Engineering · Comparison
Citric vs nitric passivation.
Both processes do the same job — remove embedded iron and form a passive chromium-oxide layer on stainless steel. The chemistry is different. Citric is biodegradable, gentler on free-machining grades, and increasingly the industry default. Nitric is older, faster on some alloys, and still common where legacy specs require it.
| Spec | Citric (Method 2) | Nitric (Method 1) |
|---|---|---|
| Active chemistry | Citric acid (4–10 wt%) + complexants | Nitric acid 20–45 vol% (Nitric 1–3; 45–55% for Nitric 4); Nitric 1 adds sodium dichromate |
| Process temperature | 21–71 °C (70–160 °F) across ASTM A967 Citric 1–3; Citric 1 runs 60–71 °C (140–160 °F) | 21–60 °C (70–140 °F) across ASTM A967 Nitric 1–4; Nitric 3 runs up to 60 °C (140 °F) |
| Dwell time | 4–30 minutes | 20–30 minutes |
| Standard | ASTM A967, AMS 2700 Method 2 | ASTM A967, AMS 2700 Method 1 |
| Effectiveness on 304/316 | Excellent | Excellent |
| Effectiveness on PH stainless | Best (gentler — no etch on H900 surfaces) | Risk of attack on aged high-strength conditions (e.g. H900) |
| Free-machining grades (303, 416) | Compatible | Use the sodium-dichromate bath (ASTM A967 Nitric 1 / AMS 2700 Method 1 Type 2) or A-A-A; plain nitric risks flash attack |
| Environmental | Biodegradable, non-toxic | Hazardous waste, NOx fumes, regulated |
| Worker safety | Mild — gloves and eye protection | Acid-resistant PPE, ventilation required |
| Cost | Higher chemistry cost; lower disposal cost | Lower chemistry cost; higher disposal cost |
Decision rule
When to use which.
- If the drawing specifies Method 1 or Method 2, use that.
AMS 2700 Method 1 = nitric. Method 2 = citric. The spec is binding.
- If the spec just says “passivate” or “ASTM A967,” default to citric.
Easier on free-machining grades, easier on workers, easier on disposal. Industry has shifted hard toward citric over the last 20 years.
- 17-4PH H900 — citric is safer.
Nitric can attack the chromium-rich aged surface of high-strength PH conditions. Citric is gentler and produces equivalent corrosion protection.
- For 303 / 416 / sulfur-bearing free-machining grades — citric or inhibited nitric.
Plain nitric can attack the sulfide inclusions and cause pitting or flash attack. Citric, nitric with sodium dichromate (ASTM A967 Nitric 1 / AMS 2700 Method 1 Type 2), and the alkaline-acid-alkaline (A-A-A) sequence are all established for these grades. Whichever method is used, the passivation supplier must control flash attack.
- Order matters when paired with electropolish.
Post-machining citric wash, then electropolish, then final passivation is the right sequence.
Keep exploring
Related tools & references
Stainless passivation in context with every other finish, with specs.
Austenitic stainless that passivates cleanly with either method.
PH stainless where citric is the safer choice on aged surfaces.
Machining notes for the most-passivated stainless we run.
How aging condition affects passivation method choice.
Machining stainless that needs passivating?
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