What is the difference between electropolishing vs passivation?

Electropolishing vs passivation involves comparing two distinct processes: electropolishing removes surface imperfections and contaminants to create a smooth surface. In contrast, passivation enhances the protective oxide layer on the metal's surface to improve corrosion resistance.

How does the electropolishing process work?

The electropolishing process uses an electrical current to dissolve the high points on a metal surface, resulting in a smooth surface finish. This chemical process removes surface contaminants and imperfections, improving the metal's surface appearance and corrosion resistance.

What is the purpose of chemical passivation?

Chemical passivation enhances the protective oxide layer on stainless steel and other metal parts. This process involves treating the surface with acids, such as citric acid or nitric acid, to remove free irons and improve the corrosion resistance of the metal's surface.

Why is mechanical polishing used before electropolishing?

Mechanical polishing removes major scratches and surface imperfections from metal surfaces. It prepares the surface for the electropolishing process, which will further smooth and clean it, ensuring optimal surface finish and corrosion.

What types of stainless steel parts can benefit from electropolishing?

Electropolishing can benefit various stainless steel parts, including food processing equipment and pharmaceutical components, and is a consideration with pharmaceutical equipment manufacturers. This process ensures high cleanliness, corrosion resistance, and a superior surface finish.

March 1st, 2024

Electropolishing vs Passivation: Key Differences and Benefits

Passivation alone can achieve a 1.0 or greater Cr/Fe ratio. If a 1.0 Cr/Fe ratio is needed, electropolishing vs. passivation shows that electropolishing is unnecessary, as chemical passivation treatments can achieve this ratio.

The 1.0 or greater Cr/Fe ratio indicated in the ASME-BPE standard is a minimal requirement. The best passive and corrosion-resistant surfaces will have a Cr/Fe ratio above 1.5/1, achievable by the passivation process alone. In most cases, the pharmaceutical industry requires a 15-25Ra value, typically achieved through a mechanical polishing procedure. This procedure can cause problems with forming “gray residue” and Class 1 rouge, a form of discoloration and corrosion.

Understanding Electropolishing

What Is Electropolishing?

Electropolishing is an electrochemical process that meticulously removes surface imperfections and contaminants from metal surfaces, resulting in a smooth, corrosion-resistant finish. Often referred to as “reverse plating,” this process uses an electric current to dissolve the high points on a metal surface, creating a featureless, mirror-like finish. Electropolishing is widely used to enhance the surface quality of stainless steel and similar alloys, making it an essential technique in industries that demand high cleanliness and corrosion resistance.

How Does Electropolishing Work?

Electropolishing involves immersing the metal part in a carefully controlled chemical bath, typically composed of phosphoric and sulfuric acids. An electric current is then applied, selectively removing surface imperfections and contaminants. This process improves the surface roughness (Ra) by up to 30-40% and ensures microscopic precision and uniformity. Electropolishing is highly effective for sizing and deburring metal surfaces. The result is a metal surface that boasts superior corrosion resistance and a pristine, smooth finish.

Mechanical Polishing

Mechanical polishing is a hand sanding process that uses various forms of abrasive media to remove scratches, gouges, and other damage from the surface material. The media is applied to the metal surface using hand-held power equipment, resulting in surfaces compliant with ASME-BPE surface finish standards. However, mechanical polishing damages the surface of stainless steel, leaving behind scratches and other surface contaminants. This damaged surface is known as the “Beilby layer” and is usually in the range of .0003” to .0005” in depth.

Electropolishing Process

Electropolishing removes surface contaminants from mechanical polishing and produces a more cleanable, featureless, smooth surface finish. The need to electropolish a surface depends on the desired surface finish requirements. All austenitic stainless steel surfaces, whether electropolished or not, require a passivation process to improve their corrosion resistance. The formation of a protective oxide layer on the surface does not require electropolishing.

Corrosion Resistance and Surface Layers

A study by J. Wulff examined three distinct surfaces: honed, ground (or mechanically polished), and electropolished. It found that the honed surface had up to three different layers, and the mechanically polished surface had up to seven distinct layers. In contrast, the electropolished surfaces demonstrated only one layer of pure austenite.

In mechanical polishing illustration, layers of the material are folded over on the surface. Studies have shown that underneath the “folds,” trapped particles of abrasives, oxides, polishing compounds, dyes, greases, and surface impurities are embedded in the distorted crystal structure. Studies further show that no cleaning or passivation can remove these surface impurities. Only when the system is placed into service, with normal operating conditions of heating and cooling cycles, does the material expand, allowing these surface contaminants to be released onto the surface and into the product.

Free Iron and Rouge Formation

Stainless steel is an alloy that contains approximately 64% iron, so the grinding dust released during this finishing process contains free iron particles that are distributed and then deposited downstream on piping and equipment walls, contributing to Class 1 rouge.

Benefits of Electropolishing

The electropolishing process offers the ultimate product contact surface by providing an optimum micro-surface finish, reducing total surface area, and providing pure alloy without contamination or damage at the material's product contact interface surface. Electropolished surfaces offer optimum cleanability, sterility, corrosion resistance, and a reduction in rouge formation.

Approximately .0005" of material is removed from the steel's surface during electropolishing. This removes the damaged layer and subsequent surface contaminants trapped under the smeared material on mechanically polished surfaces.

Protective Oxide Layer

In addition to the obvious benefits to the surface via the electropolishing process, ASTM B-912-02 specification recognizes electropolishing and electrochemical cleaning as an acceptable passivation process. Chemical passivation treatments, as defined by industry standards like ASTM A967 and AMS 2700, are crucial for ensuring optimal protection against corrosion. To meet ASTM-B-912-02, a nitric acid or citric acid and water passivation bath is applied at ambient temperature to a surface. It is a very fast and effective alternative to conventional passivation processes. A final rinse using deionized (DI) water at ambient temperature is performed after passivation. The duration of the rinsing process will be determined by testing the water to ensure that the effluent conductivity is within 1μS of the influent.

Chemical passivation is required after the electropolishing process occurs since electropolishing passivates the surface only to a condition typically attained by phosphoric acid. Chemical passivation treatments, as outlined in industry standards such as ASTM A967 and AMS 2700, provide guidelines on cleaning, descaling, and ensuring optimal protection against corrosion. Chemical passivation is passivation with citric acid-based chelant systems or nitric acid. It will effectively improve the Cr/Fe ratio and double the corrosion resistance of an electropolished surface alone.

Chemical passivation will improve the corrosion resistance of all austenitic stainless steel surfaces, regardless of the surface profile or surface roughness condition. It chemically removes the iron and iron oxide from the surface and leaves the protective oxide layer at the surface to protect the alloy. The effectiveness of the passivation process can be quantified or measured in terms of the Cr/Fe (chrome to iron) ratio. The more chromium on the surface, the more corrosion resistance will be present. Chemical passivation will attain a chrome-to-iron ratio above 1.0 (Cr/Fe) without producing any measurable change in the finish characteristics of the surface. Chemical cleaning and passivation will improve surface corrosion resistance and remove surface contamination but will not remove surface area, surface roughness, or cold work damage from mechanical polishing.

The passivation process improves the surface's chemistry (increasing the Cr/Fe ratio), while the electropolishing process removes surface damage, improves the surface profile and cleanability, and smooths the surface.

Although electropolishing passivates the surface, it doesn't meet the Cr/Fe ratio levels attained by comparative passivation processes. If achieving a set Cr/Fe ratio is the goal, passivation is the process to use. If a cleanable, corrosion-resistant surface is desired, an electropolishing process followed by chemical passivation is the best choice.

Processors must be more concerned with product contact surfaces beyond the Cr/Fe ratio. Proper material selection and surface conditions could reduce the need for repetitive passivation treatments to correct iron contamination and cleaning inefficiencies.

Electropolishing vs Passivation FAQs

What is the difference between electropolishing vs passivation?: Electropolishing vs passivation involves comparing two distinct processes: electropolishing removes surface imperfections and contaminants to create a smooth surface. In contrast, passivation enhances the protective oxide layer on the metal's surface to improve corrosion resistance.
How does the electropolishing process work?: The electropolishing process uses an electrical current to dissolve the high points on a metal surface, resulting in a smooth surface finish. This chemical process removes surface contaminants and imperfections, improving the metal's surface appearance and corrosion resistance.
What is the purpose of chemical passivation?: Chemical passivation enhances the protective oxide layer on stainless steel and other metal parts. This process involves treating the surface with acids, such as citric acid or nitric acid, to remove free irons and improve the corrosion resistance of the metal's surface.
Why is mechanical polishing used before electropolishing?: Mechanical polishing removes major scratches and surface imperfections from metal surfaces. It prepares the surface for the electropolishing process, which will further smooth and clean it, ensuring optimal surface finish and corrosion.
What types of stainless steel parts can benefit from electropolishing?: Electropolishing can benefit various stainless steel parts, including food processing equipment and pharmaceutical components, and is a consideration with pharmaceutical equipment manufacturers. This process ensures high cleanliness, corrosion resistance, and a superior surface finish.

ABOUT CSI

Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at www.csidesigns.com.