The Relationship between the Surface Treatment and Corrosion Resistance of Sanitary Fittings

The inner and outer surfaces of sanitary stainless steel fittings have good passivating layer and strong corrosion resistance. Sanitary fittings have a high finish on the inside and outside surfaces, so they rarely have medium adhesion, which is good for corrosion resistance. The less the liquid medium with the high surface finish in the pipe is retained, the easier it is to flush, especially in the pharmaceutical industry.

Electrochemical polishing

Electrolytic polishing solutions include phosphoric acid, sulfuric acid, chromic anhydride, gelatin, potassium dichromate, etc. The inner surface of the stainless steel pipe is on the anode, and the polishing fluid is flowing in and out of the stainless steel pipe with low voltage and large current for electrolytic polishing treatment. At the same time, two contradictory processes occur on the inner surface of the tube, namely the formation and dissolution of the metal surface passivation layer (including thick mucosa). Due to the different conditions for the passivation of the film forming in the convex and concave parts of the surface and the dissolution of the anode, the metal salt concentration in the anode area keeps increasing, so a kind of high resistance thick mucosa is formed on the surface. The different thickness of the film at the concave and convex parts leads to high current density on the anode surface and fast dissolution rate at the tip, it can achieve the goal of cutting the prominent micro part in a short period of time, which can achieve a high smoothness Ra≤0.2-0.4μm.

Electrochemical polishing

How to master the quality of polishing is related to electrolyte formulation, concentration, temperature, power on time, current density, electrode condition, surface treatment degree of the tube, etc. If the technology is not well mastered, the surface finish of the tube will be damaged. If the electrolysis is too high, more and more concave and convex surfaces will appear, or even the strip will be scrapped.

Mechanical polishing of the inner surface of sanitary fittings

Mechanical polishing of the inner surface of sanitary fittings includes rotary and linear polishing. Here is an example of rotating mechanical polishing. The machine polishing equipment is relatively simple, the cloth dish made of fine sand grain by grade and the cloth dish are polished repeatedly on the outside surface of the pipe, and the smoothness can reach a level of Ra≤0.2-0.4μm. Compared with electrolytic polishing, mechanical polishing and electrolytic polishing are characterized by simple equipment, low technical content, easy to master, low cost, and no scrap due to pipe damage, so they are widely used.

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