Galvanized steel pipe
Galvanized steel pipe
Features and common problems of galvanized steel pipe (ASTM A123 American Standard hot dip galvanizing standard is attached)
Galvanized steel pipe refers to the welded steel pipe with hot-dip or electro galvanized surface. That is, cold (electric) plated steel pipe and hot plated steel pipe.
Application:
Galvanizing can increase the corrosion resistance of steel pipe and prolong its service life. The galvanized pipe is widely used, not only as the pipeline for water delivery, gas, oil and other general low-pressure fluid, but also as the oil well pipe and oil pipe of the petroleum industry, especially the offshore oil field, the oil heater, condensate cooler, coal distillate wash oil exchanger pipe of the chemical coking equipment, as well as the trestle pipe pile, the support frame pipe of the mine tunnel, etc.
Hot dip galvanized pipe
Production process: the hot-dip galvanized pipe is to make the molten metal react with the iron matrix to produce an alloy layer, thus combining the matrix and the coating. Hot dip galvanizing is to pickle the steel pipe first, in order to remove the iron oxide on the surface of the steel pipe, after pickling, it is cleaned by ammonium chloride or zinc chloride aqueous solution or the mixture of ammonium chloride and zinc chloride aqueous solution tank, and then it is sent into the hot dip galvanizing tank. Hot dip galvanizing has the advantages of uniform coating, strong adhesion and long service life. Hot dip galvanized steel pipe base
The complex physical and chemical reactions with the molten bath form a zinc iron alloy layer with a close corrosion-resistant structure. The alloy layer is integrated with pure zinc layer and steel pipe matrix. Therefore, it has strong corrosion resistance.
Cold galvanized pipe
Production process: the cold galvanized pipe is galvanizing, the amount of galvanizing is very small, only 10-50g / m2, its corrosion resistance is much different from that of hot galvanized pipe. Regular galvanized pipe manufacturers, in order to ensure the quality, most do not use galvanizing (cold plating). Only those small-scale and old equipment small enterprises use galvanizing, of course, their prices are relatively cheap. At present, the Ministry of construction has officially eliminated the technologically backward cold galvanized pipes. In the future, it is not allowed to use the cold galvanized pipes as water and gas pipes. The galvanized layer of cold galvanized steel pipe is electroplated layer, and the zinc layer is separated from the steel pipe substrate. The zinc layer is thin and easy to fall off. So its corrosion resistance is poor. In newly built houses, it is forbidden to use cold galvanized steel pipes as water supply pipes.
Features of hot dip galvanized pipe
Hot dip galvanizing can be divided into blow plating (hanging plating) and dip plating (hanging plating) according to different processes.
The features of blown plating (hanging plating): the advantages are that the galvanized layer is relatively uniform, the surface is bright, and the defect is that the zinc content is low, about 200g / m2;
Features of hanging plating: the advantage is that the zinc content is high, about 500g / m2, which can meet the customer's requirements for the thickness of high zinc layer; the defect is that the surface of zinc coating is uneven, which is easy to produce zinc nodules and zinc slag, affecting the appearance.
Common problems of hot dip galvanized steel pipe
1. What are the main reasons for the fall off of zinc layer?
The main reasons for the fall off of zinc layer are: surface oxide, silicide, endless evaporation of grease, low aluminum content in zinc pot, too short residence time in liquid and thick coating.
2. What is the definition of delamination and zinc layer crack?
When the zinc layer falls off, because there is no homogeneous Fe 2Al 5 intermediate layer between the steel base and the Fe Zn alloy layer, the adhesion of the coating decreases, and the zinc layer may fall off when bending. The crack of zinc layer is caused by the super thick brittle layer of zinc alloy, which reduces the ductility of the coating, and the crack of zinc layer occurs when bending.
3. What are the causes of solvent spots (steel pipe tear marks)?
(1) The high temperature of zinc solution makes the solvent too late to diffuse and scorch;
(2) , too much zinc ash;
(3) The content of aluminum in zinc solution is too high;
(4) Too much oxide on the solvent and steel pipe;
(5) The viscosity of the solvent is too high and the fluidity is poor;
(6) The ammonium chloride in the solvent is insufficient or evaporated too early;
(7) The steel pipe rolls on the zinc level;
(8) There is no gel at the bottom of the zinc pot.
4. What causes the pitting on the surface of galvanized steel pipe?
(1) When the content of aluminum on the surface of zinc liquid is higher than 0.3%, the viscosity of zinc liquid increases, which makes the zinc slag not easy to precipitate and fall, but adhere to the surface of galvanized steel pipe;
(2) In addition to the increase of aluminum content on the surface of zinc solution, the temperature of zinc solution is often higher than that of normal production. When the temperature of zinc solution is high, the convection phenomenon is intensified, and a part of zinc slag is brought up and adhered to the galvanized steel pipe;
(3) Because of the existence of aluminum, the iron element in the zinc solution will quickly join with aluminum to form Fe-Al particles, which are suspended in the zinc solution due to its small density and adhere to the steel pipe.
5. What causes the abnormal color of steel pipe surface?
(1) The temperature of zinc solution is too high, the residence time in the air is too long, and the gas pressure of internal and external blowing is too large, which can make the pure zinc layer with silver bright color on the surface consume and present gray;
(2) When the chroma of cooling water is too high, and there are impurities such as mud and organic matters, the galvanized layer will produce a matte surface such as yellow and black;
(3) If the concentration of passivation solution is too high and it has not been rinsed well, the surface of silver bright zinc coating will be brown, red brown or yellow green in varying degrees will be produced with the decrease of concentration. In galvanized parts, yellow green is normal, while brown and red brown are abnormal;
(4) In recent years, most of the steel products produced are killed steel. The silicon content in the steel is easy to make hot zinc parts produce dark and matte coating. In serious cases, the coating becomes dark gray, which is caused by excessive growth of the iron zinc alloy layer caused by silicon. When the coating is in such color, the corrosion resistance is not affected.
6. The factors affecting the adhesion of the coating are
(1) , chemical composition of steel pipe;
(2) , surface cleanliness;
(3) , surface roughness;
(4) , chemical composition of zinc solution.
7. How to form and solve the phenomenon of flow mark and tumor?
There is a great relationship between the flow mark and the iron content in the zinc bath. The solubility of iron in zinc solution increases with the increase of temperature. The solubility of iron in zinc solution is 0.02% at 430 ℃, 0.02-0.03% at 430 ℃ - 440, 0.04-0.07% at 450 ℃ - 470 ℃, and 0.3% above 500 ℃. When the high temperature galvanizing solution is cooled down for galvanizing, the iron in the zinc solution precipitates, clarifies, bails out slag and purifies, which can ensure the fluidity of the zinc solution. But the manufacturer often can't stop production, which causes the iron suspended in the zinc liquid not to sink, or even to float up, resulting in the flow mark of the steel pipe, the phenomenon of hanging tumor is more serious, and the galvanized layer is obviously thickened.
resolvent:
(1)、The manufacturer of conversion from medium temperature galvanizing to low temperature galvanizing shall have more than 24 hours for purification of precipitated iron.
(2)、Add enough iron and zinc alloy to make the iron in the zinc solution combine with the iron in the alloy rapidly, and use the purification agent as catalyst to complete the iron removal in a short time.
(3)、During the process of iron removal, it is not allowed to produce, add aluminum and aluminum alloy to avoid affecting the iron removal.
8. What is the effect of climate on drying?
(1) Under the condition of high humidity and low air pressure, the solvent will return to moisture soon before drying to the dip coating, which will make the steel pipe meet with the zinc solution and produce shooting, resulting in missing plating spots, increasing zinc consumption and other defects;
(2) For example, when the evaporated water vapor blows into one end of the steel pipe, it makes the inner wall of the pipe hole have more water, so it is difficult to dry it. On the other end, because the cold air can not dry it, it will also shoot and miss plating;
(3) The acid mist blown from the pickling section is stained on the surface of the steel pipe, which seriously causes corrosion and also causes missing plating spots.
Zinc layer thickness grade
Hot dip galvanized pipe reference standard
ASTM A123 standard for zinc coating of iron and steel products
Plating Thickness
|
Material category |
Thickness range of zinc coating |
|||||
|
Product wall thickness (mm) |
<1.6 |
1.6~<3.2 |
3.2~≤4.8 |
4.8~<6.4 |
≥6.4 |
|
|
Structural steel and steel plate |
Thickness of zinc layer (μ m) |
45 |
65 |
75 |
85 |
100 |
|
Strip and bar |
45 |
65 |
75 |
85 |
100 |
|
|
Steel pipes and pipes |
45 |
45 |
75 |
75 |
75 |
|
|
Wire rod |
35 |
50 |
60 |
65 |
80 |
|
|
Reinforced bar |
… |
… |
… |
… |
100 |
|
Zinc layer thickness grade
|
Zinc layer thickness grade |
||||
|
Thickness class |
Mil |
Oz / SF |
Micron |
G / m2 |
|
35 |
1.4 |
0.8 |
35 |
245 |
|
45 |
1.8 |
1.0 |
45 |
320 |
|
50 |
2.0 |
1.2 |
50 |
355 |
|
55 |
2.2 |
1.3 |
55 |
390 |
|
60 |
2.4 |
1.4 |
60 |
425 |
|
65 |
2.6 |
1.5 |
65 |
460 |
|
75 |
3.0 |
1.7 |
75 |
530 |
|
80 |
3.1 |
1.9 |
80 |
565 |
|
85 |
3.3 |
2.0 |
85 |
600 |
|
100 |
3.9 |
2.3 |
100 |
705 |
