There are various misconceptions regarding the surface finishes of stainless steel and some confusion about suitability.
It is often assumed that the more visually reflective the surface i.e. mirror polished, the less friction imparted on the material flow, but this is not necessarily true and tests are best carried out to establish product behaviour. Mirror polished surfaces also appear to be more hygienic but again this is not necessarily really true
Below is an explanation of popular finishes in the process industries:
Mirror Polishing
Although often specified and thought to be hygienic, mirror polishing is one of the least hygienic finishes. If studied under a microscope it is evident that the mirror polishing actually folds over the ‘peaks’ produced during normal polishing. These peaks or folds then become bug traps or dirt traps as polishing media becomes trapped inside the folds. Mirror finish is often selected to give help to allow material flow, but this also is not really a benefit that should be considered. Mirror often looks very good but should be avoided if possible in any standard of hygienic execution.
Satin Polishing
This is the most common type of pharmaceutical finish and in fine grit the surface quality is actually quite acceptable but not really the best hygienic finish although very commonly used. This finish is adequate for any non-sterile pharmaceutical application.
Electro-Polishing
Electro-polishing is the most hygienic type of finish. The process induces preferential dissolution of the ‘peaks’ or high spots on the surface of the work piece. This results in a net smoothing of the surface, which is also beneficial in removing surface stresses left over from mechanical polishing pre-treatments.Contamination left from mechanical surface treatments is also removed by electro-polishing. However, scratches and visible surface irregularities are not likely to be removed. Unfortunately electro-polishing actually looks inferior as it highlights welded areas and surface imperfections.
Cold Rolled
Cold rolled or ‘2b’ is a finish obtained during the rolling of the sheet material at the steel mill. The cold rolled material is also acid cleaned at the mill. Despite the dull unpolished appearance of 2b stainless steel it has an acceptable level of hygienic quality for many non-sterile applications and is often used in IBC manufacture with welded areas ‘strip polished’.
Bead Blasting
Bead Blast finishes are produced by the impact of a hard, inert medium onto the steel surface result in non-directional, uniform matt surfaces with low reflectivity.
Conclusion
These finishes have aesthetic appeal and can enhance the surface properties of stainless steels by hardening. The most common material used for bead blasting of stainless steel is glass beads. This produces a light smooth surface finish. A bead blast finish is not normally suitable for product contact surfaces within the pharmaceutical and associated industries.
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