Some engineering-friendly metals, like stainless steel and titanium, form a natural oxide layer that acts as a protective barrier against inconveniences like contaminants and free iron. This oxide film forms a thin but effective shield that helps isolate the base metal from the environment. But what if you could restore and stabilize that natural oxide coating without significantly altering the geometry of the metal part?
The process of passivation does just that, enhancing the corrosion resistance of metals like stainless steel.
This article goes over the basics of passivation, looking at how it works, its key advantages, and its main subtypes. In the article, we draw on 3ERP’s many years of experience applying surface finishing treatments to metal parts, discussing passivation as well as comparable treatments like anodizing.
What is Passivation?
Passivation is a surface finishing treatment used to improve the corrosion resistance of stainless steel and other metal parts.
The ASTM A967 standard defines passivation as “the chemical treatment of a stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of acid to remove free iron or other foreign matter, but which is generally not effective in removal of heat tint or oxide scale on stainless steel.”
However, in general use, the passivation meaning can also encompass other metals besides stainless steel.
How Passivation Works
Passivation is the chemical treatment of a material to make it more corrosion resistant. During passivation, the stability of the material’s protective oxide film is improved by oxidation from the surrounding air.
During passivation, a substance like nitric acid or citric acid is used to remove surface contaminants like free iron from a metal and to stabilize the material’s outer protective layer. Once the acid is rinsed away, the surface of the decontaminated metal reacts with oxygen to form a chromium oxide layer. This layer is “passive,” meaning it is less chemically reactive and less likely to corrode, giving the material excellent corrosion resistant properties.
The difference is atomic. Since the passivating acid dissolves more iron than chromium, the top few atomic layers become enriched in chromium, increasing the chromium-to-iron ratio. The protective layer is typically a few nanometers thick.
Other surface treatments that resemble passivation include anodization, which uses an electrolytic bath instead of chemicals, and chromate conversion coating, which is a related corrosion protection treatment.
Why Passivate? Key Benefits for Manufacturers
Why passivate stainless steel and other metals? Surprisingly, passivation benefits go beyond corrosion resistance, helping to keep parts spotlessly clean and prolong their lifespan. Some of the main benefits of passivation include:
Corrosion Resistance: The primary reason for the process. By creating a strong passive layer, passivation prevents rust and oxidation, even in harsh conditions.
Improved Cleanliness: Essential for safety-critical industries. The removal of contaminants like free iron makes parts safer for use in sensitive industries like food and healthcare.
Longer Part Lifespan: Protection against oxidation makes parts less likely to degrade or break, protecting the customer’s investment.
Tight Tolerance: The ultra-thin passivation film is created by the base material; it does not add a significant extra layer of thickness like a paint coating.
Appearance: The spotless look of a newly passivated surface gives the impression of high material quality, although the appearance of the metal does not change in other ways.
Passivation Chemicals: Citric Acid vs. Nitric Acid
Passivation broadly involves the use of two different passivation agents: citric acid and nitric acid. Historically, nitric acid has been the more popular choice, but proponents of citric acid passivation (when specific conditions are met) include organizations like NASA.
Both chemicals can suffer from an issue known as “flash attack” when passivating stainless steels. This is a problem in which the acid bath aggressively etches the surface of the metal.
Nitric Acid Passivation
Nitric acid is the traditional industrial standard for passivation. Its use as a passivation agent dates back to the 18th century, when Russian polymath Mikhail Lomonosov discovered that iron does not react with it.
Because it is the more established method, it may be considered the more reliable. It is highly effective but requires strict safety and environmental controls, especially when using concentrated nitric acid. The use of sodium dichromate can reduce the likelihood of flash attack, though this presents major waste handling issues.
Key nitric acid passivation advantages include:
- Established process
- Stronger oxidizer than citric acid
- Range of variants
The ASTM A967 standard governs the passivation process and outlines different nitric acid methods, shown in the table below.
| Method | Nitric acid (vol %) | Sodium dichromate | Min time | Temperature range |
|---|---|---|---|---|
| Nitric 1 | 20–25% | 2.5 ± 0.5 wt% | 20 min | 120–130°F (49–54°C) |
| Nitric 2 | 20–45% | None | 30 min | 70–90°F (21–32°C) |
| Nitric 3 | 20–25% | None | 20 min | 120–140°F (49–60°C) |
| Nitric 4 | 45–55% | None | 30 min | 120–130°F (49–54°C) |
| Nitric 5 | Not fixed | Optional (incl. accelerants/ inhibitors) |
Not fixed | Not fixed |
Citric Acid Passivation
The modern, eco-friendly alternative to nitric acid is citric acid, like the substance found in citrus fruits like oranges. Generally produced by fermentation, this chemical is safer to handle, does not emit toxic fumes, and is increasingly preferred in the medical and food industries.
In previous years, citric acid had been less popular due to fears of potential mold growth. However, when the ASTM A967 standard is met (options shown in the table below), passivation with citric acid is a good option for most industries.
Key citric acid passivation advantages include:
- Safer to handle
- More environmentally friendly
- Suitable for a wider range of stainless steels
The ASTM A967 standard also governs the different citric acid passivation methods, shown in the table below.
| Method | Citric acid (wt %) | Min time | Temperature range | Notes |
|---|---|---|---|---|
| Citric 1 | 4–10% | 4 min | 140–160°F (60–71°C) | Standard hot citric treatment |
| Citric 2 | 4–10% | 10 min | 120–140°F (49–60°C) | Shorter time option |
| Citric 3 | 4–10% | 20 min | 70–120°F (21–49°C) | Longer time option |
| Citric 4 | Not fixed | Not fixed | Not fixed | Alternative time/temp/ concentration combinations allowed |
| Citric 5 | Not fixed | Not fixed | Not fixed | Same as Citric 4, but immersion bath must have controlled pH of 1.8–2.2 |
The Step-by-Step Passivation Procedure
The passivation process involves five main steps: cleaning, rinsing, acid bath passivation, rinsing (again), and drying. The process is carried out in a passivation tank, which may have individual stations for cleaning, rinsing, and passivation, or passivation only.
The passivation procedure does not differ greatly between nitric and citric acid applications, though citric acid passivation can be faster.
Cleaning: A clean surface produces the best passivation results, so the metal surface must be thoroughly cleaned and de-greased. Some passivation lines will have a dedicated cleaning tank, using an alkaline cleaning solution.
Rinsing: The cleaning solution and any loose debris is rinsed from the part. Industrial passivation equipment will typically use deionized water in a separate rinsing tank.
Passivation: The cleaned, rinsed parts are submerged in the nitric acid or citric acid solution in the passivation tank for a set time and at a controlled temperature. Sometimes, an acidic passivating bath is not used and the acid is applied via spray or other means; this is rare but may be helpful for large or difficult-to-manipulate parts.
Rinsing: A further rinsing stage is carried out to remove any acid or neutralizing agents like sodium bicarbonate. Again, deionized water is typically used.
Drying: In most industrial passivation lines, a dedicated drying station removes traces of water from the parts.
The exact nature of each step depends on the passivation equipment used. Some cleaning and passivation tanks use ultrasonic waves to expedite the process.
Industry Standards
Passivation processes are governed by two main passivation standards, ASTM A967 (which we have referenced throughout this article) and AMS 2700. The main differences between the two standards are the user base, with SAE International’s AMS 2700 being most applicable to the aerospace industry and ASTM A967 being more broad.
As with the ASTM tables shown in previous sections, the AMS 2700 standard also specifies different passivation types with varying acid bath concentrations and other variables. Precursors to AMS 2700 in the aerospace industry included the QQ-P-35 and AMS-QQ-P-35 passivation specifications.
Other industry standards are ASTM A380 (Standard Practice for Cleaning, Descaling, Pickling, and Passivation of Stainless Steel Parts, Equipment, and Systems), a broader standard for industrial equipment, and ISO 16048 (Passivation of corrosion-resistant stainless-steel fasteners), which relates to fasteners in particular.
Passivation Materials Beyond Stainless Steel
Passivation is mainly associated with stainless steel. Although stainless steel naturally forms a protective oxide layer, that layer can be replenished or reinforced via passivation, creating further corrosion resistance.
However, other metals can also be passivated, sometimes using similar immersion techniques, sometimes via other methods.
Aluminum
Corrosion protection of aluminum typically requires one of two metal surface finishing treatments: chromate conversion coating, which resembles the stainless steel passivation technique described previously, and anodization, an electrolytic bath process. Chromate conversion produces a thin coating (as thin as 250 nm), while anodizing creates a thicker layer.
These techniques are usually required on aluminum alloys that naturally produce a very thin oxide layer that is not especially protective. However, some alloys will naturally form a thicker and more protective layer.
Titanium
Titanium naturally forms a layer of titanium oxide when exposed to air, making it highly corrosion resistant. However, titanium passivation may be required when an extra-thick protective layer is required, or if the surface of the titanium has been contaminated with iron particles.
Passivation of titanium parts may be achieved through immersion in a chemical passivation bath or through anodizing, as with aluminum.
Other Ferrous Metals
Options for creating a passivating layer on non-stainless steels include parkerizing (phosphate conversion), which uses phosphoric acid to form a crystalline phosphate conversion layer on the steel surface, or bluing, an chemical conversion method that forms a black oxide surface.
Industry Applications
Passivation is widely used for applications where corrosion resistance is essential or where contamination must be avoided. These include:
Aerospace: Passivation is widespread in the aerospace industry, as parts subject to extreme weather conditions must be highly corrosion resistant. Manufacturers typically follow the AMS 2700 standard when passivating aerospace parts like landing gear and fuel system components. The components are often tested using methods such as salt spray testing to verify long-term corrosion resistance under extreme environmental conditions.
Medical: The medical and pharmaceutical industries regularly use passivation of stainless steel and other metals to ensure they meet strict cleanliness and safety standards. Passivation improves the corrosion resistance of parts like surgical instruments and hospital equipment, ensuring sterility and preventing contamination.
Food and beverage: The food industry, like healthcare, must use components that are corrosion resistant and free of surface contaminants. Processing equipment components and food containers are some of the parts that can benefit from passivation techniques.
Chemical: Processing of chemicals requires metals components that are highly resistant to corrosion from harsh substances. Fortifying the oxide layer on the surface of parts like pipes and valves prolongs their lifespan and increases their efficacy. Oil & gas passivation services are designed to meet similar needs.
Conclusion
Passivation is an effective and convenient surface finishing treatment that can significantly prolong the lifespan of stainless steel parts via improved corrosion resistance.
With 15+ years of experience finishing metal parts produced by CNC machining, casting, sheet metal fabrication, and other techniques, 3ERP is your ideal project partner for passivated stainless steel prototypes and end-use parts. We can even provide an engineering consultation and design for manufacturing (DFM) support to ensure that your parts are built in a way that facilitates high-quality production and finishing.
Request a quote for your next batch of stainless steel parts today.
FAQs
Is passivation necessary for stainless steel parts?
Stainless steel can be said to be autopassivating. It naturally forms a protective oxide layer upon exposure to air. However, the passivation process can remove contaminants and improve the effectiveness of this protective layer, making parts last longer.
What is the main advantage of passivation vs. other surface treatments?
Unlike coatings or plating, passivation is a non-additive process that removes surface contaminants (like free iron) to restore a material’s natural protective oxide layer. Its primary advantage is providing superior corrosion resistance without altering part dimensions.
Passivation vs. anodizing: which is best for aluminum and titanium?
While both processes enhance corrosion resistance, they serve different purposes based on the base metal:
Aluminum: Anodizing is the industry standard. It is an electrochemical process that grows a thick, durable, and porous oxide layer integrated with the substrate.
“Passivation” is rarely applied to aluminum; instead, Chemical Conversion Coating (Chromating) is used for similar protective goals.
Titanium: Both are viable. Anodizing (Type 2 or 3) is used for wear resistance and color coding, while Passivation is primarily used to remove surface contaminants and ensure biocompatibility for medical implants.
Key Difference: Anodizing is an electrolytic conversion that adds thickness, whereas passivation is a chemical cleaning process that restores the metal’s natural oxide film without adding a layer.
What is the main passivation vs. pickling difference?
Pickling and passivation both involve the use of a chemical bath to alter the surface of a metal part. The main difference is that pickling is more aggressive and capable of removing heat tint and oxide scale, as well as layers of the metal itself.
