Selection and acceptance of hot-dip galvanized steel pipe in building installation engineering
Galvanized steel pipes have been widely used in construction and installation projects for hundreds of years. For example, pipeline installation projects are used to transport water, gas, steam and other fluids. In electrical installation projects, they are used for lighting, power, telephone, fire fighting, etc. The wire casing of the electrical system is laid. Hot-dip galvanizing of steel pipes is an anti-corrosion measure that reacts molten zinc and iron to form an alloy layer, thereby combining the substrate and the plating. Hot-dip galvanizing process: First, the steel pipe is pickled to remove iron oxide on the surface of the steel pipe, and then washed in ammonium chloride or zinc chloride solution, and finally sent to the hot-dip galvanizing bath to form a galvanized layer. . In 2000, the state has banned the use of cold-galvanized (electroplated) steel pipes as water pipes. Compared with cold galvanizing, hot-dip galvanizing has the advantages of uniform plating, strong adhesion and long service life. Therefore, hot-dip galvanized steel pipes are often used as water supply and drainage pipelines and wire protection pipes in the construction and installation of hydropower. However, there are some unclear problems in the design selection of hot-dip galvanized steel pipes and the material acceptance process of construction supervision, mainly the design reference standards and the acceptance criteria of supervision. This paper intends to discuss these unclear issues.
What is hot galvanizing
Galvanizing, also known as hot dip galvanizing and hot dip galvanizing: is an effective metal corrosion protection method used primarily in metal structures in various industries. The rust-removed steel is immersed in a zinc solution melted at about 500 ° C to adhere the zinc layer to the surface of the steel member, thereby preserving the corrosion. Hot-dip galvanizing process: finished pickling-washing-assisting plating-drying-hanging-cooling-pharmaceutical-cleaning-grinding-hot galvanizing is completed. Hot-dip galvanizing is a result of the older hot-dip method. Since the application of hot-dip galvanizing in France in 1836, it has been more than 170 years old. In the past three decades, with the rapid development of cold rolled strip, the hot dip galvanizing industry has been developed on a large scale.
Typically, the electroplated zinc layer has a thickness of 5 to 15 μm, and the hot-dip galvanized layer is generally 35 μm or more, or even as high as 200 μm. Hot-dip galvanizing coverage is good, the coating is dense, and there is no organic inclusions. It is well known that the mechanism of anti-atmospheric corrosion of zinc has mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, there are ZnO, Zn(OH)2 and basic zinc carbonate protective films on the surface of the zinc layer, which slows down the corrosion of zinc to some extent. The protective film of the layer (also called white rust) is destroyed and a new film layer is formed.
When the zinc layer is seriously damaged and the iron matrix is endangered, zinc electrochemically protects the matrix. The standard potential of zinc is -0.76V, and the standard potential of iron is -0.44V. When zinc and iron form a microbattery, zinc is dissolved as an anode. Protected as a cathode. It is obvious that hot-dip galvanizing is superior to electro-galvanizing in the resistance to atmospheric corrosion of the base metal iron.
Galvanized layer formation process
The hot-dip galvanizing layer formation process is a process of forming an iron-zinc alloy between the iron matrix and the outermost pure zinc layer, and the surface of the workpiece forms an iron-zinc alloy layer during hot dip plating, so that the iron and the pure zinc layer are very For a good combination, the process can be simply described as: when the iron workpiece is immersed in the molten zinc liquid, a zinc and an alpha iron (body center) solid solution are first formed at the interface. This is a crystal formed by the dissolution of zinc atoms in the solid state of the base metal iron. The fusion between the two metal atoms is relatively small. Therefore, when zinc is saturated in the solid solution, the two elements of zinc and iron diffuse into each other, and the zinc atoms diffused into (or infiltrated into) the iron matrix migrate in the matrix lattice, gradually forming an alloy with iron, and diffusing. The iron in the molten zinc solution forms an intermetallic compound with zinc and sinks into the bottom of the hot-dip galvanizing pot, which is zinc dross. When the workpiece is removed from the zinc immersion liquid, a pure zinc layer is formed on the surface, which is a hexagonal crystal. Its iron content is not more than 0.003%.
What is cold galvanizing?
Electrogalvanizing, commonly known as cold galvanizing, is an electrochemical method in which a zinc ingot is used as an anode. After the zinc atom loses electrons, it becomes an ionic state and dissolves into the electrolyte. The steel strip acts as a cathode, and the zinc ions are electronically reduced on the steel strip. Zinc atoms are deposited on the surface of the steel strip to form a coating.
The difference between hot-dip galvanizing and cold-galvanizing
Hot-dip galvanized sheets and galvanized sheets have great differences in galvanizing. The galvanizing amount of hot-dip galvanizing cannot be too small, and the minimum is 50-60 g/m2 on both sides and up to 600 g/m2. The galvanized layer of the electrogalvanized sheet can be very thin, up to 15g/m2, but if the coating is thicker, the production line speed is very slow, and it is not suitable for the process characteristics of the modern unit, and the maximum is about lO0g/m2. Because of this, the production of electrogalvanized sheets is greatly limited.
Hot-dip galvanized sheets and electro-galvanized sheets are fundamentally different in the microstructure of the coating. There is a layer of slightly brittle compound between the pure zinc coating of the hot-dip galvanized sheet and the base of the steel strip. When the pure zinc coating is crystallized, most of the zinc is formed, and the coating is uniform and non-porous. The zinc atom of the electroplated zinc layer is only deposited on the surface of the steel strip, and is attached to the surface of the steel strip by physical action. There are many holes to dry, which is easy to cause pitting due to corrosive medium, so the hot-dip galvanized sheet is more than the electro-galvanized sheet. More resistant to corrosion.
The heat treatment process of hot-dip galvanizing and electro-galvanized sheet is also completely different. The hot-dip galvanized sheet is generally made of chilled board as raw material, and is continuously annealed and hot-dip galvanized on the galvanizing line, and the steel strip is cooled after being heated for a short time. Therefore, the strength and plasticity are affected to some extent, and the punching performance is worse than that of the same cold-hardened plate after the degreasing and annealing of the professional production line. The electrogalvanized sheet is made of cold-rolled sheet, which basically guarantees the same processing performance of the cold-rolled sheet, but its complicated process also increases the production cost. In short, the hot-dip galvanized sheet has a lower production cost and a wider application range, and has become a major variety in the galvanized sheet market.
Hot-dip galvanizing corrosion resistance is much higher than cold galvanizing (also known as electro-galvanizing). Hot-dip galvanizing does not rust in a few years, and cold-galvanizing will rust within three months.
The cold galvanizing process is used to protect the metal from corrosion. For this purpose, a coating of zinc filler is applied, which is applied to the protected surface by any coating method and dried to form a zinc filler coating in the dried coating. Has a content of zinc (up to 95%). Steel is galvanized on the surface under cooling conditions, while hot-dip galvanizing is the galvanization of the surface of the steel pipe under hot dip conditions. Its adhesion is very strong and it is not easy to fall off. Although the hot-dip galvanized pipe is also rusted, However, in a long period of time, technical and sanitary requirements can be met.
The difference between galvanized products:
1) Both the inner and outer walls of the hot-dip galvanized steel pipe are galvanized, while the cold-galvanized steel pipe is only galvanized on the outer wall.
2) The cold plating layer is uniform and the thickness is very thin; the hot plating layer is thick and the thickness is not uniform.
3) The appearance of cold plating is bright and the color of hot plating is dark.
4) Because the cold plating layer is thin, the corrosion resistance is poor; the hot plating layer is several times times thicker than the cold galvanizing, and the infiltrated layer is formed with the base metal, and the corrosion resistance is good.
5) The cold-galvanized steel pipe looks brighter and smoother; the hot-dip galvanized steel surface is not cold-plated and bright, and the surface of the hot-dip galvanized steel pipe looks like some uneven bumps, and the surface is not bright, not reflective. .
Effect of hot-dip galvanizing layer on corrosion protection of steel pipe
When hot dip galvanizing, the zinc-iron alloy layer is metallurgically bonded to the steel substrate and the zinc layer, which is stronger than the combination of paint and steel. Therefore, the number of hot-dip galvanized layers exposed to the atmosphere will not fall off for 10 years until it is completely corroded. On the one hand, zinc has a more negative electrode potential than iron. When zinc and iron constitute a microbattery, zinc is the anode and iron is the cathode. When corroded, zinc dissolves and iron is not damaged. When the galvanized layer has small cracks or damage, zinc will prevent the rust of steel from continuing to crack or damage in the form of a sacrificial anode, which is the main feature of the galvanized layer superior to other coatings. On the other hand, the zinc layer has a shielding effect on steel. Zinc forms a shielding layer on the surface of the steel substrate in the atmosphere or other environments. The corrosion process of zinc increases its ability to form a barrier layer because zinc reacts with oxygen, carbon dioxide, moisture, etc. in the air to form a corrosion-resistant byproduct zinc carbonate film. This basic zinc carbonate film is hard and insoluble, and becomes an effective shielding protective layer. It seals the corrosion of iron in the external environment, slows down the corrosion of zinc, and reduces the corrosion rate of steel to 1/25.
The zinc-iron alloy layer between the substrate and the pure zinc layer contains more than 90% zinc, which has an electrochemical protection effect similar to that of zinc. The thicker the coating, the longer the durability, and the durability is almost proportional to the thickness. The zinc-iron alloy layer contains more than 90% zinc, so the thickness of the zinc layer and the amount of zinc adhesion can be approximated. The thickness of the 1 μm zinc layer is equivalent to about 7 grams per square meter.
Selection of hot-dip galvanized steel pipe
1. Classification of hot-dip galvanized electric pipe protection
British Standard BS4568-1970 “Instrument Specification for ISO Type Threaded Steel Pipe and Fittings for Electrical Installations”, Section 4.2 stipulates: Steel pipes and fittings can be classified into the following categories depending on the type of protection used. Level 1: Mild protection (eg paint), both inside and outside walls; Level 2: Moderate protection (eg baking), both inside and outside walls; Level 3: Moderate to heavy protection ( For example: inner wall baking varnish, outer wall galvanized powder), inner wall protection is the same as level 2, outer wall protection is the same as level 4; level 4: heavy protection (eg hot dip galvanizing, galvanized powder) is protected by the inner and outer walls. In addition, the anti-corrosion performance test in 9.1.1 stipulates: For the third-stage pipelines and fittings, the inner and outer walls should be tested in different ways, and the third-stage uses 0.75% potassium ferricyanide and 0.25% ammonium persulfate solution. Class 4 uses a 2% sulfuric acid solution. It can be seen that the Class 3 wire and the Class 4 wire specified in BS4568 are differentiated according to the degree of protection. The author has inspected the manufacturer of galvanized wire tubes. According to the investigation, the production process of welded steel pipes is roughly as follows: strip steel → feeding → unwinding → butting → storage → forming → high frequency welding → removing inner welding rod → sizing → Eddy current flaw detection → Straightening → Cutting → Flat head → (Hydraulic pressure detection) → Final inspection → Steel pipe. The production process of steel pipe galvanizing is: steel pipe → degreasing → water washing → pickling → rinsing → solvent spraying → drying → hot galvanizing → outer blowing → (internal blowing) → air cooling → water cooling → passivation → car wire → printing → final Inspection → packaging. The 3-stage electric pipe is directly welded with galvanized steel strip, and the weld is treated by hot-spraying zinc. For the 4-level electric pipe, the strip is first welded into a steel pipe and then galvanized. The production process sequence becomes the dividing standard for both, which is inconsistent with BS4568.
2. Hot-dip galvanized water pipe is used as electric pipe
In general, an electric pipe refers to a thin-walled steel pipe, and a water pipe refers to a thick-walled steel pipe. However, the wires of weak current systems (such as fire alarm systems) have high requirements on the shielding performance of electric pipes. Sometimes, hot-dip galvanized thick-walled steel pipes are used, that is, water and gas pipes are used as electric pipes. It should be noted that GB3640-1988 “General Carbon Steel Wire Bushing” Technical Requirements Section 2.4 stipulates “The inner surface of the steel pipe should be smooth, and the burr with a height of not more than 1mm is allowed at the weld.” This regulation is to prevent the wire worn inside the pipe from being damaged by the burr and damage the insulation. However, GB/T3091-93 “galvanized welded steel pipes for low-pressure fluid transportation” does not have such regulations. Therefore, when water pipes are used as electric pipes, there is no clear acceptance criteria for supervision. In addition, the author found that the water quality test certificate of the water pipe material as the electric pipe submitted by the construction unit in the actual project has the test data of the hydraulic test. It is obvious that the construction unit did not indicate the use of the steel pipe in the contract when purchasing the steel pipe material.
Construction supervision acceptance of hot-dip galvanized steel pipe
When supervising the acceptance of hot-dip galvanized pipe fittings, the quality of the following aspects is generally examined.
1. Visual inspection
The main surface of the pipe should be smooth, free of nodules, zinc ash and iron, allowing a very small amount of storage and transportation spots to exist. However, when the diameter of the iron is less than 2 mm, the zinc has a sacrificial protective effect and has little effect on the corrosion resistance. GB/T 13912 “Technical requirements and test methods for hot-dip galvanized layers of steel-coated steel parts” and British standards point out that the plating of plating and inadvertent damage can be repaired, and the area allowed for repair and the thickness of repair are compared. Clear requirements. The material to be repaired shall be a fusion welded low melting point zinc alloy, thermal sprayed zinc or a special zinc-rich coating that is similar in performance to the galvanized layer. Since the steel substrate in recent years is mostly silicon-containing killed steel, it is easy to produce a dark coating with little or no pure zinc layer on the surface, and the thickness of the alloy layer is the thickness of the galvanized layer. The surface of the coating has no zinc luster and is gray. In severe cases, it is dark gray. This is caused by the exposed surface of the iron-zinc alloy layer. However, the pipe has no effect on the atmospheric corrosion resistance and only affects the appearance. Acceptance should not be rejected when used for concealed works or concealed applications.
2. Thickness of zinc layer
The thickness of the tube wall determines the thickness of the coating. Because thick pipe fittings have no difficulty in obtaining thicker coatings, thin steel pipes with smooth surfaces are difficult to obtain thicker coatings. As mentioned above, the thickness of the zinc layer can be approximately converted to the amount of zinc weight per square meter. Some owners and supervisors only pay attention to whether the appearance quality is flat and bright, neglecting the thickness of the zinc layer, or even not testing the zinc layer thickness at all, which is not appropriate. Section 2.6 of the Technical Requirements section of GB3640-1988 and Section 5.6 of the Technical Requirements section of GB/T3091-93 stipulate that: Zinc-plated steel pipes shall be tested for zinc layer uniformity. The sample shall be immersed continuously in the copper sulfate solution for 5 times without redness (copper plating). Early foreign standards also specified the use of copper sulfate test to verify the uniformity of the zinc layer. The copper sulfate test is actually a method to determine the thickness of the minimum zinc layer. It is not the difference between the minimum thickness and the maximum thickness. Therefore, the term “uniformity” is not very accurate. Moreover, the dissolution rate of the pure zinc layer and the iron-zinc alloy layer in the copper sulfate solution is greatly different, so the correspondence between the number of times of immersion and the amount of zinc attached is not very good. Therefore, any standard that mentions the copper sulfate test as a uniformity test needs to be modified. In fact, as long as the amount of zinc added meets the standard, the number of copper sulfate tests is much higher than the four times specified in BS4568 and five times specified in GB3640-1988 and GB/T3091-93, and there is no problem in corrosion resistance. The minimum zinc layer thickness can be directly controlled by the method of GB/T 13912 which specifies the minimum local thickness of the zinc layer.
3. Zinc layer adhesion strength
The hot-dip galvanized layer should have sufficient adhesion strength. When there is no external stress to bend or deform the pipe, the coating should not peel off. In fact, the galvanized layer generally does not crack or peel off when the plated parts are subjected to collision during normal handling, handling, transportation, and installation. However, when the steel pipe is deformed by external force, it is normal for the zinc layer to peel off. For example, when the groove of the clamp type connecting pipe is in the groove, the zinc layer is prone to peeling off or peeling, but this does not mean that the adhesion of the zinc layer is not it is good. It is only necessary to carry out anti-corrosion treatment at the groove where the zinc layer is peeled off or peeled, such as zinc spray or anti-rust paint, and no need to re-galvanize. The test methods for adhesion strength are not specified in the two standards of “Ordinary Carbon Steel Wire Bushing” and “Hot Galvanized Welded Steel Pipe for Low Pressure Fluid Transport”. When necessary, both the supplier and the buyer should agree that the coating does not peel off or bulge. When the galvanized parts are made of low-silicon steel, the thickness of the coating is moderate, the alloy layer is thin and the pure zinc layer is thick, and there is no problem in the hammer test. However, the popularity of silicon killed steel has led to more thick coatings. The iron-zinc alloy layer in the coating tends to be thicker and even occupies almost the entire coating. A considerable part of these tubes cannot pass the hammer test, but the corrosion resistance is not problematic when used. The American standard has cancelled the hammer test since 1989, and the adhesion test uses the hard knife test. If the zinc layer has good adhesion and the sharp blade or sharp knife is shoveled, only the chip can be scraped off, and there is no whole zinc layer. Collapsed to expose the iron matrix. The hard knife test can be applied to all types of galvanized parts, while the hammer test is only suitable for galvanized parts with flat surfaces and thick workpieces.
Suggestions for the selection and acceptance of hot-dip galvanized steel pipes
The British standard BS4568-1970 has been established for more than 30 years. The hot-dip galvanizing process technology has been relatively mature. Some standards for hot-dip galvanized steel pipes have been established in China, and the technical requirements are not lower than BS4568. Therefore, it is recommended to draw on domestic standards when designing and selecting. For visual inspection, surface smoothness is a relative term, and the use of the product should be the determining factor when specifying the smoothness. Pipes buried in or buried in concrete or suspended ceilings should not pursue surface smoothness, and the smoothness of the installed pipeline should be the focus of inspection. When contracting, pay attention to the thickness of the galvanized layer or the amount of zinc adhesion. Because the durability of hot-dip galvanized steel pipes is related to the location of the project, and the service life of different regions varies greatly. The same is 400g/m 2 with zinc. The service life of steel pipes in the suburbs, along the strips, urban areas and heavy industrial cities is 67 years, 33 years, 21 years and 9 years respectively. The demand side shall determine the necessary amount of zinc to be added according to the actual conditions of the project. According to the Standardization Law of the People’s Republic of China, standards other than mandatory standards (GB) are recommended standards (GB/T). For the recommended standards, both parties can choose according to the situation and agree in the contract. Once agreed, both parties should strictly implement the survey, design, construction and procurement. For galvanized electric tubes, the “Homogeneity” test for the mandatory standard GB3640-1988 – the copper sulphate test is easier to pass, and some companies do not implement the recommended standard GB/T13912. However, there is no mandatory standard for hot-dip galvanized water pipes. The recommended standards should be indicated in the contract. Otherwise, there is no basis for the supervision of the steel pipe materials. The supervisor should clarify the “master control project” for the acceptance of galvanized steel pipes. The inspection and compliance with the standards in accordance with the provisions of the current mandatory standards is called the main control project. Regardless of whether the demander is indicated in the contract, the inspection shall be carried out in accordance with the standards and the inspection results shall be in compliance with the standard. Such as chemical composition, mechanical properties, dimensional deviation, surface quality and flaw detection, hydraulic test or flattening, flaring and other process performance experiments, are the main control projects. According to the requirements of the purchaser, after consultation between the supplier and the buyer, after the recommended standards cited in the contract, the recommended standards become the main control items, such as zinc layer adhesion or zinc layer thickness detection, salt spray resistance test, etc. .
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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