Nitric vs Citric Passivation Methods

Stainless steel is an inherently corrosion resistant material, however when stainless steel is machined, formed or fabricated free iron can be introduced to the surface that can corrode independent of the base material.  Proper passivation of stainless steel with an oxidizing acid such as nitric or citric acid removes this free iron and promotes the growth of a thin, dense protective oxide layer which maximizes the corrosion resistance of the stainless steel. Depending on the type of stainless steel and end application certain passivation processes may perform better at passivating than others. In this article we will compare nitric vs citric acid passivation which are the two primary chemistries specified in ASTM A967 and AMS 2700.

Nitric Acid Passivation

When comparing nitric vs citric passivation, the most common method used throughout industry is nitric acid passivation. The Nitric acid passivation processes was the original passivation processed specified in QQ-P-35, the first military specification covering passivation, revision A being released in the 1960s.  Nitric acid passivation offers a range of options to customize the oxidizing potential of the acid to suit a specific grade of stainless steel. The various methods and types of nitric acid passivation include several heated options as well as options that include a sodium dichromate.

The higher nitric acid concentration and the higher the nitric acid temperature, the more oxidizing potential the passivation chemistry has.  Sodium dichromate can also be added to the nitric acid to increase the oxidizing ability of the bath making it better for less corrosion resistant stainless steels, such as precipitation hardened, martensitic and ferritic grades of stainless steel.  These grades of stainless steel have less nickel and chromium in them making them more susceptible to etching.  The higher the oxidizing potential of the chemistry, the faster and more effective the passive oxide barrier is formed on the surface, reducing the potential for etching.

A summary of the various nitric acid passivation methods per ASTM A967 is provided below:

• 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

ASTM A967 also offers a very useful reference of stainless steel grades to the recommended method of nitric acid passivation. A summary of this table is provided:

Contamination of passivation chemistry can lead to flash attack of the surface, which produce a heavily etched or darker surface. A common containment that leads to flash attack is chlorides which can come from several sources including dragging in acids or using having chloride in the water. In addition, organic buildup in passivation baths such as the drag-in of machining oils from parts that are not properly cleaned, can lead to flash attack or etching of the stainless steel.  As such, regular analytical analysis and maintenance of passivation chemistries is required. Certain passivation methods are also more resistant to flash attacks than others. For nitric acid passivation the baths with increased oxidizing potential are also more resistant to flash attacks. Nitric acid also is more resistant to flash attack compared to citric acid. ^[1]

Citric Acid Passivation

Citric Acid passivation was developed by Adolf Coors brewing company for the passivation of the inside of beer kegs. It offers an effective alternative to nitric passivation with less handling concerns and is consider environmentally friendly being on the GRAS (Generally Recognized as Safe) list for the FDA making it ideal for food and beverage applications.

When comparing nitric vs citric passivation, citric solutions can effectively passivate a wider range of stainless-steel alloys compared to any one nitric acid passivation solution, allowing for assemblies of several stainless-steel alloys to be passivated.

Passivation chemistries remove free iron from the surface but can also remove some nickel and chromium from stainless steel. Removing nickel and chrome reduces the corrosion resistant material at the surface leaving a thinner oxide layer. Citric acid passivation selectively removes iron over nickel and chromium leaving a thicker corrosion resistant oxide layer than nitric acid passivation ^[2] 

Once of the other advantages of citric acid is the bath formulation can be adjusted to reduce cycle times over nitric acid, allowing for increase throughput and reduced costs of passivation verses that of nitric acid.  Cycle times as low as 4 minutes are possible with certain citric acid passivation formulations.  A summary of the various citric acid passivation concentrations and times from ASTM A967 are provided below.

• 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

Passivation Pretreatment

A universal requirement when comparing nitric vs citric acid passivation is the need for parts to be properly pretreated. For the martensitic grade and precipitation hardened grades of stainless steel that are heat treated, there is a potential for scale on the parts after the hardening process. For machined parts there is cutting fluids and other oils. Finally, for assemblies there is weld scale and heat marks. Any of these scales or oils left on a part lower the corrosion protection of the material and in passivation will inhibit the effectiveness and can damage parts. Scales and oils should be removed before passivation. Oils can simply be cleaned or vapor degreased off parts. While scale needs to be removed either with descaling mineral acids such as hydrochloric acid, or inorganic deoxidizers such as potassium permanganate or with abrasive methods such as media blasting or vibratory polishing.  Mechanical scale removal methods are recommended for those parts that require a very uniform surface especially for parts with heat-affected zones such as weldments.

Conclusion

Passivation of stainless steel is a critical component in the manufacturing of stainless-steel components to ensure fully optimized corrosion resistance. There are many different factors when choosing a citric vs nitric passivation method and this article covered some of the basics of choosing a passivation process. For additional information and what process may be right for your application please feel free to contact a member of Advanced Plating Technologies Sales & Engineering group at sales@advancedplatingtech.com or 414.271.8138.

Blog Authored By: Will T., Process Engineer

References:

[1] Mohr, J. H. (2007, August 1). Making Stainless Steel Stainless. Retrieved from PF Online : https://www.pfonline.com/articles/making-stainless-steel-stainless

[2] R. Kremer, Stellar Solutions, Inc. (2007). Developments In Citric Acid Passivation of Stainless Steel. McHenry : NSF.

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By S Patel

Passivation is a critical processing step in the manufacturing of stainless steel components to enhance the corrosion resistance and make stainless steel parts truly “stain-less.” Proper passivation of stainless steel components can make the difference between satisfactory field performance and premature corrosion failures.  If performed improperly, the passivation process can actually attack and etch the stainless surface or induce corrosion. Passivation is often misunderstood to be a scale removal or bright dipping process when in fact it is fully removed from either of these processes.  The information within the blog serves to detail some of the key principles of passivation of stainless steel offered by Advanced Plating Technologies.

There are two primary functions of the passivation process as follows:

1. Removal of free iron particle/residue from stainless steel parts which may have deposited during machining, grinding, forming or stamping operations as well as from shop dirt and/or debris. The need for passivation arises when parts are fabricated, either by cold forming or machining where contaminants such as shop dirt or iron particles from cutting tools/fixtures may be transferred to part surface.  If not removed, the free iron particles can corrode very rapidly giving the appearance that the stainless steel itself is rusting.
2. To develop a protective passive oxide film on the surface of the components. This passive layer is 100,000 times thinner than a human hair but forms a physical barrier to corrosion or attack of the stainless steel material beneath.  It is this passive oxide barrier that the term “passivation” stems from.

  There are two primary passivation categories as follows:

1. Nitric Acid Passivation:  Nitric acid passivation processes are offered either with or without sodium dichromate at various temperatures, chemistry concentrations and exposure times/ranges.  The specific process is a function of the specific stainless steel material grade.
2. Citric Acid Passivation: Citric Acid passivation processes are offered at various temperature & pH ranges.  The specific processes are also a function of the stainless steel material grade.

Cleaning and Descaling Prior to Passivation

The material grade and as-formed surface condition are the most important factors in properly specifying a cleaning & passivation protocol.  Most commonly as-formed stainless steel components have light oil on the surface.  This film needs to be thoroughly removed by soaking parts in caustic cleaning solution or organic degreasing solvent.  The acid chemistry used for passivation process is not capable of removing oil or grease from the surface of the parts.  If not removed, the acid will not evenly wet the surface resulting in inconsistent processing.  In addition, residual oil on the surface will react with acidic chemistry and create oxygen bubbles which further interfere with the passivation process.

Some passivation components, such as precipitation-hardened grades (e.g. 17-4, 15-5 or 17-7), are heat treated after machining/forming but prior to the passivation process.  Heat treatments are performed to achieve final surface hardness that these grades offer.  However, these processes often impart an oxide or discoloration scale/iris on the surface even when performed in an “inert” environment.  These films must be removed prior to passivation operations by means of acid descaling or mechanical methods such as blasting.  The passivation chemistry is not capable of removing heat treatment oxide films and parts.  If this is not performed, the heat treatment oxide will impair the ability of the passivation acid to reach the surface of the stainless steel and will prevent proper passivation.

Contamination in either nitric or citric acid chemistry can be detrimental to the passivation process.  This is especially true of chloride which can lead to flash-attack or etching of the surface of the parts being passivated. Instead of achieving the desired oxide film with a clean and corrosion-resisting surface, flash attack will produce a heavy etched or darkened surface.   A common chloride contamination limit is no greater than 60 mg/litter in either citric or nitric acid passivation chemistries.  It is important to note that some low nickel alloys such as Martensitic 400-series grades can mildly etch during a normal passivation process.  This mild etching of the surface is not indicative of flash attack and is not representative of process contamination.

Free-machining (e.g. 303, 416, 440F), Ferritic and Martensitic grades of stainless steel (400 series) often exhibit improved passivation if processed with an AAA (Alkaline-Acid-Alkaline) passivation process. This process was originally developed by Carpenter Technology (www.cartech.com) for passivation of free machining stainless grades. It was discovered during standard passivation processes, the phosphorus or sulfur added to free machining alloys to improve machinability would etch from the surface leaving a micro voids on the surface.  These voids act as micro capillaries that can trap nitric & citric acid during the passivation process. If the trapped acid is not neutralized it can result in an acid attack on the surface and accelerate corrosion. The AAA process has the ability of neutralizing acid from the surface of the parts greatly improving the passivation and corrosion resistance of the passivated components.  As such, the AAA process is highly recommended for all free machining grades as well as Ferritic and Martensitic grades of stainless steel.  More information is available regarding the AAA method of passivation within the white papers of our technical library.

Citric acid passivation is an alternative process to nitric that is commonly used in the medical and food processing industries. Since nitric acid passivation utilizes nitric acid and in some cases sodium dichromate, a source of hexavalent chromium ions, citric acid is sometimes preferred in applications where food processing or human contact is a design consideration.

Advanced Plating Technologies offers all methods of passivation including nitric, citric and AAA methods.  Specifications including QQ-P-35 and ASTM A967 provide a table reference of recommended passivation methods as a function of material grade.  Federal specifications are available within the technical library of this site.  In addition, a member of the Advanced Plating Technologies technical sales or engineering group can provide recommended best practices based upon material grade, design concerns and the as-received condition of the parts.

Passivation of Stainless Steel – Testing of Passivated Parts    

Passivated parts can be tested via several methods as described in ASTM A380 or ASTM A967 but not all tests are suitable for all grades of stainless steel. The test methods include the following methods:

1. Water immersion test
2. High humidity test
3. Salt spray test
4. Copper Sulfate Test
5. Potassium Ferricyanide-Nitric Acid Test

Among the above tests copper sulfate is the most common & frequently used in industries. The copper sulfate is a favorable test due to the fact that it is nondestructive and quickly performed on a wide range of test samples. However, the copper sulfate test is not recommended for all alloys and can yield a false positive test on Martensitic or low chromium Ferritic grades of stainless steel.

If you would like additional information on test methods, passivation of stainless steel or passivation of medical alloys including titanium, MP35N or Elgiloy, you can contact a member of our technical sales team.

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