Most metallic elements can form oxides with oxygen in the air or dissolved oxygen in water ,such as iron oxide Fe2O3 and stainless steel containing chromium elements react with oxygen to generate rich chromium oxide (Cr2O3) which has the characteristics of compact , can isolate effectively use the corrosive medium in the environment , so as to achieve the aim of relative stainless steel pipe
Because stainless steel pipes are not completely rust free, only in the correct use of the environment and installation method as the premise , can ensure that stainless steel rust free , the service life can up to 70 years
①Chemical Corrosion (Dry Corrosion)：Corrosion caused by the reaction of chemicals in the environment with stainless steel including chloridion, oxalic acid , cement paste etc
②Electrochemical Corrosion(Humidity Corrosion):Due to stainless steel has the characteristic of electrode potential difference (such as iron,carbon steel) ,corrode of galvanic cells by contacting metal .Electrochemical corrosion includes microbial corrosion and stray current corrosion
③Intergranular Corrosion: In the condition of high temperature , such as welded , chromium element in stainless steel diffusion speed slower than that in carbon steel , chromium and carbon grain boundary diffusion to combine to form carbon chromium compounds (rather than the passivation membrane Cr2O3) grain boundary caused by poor chromium (to achieve anti-corrode, chromium content should be 12% or higher) under the influence of external corrosion medium corrosion
④Stress Corrosion：Stress is a kind of reaction force , the product needs to apply a force to make it forming and the relative stress (resist the forming force ) is restore the original state of the material.Stress corrosion is the destructive corrosion caused by the interaction of stress and corrosive medium in the service environment
- Chemical Corrosion：Avoid contacting with corrosive substances caused by chemical reactions with stainless steel .Otherwise, wash the wall with oxalic acid with plenty of clean water to reduce the concentration of oxalic acid in the environment .Avoid contacting stainless steel pipes and fittings with mortar construction
- Electrochemical Corrosion:Avoid contacting with stainless steel which has electrode potential difference directly ,mainly reflected in the connection of pipe fittings , such as stainless steel can not contact steel bracket , iron valve directly.Otherwise , adding an insulating layer at the contact position . It is recommended that when installing the stainless steel pipe , all the supporting pipe fittings should be made of stainless steel.At the same time , in the construction site , do not put the electrical pen in the stainless steel pipe ,and do not use the stainless steel pipe as a conductor of the circuit to avoid stray current corrosion
- Intergranular Corrosion&Stress Corrosion:Stainless steel raw material with low carbon content can be used for solid solution heat treatment
Chloride Permit Scale
Stainless Steel Brand No.
L means low carbon , carbon content is low in raw material from that brand
①High carbon will cause carboloy chromium compound in commissure , form sensitization zone thereby reduce corrosion resistance in commissure , low carbon content can alleviate this problem
②Carbon content will influence material’s hardness. When the material hardness is large , the force exerted during the molding of the product is large , and the corresponding stress will also increase .For ultra-low carbon brand , the stress generated in the production process is smaller.
③In addition to reducing the hardness to control stress , the clamping tool can better compress the lower hardness pipe and pipe fittings when they are installed and pressed together , so as to improve the connection performance.
Thermosetting solution treatment is a process of rapid cooling with water after heating the product at a high temperature of 1040-1080℃ under the protection of hydrogen gas .At high temperature , the carbide can be completely or basically dissolved , so that the carbon element in the austenitic saturated state can no longer form carbide, so as to improve corrosion resistance . Thermosetting solution treatment can effectively eliminate the weld sensitization zone of the product and restore the base material properties of the stainless steel.In addition , it can also reduce the hardness, improve the toughness and eliminate the product stress , which is a necessary process in the production process of pipe fitting
Argon Arc Welding Process is Are Welding under the protection of argon.Because the temperature at the arc of the torch is extremely high(≥10000℃)，it will form carbology chromium compound in commissure at high temperature and lower the corrosion resistance.Therefore internal and external argon protection is required during welding to isolate oxygen , lower the commissure’s temperature rapidly to reduce the form of carbology chromium compound.
①Because the pipe fitting’s shape is complex and various , argon gas protection is difficult to reach all aspects.It’s easy to form carbology chromium compound in commissure to lower the corrosion resistance .Therefore thermosetting solution treat is a necessary process.
②Because deformation of pipe fitting is large while forming , the force is also greater . It needs thermosetting solution treat to reduce stress corrosion and avoid the springback stress
③Thermosetting solution treat can low pipe fitting’s hardness and improve the fit and connection performance while pressing
①Inspection of appearance : insect the commissure’s color (no return color is allowed) and welded technique (no undercut,slider skips and hole are allowed )
②Flaring Test: Pipe shall be flared by 60° circular cone. Breakage and crack are not allowed.
③Flattening Test:Compress the pipe to the distance between pipe wall 4 times check whether have breakage and hole or not
④Eddy Current Inspection : The pipe is passed through the eddy current detection equipment by the conveyor belt.Due to the principle of electromagnetic induction,Magnetic field will be generated.When there are defects in the welding seam or surface of the pipe the magnetic field will change.
The weld can be cleaned by applying pickling passivation paste to the weld to restore the silver white of the base metal
The test should be divided into delivery inspection (full inspection is required) and type test(sampling inspection)
The delivery inspection of pipe: appearance inspection、dimensional inspection、hydrostatic test、air tight test、eddy current inspection、mechanical property.
The delivery inspection of pipe fittings: appearance inspection、dimensional inspection、hydrostatic test、air tight test
①The factory first manufacture or the product transition trial
②After the formal production, the structure , material, technology will have a big change , may affect the product performance.
③When production is resumed after half a year of suspension
④When there is a big difference between the delivery inspection and last type inspection
In ZhengKang’s national laboratory , except the delivery inspection items, pipes also need to be tested for corrosion resistance : intergranular corrosion tets and salt spray corrosion test
Except the delivery inspection , Salt Spray Corrosion Test and Connection Performance are also needed:1. Pull-out Test 2.Temperature Variation Test 3.... 4.Hydraulic Vibration Test 5.Hydraulic Pressure Test 6.Hiss Test 7.Pressure Kickout Test(fluctuation)
Except Thermosetting solution treat,the surface treatment of products can be divided into two types: pickling and passivation 、polishing
Pickling and passivation :The process of using acid solution to remove oil, rust, oxide, and other impurities and then using passivating agent to form passivating film on the product surface
Polishing: the process of polishing the surface of a product by physical means to make the surface of product bright and clean and to form a passivating film
The cost of passivation process is relatively low , but exists environmental pollution .The surface of the polished product is beautiful and due to the smooth surface , the pollutants in the environment are not easy to deposit on the surface of the product , which also increases the corrosion resistance of the product to some extent.
The surface treatment of the product , except changing it’s appearance , the most important purpose is to improve the corrosion resistance , whether pickling passivation or polishing , for pass the salt spray corrosion test , both their performance in corrosion resistance is equal