The relationship between surface treatment and corrosion resistance of stainless steel pipes

The surface of the stainless steel pipe (especially after electrolytic polishing and mechanical polishing) has a good passivation layer and has a strong corrosion resistance. The inner and outer surface of the surface is clean and high, and the adhesion of the medium is rarely conducive to corrosion resistance. The less the surface finish, the less the liquid medium, which is good for washing, especially in the pharmaceutical industry.

1. Internal surface electrolytic polishing (electrochemical polishing) : electrolytic polishing liquid is phosphoric acid, sulphuric acid, chromium anhydride, gelatin, potassium dichromate, etc. The surface of the stainless steel pipe is on the anode and the polishing solution is electropolishing with low voltage and high current. At the same time, there are two contradictory processes on the surface of the pipe, namely the formation and dissolution of the passivation layer of the metal surface (including the thick mucosa). Because the surface microcosmic bulge and the concave part of the film enter passivation condition is different, and because of the anode dissolving. Because of the part surface micro convex and concave part into the membrane into the passivation condition is different, and because of the anodic dissolution, anode area metal salt concentration increasing, the surface to form a high resistance of thick mucous membrane. Different thickness of the membrane in the concave and convex leads to the surface of the anode current density is big, point discharge dissolving speed, in a short time, achieve the goal of made outstanding micro parts, can achieve very high roughness Ra 0.2 to 0.4 microns or less. Under this action, the inner table increases the chromium content and increases the corrosion resistance of the metal surface passivation layer.

How to master the quality of polishing is related to electrolyte formula, concentration, temperature, current density, electrode condition and surface treatment. Technical mastery is bad it will destroy the pipe surface finish, electrolytic degree will have more greater concave and convex surface, even pipe scrap, really make good quality needs certain technology, cost is higher.

2. Internal surface mechanical polishing: rotation and linear polishing. For example, the mechanical polishing equipment is relatively simple, the power and the polishing disc, the advanced polishing equipment is relatively simple, the power and the polishing disc, advanced polishing wax. The cloth disk and cloth plate made of fine sand are used for polishing and polishing on the surface of the inner surface of the pipe for many times, and the smoothness can reach the Ra less than 0.2-0.4 mu m.

Mechanical polishing and electrolytic polishing are relatively simple and easy to master, with low cost and low cost. Therefore, it is widely used. However, it is much better to electropolish the surface printing layer to resist corrosion.

Source: China Steel Pipe Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)