Passivation and pickling of stainless steel
Passivation of stainless steel is a chemical treatment that removes contamination and free iron from the surface and regrows the chromium oxide film.
Without it, a part after welding, cutting or machining corrodes precisely in the places where the tool touched it. Pickling solves a coarser task: it removes scale and the chromium-depleted layer of metal, after which passivation restores the surface's resistance.
Why passivate
Stainless steel does not fail to rust on its own, but because in air it instantly becomes covered with a thin film of chromium oxide. The film self-repairs, but only if the surface is clean and the chromium in it is not bound.
What prevents the film from forming:
- welding scale and heat tint after welding and cutting;
- the chromium-depleted layer under the scale;
- particles of carbon steel left by a brush, rolls or a workbench;
- traces of oil, cutting fluid, paint and marking pencils;
- burrs and scratches where moisture and dirt accumulate.
Free iron on the surface is the main cause of so-called false rust. The steel itself may be 304 or 316, but the spots will be given by precisely the foreign metal that stuck during processing. Passivation dissolves this layer and restores the surface's ability to resist the environment.
The effect of welding stands apart. The weld and the heat-affected zone are heated to temperatures at which chromium goes into carbides, and a depleted layer remains under the scale. Even if the weld looks clean externally, after cleaning it requires passivation.
Pickling and acids
Pickling is the removal of scale and the depleted layer. Passivation is the restoration of the oxide film. Their solutions and regimes are different, and it is not always possible to combine them in one operation.
| Operation | What it removes | Base of the solution | Where it is used |
|---|---|---|---|
| Pickling | Scale, heat tint, the depleted layer | A mixture of nitric and hydrofluoric acids | Weld seams, heat-affected zone |
| Passivation | Free iron, contamination | Nitric acid or citric acid with additives | After pickling, after machining, after installation |
| Electropolishing | Micro-protrusions, contamination, the depleted layer | An electrolyte under current | Food and pharmaceutical equipment |
| Mechanical cleaning | Scale, burrs, scratches | Abrasive, a stainless steel brush | Large seams, accessible areas |
The order of work is usually this: mechanical cleaning, then pickling of the seam, then passivation of the whole surface, then rinsing. If the whole part cannot be pickled — for example, there are gaskets nearby, painted areas or sensitive automation — they limit themselves to a paste, which is applied only to the seam.
The paste is convenient on site: it is applied with a brush, held according to the instructions and washed off. The acids in it are the same, so the protection requirements are the same: rubber gloves, goggles, ventilation, neutralisation of spills. Hydrofluoric acid is dangerous for glass and for the skin, and its vapours must not be inhaled, so the treatment is carried out in a ventilated room or outdoors.
Citric acid is milder than nitric acid and gives no vapours, so it is chosen for food and medical circuits, where residues of a strong acid are unacceptable. It removes free iron and light contamination, but does not remove scale — that requires a separate pickling operation.
Quality control
Passivation can be checked without a laboratory — by several field methods.
| Method | What it shows | Limitation |
|---|---|---|
| Visual inspection | Scale, heat tint, deposit | Does not see free iron |
| Water test | Streaks and rust spots after holding | Time is needed for them to appear |
| Free iron test | Blue spots on the treated area | A consumable solution, needed for each check |
| Measurement of thickness and roughness | Preparation for food and pharmaceutical requirements | Does not replace a chemical test |
| Rinse check | Acid residues on the surface | Requires a pH indicator |
The practical minimum for a welded assembly: cleaning, pickling, passivation, rinsing, visual inspection and a free iron test in the seam zone. The result is recorded in a report, because for food and pharmaceutical facilities not only the treatment itself matters but also confirmation that it has been carried out.
Roughness is checked where the surface is washed and where there must be no stagnant zones. A polished and electropolished surface gives fewer places for product to stick than a ground one with coarse grit, but it too requires passivation after any repair or re-welding of a seam.
When it cannot be avoided
- Food and pharmaceutical equipment. After welding tanks, pipelines and frames, passivation is mandatory: a depleted seam becomes a centre of corrosion and product build-up.
- Swimming pools and wet areas. Handrails, steps, ladders and fasteners in chlorinated water become covered with pitting first of all without passivation of the seams.
- Medicine and laboratories. Instruments, sinks, trays and reactors after welding and machining undergo passivation or electropolishing.
- Chemical apparatus. Pickling of seams made of 316L and 321 is mandatory before start-up: under the scale the chromium has burned out, and the environment will begin to destroy precisely that area.
- Outdoor structures. Facade elements, railings and fasteners in an urban atmosphere and by the sea are treated after installation.
- Warehouses and logistics. After cutting and bending, particles of the tool and cutting fluid remain on the edges; before shipment they are removed by cleaning and passivation.
Small batches and individual items are cheaper to treat with paste on site than to take to a bath. Large batches of pipes, sheets and parts are treated by immersion, because in that way the result is more stable and the holding time is easier to control.
Frequently asked questions
What is passivation of stainless steel
A chemical treatment that removes free iron and contamination from the surface and restores the chromium oxide film. Without it, the surface after welding and machining loses its resistance.
Is passivation needed after welding
Yes. The weld and the heat-affected zone lose chromium to carbides, and a depleted layer remains under the scale. If the seam is not treated, corrosion will follow it.
What is used to pickle stainless steel
A mixture of nitric and hydrofluoric acids, as a solution for immersion or a paste for the seam. For passivation, nitric or citric acid is used separately.
Can citric acid be used for passivation
Yes, for removing free iron and light contamination. Citric acid does not remove scale, so after welding pickling is done first.
How do I check that passivation has worked
By visual inspection, a water test and a free iron test. For critical facilities the result is recorded in a report together with the treatment parameters.
Is passivation harmful to the steel itself
No, if the regime is observed. The danger is over-holding in a strong acid and poor rinsing: residues of the reagent must be removed completely.
Does passivation replace mechanical cleaning
No. Coarse scale, spatter and burrs are first removed mechanically, otherwise the chemistry will not reach the depleted layer.
What should I do if spots appear after treatment
Check whether free iron and traces of acid remain, and repeat the cleaning and passivation. The source of the spots is often not the steel but the tool or the rinsing water.
More on the topic
- Welding stainless steel: with what and how — preparation of the seam and typical defects.
- Processing stainless steel: cutting, bending, welding, polishing — the whole cycle of operations in order.
- Grinding and polishing: how to get a mirror — surfaces for food and decorative tasks.
- Food-grade stainless steel: grades and requirements — surface requirements in the food industry.
- Catalogue of stainless steel products — pipe, sheet, round bar and angle for processing.