Article
Stainless steel resists corrosion because of a very thin chromium-rich oxide layer on its surface. Welding, heat treatment, grinding and handling can damage that layer or leave contamination behind. Pickling and passivation restore it.
Pickling
Pickling uses a strong acid mixture, usually nitric and hydrofluoric acid, to remove:
- Heat tint and oxide scale from welding and heat treatment
- The chromium-depleted layer just beneath the scale
- Embedded iron particles and other contamination
It leaves a clean, uniform matt-silver surface. Pickling is done by immersion in tanks, by spraying, or with pickling paste on welds.
Passivation
Passivation uses a milder oxidising acid, typically nitric or citric acid. It does not remove scale. Instead, it dissolves free iron from the surface and encourages a thicker, more uniform passive oxide layer to form.
Comparison
| Point | Pickling | Passivation |
|---|---|---|
| Removes scale and heat tint | Yes | No |
| Removes free iron | Yes | Yes |
| Typical chemicals | Nitric + hydrofluoric acid | Nitric or citric acid |
| Surface change | Removes a thin layer of metal | Negligible |
| Standards | ASTM A380 | ASTM A967, ASTM A380 |
In practice, pickling is usually followed by passivation, and suppliers often quote the combined process as "pickled and passivated".
When it is needed
- After solution annealing of pipes and fittings
- After welding spools and fabrications
- For food, pharmaceutical, chemical and marine service
How to check the result
Common checks include visual inspection for uniform colour, the water immersion or humidity test for rust spots, and the ferroxyl test for free iron. ASTM A967 describes several of these verification tests.
Frequently asked questions
Is passivation the same as pickling?
No. Pickling removes scale and heat tint using strong acids; passivation removes free iron and strengthens the passive layer with milder acids.
Which standard covers passivation?
ASTM A967 covers chemical passivation treatments, and ASTM A380 covers cleaning, descaling and passivation practice.
Why do stainless pipes rust after welding?
Heat tint and embedded iron reduce corrosion resistance; pickling and passivation remove them.
Key takeaways
- Pickling removes scale and heat tint; passivation removes free iron.
- Usually done together after welding or heat treatment.
- Verify with visual, water, humidity or ferroxyl tests.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.