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Navigating Stainless Material and Finishes for Hygienic Vessels

Stainless Finishes for Hygienic Vessels are measured by an RA (Roughness Average) meter.  Manufacturers of custom vessels for life science, health and pharmaceutical industries adhere to strict design parameters to maintain compliance in these industries. The RA is a standard for an average of peaks and valleys on the metal’s surface, measured in microinches.  Here is the evolution of stainless material coming to Apache with RA designations as it is transformed into custom sanitary design vessels:

RA > 100 Mill Finish  A Mill Finish has an unpolished, dull-gray, matte appearance. This material type is the basis for supply for all stainless-steel flat products as well as the basis for additional finishing operations.  It is hot or cold rolled with an RA (Roughness Average) of more than 100 microinches, depending on the gauge of the material. 

RA 15 – 40 2B Mill Finish  2B Mill finish is widely used in food, chemical and pharmaceutical equipment applications. It is corrosion resistant and has a typical RA range of 40 (7 gauge) to 15 (16 gauge) microinches.

RA 29 – 58 No. 3 and No. 4 Finish  Mechanical polishing achieves these finishes. The No. 3 Finish uses a 120-grit abrasive and has a semi-polished finish with an RA range of 36 – 58 microinches.  The No. 4 Finishes uses a 150-grit abrasive and has a polished, brushed surface.  The RA range is 29 – 40 microinches.

RA 18 – 31 No. 4A  Also identified as No. 4 Dairy Finish, this finishes is required to meet the basic FDA 3-A standards. It uses a 180 grit and has a RA range of 18 – 31 microinches.

< 32 RA Pickle Passivation   Pickle Passivation often follows other manufacturing processes including welding and grinding. Also called Pickling, it is the immersion of the metal in a pickling bath or coating the material with pick­ling solution, such as nitric-hydrofluoric acid. The process removes both metallic contamination and heat-treating scales. Pickle passivated stainless steel has a matte appearance. Apache’s tests have confirmed improvements up to 25% in RA read­ings on material that has been pickle passivated.

Lowest RA Electropolishing Electropolishing is an electro-chemical process that removes surface materi­al from stainless steel. The process includes an immersion of the stainless-steel component into a temperature-controlled bath of electrolyte that is charged with a DC power supply. Electrolytes used in electropolishing are concentrated sulfuric and phosphoric acid solutions. The finish has a mirror appearance. Apache’s before/after tests have shown improvements in RA smoothness up to 50%; results vary depending on stainless material.

In the highest level of sanitary design applications the vessel is pickled after manufacturing and polishing, then electropolished.


What is the difference between pickle passivation and electropolishing?

pickle passivation and electropolishing difference

The main difference between pickle passivation and electropolishing is the appearance and RA (roughness average) of the stainless material. In the photo, an electropolished sample is shown above a pickle passivated sample. The pickle passivated sample has a white appearance, while the electropolished sample has a mirror-like, shiny finish.

Pickle Passivation is the immersion of metal in a pickling bath such as nitric-hydroflouric acid. The process removes metal such as impurities and heat marks from welds.

Electropolishing is an electro-chemical process that removes surface material from stainless steel. The process typically starts with an immersion of the component into a controlled bath of electrolyte that is charged with a DC power supply. Apache’s RA tests show improvements up to 50% smoother, depending on the material.

Do you have more questions like this one?

Check out the Contract Manufacturing FAQs page.


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