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Relationship between internal and external surface treatment and corrosion resistance of high pressure boiler tube fittings
Edit:Shenyang Sidi Ou Trading Co., Ltd.   UpDate:2018-05-29

The inner and outer surfaces of the high-pressure boiler tube (especially after electropolishing and mechanical polishing) have a good passivation layer and strong corrosion resistance. The internal and external surface finish is high, and the adhesion of the medium is less favorable for corrosion resistance. The inner surface finish of the tube is high and the less liquid medium is retained, which is beneficial for rinsing, especially in the pharmaceutical industry. 1. Electrolytic polishing of the inner surface of the tube (electrochemical polishing): The electrolytic polishing liquid is phosphoric acid, sulfuric acid, chromic anhydride, gelatin, potassium dichromate and the like. The inner surface of the stainless steel tube is on the anode, and the polishing liquid flows in and out to perform electropolishing treatment with a low voltage and a large current. At this time, the inner surface of the tube carries out two contradictory processes at the same time, that is, the formation and dissolution of the metal surface passivation layer (including the thick mucous membrane). The conditions for film formation into the passivation due to the microscopic convex portion and the concave portion of the surface are different, and the anode is dissolved. Since the conditions for the surface microscopic convex portion and the concave portion to form a film into the passivation are different, and the anode is dissolved, the concentration of the metal salt in the anode region is continuously increased to form a high-resistance thick mucous membrane on the surface. The thickness of the film at the unevenness causes the current density of the anode surface to be large, the tip discharge dissolves quickly, and the microscopic portion of the flattened protrusion is achieved in a short time, and a high degree of smoothness Ra ≤ 0.2-0.4 μm can be achieved. Under this action, the inside of the tube increases the chromium content and increases the corrosion resistance of the passivation layer on the metal surface. How to master the quality of polishing depends on the electrolyte formulation, concentration, temperature, energization time, current density, electrode condition, and tube surface treatment. If the technology is not well mastered, it will destroy the surface finish of the tube. If the degree of electrolysis is too large, more and more bumps will appear, and even the tubes will be scrapped. It takes a certain amount of technology to make good quality, and the cost is high.

2. Mechanical polishing of the inner surface of the tube of high-pressure boiler : there is rotation and straight line polishing. Here, rotary mechanical polishing is taken as an example: mechanical polishing equipment is relatively simple, power and polishing disc, advanced polishing equipment is relatively simple, power and polishing disc, advanced polishing wax. The cloth disc and the cloth disc made of step-by-step fine sand are polished on the inner and outer surfaces of the tube many times, and the finish can reach Ra≤0.2-0.4μm. Compared with electropolishing, mechanical polishing has the advantages of simple equipment, low technical content, easy to master, low cost, no damage to the tube and waste, so it is widely used. However, the surface imprinting layer has much better corrosion resistance and electrolytic polishing.