"Hot dip aluminizing of steel" is a new coating process in which steel products are immersed in molten aluminum at a certain temperature for a proper time to form an alloy aluminum layer on the steel surface.
Hot dip aluminized steel has the characteristics of high strength of steel matrix, corrosion resistance, wear resistance and high temperature resistance of aluminum layer. Its corrosion resistance is 5- 10 times that of galvanized steel; Sulfide resistance and thermal corrosion resistance. Its performance is better than that of 18-8 stainless steel. So it can bring obvious economic benefits to users.
Metallographic characteristics of hot-dip aluminized steel
Hot-dip aluminized steel enables iron and aluminum to achieve atomic bonding and close bonding through metal bonds. Microscopically, the coating is composed of alumina film, aluminum layer and alloy layer from outside to inside. In the case of carbon steel, the coating is closely connected with the base metal in a strip structure.
Coating thickness: if it is carbon steel, the aluminum layer is 0.04-0.07mm alloy layer, with an average of 0.05 mm ... However, the thickness of each layer depends on the type of steel, the amount of carbon, and the impregnation temperature and time. If the substrate is alloy steel, the coating is different from the above. (Figure omitted)
Application field of hot dip aluminized products
Aluminized steel is widely used. The initially developed I-type aluminized steel plate is mainly used for exhaust pipes and silencers of automobiles. Type ⅱ is mainly used for roof panels and side walls of buildings and some chemical containers. The aluminized steel wire produced by the new process is expected to be used as steel-cored stranded wire for power transmission to replace the current galvanized steel wire. Aluminized steel pipes have been widely used in water wall pipes, steam pipes and heat exchangers in power plants. The tip of aluminized steel structure has been widely used in transmission towers and highway guardrails.
In the long run, because zinc resources are scarce and aluminum resources are abundant, aluminized steel will become the most widely used competitor of galvanized steel.
1. Oil pipe sleeves for oil exploration, sucker rods, high-pressure pipes for valve water injection and gas injection, offshore oil production platforms, offshore and onshore oil pipelines and natural gas pipelines.
2. Petrochemical heat exchanger, industrial water pipeline, seawater pipeline, high temperature resistant pipeline, sulfide resistant pipeline and acid resistant pipeline.
Three, metallurgical industry dust removal device, heating furnace cooling pipe, blast furnace cooling circulating water pipeline, all kinds of heat-resistant anti-corrosion equipment coating drying equipment.
Four, dock equipment shore anchor, iron cable, environmental protection facilities, deck steam pipe, crude oil heating pipe, lubricating oil pipe, car muffler.
Five, traffic bridges, highway guardrails, columns, signs, signs, guardrails along the line, bridge steel structure, coastal bridges, highway silencing equipment, overhead iron towers.
Six, the municipal environment and residential areas of telephone poles, poles, park fences. Small garbage incinerator, sewage treatment facilities, high-strength manhole cover, steel grating, residential pipeline, hot spring water pipe.
Production standard of Hot Dip Aluminizing of Steel;
Aluminized products are national new products in China at present, and their technology has been rated as "national key promotion plan of scientific and technological achievements" for two consecutive years.
Refer to Japan's highest industry standard JIS 9126-1978 hot dip aluminizing standard. This standard is formulated in order to improve the resistance to harsh environment, corrosion resistance and heat resistance of iron and steel products and effectively electroplate pure aluminum.
At present, the product attachment of three products with different adaptation uses in Japan is as follows:
Schedule type, immersion thickness and schedule quantity
Species number thickness (h)5 Adhesion (g/m2)
Solution plating 1 species (resistant to harsh environment) HDA 1 60 or more10 or more.
Two kinds of solution dip plating (corrosion resistance) HDA 2 70 or more 120 or more.
Three kinds of (heat-resistant) HDA _ 3 alloys were melted and dipped with a thickness greater than 50 μm m.
Note: (1) The thickness in the table refers to the total thickness of aluminum layer and alloy layer.
(2) For HAD 2 products, the thickness of aluminum layer is above 10u in principle, but our products are not less than 0.8u
(3) Where there are special requirements, it can be diffused. The thickness of the diffusion saturated layer is greater than 70u (0.07mm).
Properties of hot dip aluminizing;
Corrosion resistance:
The corrosion resistance of aluminized steel is played by the corrosion resistance of alumina film, aluminum layer itself, alloy layer and the electrochemical corrosion protection principle between aluminum layer, alloy layer and base metal.
(1) Weather resistance
Aluminized products can be used for a long time in corrosive environments such as marine humid areas, industrial areas and sulfur hot springs.
Comparison of corrosion test results between galvanized steel and aluminized steel
Test name test conditions galvanized steel aluminized steel
Type ⅰ and type ⅱ
SO2 test air containing 16%SO2-2825-130.0-104.0
The air prototype containing 20%HNO3 disappeared in nitric acid test -2.0 -3.0.
Artificial seawater test1100 NaCl+0.3% H2 or aqueous solution-172.6-18.0-107.0.
(2) Seawater resistance
Hot dip aluminized steel has good seawater corrosion resistance, which has the same effect not only in marine environment, but also in sea surface and waves. Aluminized steel has excellent salt water corrosion resistance. The following table lists the corrosion test results of three steels in 3%NMacl aqueous solution.
Comparison of corrosion resistance of three steels in 3%NaCl aqueous solution (10℃)
material
material
Original weight
Millimeter
face
gather
Cm2 corrosion amount
24h 72h 480h
Mg mg/cm2 mg/cm2 mg/cm2 mg/cm2 mg
Carbon steel 26.6378 27.90 6.3 0.26016.7 0.600136.7 4.860
Aluminized steel 29.0568 27.901.7 0.61.70.057 2.8 0.100
13Cr steel 26.6982 27.90 3.20.1514.8 0.532 26.00. 596436
The corrosion rate of aluminized steel in brine solution is much lower than that of 13Cr billet and that of non-aluminized carbon steel.
(3) Water resistance
Because the coating of aluminized products has anticorrosion function as cathode, the aluminum oxide passivation film and alloy layer generated on aluminum surface have anticorrosion function. Therefore, the water resistance of aluminized products is extremely strong. Due to the difference of water quality, PH value, temperature and flow rate, the corrosion resistance and durability of aluminized products are also different.
(the picture is abbreviated. Chart)
Corrosion resistance of hot-dip aluminizing and hot-dip galvanizing in water
measure
Material corrosion amount
G/m2/day corrosion depth
U/ day
Hot dip aluminizing 0.007 0.007
Hot dip galvanizing 0.038 0.038
(4) Stress corrosion resistance and crack prevention
Some special steel, carbon steel, etc. If it is placed in an aqueous solution (low resistance medium) containing chloride, sulfate and nitrate, stress corrosion cracking will occur. If aluminum is plated, it can be prevented This is because in low resistance medium, the aluminized layer acts as a sex anode and will not break until the coating disappears. In the use experiments of aluminized pipes such as heat exchangers, no matter what kind of heat exchangers, they all show several times of durability.
(5) Drugs and other corrosive aqueous solutions
The aqueous solution resistance of aluminized products depends on the depth of ions and organic matter in water and the water temperature.
For organic acid, nitric acid, sulfuric acid (3), ammonia water and ammonium salt, the content is below 1% and above 20%. It has corrosion resistance to hydrogen sulfide water, sulfurous acid aqueous solution, urea water, brine, calcium chloride and sulfur-containing hot spring water.
heat resistance
(1) High temperature oxidation resistance
Ordinary steel will oxidize rapidly at a high temperature of 550 degrees Celsius, while aluminized steel has strong high temperature resistance, oxidation resistance and corrosion resistance. This is because the dense oxide film on the aluminum surface prevents the diffusion of oxygen, and the alloy layer itself (shown in the following reaction formula) also has strong heat resistance.
4Al+3O2 - >2Al2O3
2 (Fe-Al) +3O2->Fe2O3+Al2O3
Fe2O3+2Al - >2Fe+Al2O3
(Selective oxidation of aluminum can prevent oxidation of iron)
Heat resistance test of several steels (after100h) gasification increment (g/cm2)
Heating conditions: 600c 800c 1000c.
The ratio of oxidation incremental raw materials to aluminized incremental raw materials
Materials-
Carbon steel-0.075 0.001168:10.410.0055 75:1
Forged steel 0.0032 0.0074 0.0095 78:10.410.0077 52:1.
Ordinary cast steel 0.0024 0.1.0023 43:1.37 0.01231:1.
13CY steel -0.22 0.0062 35: 1
(2) Resistance to carbon impregnation
The alloy layer on the surface of aluminized steel can inhibit the invasion and diffusion of carbon. In addition, it can also prevent the base material (steel) from decarburization in the environment of high temperature reducing gas.
(3) High temperature vulcanization resistance
Hot dip aluminizing has very good sulfide resistance, even better than chromium, especially for hydrogen sulfide.
situation
Material Corrosion (g/m2/hr)
499 593 649
Aluminized steel -0.02-0.20
Carbon steel 19-
18C-8Ni steel 6.5 18-
Thermal reflectivity: the surface of aluminized steel plate is not only beautifully processed, but also has excellent reflectivity, and the stable light reflectivity is about 80% at the temperature below 600 degrees; The reflectivity under infrared ray reaches 95%, while the galvanized steel plate is only 15% at 350-450 degrees.
Welding of hot dip aluminized products
When aluminized steel needs to be welded, covered electrode (made in China) can be used, and the corrosion resistance of welded parts is the same as that of aluminized steel. When welding, the aluminum layer diffuses and soaks at high temperature, and the parent metal is also protected.
Annotation item
1) and other metal contact corrosion (electrochemical corrosion)
All kinds of metals have their own potentials in electrolyte solutions (fresh water and seawater). Due to the potential difference, different metals in contact with each other will have current flowing through them, causing electrical corrosion. Hot dip aluminized steel should also pay attention to this effect in use.
2) As heat resistance, if it is used at 600 degrees Celsius for a long time, if the aluminum layer in the coating is too thick, the residual aluminum will peel off. If the diffusion treatment of 1-2 hours is performed at a temperature of about 750 degrees Celsius in advance, peeling can be prevented and the alloy layer can be thickened.
3) Because the alloy layer of aluminized products is hard and malleable, attention should be paid to strong collision during handling and processing.
4) The volume expands after aluminizing, so the joints such as bolts and nuts need to be replenished.
Visual inspection shows that the coating is continuous and the surface coverage is intact.
Slag-free and needle-free electroplating
The marking inspection is 2.0MM on both sides of the marking.
The coating will not peel off and fall off.
Coating amount and thickness > 120g/m2.
Thickness > > 0.080mm.
Weldability: Compared with cold-rolled steel plate, the welding conditions are different. Because aluminum metal is soft and easy to combine, and because of its strong conductivity, the heat dissipation of welded parts is better than that of cold-rolled steel plates. In addition, the melting point of the coating is higher than that of galvanized steel, so it is not easy to sag, and the melting area is small, so there is no need to use as much current as galvanized steel.