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durable finish that would meet most pharmaceutical industry requirements for
cleanliness if this finish could be maintained during shipping, handling, fabrication,
construction, and service. This finish is produced by rolling sheet through highly
polished rolls but, unfortunately, this finish is not applicable for plate, pipe,
castings, and machined components. As the production processes for plate, bar
stock, and castings do not produce asimilar finish, these components need to be
finished to produce an equivalent surface.
Polishing is specified to facilitate cleaning, inhibit corrosion, and improve
appearance. It may or may not improve “cleanability” as mechanical polishing
produces an infinite number of microscopic “scratches” that may provide refuge for
microscopic particles including bacteria.
On the other hand, electropolishing will remove peaks, may remove corrosion-causing
impurities, and will reduce both surface roughness and area making
it more difficult for dirt, bacteria, or proteins to “cling” to the surface or hide in the
scratches. The reduction in surface area also reduces the number of sites where
corrosion can originate.
Polishing improves the appearance of asurface. If there isaneed to sanitize
exterior surfaces, electropolishing of the exterior may be justified but often exterior
polishing is done solely to improve the appearance and is an unnecessary cost
particularly when the exterior surface is covered by insulation.
Cost of Polishing
The following table provides acomparison of polishing costs for arange of typical
vessel sizes:
Vessel budgetary pricing (Oct 03) [150 psig, 3508 F(10 barg, 1808 C)]
Vessel volume (L) SFT 3finish SFT 5(add) Passivation (add)
100 $11,000 $14,000 $2,000
1,000 $17,000 $23,000 $3,000
10,000 $60,000 $79,000 $4,000
100,000 $300,000 $375,000 $23,000
Greene
Types of Polished Finishes
Mechanical cleaning using blasting, grinding, and/or wire brushing is the primary
cleaning method used by fabricators to clean stainless steel.
Sandblasting may be used to create auniform finish on stainless steel and
remove fabrication soils, smearing, and heat tint caused by welding. The surface
will appear dull with asilver gray appearance and have aroughened texture
that will soil easily. The force of the blast can cause buckling and warping particularly
when finishing lighter gage materials. As shot, grit, or sand might damage
the surface, they should not be used for sanitary applications.
Wire brushing with stainless brushes is the next most common method of
cleaning. Carbon steel brushes should never be used, as iron particles will become
embedded in the surface and cause rouging. Even stainless steel brushes may leave
alayer of dissimilar material on the surface that can lead to rouging. Heat tint and
other surface contamination can be removed using clean abrasive discs or flapper
wheels to lightly grind the surface but these methods may also leave asmeared
layer of lower corrosion resistance. This mechanically smeared surface layer is