What Is Passivation? How It Works and How to Passivate Stainless Steel

Key Takeaways

• Passivation is a post-fabrication process that increases the corrosion resistance inherent in a stainless steel grade.^[1] Its purpose is to remove free iron and other less noble contaminants from the surface.^[2]

• The passive film that protects the metal is only nanometers thick.^[3][4]

• Acid passivation does not remove any metal. Pickling, usually with nitric and hydrofluoric acid, removes a thin layer of metal, and electropolishing removes metal electrolytically.^[5][6]

• In NASA tests, citric acid passivated panels showed equal or less corrosion than nitric acid passivated panels after six months.^[7]

• The core specifications are ASTM A967/A967M-25, ASTM A380/A380M-25, and SAE AMS2700G, all revised in 2025.^[8][9][2]

Introduction

Passivation changes the chemistry of a metal surface without changing how it looks.^[10] In this guide you will learn what passivation is and where the name comes from, why stainless steel often needs it even though it passivates itself, how nitric acid and citric acid passivation compare, which standards apply, and how to test the result.

What Is Passivation?

In corrosion science, a metal is passive when a thin protective oxide layer separates it from its environment. For iron, it is now generally accepted that passivity comes from an oxide film 1 to 4 nm thick that isolates the metal surface from a corrosive aqueous environment.^[4]

In manufacturing, passivation typically refers to a chemical treatment that increases the corrosion resistance inherent in the particular stainless steel grade a part was made from.^[1] SAE AMS2700G defines the purpose of passivation as removal of free iron or other less noble contaminants from the surfaces of corrosion-resistant steel parts.^[2]

ASTM A967/A967M names three particular passivation treatments:^[8]

1. Immersion in nitric acid solutions

2. Immersion in citric acid solution

3. Electrochemical treatment

The passive film is not a coating. It forms from the alloy itself, and on stainless steel it reforms after damage as long as oxygen is available.^[5]

Why Is It Called Passivation?

The name comes from early experiments with iron and nitric acid. Christian Friedrich Schönbein found that iron dissolves in dilute nitric acid and produces hydrogen, but that iron first placed in concentrated nitric acid shows little or no reaction when returned to the dilute acid. He named the first state the active condition and the second the passive condition.^[11]

The theory of a protective oxide skin traces back to Schönbein and Michael Faraday, whose papers on the subject appeared in 1836.^[4][11]

How Passivation Works on Stainless Steel

The corrosion resistance of stainless steel comes from a passive, chromium-rich oxide film that forms naturally on its surface.^[5] A stainless steel has a chromium content of at least 10.5% by weight and no more than 1.2% carbon.^[5] Adding nickel, molybdenum, nitrogen, and titanium or niobium extends its corrosion resistance over a wider range of service conditions.^[5]

The passive oxide layer is extremely thin. In one study, surface analysis of the air-formed oxide on 316L stainless steel measured a total thickness of 2.2 nm, with a 0.7 nm outer layer and a 1.5 nm inner layer.^[3] Other sources say approximately 2.5 nm or 3 to 5 nm.^[1][10]

Stainless steel passivates itself whenever a clean surface is exposed to enough oxygen, and the transparent passive layer quickly reforms if damaged, allowing stainless steel to keep its corrosion resistance even after scratching or machining.^[5]

In small areas deprived of oxygen, such as mechanical joints, tight corners, and incomplete or poorly finished welds, the passive state can break down and the surface becomes active. This results in localized crevice corrosion or pitting.^[5]

Why Chemical Passivation Is Needed

If stainless steel passivates itself, why treat it? There are three reasons:

1. Free iron and surface contamination: Carbon steel tooling and handling equipment used without adequate cleaning, and fabrication in mixed-metal shops without segregation, leave iron on the surface that then rusts.^[5]

2. Speed and control: Natural passivation can take a couple of days to reach full film thickness, while chemical passivation builds the passive layer quickly under controlled conditions.^[6]

3. Surface chemistry: Chemical passivation significantly increases the chromium-to-iron (Cr/Fe) ratio in the near-surface region. Simple passivation by atmospheric exposure does not form the surface compounds needed for that enhanced protection.^[10]

Does Stainless Steel Need to Be Passivated?

Stainless steel does not always require passivation. For example, steel delivered from mills and reputable stockholders is already fully passive.[5] 

Passivation is useful when fabrication has disturbed or contaminated the surface, or in other specific situations. Examples include:

• Machined parts, especially intricate ones where oxygen reaches freshly cut surfaces slowly^[5]

• Parts exposed to contaminants like free iron from grinding dust, rust particles from nearby carbon steel work, or abrasion from tools^[6]

• Welded areas, where metal can carry high manganese and Cr/Fe ratios as low as 0.11, and passivation is used to remove the manganese and raise the ratio above one^[10] 

• Parts built to a specification that requires passivation, such as SAE AMS2700 for aerospace corrosion-resistant steel parts or, in medical devices, ASTM F86 for metallic surgical implants^[2][12]

Which Stainless Steel Grades Need Passivation?

Passivation treatments are chosen grade by grade. ASTM A967 gives non-mandatory guidelines on selecting treatments for particular grades of stainless steel in its appendices but makes no recommendation on the suitability of any grade, treatment, or acceptance criteria for a particular application.^[8]

According to metals company Carpenter Technology, the more resistant grades can be passivated in 20% nitric acid without sodium dichromate. For less resistant materials, sodium dichromate or a higher nitric acid concentration is used to reduce the chance of flash attack, in which the part turns gray or black and the surface deteriorates.^[1] A special process with a sodium hydroxide rinse may be used for free-machining grades to neutralize trapped acid.^[1]

A joint DoD and NASA study shows how much the conditions vary by alloy:^[7]

The Passivation Process, Step by Step

Whether you passivate stainless steel parts in-house or through a finishing shop, several steps are required.

1. Degrease and clean: Acid treatments alone cannot be relied upon to remove oil, grease, or inorganic contaminants.^[5] And grease left on a part reacts with the acid to form gas bubbles.^[1] In NASA’s procedure, panels received an acetone wipe, a second degreasing with a cleaner in deionized water, rinses, and a caustic cleaning.^[7]

2. Descale and pickle if needed: Before acid passivation, the surface should be free of oxide scale, and chromium-depleted layers left by heat tint should be removed by pickling.^[5]

3. Immerse in the passivation solution: Use nitric acid or citric acid at the concentration, temperature, and time your specification calls for.^[6] Common minimum immersion times are 20 minutes or 30 minutes, though some methods require as little as two minutes.^[2]

4. Rinse: Rinsing removes residual acids and stops ongoing chemical reactions, but the water must be clean.^[2] In its tests, NASA checked that the water on each rinse stayed between pH 6.0 and 8.0.^[7]

5. Verify: Test with the method required by the specification. ASTM A967 includes several alternative tests for confirming effectiveness.^[8]

Testing Passivated Parts

ASTM A967 describes qualitative tests intended to confirm the effectiveness of passivation, particularly the removal of contaminant iron and other exogenous matter.^[8] It lists seven practices:^[8]

1. Water immersion test

2. High humidity test

3. Salt spray test

4. Copper sulfate test

5. Potassium ferricyanide-nitric acid test

6. Damp cloth test

7. Boiling water immersion test

Nitric Acid vs Citric Acid Passivation

ASTM and other standards offer manufacturers the choice of nitric acid or citric acid for passivation.^[2][8]

Nitric Acid Passivation

Nitric acid is the long-established route and is widely used in commercial stainless steel passivation treatments.^[5] The longtime US military specification for passivating stainless steel, Federal Specification QQ-P-35C, specified nitric acid processing.^[7]

The drawbacks of nitric acid passivation are environmental and practical. The process produces fumes containing nitrogen oxide (NOx) emissions, which are dangerous to workers and require general area or local ventilation of the passivation tank. Its use may require permits under federal and state regulations.^[7] In the United States, wastewater from the process is regulated under the EPA Metal Finishing categorical standards (40 CFR 433).^[7]

Citric Acid Passivation

Citric acid passivation was developed for the beverage industry in Germany, which needed containers free of iron because iron gives beverages an unwanted taste.^[7] Though a weaker oxidizing acid, it can also help form the passive layer.^[5]

Advantages include no toxic fumes during processing, lower required concentrations due to its efficiency, and baths that retain their potency longer and need less frequent refilling.^[7]

A 2020 study passivated 15-5PH and 17-4PH precipitation-hardening stainless steels in 15% citric acid and 20% nitric acid, finding that both treatments gave very low corrosion rates but citric acid helped to reduce unwanted pitting corrosion.^[13] The authors concluded that citric passivation could be a green alternative to nitric passivation on those steels.^[13]

Passivation vs Pickling, Electropolishing, and Anodizing

Descaling, pickling, and passivation are often confused, but they are distinct processes.[5] The table adds electropolishing and anodizing, and shows what each process removes and what it leaves behind:

The key difference from pickling is metal removal. Pickling removes a thin layer of metal, usually with nitric and hydrofluoric acids, and it is the process for removing weld heat tint; acid passivation removes no metal.^[5] Nitric acid alone is not an effective pickling acid, and nitric-hydrofluoric preparations should be avoided where a surface only needs decontamination, because they can etch it.^[5]

Passivation also differs from most corrosion protection, which adds a layer such as paint, plating, or a phosphate conversion coating. The passive film is a few nanometers thick and forms naturally from the alloy itself.^[3][5] Because no metal is removed either, the treatment leaves dimensions unchanged.^[5]

Passivation vs Electropolishing

ASTM A967 includes electrochemical treatment alongside nitric and citric acid immersion.^[8] Electropolishing is a related but separate process: it removes metal ion by ion from a part connected as the anode, using direct current and an electrolyte of mostly sulfuric and orthophosphoric acid, usually for two to 20 minutes.^[6]

Because iron and nickel are removed more easily than chromium, electropolishing leaves a chromium-rich surface, which accelerates and improves passivation.^[6] It raises the near-surface Cr/Fe ratio, and chemical passivation treatments raise it further.^[10] Electropolishing is common in the pharmaceutical, biochemical, and food processing industries, and it can treat small, mechanically fragile parts.^[6]

Which Metals Can Be Passivated Besides Stainless Steel?

The core passivation specifications (ASTM A967, ASTM A380, and SAE AMS2700) are written for stainless or corrosion-resistant steel parts.^[8][9][2] Other metals passivate too, some naturally and some through dedicated surface treatments:

• Titanium and implant alloys: In medical devices, ASTM F86-21 covers surface preparation of metallic surgical implants made from iron, cobalt, nickel, titanium, and tantalum base materials, including treatments that promote an inert or passive surface such as a metal oxide film.^[12]

• Aluminum: Aluminum can be anodized. Anodizing converts the surface into a durable, corrosion-resistant anodic oxide finish.^[14]

• Silicon in semiconductors and solar cells: Here, passivation means reducing the number of dangling silicon bonds at the surface with a passivating layer. The electronics industry mostly relies on thermally grown silicon dioxide, and commercial solar cells commonly use dielectric layers such as silicon nitride.^[15]

Recommended reading: The Anodizing Process Explained

Passivation Standards and Specifications

The three core documents cited throughout this article were all revised in 2025.

Recommended reading: Surface Finish: Parameters, Standards, and How to Specify It

Conclusion

Passivation is less a coating than a reset. It removes free iron and other contaminants from stainless steel so that the alloy’s chromium can form the nanometer-thin passive film that gives the material its corrosion resistance.^[2][3] The treatment does not remove any metal, which is what separates it from pickling and electropolishing.^[5][6]

For most machined or fabricated parts, the practical decisions are chemistry, grade, and verification. Nitric acid is the long-established route, but citric acid offers notable benefits that could ultimately lead to it replacing nitric acid in many cases.^[7][13]

References

[1] DeBold T. Passivating stainless steel parts [Internet]. Carpenter Technology Corporation; [undated] [cited 2026 Sep 28].

[2] SAE International, AMS B Finishes Processes and Fluids Committee. AMS2700G: Passivation of corrosion resistant steels [Internet]. SAE International; 2025 May 27.

[3] Wang Z, Paschalidou EM, Seyeux A, Zanna S, Maurice V, Marcus P. Mechanisms of Cr and Mo enrichments in the passive oxide film on 316L austenitic stainless steel. Front Mater. 2019;6:232.

[4] Graham MJ. Passivity of iron [Internet]. In: Meeting abstracts, 201st Meeting of The Electrochemical Society. Institute for Microstructural Sciences, National Research Council of Canada; 2002.

[5] Crookes R. Pickling and passivating stainless steel [Internet]. 2nd ed. Materials and Applications Series, Vol. 4. Euro Inox; 2007. Hosted by the British Stainless Steel Association.

[6] Euro Inox. Electropolishing stainless steels [Internet]. 1st ed. Materials and Applications Series, Vol. 11. Euro Inox; 2010. Hosted by the International Molybdenum Association.

[7] Lewis PL, Kolody M, Curran J. Alternative to nitric acid passivation of stainless steel alloys [Internet]. NASA Kennedy Space Center; [results through 2013]. NASA Technical Reports Server document 20140002809.

[8] ASTM International. ASTM A967/A967M-25: Standard specification for chemical passivation treatments for stainless steel parts [Internet]. ASTM International; 2025.

[9] ASTM International. ASTM A380/A380M-25: Standard practice for cleaning, descaling, pickling, and passivation of stainless steel parts, equipment, and systems [Internet]. ASTM International; 2025.

[10] Kerber SJ, Tverberg J. Stainless steel surface analysis. Adv Mater Process (ASM International). 2000 Nov:33-6.

[11] Faraday M, Schönbein CF. The letters of Faraday and Schœnbein, 1836-1862: with notes, comments and references to contemporary letters. London: Williams & Norgate; 1899.

[12] ASTM International. ASTM F86-21: Standard practice for surface preparation and marking of metallic surgical implants [Internet]. ASTM International; 2021.

[13] Lara-Banda M, et al. Alternative to nitric acid passivation of 15-5 and 17-4PH stainless steel using electrochemical techniques. Materials. 2020;13:2836.

[14] Aluminum Anodizers Council. What is anodizing? [Internet]. Aluminum Anodizers Council; [cited 2026 Sep 28].

[15] Honsberg C, Bowden S. Surface recombination [Internet]. PVEducation; [cited 2026 Sep 28].

[16] ISO/TC 2. ISO 16048:2003: Passivation of corrosion-resistant stainless-steel fasteners [Internet]. 1st ed. International Organization for Standardization; 2003 [confirmed 2024].

[17] DIN Media. DIN EN 2516: Aerospace series: passivation of corrosion resisting steels and decontamination of nickel base alloys [Internet]. DIN Media.