, 2015), including the hydrofluoric acid first aid in the SDS. In the US, nitric acid has an exposure limit of 2 ppm and hydrogen fluoride 3 ppm as 8-hour time-weighted averages (29 CFR 1910.1000, eCFR 2026); elsewhere, national limits apply. Third, rinse until the final rinse water is at pH 6 to 8 for most applications (Euro Inox, 2015), and dispose of used solutions and process water safely (Euro Inox, 2015) under the SDS and local waste rules, in the US 40 CFR 261.22 (eCFR, 2026). Fourth, passivate in nitric acid, under its SDS and 29 CFR 1910.1000 or national limits, if the pickle used hydrochloric or sulfuric acid alone (Nickel Institute, 2020) or the purchase specification calls for it.

Duplex and high-alloyed austenitic grades need a more aggressive mixture, while ferritic and martensitic grades usually need milder solutions or shorter times (Euro Inox, 2015). Hardened martensitic parts can suffer hydrogen embrittlement in acids that generate hydrogen; if acid treatment is unavoidable, Euro Inox recommends heating them at 120 to 150 °C for 24 h immediately afterwards (Euro Inox, 2015). Euro Inox advises entrusting tank immersion, spray pickling and nitric acid passivation to competent specialists (Euro Inox, 2007). Lubinpla can organize a fabrication's SDS, certificates and site history into one cited treatment document for your review.

Can passivation alone remove weld heat tint on stainless steel?

Passivation alone cannot remove weld heat tint on stainless steel; heat tint caused by welding must be removed by grinding, pickling or electropolishing (Avery, Nickel Institute, 2011). Euro Inox gives the reason: nitric acid treatments alone cannot be relied on to remove sufficient metal from the surface (Euro Inox, 2007). Chemical passivation of heat tint areas improves corrosion resistance, but not to the level of clean surfaces (Avery, Nickel Institute, 2011).

The metal underneath is the problem. As heat tint forms, chromium is drawn to the surface oxide, leaving a layer at and just below the surface with a lower chromium level than the bulk steel and therefore reduced corrosion resistance (Euro Inox, 2007). Euro Inox calls it good practice to remove all visible heat tinting, including on hidden faces of welds that will be exposed to the service environment (Euro Inox, 2007).

Mechanical removal has its own limit. Brushing removes heat tint quickly, but it does not guarantee complete removal of the chromium-depleted zone (Euro Inox, 2015), and grinding not followed by pickling should leave a surface completely free of defects, heat tint and scale (Euro Inox, 2015). Euro Inox also lists grinding or abrading followed by nitric acid decontamination as one combination of techniques (Euro Inox, 2007). Electropolishing is the remaining route, since it also removes heat tint and oxide scale (ASTM B912-26, Note 1).

What do pickled and passivated stainless surfaces look like?

Pickled stainless steel surfaces look matt grey to white when pickling is done correctly, while passivated stainless surfaces look unchanged, because the passive layer is transparent (Euro Inox, 2007). A matt grey is left on annealed mill products after descaling and pickling (Euro Inox, 2007). Nitric-hydrofluoric pickling produces clean, white, scale-free surfaces on chromium-nickel grades that were thoroughly pre-cleaned and fully annealed without precipitated chromium carbides (Nickel Institute, 2020).

Pickling faults are visible. Time in the bath beyond what descaling needs causes excessive surface attack, seen as etching (Nickel Institute, 2020), and patchy, uneven pickled surfaces result when the surface is not clean before the acid treatment (Euro Inox, 2007). Surface pitting can also follow excessive contact times with heat tint removal products (Euro Inox, 2007).

Passivation shows no visible sign of success. ASTM A967/A967M-25 requires passivated parts to show a chemically clean surface with no etching, pitting or frosting on visual inspection (ASTM International, 2025), so a frosted or etched look points to a fault. Free-machining and high-carbon grades such as 303, 416 and 410 may show etching in 20 to 40 % nitric acid treatments (Nickel Institute, 2020). Confirming passivation therefore needs a free iron test such as water immersion, high humidity or copper sulfate (ASTM International, 2025). The same tests apply to the free iron left by tools on either surface, and their meaning is covered in what stainless steel passivation does, part of the pretreatment, conversion coatings and passivation guide.