With development of economic construction continually, using sanitary stainless steel tube in each industry becomes more and more wide. Through analysis of Stainless surface passivation layer corrosion resistant ability and combining with technology and device application that our company produce small sanitary stainless steel tube, manufacture high quality steel tube, and satisfy requirement of industrial fluid tube.
Keyword: Surface passivation layer corrosion resistant ability of production process and equipment tube inside and outside surface finish sanitary tubes are widely used for pharmacy, food, peer, drinking water, bioengineering, chemical engineering, air purification, aviation and nuke industry on many areas much industry of national economy construction, having many import every year. Here are introduction of our industry producing Health level (food grade) stainless steel technology and equipment and tubal property and quality.
First, surface analysis of stainless steel
Auger electric spectroscopy (AES) and X-ray optical spectroscopy (SPS) method can be used for the stainless steel surface analysis, to determine the stainless steel tube inside and outside surface corrosion resistance ability. AES method for the analysis of the very small diameter, can be less than 20 nm, its original function as recognition elements. XPS method of analysis of straight yao about 10 microns and is mainly used to determine the near surface chemical state of element. By AES and XPS detectors of mechanical polishing has been exposed to the atmosphere of 316 stainless steel surface scanning results show that the analysis on the surface of a stainless steel king kong total depth of the most typical of 15 nm, and provide the composition of the passivation layer, thickness and its corrosion resistant ability, etc.
Second, stainless steel tube surface corrosion
1, medium in the Ci – stainless steel surface passivation layer is easy to be destroyed, this is because the Ci – oxidation potential can be larger. If the passivation layer printing is changed in metal alone will continue to be corrosion. In many cases, blunt layer on the surface of the metal, only local places are destroyed, the role of the corrosion is a small hole or pit, in the distribution of the material surface produce irregular pitting is called small pit corrosion. Pitting corrosion rate increases with temperature rise and increased with the increase of concentration. The solution is to use low or low carbon stainless steel (such as 316 l, 304 l)
2, Austenitic stainless king at the time of manufacturing and welding stainless steel surface blunt with layer easy to damage. Manufacturing and welding when the heating temperature and heating rate in the region of the stainless steel sensitization temperature (about 425-815 ?), the material of the supersaturated carbon will precipitate in the grain boundary firstly, and the combination with chromium chromium Cr23C6 carbide formation, the diffusion velocity of carbon in austenite is bigger than chromium diffusion velocity, chromium to supplement the grain boundary due to the formation of chromium carbide and the loss of chromium, as a result the chromium content of grain boundary will continue to fall along with the continuous precipitation of chromium carbide, forming the so-called poor chrome area, make the electric pad can be reduced, the passivation layer corrosion resistant ability. When in contact with the medium Ci – such as corrosive medium, it could cause micro cell corrosion. Although only in grain surface corrosion, but it quickly into internal forming intergranular corrosion. Special stainless steel tube in the welding processing areas is more obvious.
3, Stress corrosion crack: is the static stress and cause crack role together with the corrosion of the metal embrittlement. Stress corrosion crack breaking destroy the environment is often quite complex. Is not only a tensile stress, the stress and the production, welding, or combination of residual stress in metal heat treatment.
Third, surface treatment of stainless steel tube inside and outside and corrosion resistance
Stainless steel tube inside and outside surface (especially if after electrolytic polishing, mechanical polishing) has good passivation, corrosion resistant ability is stronger. Inside and outside surface finish high, medium adhesion rarely for corrosion resistance. Tube surface finish high liquid medium stranded the less conducive to wash, especially in the pharmaceutical industry.
1, the tube surface electrolytic polishing electrochemical polishing (): electrolytic polishing liquid is phosphoric acid, sulfuric acid, chromic anhydride, gelatin, potassium dichromate, etc.Stainless steel tube surface at the anode, the polishing liquid flows within and with low voltage large current and electrolytic polishing processing.The tube surface with two conflicting process at the same time, the metal surface passivation layer (including thick mucous membrane) formation and dissolution.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.Because of the part surface micro convex and concave part into the membrane into the dull 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.And in this role, the tube sheet increases the chromium content, increased the corrosion resistant ability of passivation layer on the metal surface.
How to grasp the quality of the polishing with electrolyte formula, concentration, temperature, current time, current density, electrode, the degree of pipe surface treatment and so on.Technical mastery is bad it will destroy the tube surface finish, electrolytic degree will have greater concave and convex surface, even tube scrap, really make good quality needs certain technology, cost is higher.
2, the tube surface mechanical polishing: spins and linear polishing.Here in rotating mechanical polishing, for example: mechanical polishing equipment is simple, the motivation and pads, advanced polishing equipment is simple, the motivation and pads, high-grade polishing wax.Are of fine sand cloth plate and plate in many times back and forth on the tube surface multichannel polishing processing, can reach Ra finish 0.2 to 0.4 microns or less.
Mechanical polishing and electrolytic polishing compared with simple equipment, low technical content easy to grasp, cost is also low, won’t destroy tube caused scrap, so widely used.But the printing substrate surface corrosion resistant ability of electrolytic polishing.
Fourth, sanitary stainless steel pipe production process
Steelmaking, rolling round steel, perforated, cold drawing, cold rolled, bright annealing, inner surface polishing, surface polishing, inspection acceptance, packaging warehousing.
Fifth, the choose and buy of sanitary stainless steel tubes
Stainless steel is chromium containing 50% iron and 10.5% less, and also to join such as nickel, titanium, molybdenum, according to different containing suspension components, the metal internal microstructure is different also, it is markov shape, iron grain size, austenite, duplex, precipitation hardening stainless steel, etc.Sanitary stainless steel tubes also have different materials.
General industrial tube austenite size stainless steel material is more commonly used;00 0 cr19ni9 (USU304), cr19ni11 (USU 304 l), 0 cr17ni12mo2 (USU) 316, 00 cr17ni14mo2 (USU 316 l), 1 cr18ni9ti (SUS 321), etc.
By the way of health grade stainless steel tube also need stainless steel sanitary fittings.