What is passivation, and how does it work? How do I passivate stainless steel parts after machining operations? These are common questions asked by machining shops and manufacturers of various part materials: stainless steel, titanium, tantalum and more.
What is passivation?
Passivation is a non-electrolytic finishing process that makes stainless steel more rust-resistant. The passivation process typically uses nitric or citric acid to remove free iron from the surface. This results an inert, protective oxide layer that is less likely to chemically react with air and cause corrosion.
By chemically removing free irons from the surface of stainless steel, the passivation process adds a thin oxide “film” layer. Less iron at the surface means more chromium. More chromium means a thicker chromium oxide surface when the stainless steel is exposed to air (oxygen). And that thicker, chemically non-reactive surface means more protection against rust.
Per common passivation specifications ASTM A380 & A967, passivation is “the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination, but will not significantly affect the stainless steel itself.” Further, ASTM A380 states passivation is “the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing the spontaneous formation of the protective passive film.”
Therefore, a “mild oxidant”, such as a nitric acid or citric acid (mineral or organic acid solution), removes the excess iron and associated contaminants from the surface of the stainless steel and allows the formation of a chromic oxide layer when exposed to air, which thus leads to stainless steel’s corrosion-resistant properties.
Why passivate stainless steel?
To answer this question, let’s look at what stainless steel is first. Stainless steels are naturally corrosion-resistant, which might suggest that passivating them would be unnecessary; however, stainless steel is not entirely impervious to corrosion. Stainless steel derives its corrosion resistant properties from its chromium content. The chromium, in the presence of air (oxygen), forms a thin film of chromium oxide which covers the surface of the stainless steel. Chromium oxide is inert or “passive” by nature, and chromium in the material gives stainless steel its corrosion-resistant properties.
Under ideal conditions, pure, cleaned stainless steel (SS) forms an inert, oxide film when exposed to oxygen in the atmosphere which protects the SS from corrosion. Under realistic, normal conditions, any of the following can inhibit the formation of the oxide film which protects stainless steel from corrosion:
- foreign material in a manufacturing environment
- sulfides added to the stainless steel for improved machinability
- particles of iron from cutting tools transferred to the surface of the stainless steel parts
Therefore, these contaminants need to be removed down to the surface grain boundaries / structure of the stainless steel surface. This removal and oxide growth process is called passivation.
How does the passivation process work?
Many passivation specifications (ASTM A967, AMS2700, ASTM A380) exist to instruct on the proper process to passivate stainless steel, titanium and other materials, some of which are listed below. Common to nearly all the specifications are:
- cleaning the surface from any contaminants listed above
- chemical treatment via immersion in an acid bath (typically nitric or citric acid)
- testing of the newly passivated stainless steel surface to ensure effectiveness of the process steps
This chemical treatment simply augments/expedites the naturally occurring process when the material is exposed to oxygen in the atmosphere. It simply helps to “grow” the inert, oxide layer faster and thicker than found naturally.
Some specifications call for use of sodium dichromate added to the acidic baths solutions to provide more rapid formation of the oxide layer or passivation film. This practice of using sodium dichromate is becoming less and less prevalent with the significant advancements in ultrasonic machines and citric acid known as Citrisurf® which encourage oxygen formation at the surface of the material while the material is still in immersion of the acid bath.
Video: Fully Automated Citric Acid Passivation System
How To Passivate Stainless Steel Parts? What passivation equipment is needed?
Putting together everything listed above requires a process that will properly clean and acid bath passivate stainless steel. Common passivation process steps for stainless steel are listed below:
- Alkaline cleaning of the materials to remove all contaminants, oils, foreign material, etc. – Commonly uses detergent cleaners like sodium hydroxide, Micro90, Simple Green, etc.
- Water rinse – commonly with DI Water in high precision industries
- Nitric or citric (Citrisurf) immersion bath to fully dissolve any free irons and sulfides and expedite the formation of passive film or oxide layer
- Water rinse – commonly with DI Water in high precision industries
- Second water rinse – commonly with DI Water in high precision industries
- Dry parts
- Test sample parts via specification standards using: salt spray, high humidity chamber exposure, or copper sulfate testing
Types of Passivation Equipment and Passivation Lines
Best Technology is recognized as an industry leader in passivation equipment, tanks, systems and lines. Our experts understand the various passivation specifications and can design equipment to fit and meet passivation requirements and specifications.
Many high precision and critical part manufacturers such as aerospace and medical device manufacturers are required to meet additional guidelines, specifications, regulations and accreditation standards when passivating their product. One such accreditation is NADCAP or National Aerospace and Defense Contractors Accreditation Program. Such rigorous specifications and regulations typically require our automated passivation systems to ensure tight, documented process control parameters for the passivation process.
Because of the long-ago issues with citric acid, many industries developed their processes and specifications around using nitric acid. Specifically, the medical device and aerospace industries have required any manufacturers to use nitric acid as the passivation solution for their stainless steel, titanium and other alloy parts. Best Technology has seen many long-time nitric acid passivation companies start to transition to using citric acid.
ASTM A380 – Practice for Cleaning, Descaling and Passivating of Stainless Steel Parts, Equipment and Systems
ASTM A967 – Specification for Chemical Passivation Treatments for Stainless Steel Parts (based on US Defense Department standard QQ-P-35C) – one of the most common passivation specifications
AMS 2700 – Passivation of Corrosion Resistant Steels
AMS-QQ-P-35 – Passivation Treatments for Corrosion-Resistant Steel
ASTM F86 – Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants
Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants
ASTM B600– Passivation for titanium and titanium alloys is now recognized in the ASTM standard.
BS (British Standard) EN 2516 – Passivation of Corrosion Resisting Steels and Decontamination of Nickel Base Alloys
Military Specs and Standards
The following specs refer to QQ-P-35 for passivation of stainless steel and therefore allow the use of ASTM A967 and AMS 2700:
- MIL-STD-808A (section 184.108.40.206.1, Table II. finish code numbers F-200, F-201, F-202, F-203, F-204, Table VIII. finish code number D-200) – “Finishes, Materials and Processes for Corrosion Prevention and Control in Support Equipment (S/S by MIL-HDBK-808)”
The following specs refer to ASTM A380 for passivation of stainless steel:
- MIL-DTL-14072E (Table IV. Finish E300) – “Finishes for Ground Based Electronic Equipment”
The following specs refer to ASTM A967 and AMS 2700 for cleaning and passivation of stainless steel:
- MIL-S-5002D (section 3.8.6) – “Surface Treatments and Inorganic Coatings For Metal Surfaces of Weapons Systems”
- MIL-STD-171F (section 220.127.116.11, Table V. finish numbers 5.4.1 and 5.5.1) – “Finishing of Metal and Wood Surfaces”
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