METAL ANTICORROSIVE COATING, PREPARATION METHOD THEREFOR, AND USE THEREFOR

Abstract

The invention discloses a metal anticorrosive coating. The coating is an inorganic coating used for metal anticorrosion. This coating has a double-layer structure, including an outer enamel coating and an inner base oxide coating. Meanwhile, the content of the base metal oxide decreases from the inner layer to the outer layer, which causes the thermal expansion coefficient of the coating to increase from the inner layer to the outer layer, ensures that the overall thermal expansion coefficient of the coating is coordinate with various base metals. The composition of the outer layer enamel coating includes: by weight, 1-40 parts of silicon, 1-30 parts of sodium, 1-20 parts of potassium, 2-20 parts of calcium, 0.5-15 parts of fluorine, 0.3-10 parts of cobalt, 0.2-10 parts of nickel, 1-18 parts of boron, 0.5-10 parts of phosphorus, 0.1-8 parts of magnesium, and the rest is oxygen; the composition of the base oxide coating of the inner layer includes the base metal and oxygen. A preparation process of a double-layer dense metal anticorrosive coating formed by low-temperature sintering is also disclosed, including the following steps: 1) grinding; 2) preparation of mixture; 3) grinding; 4) high temperature reaction; 5) grinding; 6) coating; 7) sintering. The coating of the invention has the advantages of improving the corrosion resistance by more than 14 times, has a high ductility which can be coordinated with the reinforcing steel bar in tensile deformation, has a thermal expansion coefficient gradient which can be applied to different metals and different types of the same metal.

Claims

1. A metal anticorrosive coating, characterized by: the metal anticorrosion coating is a double-layer structure coating, which is composed of an enamel coating and a base oxide coating; in the double-layer structure, the enamel coating is the outer layer, the base oxide coating is the inner layer, and the inner layer is in contact with the base metal. The composition of the enamel coating includes, by weight, 1-40 parts of silicon, 1-30 parts of sodium, 1-20 parts of potassium, 2-20 parts of calcium, 0.5-15 parts of fluorine, 0.3-10 parts of cobalt, and 0.2-10 parts of nickel, 1-18 parts of boron, 0.5-10 parts of phosphorus, 0.1-8 parts magnesium, the rest is oxygen. The composition of the base oxide coating includes base metal element and oxygen; in the two-layer structure coating, there is a base metal oxide with a reduced concentration gradient from the inner layer to the outer layer.

2. The metal anticorrosive coating of claim 1, wherein the thermal expansion coefficient of the double-layer structure coating is coordinate with the thermal expansion coefficient of the base metal.

3. (canceled)

4. The metal anticorrosive coating of claim 1, wherein the content of the base metal in the base oxide coating is 40 to 85 parts, and the rest is oxygen.

5. The metal anticorrosive coating of claim 1, wherein the composition of the enamel coating includes 2-30 parts of silicon, 2-20 parts of sodium, 2-15 parts of potassium, 4-16 parts of calcium, 2-10 parts of fluorine, 0.5-7 parts of cobalt, 0.3-8 parts of nickel, 2-10 parts of boron, 0.8-6 parts of phosphorus, 0.2-5 parts of magnesium, the rest is oxygen.

6. The metal anticorrosive coating of claim 1, wherein the base metal is selected from iron, steel, copper, copper alloy, aluminum, aluminum alloy, magnesium, and magnesium alloy.

7. The metal anticorrosive coating of claim 1, wherein the silicon, sodium, potassium, calcium, fluorine, cobalt, nickel, boron, phosphorus, magnesium, and oxygen elements are determined by energy dispersive spectrometer (EDS), the specific method is as follows: the EDS test is operated on an energy spectrometer; firstly, the surface of the sample is ground and polished, and the gold nanoparticles are sprayed on the surface to make a gold film which is conductive, and then the sample is attached to the conductive tape; secondly, the acceleration voltage value is adjusted to 10-40 kV, the dead time is 15-45%, and the measurement time is fixed to 50-400 s; thirdly, the sample is put into the sample cavity, the parameters are set and the vacuum makes the focus clear, the area is selected that needs element analysis and the position is maintained, the point scan, area scan, line scan for element analysis are applied.

8. The metal anticorrosive coating of claim 1, wherein the enamel coating component is selected from enamel powder, thermal expansion regulator, flux, and binder. The content of the enamel powder is 40-90 parts, the content of the thermal expansion regulator is 5-40 parts, the content of the flux is 1-20 parts, the content of the binder is 0.5-12 parts, the content is based on weight.

9. The metal anticorrosive coating of claim 8, wherein the element content of the enamel powder is: 1-40 parts of silicon, 1-20 parts of sodium, 1-23 parts of potassium, 1-18 parts of calcium, 0-10 parts of boron, 0.8-10 parts of phosphorus.

10. The metal anticorrosive coating of claim 8, in the enamel powder, content of siliconoxides is 3-39 parts, content of sodium oxides is 3-28 parts, content of potassium oxides is 1-25 parts, content of boron oxide is 0-15 parts, content of phosphorus oxides is 0.5-10 parts, the content is based on weight.

11. The metal anticorrosive coating of claim 10, wherein the silicon oxides are selected from one or more of silicon oxide, silicon dioxide, and silicon peroxide.

12. The metal anticorrosive coating of claim 10, wherein the sodium oxides are selected from one or more of sodium oxide, sodium peroxide, and sodium hydroxide.

13. The metal anticorrosive coating of claim 10, wherein the potassium oxides are selected from one or more of potassium oxide, potassium carbonate, and potassium hydroxide.

14. The metal anticorrosive coating of claim 10, wherein the phosphorus oxides are selected from one or more of phosphorus trioxide and phosphorus pentoxide.

15. The metal anticorrosive coating of claim 8, wherein the thermal expansion regulators are selected from one or more of sodium silicate, potassium silicate, calcium silicate, magnesium silicate, sodium tetraborate, potassium tetraborate, calcium borate, barium borate, and lithium borate.

16. The metal anticorrosive coating of claim 8, wherein the fluxs are selected from one or more of sodium carbonate, potassium carbonate, magnesium carbonate, strontium carbonate, lithium carbonate, calcium carbonate, barium carbonate, calcium fluoride, magnesium fluoride, and potassium fluoride.

17. The metal anticorrosive coating of claim 8, wherein the binders are selected from one or more of cobalt monoxide, cobalt trioxide, nickel monoxide, and nickel trioxide.

18-21. (canceled)

22. A method for preparing a metal anticorrosive coating and a metal product with a metal anticorrosive coating, characterized in that it includes the following steps: 1) the first grinding: weight the enamel powder, thermal expansion regulator, flux, and binder; and the content of the enamel powder is 40-90 parts, the content of the thermal expansion regulator is 5-40 parts, the content of flux is 1-20 parts, the content of binder is 0.5 to 12 parts; the content is based on weight, and ground into powder; 2) preparation of the mixture: mixing the above four raw materials with water to obtain a mixture; 3) the second grinding: grind the mixture obtained in step 2) into powder after drying; 4) high-temperature reaction: the mixture obtained in step 3) is reacted in a high-temperature furnace at 520 to 720° C. for 10 to 20 minutes; 5) the third grinding: grind the mixture after high temperature reaction to obtain coating powder; 6) coating: coating the powder obtained in step 5) on the base metal; 7) sintering: the powder-coated base metal obtained in step 6) is sintered at high temperature to obtain a metal anticorrosive coating and a metal product with a metal anticorrosive coating; the metal anticorrosive coating is a double-layer structure coating, which is composed of an enamel coating and a base oxide coating. In the double-layer structure, the enamel coating is an outer layer, and the base oxide coating is an inner layer; the inner layer is in contact with the base metal; the composition of the base oxide coating includes the base metal and oxygen; the double-layer structure coating, there is a base metal oxide with a reduced concentration gradient from the inner layer to the outer layer; the composition of the enamel coating includes, by weight, 1-40 parts of silicon, 1-30 parts of sodium, 1-20 parts of potassium, 2-20 parts of calcium, 0.5-15 parts of fluorine, 0.3-10 parts of cobalt, and 0.2-10 parts of nickel, 1-18 parts of boron, 0.5-10 parts of phosphorus, 0.1-8 parts of magnesium, and the rest is oxygen. The composition of the base oxide coating includes base metal and oxygen.

23. (canceled)

24. The preparation method according to claim 18, wherein the sintering parameters of step 7) are: a temperature of 500-620° C., a sintering time of 10-20 minutes, and a temperature increase rate of 5-15° C. per minute.

25-37. (canceled)

38. A metal product, characterized in that the metal product comprises the metal anticorrosive coating according to any one of claim 1.

39-40. (canceled)

41. The metal anticorrosive coating of claim 1, wherein when the base metal is iron or steel, the thermal expansion coefficient of the double-layer structure coating is 10×10.sup.−6/° C.˜16×10.sup.−6/° C. when the base metal is copper or copper alloy, the thermal expansion coefficient of the double-layer structure coating is 13×10.sup.−6/° C.˜16×10.sup.−6/° C. when the base metal is aluminum or aluminum alloy, the thermal expansion coefficient of the double-layer structure coating is 20×10.sup.−6/° C.˜26×10.sup.−6/° C., when the base metal is magnesium or magnesium alloy, the thermal expansion coefficient of the double-layer structure coating is 23×10.sup.−6/° C.˜29×10.sup.−6/° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] FIG. 1a shows a picture of electrostatic sprayed round steel, FIG. 1b shows a picture of electrostatic sprayed rebar.

[0070] FIG. 2 shows a partial electron micrograph of Embodiment 1 (the scale is 250 μm).

[0071] FIG. 3 shows a partial electron micrograph of Embodiment 1 (scale is 100 μm).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] The following examples are only used to illustrate the present invention and not limit the scope of the present invention. In addition, it should be understood that, after reading the content of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of the present invention.

Embodiment 1: Preparation of Double-Layer Thermal Expansion Coefficient Gradient Structure Coating

[0073] 1) The first grinding: weight the enamel powder, thermal expansion regulator, flux, binder. The content of the enamel powder is 40-90 parts, preferably 60-75 parts; the content of the thermal expansion regulator is 5-40 parts, preferably 10-25 parts; the flux content is 1-20 parts, preferably 5-12 parts; the binder content is 0.5-12 parts, preferably 2-6 parts. The stated content is by weight. And grind into powder.

[0074] 2) Preparation of the mixture: mixing the above four raw materials with water to obtain a mixture;

[0075] 3) Second grinding: grind the mixture obtained in step 2) into powder after drying;

[0076] 4) High-temperature reaction: the mixture obtained in step 3) is reacted in a high-temperature furnace at 600° C. for 15 minutes;

[0077] 5) Third grinding: grind the mixture after high temperature reaction to obtain coating powder;

[0078] 6) Coating: coating the powder obtained in step 5) on the base metal using an electrostatic spray method, in which the electrostatic voltage is 80 kV, the current is 20 μA, the powder output is 500 g per minute, and the spray distance is 15 cm.

[0079] 7) Sintering: the powder-coated base metal obtained in step 6) is sintered at 530° C. The sintering time is 15 minutes, and the heating rate is 7.5° C. per minute. At the end, the temperature is naturally cold to room temperature to obtain a metal anticorrosive coating and metal products with a metal anticorrosive coating.

[0080] The specific steps of embodiments 1-8 and comparison embodiments 1-3 are as in embodiment 1, and the specific ratio (weight ratio) is shown in Table 1.

TABLE-US-00001 TABLE 1 Specific composition ratio (weight ratio) and production process parameter settings of embodiments 1-8 and comparison embodiments 1-3. Embodi- Embodi- Embodi- Embodi- Embodi- Embodi- ment 1 ment 2 ment 3 ment 4 ment 5 ment 6 Base metal Copper or Iron or Iron or Iron or Iron or Iron or copper steel steel steel steel steel alloy Enamel powder Total 67 40 90 60 75 67 Thermal expansion Sodium silicate 12 3 3 12 coefficient regulator Potassium silicate 12 1 3 10 Calcium silicate 8 1 4 Sodium tetraborate 10 Potassium tetraborate 5 10 5 Calcium borate 8 Flux Sodium carbonate 5 5 5 Potassium carbonate 3 5 Magnesium carbonate 0.5 10 Calcium fluoride 5 0.5 2 5 Potassium fluoride 5 Magnesium fluoride 1 3 1 Binder Cobalt monoxide 3 3 2 3 3 Cobalt trioxide 3 3 1 Nickel oxide 2 3 2 2 Nickel trioxide 3 2 2 1 Preparation process High temperature 600 520 720 720 520 600 reaction temperature (° C.) High temperature 15 10 20 13 17 15 reaction time (min) Voltage (kV) 80 40 60 30 90 80 Current (uA) 20 20 40 30 80 20 Air output (g/min) 500 400 600 700 200 500 Spraying distance 15 20 25 10 30 15 (cm) Sintering temperature 530 530 530 600 550 530 (° C.) Sintering time 15 10 20 13 17 15 (min) Sintering temperature 7.5 5 10 12.5 15 7.5 increase rate (° C./min) Compar- Compar- Compar- ison ison ison Embodi- Embodi- embodi- embodi- embodi- ment 7 ment 8 ment 1 ment 2 ment 3 Base metal Aluminum or Magnesium or aluminum magnesium Iron or Iron or Iron or alloy alloy steel steel steel Enamel powder Total 67 67 30 50 67 Thermal expansion Sodium silicate 12 12 5 12 coefficient regulator Potassium silicate 10 Calcium silicate 10 Sodium tetraborate 10 Potassium tetraborate 5 5 10 5 Calcium borate 10 Flux Sodium carbonate 5 5 5 10 5 Potassium carbonate 5 Magnesium carbonate 5 Calcium fluoride 5 5 5 5 Potassium fluoride 10 Magnesium fluoride 1 1 10 1 Binder Cobalt monoxide 3 3 5 3 Cobalt trioxide 5 Nickel oxide 2 2 2 Nickel trioxide 5 Preparation process High temperature 600 600 600 520 600 reaction temperature (° C.) High temperature 15 15 15 10 10 reaction time (min) Voltage (kV) 80 80 50 40 30 Current (uA) 20 20 30 20 10 Air output (g/min) 500 500 500 400 900 Spraying distance 15 15 15 20 10 (cm) Sintering temperature 530 530 620 530 680 (° C.) Sintering time 15 15 15 10 15 (min) Sintering temperature 7.5 7.5 7.5 5 10 increase rate (° C./min)

TABLE-US-00002 TABLE 2 Composition ratio (mass ratio) of enamel powder in embodiments 1-8 and comparison embodiments 1-3. Embodi- Embodi- Embodi- Embodi- Embodi- Embodi- ment 1 ment 2 ment 3 ment 4 ment 5 ment 6 Silicon oxides Silicon oxide 30 15 15 10 9 30 Silicon dioxide 5 4 20 10 15 5 Silicon peroxide 4 3 4 19 15 4 Sodium oxides Sodium oxide 20 4 4 10 15 20 Sodium peroxide 4 20 4 10 3 4 Sodium hydroxide 4 4 3 8 10 4 Potassium oxides Potassium oxide 10 5 10 3 8 10 Potassium carbonate 10 10 5 3 8 10 Potassium hydroxide 5 10 10 2 9 5 Phosphorus oxides Phosphorus trioxide 4 5 7 10 3 4 Phosphorus pentoxide 3 5 3 0 5 3 Boron oxide 1 15 10 15 0 1 Compar- Compar- Compar- ison ison ison Embodi- Embodi- embodi- embodi- embodi- ment 7 ment 8 ment 1 ment2 ment 3 Silicon oxides Silicon oxide 30 30 40 30 Silicon dioxide 5 5 30 5 Silicon peroxide 4 4 4 Sodium oxides Sodium oxide 20 20 10 5 20 Sodium peroxide 4 4 10 15 4 Sodium hydroxide 4 4 10 4 Potassium oxides Potassium oxide 10 10 10 Potassium carbonate 10 10 20 10 Potassium hydroxide 5 5 5 Phosphorus oxides Phosphorus trioxide 4 4 7 10 4 Phosphorus pentoxide 3 3 8 3 Boron oxide 1 1 15 20 1

TABLE-US-00003 TABLE 3 Element content (mass ratio) of enamel coating in embodiments 1-8 and comparison embodiments 1-3. Compar- Compar- Compar- ison ison ison Embodi- Embodi- Embodi- Embodi- Embodi- Embodi- Embodi- Embodi- embodi- embodi- embodi- ment 1 ment 2 ment 3 ment 4 ment 5 ment 6 ment 7 ment 8 ment 1 ment 2 ment 3 Silicon 7 3 30 4 15 7 7 7 1 50 8 Sodium 12 7 16 7 2 12 12 12 22 35 5 Potassium 7 15 10 10 3 7 7 7 17 2 3 Calcium 8 11 4 16 10 8 8 8 18 1 5 Fluorine 5 3 2 5 7 5 5 5 1 2 9 Cobalt 2 4 1 2 0.5 2 2 2 0 1 15 Nickel 2 4 0.5 2 8 2 2 2 0 1 15 Boron 5 8 2.5 2.5 10 5 5 5 14 1 1 Phosphorus 2 4 1 1.5 6 2 2 2 8 1 1 Magnesium 1 2 0.5 5 0.2 1 1 1 0 5 1 Oxygen 49 39 32.5 45 38.3 49 49 49 19 1 37

[0081] In order to verify the effect of the coating and coating method for anti-corrosion of steel bars of the present invention, the following tests were conducted.

[0082] 1) Wear Resistance Test

[0083] According to the coating process of embodiment 1 and comparison embodiment 1, the coatings of the present invention were fabricated on steel plates, respectively, with two replicate samples per experimental group, and a total of four samples. The contents of silicon, sodium, potassium, calcium, fluorine, cobalt, nickel, boron, phosphorus, magnesium, and oxygen in comparative embodiment 1 are not within the scope of the claims. The wear resistance of the coating was tested according to the falling sand erosion test method of ASTM D968-93. The sand used was Chinese ISO standard sand. After the coating surface flushed out an area with a diameter of 2 mm, the sand falling was stopped and the volume of sand falling was recorded. When the falling sand volume is larger, the wear resistance of the coating is better.

TABLE-US-00004 TABLE 4 Wear resistance test data. Comparison Embodiment 1 embodiment 1 Sample Sample Average Sample Sample Average 1 2 value 3 4 value Falling 12.6 12.4 12.5 4.0 4.2 4.1 sand volume (L)

[0084] From the volume of falling sand in Table 4, the average value of the falling sand volume of embodiment 1 of the present invention is 12.5 L, and the average value of the falling sand volume of comparison embodiment 1 is 4.1 L. It can be seen that the wear resistance of the coating of embodiment 1 is far superior to that of comparison embodiment 1.

[0085] 2) Tensile Test

[0086] Six groups (Embodiments 1, 2, 3 and comparison embodiments 1, 2, 3) were selected with three replicate samples in each group, and three resistance strain gauges were attached to each coated steel bar. At the beginning of the experiment, the steel bar was placed on a tensile testing machine to measure the change of strain with load, and the resistance strain gauge was connected to a strain gauge to measure the strain change on the coated steel bar.

TABLE-US-00005 TABLE 5 Rebar tensile test. Coating cracking strain value (micro strain) Measuring Measuring Measuring Average point 1 point 2 point 3 value Embodi- #1 tested 2099 2100 2090 2096 ment 1 rebar #2 tested 2050 2100 2076 2075 rebar #3 tested 2071 2090 2046 2069 rebar Embodi- #1 tested 1808 1888 1854 1850 ment 2 rebar #2 tested 1789 1812 1822 1807 rebar #3 tested 1866 1843 1823 1844 rebar Embodi- #1 tested 1477 1423 1456 1452 ment 3 rebar #2 tested 1508 1488 1478 1491 rebar #3 tested 1400 1478 1492 1457 rebar Compar- #1 tested 865 872 828 855 ison rebar embodi- #2 tested 823 852 843 839 ment 1 rebar #3 tested 847 821 853 840 rebar Compar- #1 tested 789 777 761 776 ison rebar embodi- #2 tested 750 756 781 762 ment 2 rebar #3 tested 743 762 771 759 rebar Compar- #1 tested 698 666 645 670 ison rebar embodi- #2 tested 687 677 690 685 ment 3 rebar #3 tested 669 650 666 662 rebar

[0087] According to the experimental results in Table 5, when the coating was cracking, the average strain value of the three groups of coated steel bars of embodiment 1-3 was 1200-2300 micro strain, and the average strain value of the coated steel bars of comparison embodiments 1, 2, and 3 was in the range of 650-850 micro strain. Therefore, the steel rebar coated with the tough coating for reinforcing steel anticorrosion of the present invention can be coordinated with the building steel rebar. If the coating was fabricated not according to the specific material ratio and specific preparation process parameters, the performance of the coating could not meet the demand.

[0088] 3) Rebar Corrosion Test

[0089] Six experimental groups and six control groups were selected, and the experimental group was coated steel bars (Embodiments 1, 4, 5 and comparison embodiments 1, 2, 3). The control group 1 was the coating data of the group 2 in the accelerated corrosion test of the steel bar in Table 1 of CN105670366B patent, and the control group 2 was the coating data of the group 1 in the accelerated corrosion test of the steel bar in the patent CN105819691A. The control group 3 was the data of coated round steel bar in the accelerated corrosion test of steel bar in Table 1 of CN105585883B patent. The control group 4 was the coating data of the group 3 in the accelerated corrosion test of steel bar in Table 4 of CN105238105B patent. The control group 5 was the data of the coating without fiber in the accelerated corrosion test of steel bar in Table 1 of CN105131660B patent. The control group 6 was uncoated bare steel. Place samples in a 3.5% sodium chloride solution and conduct an accelerated corrosion test after energizing.

TABLE-US-00006 TABLE 6 Accelerated corrosion test of steel bars. Corrosion time (min) # 1 # 2 # 3 Average Group steel bar steel bar steel bar value Embodiment 1 1523 1521 1511 1518 Embodiment 4 1576 1556 1566 1566 Embodiment 5 1423 1511 1486 1473 Comparison 566 513 542 540 embodiment 1 Comparison 500 540 532 524 embodiment 2 Comparison 444 456 434 445 embodiment 3 Control group 1 1036 1045 1048 1043 Control group 2 1176 1021 1078 1091 Control group 3 673 778 798 750 Control group 4 576 610 613 599 Control group 5 273 349 336 319 Control group 6 110 108 112 110

[0090] It can be seen from Table 6 that the uncorroded time of coated steel bars of embodiments 1, 4, and 5 is about 14 times longer than uncoated steel bars, 5 times of the corrosion resistance time of steel bar of CN105131660B, 2.5 times of the corrosion resistance time of steel bar of CN105238105B, 2 times of the corrosion resistance time of steel bar of CN105585883B, 1.5 times of the corrosion resistance time of steel bar of CN105670366B, 1.4 times of the corrosion resistance time of steel bar of CN105819691A. At the same time, it can be seen that the corrosion resistance of comparison embodiments 1, 2, and 3 is only one-third of that of embodiments 1, 4, and 5. Indicating that if the specific material ratio and specific preparation process parameters are not followed, the performance of the coating could not meet the demand.

[0091] 4) Corrosion Resistance Test of Metal Plate

[0092] Take four experimental groups and a control group respectively, the experimental group was coated metal plate (Embodiments 1, 6, 7, 8), the control group was uncoated steel plate, uncoated copper plate, uncoated aluminum plate. The total number of test steel plates is 15. Place the samples in a 3.5% sodium chloride solution and conduct an accelerated corrosion test after energizing.

TABLE-US-00007 TABLE 7 Accelerated corrosion test of metal plates. Group Corrosion Embodi- Embodi- Embodi- Embodi- time (min) ment 1 ment 6 ment 7 ment 8 Coated plate 1 1554 1557 1583 1565 Coated plate 2 1501 1499 1511 1504 Coated plate 3 1430 1467 1493 1463 Average of 1495 1508 1529 1511 three plates Uncoated 107 113 112 111 plates

[0093] It can be seen form Table 7 that the uncorroded time of coated metal plates is around 14 times that of uncoated metal plates, it has an excellent corrosion resistance for steel plate, aluminum plate, magnesium plate, and copper plate.

[0094] 5) Thermal Expansion Coefficient Test of Metal Plate

[0095] Take the embodiments 1-8 and comparison embodiments 1-3 for the thermal expansion coefficient test. The base metals of embodiments 1 to 5 and comparison embodiments 1, 2, and 3 are steel plates, the base metal of embodiment 6 is copper plate, the base metal of embodiment 7 is aluminum plate, and the base metal of embodiment 8 is magnesium plate. And for the steel plate, copper plate, aluminum plate and magnesium plate, three sets of plates with different thermal expansion coefficients were taken, and a total of 33 test pieces were tested for the coating thermal expansion coefficient. Among them, the enamel coating is C1, the base oxide coating is C2, and the enamel coating and the base oxide coating are collectively referred to as a double-layer structure coating, which is C1+C2. The thermal expansion coefficients of the enamel coating, the base oxide coating, and the double-layer structure coating were measured separately.

TABLE-US-00008 TABLE 8 Test for determination of thermal expansion coefficient of metal plates. Group Thermal Compar- Compar- Compar- expansion ison ison ison coefficient Embodi- Embodi- Embodi- Embodi- Embodi- Embodi- Embodi- Embodi- embodi- embodi- embodi- (×10.sup.−6/° C.) ment 1 ment 2 ment 3 ment 4 ment 5 ment 6 ment 7 ment 8 ments 1 ments 2 ments 3 Coating C1 12.8 13.0 13.1 13.0 13.1 17.3 24.5 26.5 12.1 12.0 12.4 1 C2 12.6 12.7 12.6 12.5 12.8 17.0 24.0 26.2 12.0 — — C1 + 12.7 12.8 12.8 12.7 12.9 17.2 24.2 26.3 12.1 — — C2 Base 12.5 12.5 12.5 12.5 12.5 16.8 23.9 26.1 12.5 12.5 12.5 metal 1 Coating C1 11.2 11.2 11.3 11.0 11.2 13.7 20.6 23.8 10.0 10.5 10.8 2 C2 10.8 10.9 11.0 10.8 10.8 13.5 20.4 23.6 10.0 — — C1 + 10.9 11.0 11.1 11.0 10.9 13.6 20.5 23.7 10.0 — — C2 Base 10.8 10.8 10.8 10.8 10.8 13.5 20.4 23.5 10.8 10.8 10.8 metal 2 Coating C1 15.6 12.8 15.8 16.0 15.6 19.4 25.7 28.9 14.9 15.1 15.5 3 C2 15.5 15.5 15.4 15.6 15.5 19.2 25.6 28.7 14.9 — — C1 + 15.5 15.6 15.6 15.7 15.5 19.3 25.6 28.8 14.9 — — C2 Base 15.4 15.4 15.4 15.4 15.4 19.2 25.5 28.6 15.4 15.4 15.4 metal 3

[0096] It can be obtained from Table 8 that the thermal expansion coefficient of the double-layer structure coating (C1+C2) in the present invention would change with the change of the base metal, so the whole double-layer structure coating and the base metal are coordinated, thus proving that this coating is universal and can be applied to various metals. Meanwhile, it is also observed that when the base metal is iron or steel, the thermal expansion coefficient of the double-layer structure coating ranges from 10×10.sup.−6/° C. to 16×10.sup.−6/° C.; when the base metal is copper or a copper alloy, the thermal expansion coefficient of the double-layer structure coating ranges from 13×10.sup.−6/° C. to 20×10.sup.−6/° C.; when the base metal is aluminum or aluminum alloy, the thermal expansion coefficient of the double-layer structure coating ranges from 20×10.sup.−6/° C. to 26×10.sup.−6/° C.; when the base metal is magnesium or a magnesium alloy, the thermal expansion coefficient of the double-layer structure coating ranges from 23×10.sup.−6/° C. to 29×10.sup.−6/° C. At the same time, it can be seen that the thermal expansion coefficients of the base metal, the base oxide coating (C2), and the enamel coating (C1) are gradually increased, therefore it can be found that the coating has a gradient of thermal expansion coefficient. At the same time, it can be seen that the thermal expansion coefficients of comparative embodiments 1, 2, and 3 are not satisfied with the harmonized requirements, and none of comparative embodiments 1, 2, and 3 have a base oxide coating. It shows that if the specific material ratio and specific preparation process parameters are not followed, the performance of the coating would not meet the demand.

[0097] As can be seen from Table 3 to Table 8, the compositions of the coating are silicon, sodium, potassium, calcium, fluorine, cobalt, nickel, boron, phosphorus, magnesium, and oxygen meet the specific composition ratio. Combined with specific preparation process parameters, the metal coating of the specific double-layer structure of the present invention can be obtained. In the two-layer structure coating, there is a base metal oxide with a reduced concentration gradient from the inner layer to the outer layer.

[0098] 6) Optical and Scanning Electron Microscope Images of Coating

[0099] FIG. 1a is a picture of electrostatically sprayed round steel, using the raw material ratio of embodiment 1. It can be seen that the coating is very glossy from both macro and micro, the enamel gloss indicates that the coating has a higher density. There are no cracks in the coating due to too low thermal expansion coefficient, nor flaking due to excessive thermal expansion coefficient. It means that the thermal expansion coefficients of the coating and the base metal are in good agreement. This dense structure also means that the coating has a good corrosion resistance.

[0100] The detection method of the energy spectrometer (EDS) is: EDSmeasurement adopts the energy spectrometer to test. First, the surface of the sample is ground and polished, and the gold nanoparticles are sprayed on the surface to make a gold film which is conductive, and then the sample is attached to the conductive tape. The acceleration voltage value is adjusted to 10-40 kV, the dead time is 15-45%, and the measurement time is fixed to 50-400 s. The sample is put into the sample cavity, the parameters are set and the vacuum makes the focus clear, the area is selected that needs element analysis and the position is maintained, the point scan, area scan, line scan for element analysis are applied. Through the detection of EDS technology, it is obtained that the silicon content is 7%, the sodium content is 12%, the potassium content is 7%, the calcium content is 8%, the fluorine content is 5%, the cobalt content is 2%, the nickel content is 2%, the boron content is 5%, the phosphorus content is 2%, the magnesium content is 1%, and the oxygen content is 49%.

[0101] FIG. 1b is a picture of electrostatically sprayed rebar, using the raw material ratio of embodiment 1. It can also be seen that the coating has a dense structure and an enamel gloss. It can be seen that no cracking occurs at the boundary between the convex surface and the plane, indicating that the coating does not crack during the high-temperature sintering process. Furthermore, it illustrates that the thermal expansion coefficient gradient double-layer coating of the present invention can deform collaboratively with the base metal at high temperatures.

[0102] FIG. 2 is an electron micrograph of embodiment 1, which is similar to embodiments 2 and 3, so embodiment 1 is taken as a representative. It can be seen that the thickness of the coating is about 200 μm, the density is very high. There are no through holes, only a small number of closed pores. The area of closed pores is calculated to obtain a closed pore rate of 4.3%. The presence of a small number of closed pores can make the coating have a certain ductility. At the same time, the coating consists of two parts, namely enamel coating (C1) and base oxide coating (C2). The thickness of C1 is about 180 μm and the thickness of C2 is about 20 μm. C1, C2 and the steel bar form a sandwich structure. Because of sintering in an oxidizing atmosphere, C2 are formed. Besides iron oxide, Fe—Co and Fe-Ni mixed crystals also exist in C2, which makes the coating more tightly bonded to the steel bar. And the thickness of C2 is controllable, and its thickness would increase with the increase of sintering temperature. At the same time, due to the presence of the outer coating C1, it would inhibit the contact between the external oxygen and the C2 layer, so the thickness of the coating is controlled to around 20 μm. In addition, the presence of the C2 transition layer in the coating can not only increase the adhesion, but also effectively improve the corrosion resistance.

[0103] FIG. 3 is a partially enlarged electron micrographfrom FIG. 2. In FIG. 3, it can be clearly seen that the coating near the steel bar is significantly different from the coating away from the steel bar. The area of white spots in the coating nearer to the steel bar is getting larger and larger. The EDS results show that the white spots are iron oxide components. The iron element of the whole coating (C1+C2) gradient changes from more to less from inside to outside, therefore iron oxide also gradient changes from more to less from inside to outside. This change also causes a gradient change in the thermal expansion coefficient of the coating. As the iron oxide decreases, the thermal expansion coefficient of the coating gradually increases. The double-layered thermal expansion coefficient gradient structure of this sandwich structure makes the coating and the steel bar has an extremely strong adhesion and 14 times better corrosion resistance than ordinary bare steel.