Citric Acid Passivation

What is Citric Acid Passivation?

Citric Acid Passivation is the removal of contaminants or “free irons” from the surface layer of metals such as Stainless Steel to allow the growth of a natural, inert, oxide layer on the metal that restores its natural corrosion resistance by immersing parts in a bath of Citric Acid to make the metal “passive”. The passivation of Stainless Steel can be accomplished by other means, including Electropolishing and Nitric Acid Passivation. Citric Acid Passivation offers an environmentally friendly alternative to traditional methods while providing excellent results.

Understanding Passivation

Contaminants left behind from the manufacturing process inhibit the formation of the oxide layer that forms on the outside layer of metals and protects metals like stainless steel from corrosion. Although stainless steel is inherently passive, these impurities left behind combine with the metal resulting in a corrosive reaction.

Manufacturers in industries that require a high level of cleanliness such as medical, aerospace, laboratory, and food manufacturing often rely on chemical passivation to improve performance, durability, and corrosion resistance.

Why Citric Acid Passivation

Citric Acid Passivation has become more widely accepted by industries such as Medical and Aerospace in recent years. It offers an effective alternative to nitric acid passivation with less handling concerns and is considered environmentally friendly being on the GRAS (Generally Recognized as Safe) list for the FDA making it ideal for both the medical and food manufacturing industries.

Additionally, citric solutions can effectively passivate a wider range of stainless-steel alloys compared to traditional nitric acid methods such as ASTM A967, Nitric 2.

The Citric Acid Passivation Process

Step 1: Pre-Cleaning: Prior to the passivation process, stainless steel parts are subjected to an extensive pre-cleaning cycle to remove oils, grease, and metallic residues. The degreasing and cleaning of stainless-steel parts can be accomplished by a variety of commonly accepted methods, including acid pickle and an alkaline soak followed by a water rinse.

Step 2: Immersion Bath: After the removal of the organic and metallic residues from the surface of the part, they are then placed into a Citric Acid Passivation bath, specifically calibrated to meet ASTM A967 Citric 3 finishing standards. The three primary variables of Citric Acid Passivation to meet ASTM parameters are time, temperature, and concentration of acid.

Step 3: Final Cleaning & Dry: After the Citric Acid Passivation, parts are put through an extensive rinsing process concluding with a two-stage deionized rinsing process.

Step 4: Quality Testing & Inspection: Sample parts are tested to ensure they meet customer qualifications before being packaged for shipping.

Citric Acid Passivation Industry Standards

Most manufacturers’ standards meet two primary industry standards for citric acid passivation as set forth by ASTM International. ASTM International is a widely recognized and reputable standards organization charged with developing and publishing voluntary consensus technical data and standards for use in a wide range of materials and products. ASTM has two widely accepted standards,  ASTM A967 and AMS 2700. New England Electropolishing’s processes adhere to the ASTM A967 Citric 3 standard.

If you have any further questions about Citric Acid Passivation and whether it is right for your stainless-steel passivation needs, please contact us today and we would be happy to speak with you.

To learn more about how Electropolishing can passivate, improve surface finish, and enhance the aesthetics of stainless steel, please click here.

Resources

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