Surface Treatment and Finish

The surface treatment of a vessel is required for any surface that comes in contact with the product. It is imperative that all stainless steel surfaces are treated and cleaned in a way that prevents corrosion under the operating conditions. Stainless steels are corrosion resistant due to the formation of a microscopically thin, invisible chromium oxide layer, which occurs on clean metal and polished surfaces only. The three main surface treatment methods used are mechanical, chemical, and electrochemical.

Mechanical Surface Treatment: It is possible to improve the surface quality of steel parts in contact with the product by means of mechanical treatment. The surface finish qualities of the vessel exterior are governed more by aesthetic rather than functional attributes. The quality and smoothness of the mechanically treated surfaces are defined by values in terms of arithmetic mean roughness, Ra. Minimal surface qualities of Ra ~0.6 |im (220 grit) are generally accepted and approved in biotechnology. The finest polish or abrasive belt (400 grit) can be used to achieve smoothness values of Ra 0.28-0.34 | m. The best possible smoothness values are achieved with steel grades, SS 304L and SS 316L.

Chemical surface treatment: The two main types of chemical surface treatment are cauterization and passivation. Cauterization is the dissolution of the flaws on the vessel surface with acid mixtures, such as 15-25% (v/v) HNO3, and 1-8% (v/v) HF, for steel with chromium content above 15.5%. Surface imperfections in the interior of vessel prevent the formation a perfect passive layer. The impurities that require cauterization may be interior surfaces with oxidation tints, welding slag residues, and fine scales or overlaps generated during the working process.

Passivation is used either after cauterization or as a final treatment of ground, brushed, or polished steels with special surface structures. A natural passive layer is invariably formed when stainless steel is exposed to air. Artificial passivation with dilute nitric acid is frequently used for corrosion resistance at critical condition. The oxidizing effect of nitric acid accelerates the formation of a dense passive layer. Reports on passivation show that organic acid chelating agents can produce a more long lasting passive layer (199).

Electrochemical surface treatment: Electropolishing, or mirror polishing, is an electrochemical treatment which smoothes and polishes rough, dull surfaces. The vessel is immersed in an electrolyte and serves as an anode. The quality of the mirror polishing depends mainly on the source material and its mechanical pretreatment. Vessels with good Ra values may sometimes have microscopic surface roughness, which can be smoothed electrolytically. However, there is likelihood that existing imperfections on the material surface may be accentuated after mirror polishing. An electropolished (EP) vessel requires polishing to 320 grit. Finishes commonly used are shown in Table 3.8.

Inspection of Surface Treatment: The quality of the surface and welds after surface treatments are checked with following methods:

1. Visual inspection

2. Detection of ferric contaminations with the ferroxly test

3. Palladium testing to check the surface passivation

4. Detection detergent residues

5. Detection of chloride or sulfur contamination

6. Detection of pickling damage

7. Scanning needle electronic surface measurement

Table 3.8

Finish represented in Grit and Microns

Table 3.8

Finish represented in Grit and Microns

Grit Designation

Microns (|lm)

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