APT BLOG

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

Passivation of Stainless Steel

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.

Please click on the image below to download a PDF copy:

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.

Gold Plating Thickness of Connectors and Contacts

Proper specification of gold plating thickness for connector and contact applications is a key design consideration.  Gold plating is an exceptional finish for connectors of that demand both high reliability and durability; however, the thickness of the gold plating will impact the durability and ultimate cycle life of the connector.  Gold plated connectors have low contact resistance which is suitable for applications with low signal voltages and current in the millivolt and milliamp range.  Because gold is a noble metal, it does not readily react with chemicals in most environments, meaning that gold plated connectors will retain their conductivity over time provide the thickness of the gold provides a sufficient barrier to the substrate from the environment.    Continue reading →

Silver Plating of Stainless Steel – Silver Properties:

Silver plating on stainless steel and other high temperature alloys such as Inconel®, Nitronic® and Hastelloy® is a common silver plating service for nuts, fasteners, slip-rings, thrust-washers, bushings and other bearing surfaces that benefit from the lubricity of silver at high temperatures allowing parts to exhibit anti-galling and anti-seizing properties.  Silver is a unique precious metal that exhibits many desirable properties for utilization across a broad range of engineered applications. Of all metals, silver has the highest thermal conductivity, electrical conductivity, and optical reflectivity in the visible portion of the electromagnetic spectrum; silver has outstanding temperature resistance with a melting point of 962° C (1763° F). Additionally, silver is a soft, ductile metal with good embeddability that performs well under high torque and loads.  Silver also provides excellent solderability and brazing characteristics for joining applications of stainless steel and other high temperature alloys.  The unique combination of lubricity, high temperature resistance and thermal conductivity make silver plating on stainless steel and other high temperature alloys an outstanding combination for high temperature fastening or bearing applications where heat transfer high temperature lubricity are the principle design considerations.

Continue reading →

Silver Tarnish and Its Properties

Silver Tarnish (Left) vs No Tarnish (Right)

Silver plating is often used for cosmetic applications and is found on items such as silverware and jewelry. While silver provides value and an aesthetic appearance to these items, it is also used in multiple subsectors of manufacturing – Power Transmission, Oil & Gas, Medical, Telecommunications, Aerospace, Electronics and many more. The reasons silver plating is used is vast: ductility, electrical and thermal conductivity, solderability, high temperature lubcricity, as well as excellent optical reflectivity. Although there are many positive attributes to silver plating, silver tarnish is one is a common occurrence when the proper steps are not taken.

Continue reading →

Choosing Types of Powder Coating for any Industry

By: B. Bondhus, Process & Estimating Engineer

Powder coating is a surface finishing option that applies a relatively thin film to provide excellent corrosion protection and chemical resistance in a highly cosmetic manner.  While parts are often designed with specific colors, gloss, and textures – the types of powder coating are often overlooked, yet a critical component to every powder coating job.

Powder coatings are applied in a variety of types. Each resin system has specific attributes that are able to better suit needs of specific environments. Some of the most popular types of powder coating include: Epoxy Powder Coatings; Polyester Powder Coatings; Hybrid Powder Coatings.

Continue reading →

Deposit Thickness in Surface Finishing

By: J. Lindstedt, President. 

In ordering a surface coating system to enhance the performance of an article in service, the use of standard finishing specifications is the established procedure employed by most manufacturing entities. The specifications most commonly referenced are ASTM, MIL Specs, AMS and unique corporate specifications.

Surface finishing specifications identify a number of parameters which evaluate the ability of the surface coating to perform its intended function. The most common coating requirements used to qualify a coating system are:
Continue reading →

By Matt Lindstedt, Technical Sales Manager

Electroless Nickel Plating: Superior Corrosion Protection

Raw metals other than precious metals can easily oxidize and corrode over time when exposed to various environmental conditions and corrosive applications.  The proper design of any component must begin with a surface engineering evaluation to ensure that the product will function reliably over the intended service life.  Even in relatively mild service applications, corrosion can result in functional issues and shortfalls.  Electroless nickel plating can provide a robust solution to corrosive attack across a range of corrosive mechanisms including galvanic corrosion, chemical attack and erosion. Electroless nickel plating (EN) can be applied to a wide range of basis metals including steel, copper, brass and aluminum alloys. Electroless Nickel plating is currently utilized to promote corrosion protection performance across a diverse range of industries including heavy equipment, oil & gas, power transmission & distribution, automotive, marine and railway to name a few. Continue reading →

By James Lindstedt, Manufacturing & Process Engineer

Rack Plating Introduction – A Metal Finishing Job Shop

Advanced Plating Technologies is a metal finishing job shop. What does that mean? A metal finishing job shop encounters the metal finishing demands of any industry that has a need for metal finishing. Metal finishing is a highly diverse industry serves the needs  of  a myriad of other industries. Although there is always common ground, every industry has its own unique set of needs and criteria for metal finishing. The conscientious metal finisher must be able to recognize these unique requirements, and reconcile them with the nuances of the plating methods and processes.

Rack Plating & Tooling

When quoting at a prospective job, some of the key factors to consider are:

• Part geometry
• End use
• Base material
• Part volume (EAU)
• Throughput
• Type Of Process (Electroless, Electrolytic, Immersion Only, etc)

Continue reading →