Passivation is a chemical treatment, usually with nitric or citric acid, that removes free iron and contaminants from the surface of stainless steel so its chromium oxide passive film can re-form evenly. ASTM A967 defines the treatments and the verification tests; ASTM A380 covers cleaning, descaling and passivation of parts, equipment and systems. Pickling is a different step: it removes scale and weld heat tint, and comes before passivation when those defects are present.
Stainless steel is not stainless under all conditions. A new tank that shows rust spots shortly after start-up was almost always contaminated during fabrication or installation. This guide is written for maintenance and project leads in food, pharmaceutical, pulp and paper and chemical plants in Quebec and Ontario who need to specify, have performed or verify a passivation job, and decide when stainless steel should be painted instead.
How does stainless steel protect itself from corrosion?
Stainless steel protects itself with a very thin passive film, rich in chromium oxide, that forms on its own in contact with oxygen. The film isolates the metal from the environment and re-forms when scratched, as long as the surface is clean and the environment supplies enough oxygen.
The film can be weakened in three ways:
- Surface contamination. Iron particles, oils, grinding residue: they stop the film from forming evenly.
- Chromium-depleted zones. Under weld heat tint or scale, the surface has lost chromium and protects itself poorly.
- Aggressive environment. Chlorides, crevices and deposits starve the surface of oxygen locally; see our articles on pitting corrosion and crevice corrosion.
Passivation addresses the first point, pickling the second. The third depends on grade selection and design.
Where does iron contamination come from?
Iron contamination almost always comes from contact with carbon steel during fabrication, handling or nearby work. Embedded iron particles rust, and that rust starts local corrosion of the stainless underneath.
- Shared tools. Discs, brushes and files already used on carbon steel.
- Contaminated abrasives. Recycled abrasive used on steel, or steel shot; see our guide to choosing a blasting abrasive.
- Handling. Unprotected steel forks, slings, chains and supports.
- Nearby work. Carbon steel grinding dust and weld spatter landing on stainless.
- Storage. Stainless stored on the floor or against steel in a mixed shop.
Signs include isolated rust spots, brown streaks or a yellowish haze. A free-iron detection test, described in ASTM A380, confirms contamination.
Pickling or passivation: what is the difference?
Pickling removes a layer of metal and oxides; passivation removes only surface contamination and leaves the metal intact. The two often go together, in that order.
| Treatment | What it removes | When to use it |
|---|---|---|
| Cleaning and degreasing | Oils, grease, dirt | Always first |
| Chemical pickling | Scale, weld heat tint, chromium-depleted layer | After welding or heat treatment |
| Passivation | Free iron and surface contaminants | After fabrication, machining, grinding or pickling |
| Electropolishing | Thin metal layer, micro-roughness | High cleanliness or finish requirement |
| Non-ferrous mechanical cleaning | Contamination and light deposits | Preparation, uniform finish |
On welds, pickling gels or pastes treat the area locally. Passivation alone on a heat-tinted weld is not enough: the depleted layer stays in place.
What do ASTM A967 and ASTM A380 cover?
ASTM A967 defines chemical passivation treatments for stainless steel parts, based on nitric or citric acid, and the tests that verify the result. ASTM A380 is a broader practice covering cleaning, descaling and passivation of parts, equipment and installed systems.
- Treatments (ASTM A967). Several nitric and citric variants; the stainless grade and the specification set the variant, concentration, temperature and time.
- Verification tests (ASTM A967). Water immersion, high humidity, salt spray, copper sulfate or potassium ferricyanide; the specification selects the test by grade and use.
- Field practices (ASTM A380). Cleaning, descaling and passivation methods suited to installed equipment, with rinsing and handling precautions.
Exact parameters come from the standard and the product data sheet: they should never be improvised, since a poorly run treatment can attack some grades.
How is passivation done on installed equipment?
Field passivation follows a fixed sequence: clean, pickle if needed, passivate, rinse, dry, verify. Rinsing is the step most often rushed, yet acid or chloride residue cancels the treatment.
- Cleaning. Alkaline degreasing and deposit removal.
- Local pickling. Welds and tinted areas, with gel or paste.
- Passivation. By immersion, circulation, spraying or gel depending on geometry.
- Rinsing. With water of the quality set by the specification, often demineralized.
- Drying and verification. Selected test, visual inspection, records.
Acids require protective equipment, rinse water management and, in confined spaces, an entry plan. Citric formulations are often preferred on site because they are simpler to handle.
The best moments to passivate are known in advance: on new fabrications before start-up, after a weld repair or grinding, after mechanical cleaning that may have embedded particles, and after any work where carbon steel was handled nearby. Planning the treatment into the shutdown schedule avoids a second entry into the vessel. Recording the method, the product, the rinse water quality and the test result in the equipment file also makes the next inspection easier to compare.
Is glass bead blasting suitable for stainless steel?
Yes, when done with clean non-ferrous media reserved for stainless. Glass bead gives a uniform satin finish and removes light deposits without embedding iron. It does not replace passivation where the specification calls for it, but it prepares the surface well.
- Glass bead. Uniform finish, removal of light tint and deposits; media never used on carbon steel.
- Dry ice. Cleaning without abrasion or secondary residue, useful around sensitive equipment; see our comparison of dry ice, soda or abrasive blasting.
- Avoid. Steel shot, carbon steel brushes, recycled abrasive of unknown origin.
Our glass bead and dry ice blasting service covers these preparations.
When should stainless steel be painted?
Stainless steel is painted when the environment exceeds what its grade can handle alone, or when it must be isolated from another metal. In normal service, clean and passivated stainless needs no paint.
| Situation | Paint it? | Reason |
|---|---|---|
| Insulated austenitic stainless | Often recommended | Chlorides concentrate under insulation; see corrosion under insulation |
| Contact with carbon steel or aluminum | Often | Isolate the couple; see the galvanic series |
| Marine atmosphere or de-icing salts, with crevices | Depends on grade | Risk of pitting and crevice corrosion |
| Identification or marking | As needed | Safety or identification colour |
| Food tank interior in good condition | Generally not | Grade and passivation are enough |
Preparation follows SSPC-SP 16, which targets non-ferrous metals and stainless steel with a non-metallic abrasive to create a profile. The primer must be compatible with stainless; zinc-rich primers are generally not used on it. These systems fall under our protective coatings and industrial painting services.
Frequently asked questions
Does passivation remove weld heat tint?
No. Heat tint and the chromium-depleted layer are removed by chemical or mechanical pickling. Passivation comes after.
Nitric or citric acid?
Both are covered by ASTM A967. The choice depends on the grade, the specification and site conditions; citric is often simpler to handle in the field.
How do you know whether a surface is iron-contaminated?
With a free-iron detection test, such as those described in ASTM A380 and ASTM A967, or with a humidity test that makes rust appear on the particles.
Does passivated stainless need treatment again after a repair?
Yes, in the affected area. Grinding, welding or contact with steel re-contaminates the surface, which must be cleaned, pickled if needed and passivated again.
Do you have stainless equipment that rusts, or a fabrication to restore before start-up? Our glass bead and dry ice blasting service prepares and treats the surface with our crews and qualified subcontractors, in Montreal, across Quebec and in Ontario. Send photos, stainless grade and dimensions through the request a quote form: pricing on consultation, reply within 24 business hours.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.