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Stainless Steel Finishes: Passivation vs. Electropolishing

July 7, 2020

Passivation and electropolishing are two widely used finishing operations within our industry. Not knowing their differences often leads to confusion between specification callouts and a true understanding of each process. In this blog we will briefly explain some of the differences between the two.

EP Manifold

Holland Applied Technologies offers a broad range of sanitary fabrication capabilities, including electropolishing and passivation services.

Passivation is a chemical process designed to increase the corrosion resistance of stainless steel parts. It is a non-electrolytic process typically using nitric or citric acid which removes free iron from the surface and forms an inert, protective oxide layer that in turn renders the stainless steel more rust-resistant due to the lack of iron to react with the atmosphere. During a machining processes, stainless steel parts may have imperfections from iron embedded onto the surface from the machining tool ends. These free irons on the surface should be removed to prevent a corrosive reaction between the different metals.

Parts are placed in a basket and submerged in the nitric or citric acid bath. Depending on the stainless steel alloy, the acid concentration, temperature, and time are set to effectively remove free iron and other foreign materials on the part’s surface. Passivation ultimately leaves the part clean and more corrosion resistant due to the passive oxide “film” layer but does not change the aesthetics of the part.

Passivation processes in the pharmaceutical industry are generally regulated by the ASTM, specifically ASTM guideline ASTM A967. The most common type of passivation used throughout the biotech industry is citric acid passivation because it is a low hazard cleaning agent and is biodegradable.

Electropolishing is an electrochemical process that provides a variety of benefits, including deburring, micro finish improvement, and improved corrosion resistance by eliminating embedded contaminants and surface imperfections. Through the electropolishing process, surface material is removed by anodic dissolution using electrical current passing through electrolytic solution.

In the electropolishing process, a part is submerged in a chemical bath treated with electricity, making it shed a uniform layer of surface material. This strips away contaminants and burrs while smoothing out imperfections like microscopic cracks and jagged edges. The part is left visibly bright and shiny, and it retains these characteristics even in highly corrosive environments.

Electropolishing is used in the pharmaceutical industry to improve surface uniformity, cleanability, and prevents attack from aggressive chemicals by rendering parts passive. While small surface finish improvements are achieved, electropolishing is meant to complement, not replace, mechanical polishing. At Holland, we work with subcontractors for our electropolishing that polish in accordance with ASTM B912 and ASME BPE specifications.

So what is the difference between electropolishing and passivation? Both are methods for increasing corrosion resistance, but their similarities end there. Passivation does not change the surface appearance nor does it offer a microfinish improvement. Additionally, passivation is not an effective method for removing surfaces that have been welded with oxide heat effect scale or heat treated.

Oftentimes both electropolishing and passivation are called out together on specifications. The proper process then is to electropolish to utilize the surface removal and polishing advantages of electropolishing and then follow it up with passivation to remove any possible free irons on the surface which were exposed during electropolishing.

Holland Applied Technologies has offered electropolishing and passivation services for quite some time. For any questions or guidance with your sanitary surface finish application, please contact a Holland Sales Engineer.

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