Electropolishing stainless steel: when the electrochemical fine finish pays off

At a glance

Electropolishing is an electrochemical process that smooths, deburrs and passivates stainless steel in the micro range. It reduces the effective surface area by up to 80 percent, improves corrosion resistance and makes parts easier to clean, particularly sought after in pharmaceutical, food and beverage technology, medical technology and tank construction. INOX COLOR processes parts up to 6 x 1.2 x 2 m and small parts as bulk material, with pickling, polishing and passivation from a single source.

 

What is electropolishing?

Electropolishing is an electrochemical surface treatment for metals that smooths a matt, rough surface and makes it shine. The part is connected anodically, that is to the positive pole of a direct-current source, and immersed in a special electrolyte. Under the action of the direct current, the electrolyte dissolves a thin layer of material from the surface. In principle it is the reversal of electroplating: instead of adding material, material is removed in a targeted way.

The decisive point is that material is removed preferentially at the roughness peaks. Exactly where the surface is most uneven in the micro range, the most is removed. The result is a levelled, glossy and metallically clean surface.

 

As often the problem lies in the micro range

Stainless steel is considered robust, hygienic and corrosion-resistant. Nevertheless, problems keep occurring in everyday use that really should not happen: a tank builds up deposits after every batch, a pump in the clean room shows the first signs of corrosion, a weld seam becomes a weak point. In many cases the cause lies not in the material, but in the surface.

Even a cleanly ground stainless steel surface is anything but smooth under the microscope. It shows fine grooves, burrs, micro-cracks and structural disturbances. Exactly in these depressions, particles, product residues and moisture collect. Germs find a hold, deposits form, and corrosion can begin at edges and stress peaks. What looks smooth and clean to the naked eye is a rugged landscape in the micro range.

 

What happens during electropolishing

Direct current removes a thin layer of material, preferentially where the current density is highest: at peaks, burrs and micro-roughness. The process works without mechanical, thermal or chemical stress on the part. Damaged material layers are removed along with it.

The result is a surface that is smooth and closed in the micro range, while the macro geometry is retained. Edges are reliably deburred, contamination and structural disturbances disappear, and a metallically clean, passive surface is created. Because the process can be controlled via current, time and electrolyte, the results are reproducible to tight tolerances.

 

Up to 80% less effective surface area

The perhaps most important effect can be expressed in a single figure: by removing the micro-roughness, the effective surface area of a component shrinks by up to 80 percent. Less contact area means less adhesion, less deposit formation and significantly better cleaning behaviour. For industries where hygiene and cleanability decide on quality and approval, this is often not a comfort feature but a precondition.

In concrete terms, components benefit from:

• metallic cleanliness and chemical passivity
• fine and ultra-fine deburring in the micro and macro range
• higher corrosion resistance and fatigue strength
• low germ adhesion and reduced cross-contamination
• favourable cleaning and sterilisation behaviour
• reduced friction and reduced wear
• a decorative, high-gloss appearance

Where electropolishing is used

The process is especially sought after in pipework and tank construction for the pharmaceutical, food and beverage industries, where components come into contact with critical substances and have to stay clean over long periods. But electropolishing also plays to its strengths in mechanical and plant engineering, in medical and laboratory technology, in vacuum technology, in water treatment or in decorative applications in architecture and design, everywhere that cleanliness, corrosion protection or a high-quality finish are required.

 

Electropolishing or pickling, what is the difference?

Both processes create metallically clean surfaces, but they work differently. Pickling is a purely chemical process: a pickling acid removes scale, heat tint and contamination, for example after welding, and creates the precondition for a uniform passive layer. However, it does not smooth the surface and produces no gloss.

Electropolishing goes further: it removes the micro-roughness in a targeted way, smooths, deburrs and creates a glossy, surface-minimised and passive surface. The two processes are often combined, first pickle, then electropolish, finally passivate. Anyone who needs gloss, lowest adhesion and highest cleanability is right with electropolishing; anyone whose main aim is to remove weld scale and corrosion starting points is often already right with pickling.

 

Why our plant makes the difference

Electropolishing is not an off-the-shelf product. The result depends on material, component shape and requirement, and on whether the service provider can tune the process to them. At INOX COLOR, what is often spread across several operations comes together: we process components up to 6 x 1.2 x 2 m and, in the same works, small parts as bulk material, from a single piece to a series. Pickling, electropolishing and passivation run from a single source rather than across three providers. This keeps the process chain short, makes delivery dates plannable and ensures a consistently controlled result. We have produced Made in Germany for 40 years and are certified to ISO 9001.

 

Frequently asked questions

Which materials are suitable for electropolishing?

Primarily stainless steels (e.g. 304 and 316L). Depending on the process, other metals can also be treated. Which electrolyte and parameters fit depends on the alloy and requirement.

 

How large may the components be?

At INOX COLOR up to 6 x 1.2 x 2 m. Small parts such as screws, springs or bolts are processed as bulk material in drums, larger parts individually on contact racks.

 

Does electropolishing change the dimensions of the part?

Material removal is in the micrometre range and is controllable. The macro geometry is retained, which is why even parts with tight tolerances can be processed reliably and reproducibly.

 

Does electropolishing replace passivation?

Electropolished surfaces are already chemically passive. In many cases passivation is still carried out as a safeguard, especially for high corrosion requirements.

 

Can weld seams be treated as well?

Yes. Complete welded constructions are electropolished as a whole when seams in the visible area are also to shine. Heat tint and scale are usually pickled off beforehand.