Summary. Passivation removes free iron and contamination from stainless steel and lets the chromium-rich surface re-form its protective oxide film. Nitric acid is the long-established passivating acid; citric acid is a milder non-oxidising alternative for many grades. The process details below are general knowledge, not SPC specifications. For supply, SPC lists a 60-70% commercial grade, with a batch Certificate of Analysis supplied with every delivery and the TDS and SDS available on request.
What does passivation actually do to stainless steel?
Stainless steel resists corrosion because chromium in the alloy forms a thin, invisible, self-healing chromium-oxide film on the surface. Fabrication damages that picture. Cutting, grinding, forming and welding leave embedded iron particles from tooling, scale and heat tint at welds, and any of these can become a rust spot. Passivation is the chemical clean-up that follows: an acid treatment that dissolves free iron and contaminants without attacking the stainless steel itself, so that the surface can re-form a continuous passive film in air.
It is not a coating and it does not add metal. Three words are often mixed up: pickling removes heavier oxide scale and weld discolouration and does remove a measurable layer of metal; passivation is the lighter treatment that cleans free iron and supports film formation; and electropolishing is an electrochemical process, outside the scope of this guide. For the wider picture of pickling and other uses, see our nitric acid uses and handling guide.
Why is nitric acid the classic passivating acid?
General knowledge: nitric acid is a strong oxidising acid. In a nitric bath, free iron on the surface dissolves, while the oxidising conditions favour the chromium-oxide film on the underlying steel. It is the long-established passivating acid, and many older drawings, procedures and customer specifications name it. Some grades and specifications still require it, while citric acid is accepted for many but not all grades, so check the governing specification for your steel grade.
Commercial nitric acid is normally diluted before use. SPC lists one grade, nitric acid commercial grade at 60-70% HNO3, a colourless to pale yellow liquid with a specific gravity of 1.37-1.42 at 27 °C. It is a technical grade, not a reagent grade.
How is a nitric acid passivation bath run?
The process notes in this section and the next are general knowledge, not SPC specifications. The governing specification, whether it is a customer standard or a published practice, sets the bath strength, temperature and immersion time for each steel grade. As general guidance only, the sequence looks like this.
- Degrease first. Oil, coolant and fingerprints block the acid and cause patchy results.
- Remove heavy scale or weld tint separately. Pickling, mechanical cleaning or a dedicated pickling step comes before passivation.
- Immerse in the diluted nitric acid bath. Strength, temperature and time are set by the specification for the steel grade; do not improvise them.
- Rinse thoroughly in clean water, ideally low in chloride, and dry the parts. Residual acid or chloride left on the surface defeats the treatment.
- Test the result as the specification requires. The test method is named by the specification, not chosen by the supplier of the acid.
Two practical points. Dilute only in the order the safety data sheet states, acid into water, never water into acid. And keep chlorides out of the bath: the commercial grade datasheet limits chloride to 0.005% mass max in the acid itself, but chloride from tap water or earlier baths can matter more in practice.
How does citric acid passivation compare?
General knowledge: citric acid is an organic acid that works mainly by chelating, meaning it binds iron and lifts it off the surface, rather than by oxidising. In many shops it has become a preferred passivating chemistry for common austenitic grades. The comparison below is general chemistry and practice, not a performance claim about any product.
| Factor | Nitric acid | Citric acid |
|---|---|---|
| Chemistry | Strong oxidising mineral acid | Weak organic acid, chelating action |
| Fumes | Can release nitrogen oxide fumes, especially warm or contaminated | No nitrogen oxide fumes |
| Operator hazard | High: corrosive oxidiser, toxic fumes, strict controls | Considerably milder |
| Waste | Acidic, nitrate-bearing; needs neutralisation and treatment per local rules | Biodegradable organic, still needs pH control and local compliance |
| Specification coverage | Named in many specifications; confirm for your grade | Accepted for many but not all grades; confirm in the specification |
| Bath control | Concentration and temperature are the main controls | Concentration, temperature and time; sensitive to contamination |
If you are weighing the two, the deciding questions are the steel grade, the specification you must meet, the fume and waste controls you already have, and the operator risk your site will accept. Where a customer specification names nitric acid, the specification wins. For citric acid supply, see citric acid anhydrous.
What are the safety points for handling nitric acid?
The safety data sheet for SPC nitric acid carries the signal word Danger, with oxidiser, corrosive, acute-toxicity and irritant pictograms.
- Handle in a well-ventilated area or under local exhaust ventilation.
- Use acid-resistant equipment; the data sheet names glass, PTFE and 316L stainless steel.
- Keep nitric acid away from combustibles, organic substances, reducing agents, bases, metal powders, concentrated hydrogen peroxide and chlorine-containing compounds.
- Store at ambient temperature, below 30 °C recommended, in acid-resistant containers (our datasheet adds vented) away from the materials above.
Exposure limits in the data sheet are OSHA and ACGIH values, not Indian limits; confirm the current Indian limit with the competent authority. A fuller storage and incompatibility table sits on the nitric acid product page.
What about disposal?
General knowledge, not a compliance instruction: spent passivation baths are acidic and carry dissolved metals from the steel plus nitrate, so they should not go straight to drain. Local rules differ, so take advice from your environmental consultant on neutralisation and effluent handling.
What should you specify when buying nitric acid for passivation?
Buy against a written specification, not a nickname. State the following on the enquiry:
- Strength: our listed grade is 60-70% HNO3. If you need a different strength or a reagent grade, say so; those are not in our listed range.
- Quantity and pack: carboys or road tanker.
- Documents: ask for the technical datasheet (SPC-TDS-ACD-016) and safety data sheet in advance; a batch Certificate of Analysis is supplied with every delivery.
- Impurity limits that matter to you. The datasheet lists chloride 0.005% max, sulphate 0.01% max, iron 10 ppm max, residue on ignition 0.02% max and nitrous acid 0.05% max.
Supreme Petro Chemicals is an importer and bulk distributor in Chennai, not a manufacturer. We supply chemicals for metal-finishing and fabrication users from our Redhills warehouse. Send your requirement to +91 86087 80096 by WhatsApp or call, and we quote within 4 working hours. The full specification table is on the nitric acid product page; for the wider range of industrial acids, see our guide to sulphuric, nitric and hydrochloric acid.
FAQ
Is passivation the same as pickling?
No. Pickling is the more aggressive step that removes heavy scale and weld discolouration and takes off a layer of metal. Passivation is the lighter clean-up that removes free iron and lets the passive oxide film re-form. Many procedures do pickling first and passivation after.
Does passivation need 60-70% acid?
Not as a bath. General knowledge: passivation baths are diluted from the commercial acid, and the right strength depends on the steel grade and the specification. Always add acid to water.
Can I use citric acid instead?
For many stainless grades, yes, and it is milder to handle. Some grades and some customer specifications still require nitric acid, so check the specification before you change chemistry.
Sources: SPC data (src/data/productSpecs.ts, scripts/tds-data/nitric-acid.json, scripts/sds-batch/data/nitric-acid.sds.json) for grade, specification, storage and safety statements; all process, bath and comparison statements are general knowledge, not SPC specifications.