Features of stainless steel pipe!

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Features of stainless steel pipe!

2020-06-17 H:39:51
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1. Corrosion resistance

Stainless steel contains chromium. When chromium is oxidized, a dense chromium oxide layer is formed on the surface of stainless steel, which is called passivation layer. The thickness is (3-5) × 10-6mm. It can prevent the oxidation of metals the oxidation of oxygen iron. The loose iron oxide after carbon steel is oxidized, the loose iron oxide of carbon steel is continuously oxidized, rusted peeled. Due to the presence of chromium, a dense chromium passivation layer is produced in stainless steel to protect the metal further oxidation. The passivation film of stainless steel has the function of automatic recovery after damage, so the content of chromium is the key to affect the corrosion resistance of stainless steel pipe. Copper aluminum also have dense passivation layers, but the strength of copper aluminum is lower than that of stainless steel.

2. Solid solution is the key to improve the corrosion resistance toughness of stainless steel pipe

Most stainless steel tubes are made of austenitic stainless steel containing more than 8% nickel. The ferrite content in austenitic stainless steel is usually less than 8%. The left figure of Fig. 1 shows the body centered cubic ferrite lattice, the right figure shows the face centered cubic austenite lattice. The eight corners of the cube are iron atoms, the volume face center are carbon atoms. At high temperature, carbon steel, alloy steel stainless steel are FCC austenite lattice with three carbon atoms. For carbon steel low alloy steel, after austenitizing cooling, the spacing between iron atoms decreases, austenite shrinks, thus transforming into a ferrite lattice containing only one atomic carbon. Carbides are easily precipitated austenite. Because the nickel content of austenitic stainless steel is more than 8%, the atomic weight of nickel is 58.71, which is greater than that of iron (55.847). The austenite lattice expands is heated in solution after nickel atom replaces iron atom in austenite lattice. When the spray water cools rapidly, the lattice shrinks rapidly, there is no time for carbon to precipitate out. The carbon atoms are fixed in the austenite lattice.

If the austenitic stainless steel is cooled rapidly after heating, the carbon atoms dissolve in the austenite lattice, the carbide will precipitate at the grain boundary, so as to avoid the formation of chromium carbide, the exhaustion of chromium in the grain boundary, the combined oxidation of oxygen iron, so as to avoid intergranular corrosion. Similarly, austenitic stainless steel rapidly cools, carbon atoms dissolve in the austenite lattice, there is no carbide on the grain boundary, there is no obstacle to metal deformation, so the stainless steel becomes very soft, with large elongation good toughness; after quenching of carbon steel with ferrite matrix, the carbide precipitates during quenching, but the toughness decreases. The faster the cooling, the coarser the carbide precipitated the lower the toughness. Therefore, the solid solution of austenitic stainless steel should be heated cooled rapidly to avoid carbide precipitation.

3. Dual phase stainless steel pipe is resistant to chloride ion corrosion

The passivation film on the surface of stainless steel is more brittle in these areas due to defects, impurities solute inhomogeneity in the steel. When the etching pit exceeds the critical size (tens of microns), it is easy to corrode in the corrosive solution containing chlorine. The damaged part becomes the activated anode, the surrounding area is the cathode area. The area of the anode is very small. The current density of the anode is very high, which leads to active dissolution. Then, it becomes many pinprick holes becomes "pitting". Due to its small radius strong penetration, Cl ions can easily enter the passive film adsorb on the metal surface, then form strong acid weak basic salt with Fe2 ion produced by corrosion, which makes the microenvironment more acidic. So as to accelerate the corrosion process. Duplex stainless steel is half of austenite ferrite in stainless steel. Figure 2 is a metallographic picture of duplex stainless steel. Black is ferrite white is austenite. These two stages are distributed in layers. The existence of one phase can prevent cracks in the other phase. The propagation of cracks in ferrite phase needs to bypass the island austenite phase as a barrier layer. Therefore, the duplex stainless steel pipe is resistant to chloride ion corrosion.



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