What is Passivation?
How Does Passivation Process Work?
How To Passivate Stainless Steel Parts?
What is passivation, how does passivation work, how do I passivate stainless steel parts after installing? Don't be alone as these are questions commonly asked by manufacturers, installers and homeowners.
What is Passivation?
Passivation is a widely-used metal finishing process to prevent corrosion. In stainless steel, the passivation process uses nitric acid or citric acid to remove free iron from the surface. The chemical treatment leads to a protective oxide layer that is less likely to chemically react with air and cause corrosion.
ASTM A967 - “the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of the removal of free iron or other foreign matter.”
ASTM A380 -“removal of exogenous iron or iron compounds from the surface of a 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 … for the purpose of enhancing the spontaneous formation of the protective passive film.”
AMS 2700 - “dissolve tramp metallic elements from the surfaces of corrosion resistant steels to improve their corrosion resistance“
Passivation has a history that dates back to the mid 1800s when a chemists named Christian Friedrich Schönbein found when he exposed iron to dilute nitric acid it dissolved and produced hydrogen, but if the iron is placed in concentrated nitric acid and then returned to the dilute nitric acid, little or no reaction will take place. In the 1950s when Bell Telephone laboratories developed surface passivation which accidentally led to the discovery that silicon dioxide could be grown on silicon. This lead the way to integrated circuits and silicon devices.
The practice of passivation of stainless steel with nitric acid became a widespread practice in the 1900s, but the environmental and safety issues with nitric acid became apparent. In the 1950s research done by the Adolf Coors Brewing Company in Germany identified citric acid as an effective alternative and by the 1990s manufacturers began to adopt citric acid as a safer and more environmentally friendly alternative to nitric acid.
Today the industry standards for passivation offer methods for nitric acid or citric acid, or nitric acid with sodium dichromate. Choice of method often depends on project requirements with each method having its own advantages and disadvantages.
Stainless steel is corrosion-resistant, but is not completely impervious to rusting. A common form of corrosion in corrosion-resistant steels is when small spots on the surface begin to rust because the grain boundaries or embedded bits of foreign matter allow water molecules to oxidize some of the iron in those spots despite the present of the alloy chromium. This is the most common phenomenon with our stainless steel products.
Passivation is a post-fabrication best practice for all newly-machined stainless steel parts and components. The benefits include:
- Chemical film barrier against rust
- Extended life of the product
- Removal of contamination from product surface
- Reduced need for maintenance.
So how does passivation work?
Stainless steel being iron-based alloy is typically composed mainly of iron, nickel and chromium. Stainless steel derives its corrosion-resistant properties from the chromium content. Chromium, when exposed to oxygen (air), forms a thin film of chromium oxide that covers the stainless steel surface and protects the underlying iron from rusting. The purpose of passivation is to assist and optimize formation of this chromium oxide layer.
The immersion of stainless steel in an acid bath dissolves free iron from the surface while leaving the chromium intact. The acid chemically removes the free iron, leaving behind a uniform surface with a higher proportion of chromium than the underlying material.
Upon exposure to oxygen after the acid bath, the stainless steel forms the chromic oxide layer over the next 24 to 48 hours. The higher proportion of chromium at the surface allows for the formation of a thicker, more protective chromium oxide layer. Removal of free iron from the surface thus removes opportunities for corrosion to start.
The resulting chromic oxide layer provides a chemically non-reactive surface that protects against rust.
How To Passivate Stainless Steel Parts
Many passivation specifications (ASTM A967, AMS 2700) exist to instruct on the proper process to passivate stainless steel. The following phases are common to nearly all the specifications:
- Clean – Remove any contaminants from the surface, such as grease and oils.
- Passivate – Perform chemical treatment via immersion in an acid bath, typically citric acid.
- Test – Test the newly passivated stainless steel surface to ensure effectiveness of the process steps.
To promote the process and encourage oxygen formation on the metal surface citric acid like found in CitriSurf® will start the process while still immersed in the acid bath. Length of time of immersion of the product is typically 20 – 30 minutes.
Process steps for passivating stainless steel parts
Putting together a passivation process requires a process that will both clean and passivate stainless steel. Common process steps for passivating stainless steel are as follows:
- Alkaline cleaning of the materials to remove all contaminants, oils, and foreign materials. Commonly uses detergent cleaners like sodium hydroxide, Micro-90, or Simple Green.
- Water rinse – Clean
- Citric acid (CitriSurf) applied to create a wet surface ;to fully dissolve any iron ions and sulfides and expedite the formation of passive film or oxide layer
- Water rinse –Clean
- Second water rinse – Clean
- Air Dry
Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts
- Based on U.S. Defense Department standard QQ-P-35
- One of the most common passivation specifications
Passivation of Corrosion Resistant Steels
- Aerospace Material Specification
Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems
- Refers to ASTM A967 for specifics of how to perform passivation
(superseded) Passivation Treatments for Corrosion-Resistant Steel
- Takes the place of MIL-QQ-P-35, but has since been replaced with AMS 2700