This blog is the first in a series of posts about sanitary surface finishes. In this post, we will cover surface finishes as they pertain primarily to sheet metal and subsequent posts will provide further detail on surface finish standards for fluid process components.
How is a specific level of finish defined and how can that level of finishing be achieved? For our industry, surface finishes are a standard set by regulatory agencies (3A, ASME BPE, etc.) and manufacturers who specify a certain level of finishing on processing equipment in order to ensure cleanability and sanitation. These bodies require sanitary finishes to have a minimum Ra (roughness average) typically measured in microinches (µin) or micrometers (µm). But what is considered clean in one application will likely be different from the surface finish requirement in another application. For example, the 3A requirement is generally equivalent to, or smoother than a 32 Ra (microinch) while the ASME BPE standard categorizes several different surface finishes ranging from 0 to 32 Ra through the means of mechanical polishing and electropolishing.
You may know what type of finish you need for your application, but if you want to really understand the various terms and definitions used for stainless steel surface finishes then you can read about them here where we define important terms such as Ra, roughness, and grit.
Surface finishes can be enhanced through various means of mechanical and chemical treatments. In our industry we typically see these treatments achieved via the following methods.
Sanitary Tubing with an Electropolish Finish
Mill Finish is the baseline finish used for comparison. Mill finishes will have a dull or matte non-uniform appearance with a Ra of at least 100 µin. This finish does not meet sanitary processing finishing requirements.
2B Mill Finish will have a more reflective appearance—almost like a cloudy mirror. This finish is more common in industrial, chemical and food applications and will have a large range of Ra values.
No. 4 Finish will have a Ra of 29 to 40 microinches. Although it can be found in clean rooms and in food processing equipment, it is not compliant for 3A standards (32 Ra or less).
No. 4A Finish is similar to the no. 4 finish in that is characterized by short, polished brush lines but uses a finer grit polish. This finish is required to meet the basic 3A standards and will have a Ra of 18 to 31 microinches.
Bead blasting uses a fine bead material, such as glass, at a high pressure to provide a certain level of finish. Ra values are typically greater than 45 Ra, depending on the blasting process.
Passivation means removing the excess iron or iron compounds from the surface of the metal using an acid solution (nitric or citric acid). Removing these impurities allows the formation of an oxide layer that protects the rest of the steel from corrosion. Passivation has little effect on Ra values.
Electropolishing is an electrochemical process where the surface metal is dissolved and the embedded contaminants are removed. This creates a “smooth” mirror finish and can result in increased smoothness.
As you can see, there are many manufacturing and chemical processes used to achieve various levels of finishes. When selecting equipment with higher-end finishes, often times we see processors elect to exceed the minimal standards set by regulatory agencies in order decrease bacteria risk. Hopefully this post proves as a useful reference when weighing the various stainless steel finishes on sanitary equipment.