Nitric vs. Citric Acid Passivation | Which is Better?

Our customers often ask about the advantages and disadvantages of citric acid passivation compared to nitric acid passivation and which they should choose. While both effectively passivate stainless steel and other alloys, which one you select depends on the alloy itself and benefits you value the most.

 Citric and nitric acid passivation is effective for stainless steel parts and is detailed in ASTM A967 and AMS 2700. However, alloys such as titanium should only use nitric acid passivation per ASTM F86.

Why is citric and nitric acid passivation important?

While stainless steel has natural corrosion resistance, free iron may remain on the part surface after machining and fabrication. This free iron can corrode, so removal is necessary. The citric and nitric passivation process removes these contaminants and promotes the growth of a protective oxide layer which adds another layer of corrosion protection to the surface.

citric-passivation-equipment
Image Source: Production Machining Magazine

Nitric vs. citric acid passivation

Before diving into nitric and citric acid passivation, we've summarized the key points in the table below.

  Nitric Acid Citric Acid
Handling Special handling required Safer to use
Hazards Toxic and corrosive gases
Special disposal required
Dangerous at high temperature
No toxic gas emissions
Easier disposal
No dangers at high temperature
Ventilation Required Not Required
Process Time 20+ minutes 5 - 20 minutes
Temperature High temperature is required for most applications Room temperature for many applications
Effects on Equipment Corrosive degradation of non-stainless steel metal or polymer-based equipment No corrosive degradation
Cost Low chemistry costs
High disposal costs
High maintenance costs
High chemistry costs
Low disposal costs
Low maintenance costs

 

The citric acid passivation process

Citric acid passivation is the newer of the two processes and was initially developed by the Coors Brewing Company to passivate the inside of its beer kegs. Since citric acid is GRAS ("Generally Recognized As Safe") by the FDA, manufacturers can use it in food and beverage applications. 

Citric acid is the same non-toxic, biodegradable natural acid found in oranges and other citrus fruits, making its use in passivation an environmentally friendly alternative to nitric acid. It also has fewer handling concerns than nitric acid. And unlike nitric acid, citric acid can typically be disposed of with minimal waste treatment. 

Citric acid passivation can passivate a wider variety of stainless-steel alloys than nitric acid passivation. A citric acid passivation bath also takes far less time than nitric acid, speeding up the cleaning process considerably. 

However, for all these benefits, citric acid passivation is considerably more expensive, which is why many choose nitric acid to keep costs down. 

passivated-parts
Image Source: Electro-Spec

ASTM A967 details five different citric acid passivation methods: 

  • Citric 1: 4-10 w% Citric Acid, 140-160F, 4 Mins minimum
  • Citric 2: 4-10 w% Citric Acid, 120-140F, 10 Mins minimum
  • Citric 3: 4-10 w% Citric Acid, 70-120F, 20 Mins minimum
  • Citric 4: Other combinations of temperature time and concentration of citric acid with or without chemicals to enhance cleaning, accelerants, or inhibitors capable of producing parts that pass the specified test requirements.
  • Citric 5: Other combinations of temperature time and concentration of citric acid with or without chemicals to enhance cleaning, accelerants, or inhibitors capable of producing parts that pass the specified test requirements. Immersion bath to be controlled at pH of 1.8-2.2

The nitric acid passivation process

Nitric acid passivation is the traditional method of passivation and is also the most used. It has been used since the 1960s, when the U.S. military first detailed specifications in QQ-P-35. 

However, lower grades of stainless steel risk etching during the passivation process, also known as a "flash attack." This can be limited by adding sodium dichromate to the nitric acid, using a higher nitric acid concentration, or heating the nitric acid to a higher temperature. 

Nitric acid chemistries with high oxidizing potential are best, as the passive film formed on the surface forms faster and is more effective, reducing the potential for etching. 

While nitric acid passivation is cheaper to run, environmental hazards exist. Nitric acid is naturally hazardous and emits both toxic and dangerous fumes. It also requires special handling and disposal, making it more expensive than using citric acid passivation in some situations. 

ASTM A967 details five different nitric acid passivation methods: 

  • Nitric 1: 20-25 v% Nitric Acid, 2.5 w% Sodium Dichromate, 120-130F, 20 Mins minimum
  • Nitric 2: 20-45 v% Nitric Acid, 70-90F, 30 Mins minimum
  • Nitric 3: 20-25 v% Nitric Acid, 120-140F, 20 Mins minimum
  • Nitric 4: 45-55 v% Nitric Acid, 120-130F, 30 Mins minimum
  • Nitric 5: Other combinations of temperature, time, and acid with or without accelerants, inhibitors, or proprietary solutions capable of producing parts that pass the specified test requirements

Nitric vs. citric acid passivation: which is better?

From a purely environmental standpoint, citric acid passivation is a far better option. There are fewer handling concerns and less to worry about when it comes to disposal. But there are cases where nitric passivation works better.

The risk of flash attack is elevated with nitric acid, however, taking the precautions we discussed earlier lessens your risk. Flash attack can occur with citric acid too, but there's far less of a risk.

Early citric acid formulations suffered from organic growth and mold issues, but today's formulations include biocides that have solved that issue. Citric acid removes only the free iron on the surface, whereas nitric acid removes some metals on the alloy itself.

However, citric acid is far more expensive since it is naturally derived and can't be created in a laboratory. This results in higher chemistry costs, even as it saves money in just about every other area, including labor, equipment, maintenance, and disposal costs.

We find that citric acid passivation is often the better choice for various reasons. In precision applications such as aerospace and medical, proper passivation is vital for performance and durability.

Since citric acid only removes the free iron without altering the alloy, it is the better choice. And since citric acid is easily disposed of, it has allowed some companies to bring their passivation processes in-house rather than depending on a third-party shop.

These days there really isn't much reason not to switch over to citric acid, especially considering modern chemistries have addressed some of the early issues. While your chemistry cost will rise, the long-term savings of using citric acid and the environmental and disposal advantages make it a wise decision.

Our AquaVantage® line includes a citric acid chemistry for the passivation of stainless steel and is specifically formulated for use in the space, aerospace, and medical manufacturing industries. And with nine decades of experience developing dependable cleaning chemistries, proven results our clients count on.

Citric acid passivation with Brulin

Brulin's AquaVantage® Passivation is our citric acid chemistry designed for stainless-steel passivation. Gentle on part surfaces yet effective, AquaVantage® Passivation conforms to the AMS 2700 Method 2, ASTM 380 & ASTM 967 standards and is compatible with a wide range of stainless-steel alloys.

Our customers in the space, aerospace, and medical manufacturing industries count on AquaVantage® Passivation's specifically formulated fast-acting and highly dilutable chemistry to effectively clean a wide variety of parts in a wide range of bath temperatures. For the best results, we recommend you use AquaVantage® Passivation in conjunction with our AquaVantage® branded immersion and ultrasonic detergents. If you'd like to learn more about the benefits of using (or transitioning) to citric acid in your passivation processes, we'd be glad to answer any questions you might have.

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