ASTM A967 covers both nitric and citric passivation. Passivation removes free iron and restores the passive film — it is not a substitute for pickling mill scale.
Passivation is a common post-fabrication step for stainless steel: it removes free iron and contamination so the chromium passive film can re-form. ASTM A967 is a common reference and covers both nitric and citric acid methods.
Choosing a route:
• Nitric passivation: the traditional method, stable results, suitable for most 300-series grades; higher demands on handling and waste treatment.
• Citric passivation: milder and more environmentally friendly, suitable for general fabricated parts and food/medical contexts; less effective on heavy oxide scale.
Key points:
1) Passivation is not pickling. Weld scale and heat-treatment scale must first be removed by pickling or mechanical means, then passivated.
2) Pre-cleaning matters: degrease, remove rust and embedded iron particles (verify with an iron-detection test).
3) Follow the standard and supplier procedure: concentration, temperature and time strongly affect the result.
4) Acceptance can use the ferroxyl (blue dot) test, or salt-spray/cyclic tests per customer specification.
When purchasing, ask for the passivation procedure and inspection records — avoid accepting "it was only cleaned" as passivation.
