As we’ve written many times before in this blog, stainless steel is far more than just “corrosion resistant metal.” While it is true that its material strength and resistance to corrosion have made it versatile and the material of choice for many applications, it is also because it can be adapted to fit specific conditions. This applies to the various chemical makeups that comprise “stainless steel” and the variety of surface finishes and treatments that can be applied depending on the intended use.
2B or Not 2B?
One of the most widely used finishes within the stainless steel industry is known as Grade 2B. While not a mirror finish, it is semi-reflective, uniform and smooth. The surface is the last step in a process that begins with the steel plate being formed first by being pressed between rollers as it leaves the furnace, and then softened by annealing and then repeatedly passed through the rollers. The surface is pickled with acid to remove surface impurities, and then it is passed between polished rollers on the last of several passes to achieve the desired thickness. It is this final pass which that results in the 2B finish.
The 2B finish is a standard finish for common stainless steel grades including 201, 304, 304L and 316L. In addition to being economical and having increased corrosion resistance, what makes the 2B finish popular is that it is easy to buff using cloth wheels and compounds to make it very smooth and shiny. Steel with a 2B finish is typically found in baking equipment, food processing, tanks, vessels and pharmaceutical equipment and it meets USDA standards for those industries.
However, this approach is not acceptable when the final product is an injectable or otic (such as an antibiotic) solution. This is because the buffing process can create gaps or pockets in the metal’s surface. These voids can trap contaminants in the metal or below the surface of the polish. This foreign material will eventually leach out and contaminate the product. For such applications, electropolishing the surface is the recommended way to increase the smoothness of the surface.
Smoother than Smooth
Electropolishing uses chemistry and electricity to smooth the surface of stainless steel by removing the raised areas on the surface. Even with a factory-applied 2B smooth surface, the actual surface of the stainless steel does not appear smooth under magnification. The smoothness of the metal’s surface is expressed through the term Roughness Average (Ra), which compares the average difference between the high and low points across a section of the surface. Depending on its gauge (thickness), factory-fresh 2B-finished stainless steel typically has an Ra value ranging from 0.3 micrometers (.0003mm) to 1 micrometer (.001mm). Proper electropolishing can decrease the Ra of the surface to between 4 and 32 microinches depending on the gauge (thickness) of the metal.
The Same But Different
It is important to keep in mind that a Grade 2B finish is applied at the factory by being compressed through two rollers. After having a vessel or other equipment modified or repaired, some operators have asked for the finish itself to be “repaired.” While it is impossible to replicate the finish using mechanical or electropolishing, it is possible to come very close, especially in terms of RA values. In fact, the end result of a proper electropolishing treatment can be superior performance in material handling than the original, untreated 2B finish.
In that regard, a 2B finish can be seen as a good starting point. It’s an economical finish with well-known properties that can then be significantly further enhanced by electropolishing for a smoother surface that meets higher standards and that yields several long term benefits.