Two Types of Passivation

Nitric acid is the original and most common chemical used for passivating. Unfortunately, it poses significant environmental and safety hazards. That’s why citric acid passivation was developed. Adopted in the 1990s, citric acid does not produce noxious nitrogen oxide vapors and doesn’t require special handling or regulatory oversight because it’s not hazardous.

While the military originally used nitric acid passivation, citric acid passivation was originally adopted by the aerospace industry under the QQ-P-35 standard. In the 2000s, this standard was replaced by AMS 2700 and implemented by the medical device industry.

Today both nitric acid and citric acid are widely used, industry-accepted methods of passivation of stainless steel.

Nitric Passivation

The original passivation chemistry — QQ-P-35 heritage, now run to AMS 2700 and ASTM A967 at both Texas plants.

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Citric Passivation

The safer, environmentally friendly chemistry — AMS 2700 and ASTM A967, run at our Rowlett plant.

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The Passivation Process

Passivation is a relatively simple process that involves three steps:

Passivation Testing

Following are the most common testing methods for passivation:

Copper sulfate testing: Placing the passivated piece into a copper sulfate path for a minimum of 6 minutes.
Salt spray testing: Placing the passivated piece into a salt-spray testing chamber for a minimum of 2 hours.
Water immersion testing: Placing the passivated piece into a tank of distilled water for a minimum of 24 hours.
High-humidity tested: Placing the passivated steel into a humidity chamber for a minimum of 24 hours.

The Benefits of Passivation

In addition, citric acid has the added benefit of being environmentally safe and free from regulatory compliance issues.

If you have a stainless steel part or product that is vulnerable to corrosion and rust, contact the plating experts at Gleco Plating today to learn more about our Passivation options.

FAQ

Frequently Asked Questions

What’s the difference between citric and nitric acid passivation?

Both remove free iron and other surface contaminants so the stainless steel’s chromium-rich passive layer can form — the end result on the part is chemically equivalent, and both are permitted under ASTM A967 and AMS 2700. The difference is the process: nitric acid is the traditional method but produces hazardous nitrogen oxide fumes and requires more stringent handling; citric acid, adopted broadly since the 1990s, is safer, more environmentally benign, and gentler on sensitive grades. Unless the drawing mandates a specific method, either can be certified to the spec.

What specifications cover passivation of stainless steel?

The two active specs are ASTM A967 (with defined nitric and citric treatments plus test requirements) and AMS 2700 (Method 1 = nitric, with Types 1–8; Method 2 = citric). The old federal spec QQ-P-35 was canceled and is superseded by AMS 2700 — drawings still calling QQ-P-35 are normally processed and certified to the AMS 2700 equivalent. Gleco passivates to ASTM A967 and AMS 2700 in both citric and nitric chemistries under its AS9100D quality system.

Does passivation change part dimensions or appearance?

No — passivation is a cleaning and surface-chemistry treatment, not a coating, so it adds no measurable thickness and does not change dimensions, and on properly processed parts it produces little to no visible change. If a part comes back visibly etched, frosted, or “flash attacked,” that indicates a process problem (wrong bath for the grade), not normal passivation.

Can 303 and 400-series stainless steels be passivated?

Yes, but they need the right chemistry — free-machining grades like 303 (with sulfur) and martensitic 400-series grades are prone to flash attack in standard nitric baths, so they are typically run in nitric-with-dichromate or, increasingly, citric baths that are gentler on these alloys. This is exactly the kind of grade-versus-bath matching that should happen at order review. Gleco’s in-house chemists match the treatment (per ASTM A967/AMS 2700 tables) to the specific alloy before parts are processed.

How is passivation verified?

ASTM A967 and AMS 2700 define acceptance tests including water immersion (typically 24 hours), high-humidity exposure (24 hours minimum), salt spray (commonly 2 hours minimum, longer if specified), and copper sulfate testing — the part passes if no free-iron rust or copper deposition appears. The drawing or PO should state which test applies, since the specs allow several.

Why do machined stainless parts rust if stainless is “stainless”?

Because machining, tooling contact, and handling embed free iron and sulfide contaminants in the surface, and those particles rust and can initiate pitting of the stainless itself. Passivation dissolves that free iron and lets the chromium oxide passive layer reform uniformly. It is cheap insurance — the cost of passivating a lot of machined parts is trivial next to a corrosion complaint from the field.

Passivation

Two Types of Passivation

Nitric Passivation

Citric Passivation

The Passivation Process

Passivation Testing

The Benefits of Passivation

Frequently Asked Questions

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