CN114807815B

CN114807815B - Anti-corrosion alloy coating for steel hot dip coating and steel hot dip coating method

Info

Publication number: CN114807815B
Application number: CN202210520934.2A
Authority: CN
Inventors: 唐艳明; 李国喜
Original assignee: Fushun City Mingyao Petroleum Machinery Co ltd
Current assignee: Fushun City Mingyao Petroleum Machinery Co ltd
Priority date: 2022-05-12
Filing date: 2022-05-12
Publication date: 2023-02-03
Anticipated expiration: 2042-05-12
Also published as: CN114807815A

Abstract

The invention relates to an anticorrosive alloy coating for steel hot dip coating and a hot dip coating method thereof, belonging to the technical field of novel metal functional materials. The anticorrosive alloy coating for hot dip coating of steel comprises, by weight, 80-98 parts of aluminum, 0.2-2 parts of ferrum, 0.01-10 parts of manganese, 0.01-2 parts of boron, 0.2-3 parts of chromium and 0.1-5 parts of rare earth metal. The high corrosion-resistant multi-element alloy coating without pinholes, iron phase spheroidization and grain refinement is obtained after the hot dip coating method for the corrosion-resistant alloy coating for steel hot dip coating is adopted for treatment, the electrochemical corrosion speed can be reduced by at least one order of magnitude, and the corrosion resistance of the steel coating is greatly improved.

Description

Anti-corrosion alloy coating for steel hot dipping and steel hot dipping method

Technical Field

The invention relates to an anticorrosive alloy coating for steel hot dip coating and a steel hot dip coating method, belonging to the technical field of novel metal functional materials, steel surface anticorrosive materials and treatment.

Background

Although aluminum alloy materials have excellent resistance to corrosion by industrial water, seawater, ammonia water, hydrogen sulfide solution and gas, and various organic acid solutions, their application in these fields is limited due to insufficient mechanical properties. Since various iron-based alloys containing aluminum have been studied, but the strength of the alloy is rapidly decreased when the content of aluminum in the iron-aluminum alloy exceeds 6%, various techniques for coating aluminum on the surface of steel have been studied in order to combine the strength of steel with the corrosion resistance of aluminum, such as powder pack aluminizing, thermal spraying aluminum, and hot dipping aluminum.

Aluminizing can form only a layer of iron-aluminum alloy (aluminum content is generally 20 to 50 wt%) on the surface of a steel material, and has excellent corrosion resistance in a vapor phase corrosion medium, but the corrosion resistance in a liquid phase corrosion medium is uncertain compared with that of an aluminum alloy material and needs to be determined by specific tests (w.a. mcgill.m.j.weinbaum. Chemical refinery, 1979, no. 1. The thermal spraying aluminum is not limited by production conditions and can process workpieces with any size, but the thermal spraying aluminum coating has pores which are matched with the substrateThe mechanical bonding is low, so that the corrosion resistance is obviously different from that of an aluminum alloy material, and the aluminum alloy material is mainly used for large pieces which cannot be treated by other methods and field construction, or for repairing defects locally existing by other methods. Hot dip aluminizing is to immerse a steel material in molten aluminum, firstly, aluminum atoms are infiltrated into the steel to form an iron-aluminum alloy layer, then a layer of pure aluminum or aluminum alloy is adhered to the surface of the iron-aluminum alloy layer when aluminum liquid is extracted, and finally a composite layer consisting of the iron-aluminum alloy layer and the pure aluminum layer is formed. However, in practical application, the corrosion resistance of the hot-dipped aluminum layer produced by the hot-dipped aluminum technology used in the current production is still inferior to that of the aluminum alloy material. The reasons for this are mainly from three aspects: (1) pinholes exist in the plating layer; (2) The coating contains a large amount of acicular iron-aluminum intermetallic compound phase (FeAl) ₃ ) (ii) a And (3) coarse grains of the plating layer.

First, since aluminum is an extremely reactive metal element, even a very small amount of oxidizing substances present on the surface of a steel substrate before hot dipping aluminum oxidizes liquid aluminum atoms in contact to produce aluminum oxide particles, which reduce the wettability of the liquid aluminum, thereby causing pinholes in the coating layer, i.e., the aluminum oxide particles present at the interface between the steel substrate and the aluminum liquid, which are responsible for the pinholes in the hot-dipped aluminum layer, it has been found as early as 1962 (Sagatran. Metal surface technology, 1962, 25 (3): 20). Since the detection of defects in the coating according to the standards of hot dip aluminum plated products in practical applications only provides for macroscopic visual inspection, the microscopic degree of compactness of the coating and the effect thereof on the service life are not paid sufficient attention. Through carrying out 15% nitric acid solution corrosion research on hot-dip aluminum layers produced by various hot-dip aluminum technologies such as a gas protection method, a passivation method, a flux-bath method and the like used in the actual production at present, the appearance of the hot-dip aluminum layers produced by different hot-dip aluminum technologies is similar, but the micro-scale compactness degree of the hot-dip aluminum layers is obviously different. Furthermore, none of these prior art processes can ensure complete removal of the oxidizing species from the surface of the steel substrate, i.e., no pin holes are formed in the coating, resulting in corrosion resistance of prior hot dip aluminum plated products that is not at the theoretical level for aluminum alloy materials.

Secondly, in the hot-dip aluminum production process, because steel parts need to be continuously immersed in the aluminum liquid, iron atoms are inevitably dissolved in the aluminum liquid, along with the extension of the production time, the iron atoms can be accumulated to the maximum level of 2 percent in the aluminum liquid, which is far more than the maximum content of 0.052 percent in the aluminum solid melt, and FeAl is used for cooling the aluminum solid melt ₃ The needle-like phase is present in the coating. FeAl ₃ The aluminum matrix is a cathode phase, and the shape of the aluminum matrix is thick and needle-shaped, so that the electrochemical nonuniformity of the plating layer is increased, and the pitting corrosion is easily caused. However, all the patent documents related to hot dipping aluminum do not relate to how to effectively control the content and the form of iron in aluminum liquid and a coating.

Thirdly, all the existing hot-dip aluminum plating technologies are specified to adopt air cooling after hot-dip aluminum plating, the low cooling speed causes coarse grains of a plating layer, the surface temperature is lower than that of a matrix, and a high-melting-point phase is preferentially precipitated and solidified on the surface, so that the electrochemical nonuniformity of the surface of the plating layer is further aggravated, and the corrosion resistance is reduced.

In summary, the existing hot dip aluminum plating techniques have more than one of the above problems, for example, CN101649440A is a method for assisting in plating steel by hot dip aluminum plating, CN106148868A is a novel plating assistant for hot dip aluminum plating and a use method thereof, CN107365954A is a plating assistant for hot dip aluminum plating and a hot dip aluminum plating process of steel structure, CN103266291A is a method for plating a manganese alloy layer by hot dip aluminum plating, CN110257750A is an aluminum alloy plating layer by hot dip aluminum plating and a hot dip plating method thereof, which all use an aqueous solution plating assistant and inevitably cause pinholes in the plating layer; the molten salt assistant plating can eliminate coating pinholes, but the molten salt assistant plating has narrow component adaptability, high aluminum fluoride content, high melting point, high viscosity and difficult removal, and iron phase in the coating is not controlled. Other recent patents, such as CN111485189A, like Al-Mg-Si-Er-In hot dip coating anode alloy coating and its preparation method, although the coating is alloyed, the coating is not treated aiming at the inevitable iron phase with the largest corrosion influence, and does not play the due role In the actual production at all; CN102268625A is a hot dip aluminum plating method of a steel structural part, which provides a method for refining a plating layer by pulse, but the industrial production of large-size workpieces such as steel pipes is difficult to realize.

Disclosure of Invention

Technical problem to be solved

In order to solve the problems of the existing hot dip aluminum plating technology, the invention provides an anti-corrosion alloy plating layer for steel hot dip plating and a hot dip plating method thereof.

(II) technical scheme

In order to achieve the purpose, the invention adopts the main technical scheme that:

on one hand, the invention provides an anticorrosive alloy coating for hot dip coating of steel, which comprises 80-98 parts by weight of aluminum, 0.2-2 parts by weight of ferrum, 0.01-10 parts by weight of manganese, 0.01-2 parts by weight of boron, 0.2-3 parts by weight of chromium and 0.1-5 parts by weight of rare earth metal.

Preferably, the rare earth metal is any one or combination of lanthanum, cerium and yttrium.

Furthermore, the anti-corrosion alloy coating comprises 90 to 98 parts of aluminum, 0.5 to 1 part of iron, 0.05 to 5 parts of manganese, 0.01 to 2 parts of boron, 0.3 to 2 parts of chromium and 0.19 to 2.8 parts of rare earth metal.

On the other hand, the invention also provides a method for hot-dip coating of anticorrosive alloy on steel, which comprises the steps of carrying out molten salt bath assistant coating on the steel in the hot-dip coating process, hot-dipping the multi-component alloy, heating the surface of a coating by adopting an induction heating mode, and then rapidly cooling; the multi-element alloy comprises, by weight, 80-98 parts of aluminum, 0.2-2 parts of iron, 0.01-10 parts of manganese, 0.01-2 parts of boron, 0.2-3 parts of chromium and 0.1-5 parts of rare earth metal, wherein the rare earth metal is any one or combination of more of lanthanum, cerium and yttrium.

Further, the multi-element alloy comprises 90-98 parts of aluminum, 0.5-1 part of iron, 0.05-5 parts of manganese, 0.01-2 parts of boron, 0.3-2 parts of chromium and 0.19-2.8 parts of rare earth metal.

In the method, the molten salt bath preferably comprises 20 to 40 parts by weight of sodium chloride, 20 to 48 parts by weight of potassium chloride, 10 to 20 parts by weight of cryolite, 1 to 10 parts by weight of aluminum fluoride, 0.5 to 6 parts by weight of potassium fluoroaluminate and 0.1 to 8 parts by weight of manganese chloride.

Further, the components of the molten salt bath comprise 32-40 parts of sodium chloride, 40-48 parts of potassium chloride, 10-17 parts of cryolite, 2-8 parts of aluminum fluoride, 1-4 parts of potassium fluoroaluminate and 0.2-1 part of manganese chloride.

In the method, the working temperature of the molten salt bath is preferably 700 to 800 ℃, and the treatment time is preferably 5 to 60 minutes.

In the above method, the multi-component alloy is hot-dipped at a temperature of 700 to 800 ℃ for 3 to 10 minutes.

In the method as described above, preferably, the operation of induction-heating the surface of the plated layer is: the plating layer is heated to more than 750 ℃ by an induction heating coil.

In the method, preferably, the rapid cooling is spray cooling at 200-300 ℃ and then water cooling; or directly cooling in a nitrate bath at 200-300 ℃ and then cooling with water.

In the method, the steel material is preferably a steel pipe.

In another aspect, the invention provides a plating assistant agent for hot dip plating of steel, which comprises 20 to 40 parts by weight of sodium chloride, 20 to 48 parts by weight of potassium chloride, 10 to 20 parts by weight of cryolite, 1 to 10 parts by weight of aluminum fluoride, 0.5 to 6 parts by weight of potassium fluoroaluminate and 0.1 to 8 parts by weight of manganese chloride.

The plating assistant for hot dip plating of steel products as described above preferably comprises 32 to 40 parts of sodium chloride, 40 to 48 parts of potassium chloride, 10 to 17 parts of cryolite, 2 to 8 parts of aluminum fluoride, 1 to 4 parts of potassium fluoroaluminate and 0.2 to 1 part of manganese chloride.

The absence of pinholes is a prerequisite for the corrosion resistance of any plating layer, and the addition of any corrosion-resistant alloy element in the presence of pinholes is futile. Compared with the water solution plating assistant agent, the salt bath plating assistant method and the molten salt bath provided by the invention can thoroughly eliminate plating layer pinholes, purify aluminum liquid and reduce oxide inclusions. In the components of the salt bath provided by the invention, 4 combinations of sodium chloride, potassium chloride, cryolite (sodium fluoroaluminate) and aluminum fluoride are taken as a basis, potassium fluoroaluminate is added on the basis to adjust the melting point and reduce the viscosity, the plating assistant effect and the application range of the melting plating assistant agent are further improved, and the manganese chloride is added to purify the multi-element alloy liquid in real time by generating a small amount of reaction gas, so that the function of molten salt plating assistant is greatly improved, and the defects of the existing salt bath plating assistant formula are overcome.

The common industrial pure aluminum plating solution in the prior art is not added with alloy elements, the iron content can be accumulated to 3 percent at most, and the iron exists in a needle-shaped second phase in a plating layer, which seriously reduces the corrosion resistance of the plating layer. However, the anti-corrosion alloy plating layer for hot dip plating of steel provided by the invention can improve the corrosion resistance of the plating layer by further adjusting the quantity, the form, the potential and the passivation capability of the iron phase in the plating layer on the basis of eliminating pin holes and oxide inclusions. The invention reduces the content of iron in the molten aluminum to below 2 percent by adding manganese and boron, and correspondingly reduces the number of iron phases in the plating layer; the form of the iron phase is changed from a needle shape to a spherical shape or a block shape by adding manganese and chromium, so that the area occupied by the iron phase and the number of needle-shaped second phases are reduced, meanwhile, the potential difference between the iron phase and the aluminum matrix is reduced by manganese, and the passivation capability of the iron phase is improved by chromium; by adding rare earth to refine the iron phase and the matrix aluminum phase, and reducing oxide inclusions, the refined crystal grains can further improve the corrosion resistance, uniformity and passivation capability of the plating layer. By combining the functions of the metals in the multi-element alloy plating layer, the invention effectively improves the corrosion resistance, uniformity and passivation capability of the plating layer on the basis of eliminating pinholes and improving the iron phase form.

Further, the invention also refines the crystal grains of the coating by rapid cooling. Because the surface temperature of the plating layer is easy to drop and the high-melting-point phase is easy to precipitate even in a heat preservation state, the invention also reduces the second phase and refines the crystal grains to a greater extent by means of induction heating of the plating layer. The specific operation is that the steel plated piece is lifted out of the multi-component alloy liquid and transferred to the upper part of a cooling pool, at the moment, the temperature of the plating layer is raised through induction heating of the plating layer, the high-melting-point phase is completely dissolved, and then the steel plated piece is rapidly cooled, so that the second phase and refined grains are reduced to a greater extent. The cooling method is divided into two types: firstly, spray cooling is carried out, and then water cooling is carried out; secondly, directly cooling in a nitrate salt bath at the temperature of 200-300 ℃ and then cooling by water.

(III) advantageous effects

The invention has the beneficial effects that:

the anti-corrosion alloy coating for hot dip coating of steel provided by the invention can effectively eliminate pinholes and oxide inclusions, the quantity of iron phases in the coating is reduced by adding manganese and boron, the shape of the iron phases is changed from needle shape to spherical shape or block shape by adding manganese and chromium, the area occupied by the iron phases is reduced, meanwhile, the potential difference between the iron phases and an aluminum matrix is reduced by manganese, and the passivation capacity of the iron phases is improved by chromium; the rare earth is added to refine the crystal grains of the iron phase and the matrix aluminum phase and reduce oxide inclusions.

According to the anti-corrosion alloy coating for steel hot dip plating and the hot dip plating method thereof, provided by the invention, the high anti-corrosion multi-element alloy coating without pinholes, spheroidized iron phase and refined crystal grains can be obtained by the hot dip plating method, the electrochemical corrosion speed can be reduced by at least more than one order of magnitude, and the anti-corrosion performance is greatly improved. The electrochemical corrosion rate of the obtained coating in 3.5% NaCl solution under the optimum process conditions can be reduced from 3.60X 10% of that of ordinary Al-0.2-3% Fe coating ^-6 A/cm ² Reduced to 2.36 × 10 ^-9 A/cm ² (ii) a The corrosion speed is reduced by 3 orders of magnitude, and the effect is very obvious.

Drawings

FIG. 1 is a surface topography of a multi-component alloy water-cooling coating obtained in example 1;

FIG. 2 is a surface morphology of the multi-element alloy plating layer obtained in comparative example 1;

FIG. 3 is a surface topography of the multi-alloy plating layer obtained in comparative example 2.

Detailed Description

The method for hot dip coating the steel with the multi-element alloy comprises the following processes: the steel is subjected to oil removal, water washing, rust removal, water washing, air drying, molten salt bath assistant plating, hot dipping of multi-element alloy, induction heating of the surface of a plating layer, rapid cooling and post-treatment.

Preferably, the oil is removed by using a commercially available water-based cleaning agent, the rust is removed by using 15-20% hydrochloric acid, and the steel plate is washed by hot water and dried.

Then the steel is immersed in a molten salt bath (plating assistant) for treatment. The salt bath treatment can thoroughly remove the oxide on the surface of the steel base and the oxide in the aluminum liquid, eliminate the pinholes of the coating and reduce the oxide inclusions in the coating. Wherein the salt bath consists of 20 to 40 parts of sodium chloride, 20 to 48 parts of potassium chloride, 10 to 20 parts of cryolite, 1 to 10 parts of aluminum fluoride, 0.5 to 6 parts of potassium fluoroaluminate and 0.1 to 8 parts of manganese chloride as plating assistant. Furthermore, the salt bath is composed of 32-40 parts of sodium chloride, 40-48 parts of potassium chloride, 10-17 parts of cryolite, 2-8 parts of aluminum fluoride, 1-4 parts of potassium fluoroaluminate and 0.2-1 part of manganese chloride, the salt bath temperature of the plating assistant agent is preferably 700-800 ℃, and the treatment time is preferably 5-60 minutes. The salt bath temperature is lower than 700 ℃ and has poor purification effect on the surface of a steel matrix, and the volatilization is increased when the temperature is higher than 800 ℃. The processing time is mainly determined by the weight of the hot-dip workpiece, the time required for the weight is long, and the time required for the weight is short.

The steel is extracted from the salt bath and then dipped into the molten multi-component alloy of the anti-corrosion alloy coating layer for hot dip coating of the steel. The alloy composition of the anti-corrosion alloy coating comprises, by weight, 80-98 parts of aluminum, 0.2-2 parts of iron, 0.01-10 parts of manganese, 0.01-2 parts of boron, 0.2-3 parts of chromium and 0.1-5 parts of rare earth metal, wherein the rare earth metal comprises any one or combination of more of lanthanum, cerium and yttrium. Further, it is preferably 90 to 98 parts of aluminum, 0.5 to 1 part of iron, 0.05 to 5 parts of manganese, 0.01 to 2 parts of boron, 0.3 to 2 parts of chromium, and 0.19 to 2.8 parts of rare earth metal. Wherein, manganese and boron can reduce the solubility of iron in the aluminum liquid. Chromium and manganese can form a composite compound with the iron-aluminum compound, so that the iron-aluminum compound is spheroidized, the electrochemical performance of the plating layer is more uniform, and the corrosion resistance of the plating layer is further improved. The rare earth metal can refine grains and reduce oxide inclusions. The dosage of the alloy element determines the corrosion resistance of the plating layer, too low can not change the form of an iron phase and can not improve the corrosion resistance of the plating layer, and too high can form too much second phase to aggravate the electrochemical nonuniformity of the plating layer and reduce the corrosion resistance of the plating layer. The temperature of the multi-component alloy liquid is preferably controlled to 700 to 800 ℃, and the hot dipping time is preferably 3 to 10 minutes, and such a range of process conditions contributes to control of the iron phase in the plating layer, the thickness of the plating layer, and the strength of the steel substrate. After being extracted from the multi-element alloy liquid, the steel is quickly transferred to the upper part of a cooling pool, an induction heating ring which is matched with the shape of a plated piece is arranged at an inlet of a plated piece of the steel on the cooling pool, when the plated piece passes through the induction heating ring, the steel is immediately electrified to carry out supplementary heating and temperature rise on a plating layer on the surface of the plated piece, the temperature is controlled between 750 and 800 ℃, then the plating layer is sprayed and cooled to 200 to 300 ℃, and then water cooling is carried out, or the steel is directly cooled in a nitrate bath at the temperature of 200 to 300 ℃ and then water cooling is carried out; the rapid cooling can refine the grains of the plating layer and improve the electrochemical uniformity and passivation capability. The plating layer is subjected to induction heating to raise the temperature, so that the supercooling degree of the multi-element alloy liquid is improved, and the grain refinement is facilitated. Compared with the method of raising the temperature of hot-dip multi-component alloy and then directly and rapidly cooling, the method of the invention is not only beneficial to reducing the amount of dissolved iron (basically eliminating iron phase) of the steel plated part in the multi-component alloy liquid, but also beneficial to controlling the strength of the steel.

And finally, after cooling, washing and airing the steel pipe with water.

For the purpose of better explaining the present invention and to facilitate understanding, the present invention will be described in detail by way of specific embodiments with reference to the accompanying drawings. In the following examples the plated part is a steel pipe.

Example 1

The steel pipe of the plated part is degreased by a commercially available water-based cleaning agent, washed by water, derusted by 15-20% hydrochloric acid, washed by water and dried. Then, the steel pipe is immersed into 700 ℃ molten salt bath plating assistant agent comprising 40 parts by weight of sodium chloride, 40 parts by weight of potassium chloride, 10 parts by weight of cryolite, 2 parts by weight of aluminum fluoride, 6 parts by weight of potassium fluoroaluminate and 2 parts by weight of manganese chloride for treatment for 5 minutes. The steel pipe is then extracted from the molten salt bath and dipped into the molten multi-component alloy. The alloy consists of, by weight, 80 parts of aluminum, 2 parts of iron, 10 parts of manganese, 0.2 part of boron, 3 parts of chromium and 4.8 parts of lanthanum. The temperature of the multi-component alloy liquid is controlled at 750 ℃, and the hot dipping time is 3 minutes. After the steel pipe is extracted from the multi-element alloy liquid, the steel pipe is quickly transferred to the upper part of a cooling pool, the steel pipe passes through an induction heating ring and is heated to be higher than 760 ℃, then the coating is subjected to water spray cooling to 280 ℃, and then water cooling is carried out. And finally, washing and drying the cooled plated part. The coating with smooth and compact surface and uniform and fine crystal grains is obtained, and the scanning electron microscope result is shown in figure 1.

The resultant plating layer was subjected to an electrochemical workstation, and its polarization curve in 3.5% NaCl solution was measured to obtain its corrosion rate of 1.47X 10 ^-7 A/cm ² 。

Example 2

The plated part is degreased by a commercially available water-based cleaning agent, washed by water, derusted by 15-20% hydrochloric acid, washed by water and dried. Then, the plating piece is immersed in 710 ℃ salt bath plating assistant agent consisting of 30 parts of sodium chloride, 40 parts of potassium chloride, 15 parts of cryolite, 5 parts of aluminum fluoride, 8 parts of potassium fluoroaluminate and 2 parts of manganese chloride for treatment for 10 minutes. The plated part is then withdrawn from the salt bath and immersed in the molten multi-component alloy. The alloy consists of 85 parts of aluminum, 2 parts of iron, 10 parts of manganese, 0.2 part of boron, 2 parts of chromium and 0.8 part of cerium. The temperature of the multi-component alloy liquid is controlled at 720 ℃, and the hot dipping time is 5 minutes. After being extracted from the multi-element alloy liquid, the plated part is quickly transferred to the upper part of a cooling pool, after the steel pipe passes through an induction heating ring and is heated to 760 ℃, the plating layer enters a nitrate bath with the temperature of 300 ℃ for cooling and then is cooled by water. And finally, washing and airing the cooled steel pipe.

The obtained coating was found to have a corrosion rate of 3.48X 10% in a 3.5% NaCl solution ^-7 A/cm ² 。

Example 3

The plated piece is degreased by a water-based cleaning agent sold in the market, washed by water, derusted by 15% -20% hydrochloric acid, washed by water and dried. Then, the plating piece is immersed into a 720 ℃ salt bath plating assistant agent which comprises 40 parts of sodium chloride, 35 parts of potassium chloride, 17 parts of cryolite, 2 parts of aluminum fluoride, 5 parts of potassium fluoroaluminate and 1 part of manganese chloride for treatment for 10 minutes. The plated part is then extracted from the salt bath and dipped into the molten multi-component alloy. The multi-element alloy consists of 90 parts of aluminum, 2 parts of iron, 5 parts of manganese, 0.1 part of boron, 1.5 parts of chromium and 1.4 parts of yttrium. The temperature of the multi-component alloy liquid is controlled at 720 ℃, and the hot dipping time is 8 minutes. After the steel pipe is extracted from the multi-component alloy liquid, the steel pipe is quickly transferred to the upper part of a cooling pool, and after the steel pipe passes through an induction heating ring and is heated to 760 ℃, the steel pipe is sprayed and cooled to 280 ℃ and then is cooled by water. And finally, washing and drying the cooled steel pipe.

The corrosion rate of the obtained plating layer in 3.5% NaCl solution was 1.61X 10 ^-7 A/cm ² 。

Example 4

The plated part is degreased by a commercially available water-based cleaning agent, washed by water, derusted by 15-20% hydrochloric acid, washed by water and dried. Then, the plating piece is immersed into a 740 ℃ salt bath plating assistant agent consisting of 32 parts of sodium chloride, 48 parts of potassium chloride, 10 parts of cryolite, 5 parts of aluminum fluoride, 4 parts of potassium fluoroaluminate and 1 part of manganese chloride for treatment for 7 minutes. The plated part is then withdrawn from the salt bath and immersed in the molten multi-component alloy. The alloy composition is 95 parts of aluminum, 1 part of iron, 1 part of manganese, 0.1 part of boron, 1 part of chromium and 1.9 parts of lanthanum. The temperature of the multi-component alloy liquid is controlled at 750 ℃, and the hot dipping time is 10 minutes. After the plated part is extracted from the multi-component alloy liquid, the plated part is quickly transferred to the upper part of a cooling pool, the steel pipe passes through an induction heating ring and is heated to 780 ℃, and then the plated layer enters a 280 ℃ nitrate bath for cooling. And finally, washing and airing the cooled steel pipe with water.

The corrosion rate of the obtained plating layer in 3.5% NaCl solution was 8.39X 10 ^-8 A/cm ² 。

Example 5

The plated part is degreased by a commercially available water-based cleaning agent, washed by water, derusted by 15-20% hydrochloric acid, washed by water and dried. Then, the plating piece is immersed into 780 ℃ salt bath plating assistant agent consisting of 36 parts of sodium chloride, 37 parts of potassium chloride, 15 parts of cryolite, 2 parts of aluminum fluoride, 9 parts of potassium fluoroaluminate and 1 part of manganese chloride for treatment for 10 minutes. The plated part is then extracted from the salt bath and dipped into the molten multi-component alloy. The alloy composition is 98 parts of aluminum, 1 part of iron, 0.3 part of manganese, 0.01 part of boron, 0.3 part of chromium and 0.39 part of cerium. The temperature of the multi-component alloy liquid is controlled at 760 ℃, and the hot dipping time is 3 minutes. After being extracted from the multi-element alloy liquid, the plated part is quickly transferred to the upper part of a cooling pool, passes through an induction heating ring and is heated to 760 ℃, and then is sprayed and cooled to 250 ℃ and then is cooled by water. And finally, washing and airing the cooled steel pipe.

The corrosion rate of the obtained plating layer in 3.5% NaCl solution was 4.34X 10% ^-8 A/cm ² 。

Example 6

The plated piece is degreased by water-based cleaning agent, washed by water, derusted by 15% -20% hydrochloric acid, washed by water and dried. Then, the plating piece is immersed in a 720 ℃ salt bath with the components of 40 parts of sodium chloride, 40 parts of potassium chloride, 10 parts of cryolite, 8 parts of aluminum fluoride, 1 part of potassium fluoroaluminate and 1 part of manganese chloride for treatment for 20 minutes. The plated part is then extracted from the salt bath and dipped into the molten multi-component alloy. The composition of the multi-component alloy is 93 parts of aluminum, 1 part of iron, 5 parts of manganese, 0.01 part of boron, 0.8 part of chromium and 0.19 part of yttrium. The temperature of the multi-component alloy liquid is controlled at 730 ℃, and the hot dipping time is 5 minutes. After the steel pipe is extracted from the multi-element alloy liquid, the steel pipe is quickly transferred to the upper part of a cooling pool, the steel pipe passes through an induction heating ring and is heated to 770 ℃, and then a plating layer enters a nitrate bath at 250 ℃ for cooling. And finally, washing and airing the cooled steel pipe with water.

The obtained coating had a corrosion rate of 5.85X 10 in 3.5% NaCl solution ^-8 A/cm ² 。

Example 7

The plated part is degreased by a commercially available water-based cleaning agent, washed by water, derusted by 15-20% hydrochloric acid, washed by water and dried. Then, the plating piece is immersed into 700 ℃ salt bath plating assistant agent comprising 40 parts of sodium chloride, 40 parts of potassium chloride, 16 parts of cryolite, 2 parts of aluminum fluoride, 1.8 parts of potassium fluoroaluminate and 0.2 part of manganese chloride for treatment for 30 minutes. The plated part is then extracted from the salt bath and dipped into the molten multi-component alloy. The multi-element alloy consists of 93 parts of aluminum, 1 part of iron, 2 parts of manganese, 0.2 part of boron, 1 part of chromium and 2.8 parts of lanthanum. The temperature of the multi-component alloy liquid is controlled at 710 ℃, and the hot dipping time is 10 minutes. After being extracted from the multi-element alloy liquid, the plated part is quickly transferred to the upper part of a cooling pool, passes through an induction heating ring and is heated to 750 ℃, and then is sprayed and cooled to 250 ℃, and then is cooled by water. And finally, washing and airing the cooled steel pipe with water.

The corrosion rate of the obtained plating layer in 3.5% NaCl solution was 3.28X 10 ^-8 A/cm ² 。

Example 8

The plated part is degreased by a commercially available water-based cleaning agent, washed by water, derusted by 15-20% hydrochloric acid, washed by water and dried. Then, the plating piece is immersed into 700 ℃ salt bath plating assistant agent comprising 35 parts of sodium chloride, 45 parts of potassium chloride, 15 parts of cryolite, 3 parts of aluminum fluoride, 1.5 parts of potassium fluoroaluminate and 0.5 part of manganese chloride for treatment for 5 minutes. The plated part is then extracted from the salt bath and dipped into the molten multi-component alloy. The alloy composition is 90 parts of aluminum, 0.5 part of iron, 5 parts of manganese, 0.05 part of boron, 2 parts of chromium and 2.45 parts of cerium. The temperature of the multi-component alloy liquid is controlled at 730 ℃, and the hot dipping time is 4 minutes. After the plated part is extracted from the multi-element alloy liquid, the plated part is quickly transferred to the upper part of a cooling pool, and after the plated part passes through an induction heating ring and is heated to 750 ℃, the plated layer enters a nitrate bath at 250 ℃ for cooling. And finally, washing and drying the cooled plated part with water.

The obtained coating had a corrosion rate of 2.36X 10% in 3.5% NaCl solution ^-9 A/cm ² 。

Comparative example 1

The oil and rust removing operation of the plated part adopts the same conditions as in the example 1, the plated part is treated in a 700 ℃ melting plating-assisting salt bath consisting of 40 parts of potassium chloride, 40 parts of sodium chloride, 12 parts of cryolite and 8 parts of aluminum fluoride for 5 minutes, then the plated part is immersed in 750 ℃ aluminum-dipped liquid containing 1% of iron for 3 minutes, the dipped part is directly air-cooled after being taken out, and after being washed and dried by water, the scanning electron microscope result of the obtained plating layer is shown in figure 2. The corrosion rate of the pure aluminum plating in the NaCl solution was measured to be 3.60X 10% ^-6 A/cm ² . By comparing comparative example 1 with example 1, it can be seen that the plating layer obtained by the method used in the present invention substantially eliminates the precipitation of iron phase.

Comparative example 2

The plated article was subjected to the same treatment conditions as in the previous part of example 1 except that air cooling was directly performed after hot dipping, and the surface morphology of the obtained plated layer as a result of scanning electron microscopy was shown in FIG. 3. By comparing comparative example 2 with comparative example 1, it can be seen that the proportion of the iron phase is significantly reduced and the iron phase changes from a coarse long needle shape to a fine short rod shape after the addition of the alloy element for changing the iron phase; the plating obtained by the method of example 1 of the present invention substantially eliminates the precipitation of iron phases.

Comparative example 3

The same conditions as in example 1 were adopted for degreasing and rust removal of the plated parts, the plating assistant agent was an aqueous solution composed of 25g/L sodium chloride, 25g/L potassium chloride, 25g/L sodium fluoride and 25g/L potassium fluoride, treated at 85 ℃ for 3 minutes, then immersed in a 750 ℃ aluminum-impregnated liquid containing 1% iron for 5 minutes, extracted and air-cooled. The obtained plating layer and the plating layer obtained in comparative example 1 were immersed in 15% nitric acid at 25 ℃ for 4 hours, and the weights before and after etching were measured, and then the corrosion weight loss was calculated. As a result, comparative example 1 lost weight of 0.3mg/cm ² Comparative example 3 weight loss of 1.2mg/cm ² The large weight loss represents more pinholes, which indicates that a plating layer of the aqueous solution plating assistant process has a plurality of pinholes. Precision measurement calculation example 1 weight loss of 0.001mg/cm ² It is demonstrated that the plating assistant agent of the method of the invention can effectively eliminate the pinholes in the plating layer.

Comparative example 4

This comparative example applied the same conditions as in example 1 for the operation of the steel pipe plated with the steel, except that the molten multi-component alloy used was not added with manganese and chromium, and the remaining conditions were not changed. The resultant coating was subjected to an electrochemical workstation, and its polarization curve in 3.5% NaCl solution was measured to obtain its corrosion rate of 0.92X 10 ^-7 A/cm ² 。

Comparative example 5

This comparative example employed the same conditions for the operation of the plated steel pipe as in example 1, except that the plating assistant salt bath used was not added with manganese chloride, and the remaining conditions were unchanged. The resultant coating was subjected to an electrochemical workstation, and its polarization curve in a 3.5% NaCl solution was measured to obtain its corrosion rate of 1.08X 10 ^-7 A/cm ² 。

Compared with the comparative example 1, the corrosion rate of the plating can be effectively reduced under the same corrosion condition when the multi-element alloy contains manganese and chromium through the comparative example 4 and the comparative example 5; and when the manganese chloride is added into the plating assistant agent salt bath, the plating assistant agent salt bath also helps to reduce the corrosion speed of the plating. As can be seen from comparison of comparative examples 4 and 5 with example 1, the use of the components of the corrosion-resistant alloy coating used in the present invention in combination with the molten salt bath components provides a synergistic effect on the corrosion resistance of the coating.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention in other forms, and any person skilled in the art can change or modify the technical content disclosed above into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification of the above embodiments according to the technical essence of the present invention are within the protection scope of the technical solution of the present invention.

Claims

1. A method for hot-dip coating of steel with an anti-corrosion alloy, comprising: in the hot dipping process, after molten salt bath assistant plating is carried out on steel, the multi-element alloy is hot dipped, and the surface of a plating layer is heated by adopting an induction heating mode and then is rapidly cooled;

the components of the molten salt bath comprise 35 parts of sodium chloride, 45 parts of potassium chloride, 15 parts of cryolite, 3 parts of aluminum fluoride, 1.5 parts of potassium fluoroaluminate and 0.5 part of manganese chloride in parts by weight;

the multi-element alloy comprises 90 parts of aluminum, 0.5 part of iron, 5 parts of manganese, 0.05 part of boron, 2 parts of chromium and 2.45 parts of rare earth metal cerium in parts by weight;

the operation of heating the surface of the coating by the induction heating mode comprises the following steps: the plating layer is heated to more than 750 ℃ by an induction heating coil;

the working temperature of the molten salt bath is 700 ℃, and the plating assisting treatment time is 5 minutes;

when the multi-component alloy is hot dipped, the temperature is 730 ℃, and the hot dipping time is 4 minutes;

the rapid cooling is to directly enter a nitrate bath at 250 ℃ for cooling and then cool by water.