CN111334748A

CN111334748A - Protective layer of steel product, preparation method of protective layer and steel product

Info

Publication number: CN111334748A
Application number: CN202010266233.1A
Authority: CN
Inventors: 张洪霞
Original assignee: Tianjin Lianyou New Material Technology Co ltd
Current assignee: Tianjin Lianyou New Material Technology Co ltd
Priority date: 2020-04-07
Filing date: 2020-04-07
Publication date: 2020-06-26

Abstract

The invention provides a protective layer of a steel product, a preparation method of the protective layer and the steel product, wherein the protective layer comprises the following components: 1.27-41.32 wt% Fe, 1.93-34.05 wt% Mg, and the remainder Zn and unavoidable impurities; wherein the protective layer contains a Zn-Fe-Mg ternary alloy phase; the protective layer is formed by penetrating metallic Mg atoms and metallic Zn atoms in a penetrating agent into the surface of a steel product in a heat diffusion mode in a closed container. The steel part protective layer prepared by the embodiment of the invention has a sacrificial anode protection effect on the steel part, has extremely excellent corrosion resistance, and can endow the steel part with better corrosion resistance. The protective layer can be formed on the surface of steel with a complex shape, and can uniformly cover steel parts, thereby realizing excellent protective performance.

Description

Protective layer of steel product, preparation method of protective layer and steel product

Technical Field

The invention relates to the technical field of metal and surface treatment, in particular to a protective layer of a steel product, a preparation method of the protective layer and the steel product with the protective layer, especially the steel product with the protective layer with excellent corrosion resistance.

Background

One common method of protecting the surface of steel articles from corrosion is to apply a zinc (Zn) layer to the surface. Compared with other anti-corrosion methods, the zinc coating belongs to a method with high economical efficiency and effectiveness, and is widely applied. Over 50% of the steel sheets worldwide are protected with zinc coatings.

The neutral salt spray resistant time of the common zinc coating is between 150 and 240 hours (ASTM B117). With the progress of the demands of the industrial field and the expansion of the use environment, there is a further demand for the corrosion resistance of the zinc plating layer. Such as offshore wind power, underground engineering, coastal engineering and the like, the corrosion resistance of the plating layer is required to be more than 1000 hours. Therefore, the coating with high corrosion resistance has wide market demand.

High corrosion resistance of the coating is generally achieved by adding alloying elements to the coating. A method for preparing a zinc coating with high corrosion resistance is to electroplate an alloy coating of zinc-iron (Zn-Fe), zinc-nickel (Zn-Ni), zinc-cobalt (Zn-Co), zinc-tin (Zn-Sn) and the like. After the alloy coatings are passivated, the time for resisting neutral salt fog can reach 500-700 hours. However, depending on the electrolytic potential, complex and expensive additives are required to maintain the stability of the solution when heavy metal ions such as Fe, Ni, Co, Sn, etc. are Co-deposited with Zn ions. This necessarily increases the manufacturing cost. Meanwhile, the development of the electrogalvanizing alloy coating is limited due to the environmental protection problem of heavy metal ions such as Ni, Co, Sn and the like.

Another way to produce a highly corrosion resistant coating is to hot plate an alloy coating. In the last 70 s and 80 s, the hot-dip alloy plating layer mainly developed was a zinc-aluminum alloy plating layer. A hot-dip Zn-55% Al plating (called Galvalume plating) and a hot-dip Zn-5% Al plating (called Galfan plating) were successively developed. Compared with a zinc coating, the corrosion resistance of the Galvalume coating is improved by 4-6 times, and the corrosion resistance of the Galfan coating is improved by 2-3 times.

The hot-dip method can obtain the zinc alloy coating with good corrosion resistance in a relatively economic mode. The method is suitable for plating the surface of a large-size steel structural part with a zinc alloy coating or plating the surface of a steel plate or a steel wire with the zinc alloy coating in a continuous mode (continuous hot galvanizing).

However, the hot-dip method has difficulty in obtaining a zinc alloy coating of uniform thickness on a steel article of complicated shape. For some steel products with blind holes, screw threads and internal grooves, the hot-dip method is difficult to plate zinc alloy coating on the parts.

Another disadvantage of the coating obtained by the hot-dip or electroplating method is that the hardness is HV 50-70, the wear resistance is low, and the coating is worn quickly in some application occasions and is difficult to provide continuous protection.

The zinc alloy coating with better corrosion resistance can be obtained by using a vacuum deposition method, but the problem that uniform coating is difficult to obtain on steel products with complex shapes is also existed, and the coating can only be deposited on steel plates or steel strips with simple shapes. Further, from the economical point of view, the method requires a large investment in equipment, is expensive, and is difficult to be widely used.

How to prepare a protective layer with excellent corrosion resistance for protecting steel products is a problem to be solved urgently.

Disclosure of Invention

Accordingly, embodiments of the present invention provide a protective layer for a steel product, a method for preparing the protective layer, and a steel product, so as to obviate or mitigate one or more of the disadvantages of the prior art.

The technical scheme of the invention is as follows:

in one aspect, the present invention provides a protective layer for coating a surface of a ferrous article, the protective layer comprising:

1.27-41.32 wt% iron (Fe),

1.93-34.05 wt.% magnesium (Mg), and

residual zinc (Zn) and inevitable impurities;

wherein the protective layer contains a Zn-Fe-Mg ternary alloy phase.

In some embodiments, the protective layer is formed by infiltrating metallic Mg atoms and metallic Zn atoms in an infiltrant into the surface of the ferrous part by means of thermal diffusion in a closed vessel.

In some embodiments, the protective layer contains a large amount of a Zn-Mg binary alloy phase while being distributed and intercalated with a Zn-Fe-Mg ternary alloy phase when the Fe element content in the protective layer does not exceed 1.3 wt%; when the content of Fe element in the protective layer exceeds 1.3 wt% and does not exceed 3.2 wt%, the protective layer forms a Zn-Fe-Mg ternary alloy phase layer at the interface with the steel article. A mixed structure comprising a Zn-Mg binary alloy phase and a Zn-Fe-Mg ternary alloy phase which coexist exists on the complete ternary alloy phase layer; when the content of Fe element in the protective layer exceeds 3.2 wt% and does not exceed 41.32 wt%, the protective layer includes a dense and complete Zn-Fe-Mg ternary alloy phase layer. When the amount of Fe element diffused into the protective layer from the steel product in the protective layer is gradually increased, the Zn-Mg binary alloy phase in the protective layer is gradually changed into the Zn-Fe-Mg ternary alloy phase.

In some embodiments, the protective layer comprises: 4-41.32 wt% of Fe, 5-30 wt% of Mg, and the remainder Zn and unavoidable impurities.

In some embodiments, the protective layer has a thickness greater than 5 μm and no more than 100 μm.

In some embodiments, the protective layer has a thickness greater than 15 μm and no more than 100 μm.

In some embodiments, the surface of the protective layer has an oxide layer with a thickness of 0.2-3 μm.

In some embodiments, the protective layer comprises more than 90 wt% Fe, Mg, and Zn; the inevitable impurities include oxygen.

In some embodiments, the infiltrant includes a metal powder, a dispersant, and/or a catalyst; the metal powder includes a powder containing Mg in a metallic state and a powder containing Zn in a metallic state.

In some embodiments, the metallic powder contains 5 to 30 wt% of metallic Mg element by weight and 70 to 95 wt% of metallic Zn element by weight.

In another aspect, the present invention also provides a method for preparing a protective layer for a steel article, the method comprising:

penetrating metallic Mg atoms and metallic Zn atoms in the penetrating agent into the surface of the steel product by a thermal diffusion mode in a closed container to form the protective layer; wherein the protective layer comprises:

1.27-41.32 wt% Fe,

1.93-34.05 wt.% Mg, and

zn and inevitable impurities remain.

In some embodiments, the infiltrant includes a metal powder, a dispersant, and/or a catalyst.

In some embodiments, the metal powder comprises a powder comprising Mg in metallic form and a powder comprising Zn in metallic form.

In some embodiments, the metallic powder comprises 5-30 wt% of metallic Mg element and 70-95 wt% of metallic Zn element, and the particle size of the metallic powder is not more than 150 μm.

In some embodiments, the dispersant comprises one or more of sodium carbonate, potassium carbonate, calcium carbonate, magnesium carbonate, barium carbonate, calcium oxide, magnesium oxide, barium oxide, sodium oxide, and potassium oxide.

In some embodiments, the dispersant is in the following proportions by weight: the dispersing agent is metal powder which is 10: 90-50: 50; and the particle size of the dispersant is not more than 0.3 mm.

In some embodiments, the dispersant particle size is no more than 0.125 mm.

In some embodiments, the catalyst is chloride, and the chloride is present in a proportion of 0.02 to 0.2 wt% of the metal powder.

In some embodiments, the inner space of the closed vessel is in a vacuum state or an atmosphere state.

In some embodiments, when the state of the internal space of the closed container is a vacuum state, the vacuum degree is not more than 100 Pa; when the internal space state of the closed container is an atmosphere state, the atmosphere is a protective atmosphere or a reducing atmosphere; the protective atmosphere comprises one or more than two of nitrogen, helium, neon, argon, krypton and xenon; the reducing atmosphere comprises one or two of hydrogen and ammonia.

In some embodiments, the temperature of the thermal diffusion is between 390-450 ℃ and the time of the thermal diffusion is between 1-5 hours.

In some embodiments, the method further comprises: and further coating a coating, such as a surface chromate treatment coating, a sealant treatment coating and/or an organic coating, on the surface of the generated protective layer.

In some embodiments, the method comprises the steps of: putting a steel product, metal powder consisting of Zn-containing metal or alloy powder and Mg-containing metal or alloy powder, a dispersing agent and a penetrating agent consisting of a catalyst into a closed container; heating the closed container in which the steel part and the penetrant are put, and rotating the closed container; heating the sealed container for a predetermined time and then cooling the container.

In another aspect, the invention also provides a ferrous object comprising a protective layer as described above, said ferrous object comprising a ferrous object with a regular shape and a ferrous object with an irregular shape.

According to the protective layer prepared by the method, the main alloy elements consist of three elements of Zn, Fe and Mg, the protective layer has a sacrificial anode protection effect on steel products, has extremely excellent corrosion resistance, and can endow the steel products with better corrosion resistance. The protective layer can uniformly cover the surface of a steel product with a complex shape.

In addition, the surface of the protective layer prepared by the method provided by the embodiment of the invention has a micro-concave-convex structure, so that the protective layer can be endowed with good bonding force with organic matters or inorganic matters, and the method is suitable for secondary coating treatment. One or more inorganic layers, organic layers or inorganic-organic mixture layers can be coated on the surface of the protective layer, so that the protective layer provided by the invention has special functions such as insulation, heat insulation and aesthetic property.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

It will be appreciated by those skilled in the art that the objects and advantages that can be achieved with the present invention are not limited to the specific details set forth above, and that these and other objects that can be achieved with the present invention will be more clearly understood from the detailed description that follows.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principles of the invention. In the drawings:

fig. 1A and 1B are a microstructure diagram of a protective layer in example 1 of the present invention and its chemical composition (1.27 wt% Fe, 34.05 wt% Mg, 64.67 wt% Zn), respectively.

Fig. 2A and 2B are a microstructure diagram of the protective layer and its chemical composition (2.69 wt% Fe, 31.1 wt% Mg, 66.21 wt% Zn) in example 2 of the present invention, respectively.

Fig. 3A and 3B are a microstructure diagram of a protective layer in example 3 of the present invention and its chemical composition (3.88 wt% Fe, 22.26 wt% Mg, 73.86 wt% Zn), respectively.

Fig. 4A and 4B are a microstructure diagram of a protective layer in example 4 of the present invention and its chemical composition (8.62 wt% Fe, 18.49 wt% Mg, 72.89 wt% Zn), respectively.

Fig. 5A and 5B are a microstructure diagram of a protective layer in example 5 of the present invention and its chemical composition (18.47 wt% Fe, 15.92 wt% Mg, 65.61 wt% Zn), respectively.

Fig. 6A and 6B are a microstructure diagram of the protective layer in example 6 of the present invention and its chemical composition (21.19 wt% Fe, 25.23 wt% Mg, 53.59 wt% Zn).

Fig. 7A and 7B are a microstructure diagram of the protective layer in example 7 of the present invention and its chemical composition (19.92 wt% Fe, 8.89 wt% Mg, 71.19 wt% Zn).

Fig. 8A and 8B are a microstructure diagram of the protective layer in example 8 of the present invention and its chemical composition (16.33 wt% Fe, 1.93 wt% Mg, 1.32 wt% Al, 80.43 wt% Zn).

Fig. 9A and 9B are a microstructure diagram of the protective layer in example 9 of the present invention and its chemical composition (41.32 wt% Fe, 16.12 wt% Mg, 42.56 wt% Zn).

The reference numbers illustrate:

① protective layer, ② steel matrix, ③ Zn-Fe-Mg ternary alloy phase;

④ Zn-Mg binary alloy phase.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the following embodiments and accompanying drawings. The exemplary embodiments and descriptions of the present invention are provided to explain the present invention, but not to limit the present invention.

It should be noted that, in order to avoid obscuring the present invention with unnecessary details, only the structures and/or processing steps closely related to the scheme according to the present invention are shown in the drawings, and other details not so relevant to the present invention are omitted.

It should be emphasized that the term "comprises/comprising" when used herein, is taken to specify the presence of stated features, elements, steps or components, but does not preclude the presence or addition of one or more other features, elements, steps or components.

The inventor of the invention finds that the thermal diffusion method can solve the problems of poor coating uniformity and low coating wear resistance caused by the process methods of electroplating, hot-dipping, vacuum deposition and the like in the process of preparing a coating on the surface of a steel product, particularly a steel product with a complex shape.

The main principle of the conventional thermal diffusion zincing method is to bury the steel parts in zinc powder and inert filler (alumina or silica), and then to put the steel parts, zinc powder and inert filler into a slowly rotating vessel. The container is heated to 380-450 ℃. Under the action of the thermal field, zinc powder is contacted with Fe on the surface of the steel part, and mutual diffusion and infiltration of atomic level layers are generated, so that a Zn-Fe alloy layer is formed. In general, a Zn-Fe alloy layer obtained by the thermal diffusion zincing method has a two-layer structure, wherein one layer close to a steel matrix is a Zn-Fe alloy phase (gamma phase) containing 20 wt% of Fe, and the other layer is a Zn-Fe alloy phase (delta phase) containing 8 to 12 wt% of Fe.

If the steel part is driven by the rotary container to roll continuously, the zinc powder and the inert filler can be uniformly distributed and adhered to each surface of the steel part, so that a Zn-Fe alloy layer with uniform thickness can be obtained on each surface of the steel part. By controlling the time and temperature of the diffusion, a predetermined thickness of the Zn-Fe alloy layer can be obtained.

Compared with electroplating, hot-dip and vacuum deposition methods, the hot-diffusion zinc-impregnation method has incomparable advantages for steel products with complex shapes, especially blind holes, threads and internal grooves. The reason is that the zinc powder and the inert filler can penetrate into the inner surfaces of complex shapes such as blind holes, threads, inner grooves and the like under the rolling action. Furthermore, the coating prepared by the thermal diffusion zincing method has uniform thickness at each position on the surface of the steel product, and is greatly helpful for improving the dimensional precision of the steel product.

According to the Zn-Fe alloy layer obtained by the thermal diffusion zinc infiltration method, the hardness of the alloy layer is HV 280-370 according to the different Fe contents, the alloy layer can resist abrasion to a certain degree, and the alloy layer has good abrasion resistance. The thickness of the Zn-Fe alloy layer is usually 10-100 microns, and the alloy layer plays a role in protecting steel products.

However, the inventors have found that the degree of corrosion resistance of the Zn-Fe alloy layer protective layer obtained by the thermal diffusion zincing method is limited. Under the condition of a neutral salt spray test (ASTM B117), the time for the protective layer of the Zn-Fe alloy layer obtained by the thermal diffusion zincing method to generate red rust is 250-500 hours. In practice, to improve corrosion resistance, a passivation or sealing agent (secondary coating), such as Dacromet (Dacromet), is applied to the surface of the Zn — Fe alloy layer protective layer for a period of about 600 hours to eventually achieve white rusting under the neutral salt spray test.

The surface of the Zn-Fe alloy layer protective layer is coated with the passivating agent or the sealing agent, the passivating agent or the sealing agent is composed of inorganic matters or organic matters, the Zn-Fe alloy layer protective layer is easy to fall off under the action of mechanical external force, the ultraviolet aging resistance is weak, and the corrosion resistance expressed in the actual environment can not meet the requirement. In addition, in the process of coating the passivating agent or the sealing agent on the surface of the Zn-Fe alloy layer protective layer, certain Volatile Organic Compounds (VOC) emission is generated, and negative effects are caused on the environment.

The invention provides a protective layer of a steel product and a preparation method thereof. The protective layer has excellent corrosion resistance, and has corrosion resistance which is several times that of electrogalvanizing, hot galvanizing and hot galvanizing under the condition that the surface of the protective layer is not coated with a secondary coating (passivation treatment or sealing treatment). Meanwhile, the preparation method of the protective layer has the advantages of strong adaptability, environment-friendly process and the like.

The method for the preparation of the protective layer of the steel object of the invention is described below.

The invention provides a method for preparing a protective layer on the surface of a steel product, belonging to an improved thermal diffusion process, which forms the protective layer by permeating metallic Mg atoms and metallic Zn atoms in a penetrant into the surface of the steel product in a thermal diffusion mode in a closed container, and controls the content of the metallic Mg atoms and the metallic Zn atoms in the penetrant to ensure that the protective layer contains 1.27-41.32 wt% of Fe and 1.93-34.05 wt% of Mg, and the rest is Zn and inevitable impurities, wherein the inevitable impurities can comprise oxygen elements, and the oxygen elements are oxide films or layers formed on the surface of the protective layer by the elements such as Mg, Zn and the like in the protective layer. By way of example, the method for preparing the protective layer provided by the embodiment of the invention may include the following basic processes:

(a) putting a steel product with a clean surface, metal powder consisting of Zn-containing metal or alloy powder and Mg-containing metal or alloy powder, a dispersing agent and a penetrating agent consisting of a catalyst into a closed container.

Before the step, the step of cleaning the surface of the steel product can be further included, namely the surface of the steel product which is not coated with the protective layer is cleaned, and oil and rust removal is carried out on the surface of the steel product.

(b) The closed vessel containing the steel and iron product and the mixture was heated while the closed vessel was slowly rotated. Under the action of the thermal field, the surface-cleaned steel part exposed to the penetrant can form a protective layer through diffusion.

(c) And heating the closed container for a preset time, and then cooling the closed container to a proper temperature.

(d) And taking out the steel product with the protective layer in the closed container. And (3) carrying out surface ash removal treatment on the steel part with the protective layer.

The method of the present invention may be carried out by a process including a "direct" method or an "indirect" method of the thermal diffusion process of the basic process.

First, a "direct" method including the above-described basic process is explained, which is realized by the following processes:

cleaning the surface of the steel product, namely obtaining a bare pure metal surface on the steel product, and providing a matrix for obtaining a protective layer by thermal diffusion treatment. In practice, during the forming or heat treatment of the steel part, an oil film or an oxide layer is formed on the surface of the steel part. The oil film on the surface of the steel product generally plays a role in lubrication or rust prevention in processing, and the oxide layer on the surface of the steel product is derived from an oxide film formed by the reaction of surface Fe and oxygen in the air in the heat treatment process, or the surface Fe is slowly oxidized in the natural placing process of the steel product.

The oil film or oxide layer must be removed before the thermal diffusion process is performed, otherwise the diffusion cannot occur or the phenomenon of poor diffusion occurs. In general, the oil film cleaning process is usually carried out in a hot alkaline cleaner, usually according to the recipe provider's recommended procedures. Or the cleaning treatment of the oil film is carried out in a thermal burning mode, namely the oil film on the surface of the steel workpiece is converted into carbon by utilizing the characteristic that the oil film is thermally cracked into carbon at high temperature, and then the carbon is removed in the subsequent treatment process of cleaning the oxide layer.

The oxide layer can be removed by dissolving the oxide layer with an acid solution or by sandblasting. The method provided by the invention preferably adopts a sand blasting method to remove the oxide layer. On one hand, carbon in the previous process can be removed, on the other hand, the use of acid is avoided, and the environmental burden is reduced.

According to the method proposed by an embodiment of the present invention, the substances of Zn element and Mg element required for providing diffusion are a Zn-containing metal or alloy powder, a Mg-containing metal or alloy powder, respectively. In the process of thermal diffusion, Zn element and Mg element enter the matrix of the steel product in an atomic form through a solid-solid diffusion or a gas-solid diffusion path, and any metal or alloy powder containing Zn or Mg in a metal state can be used as a Zn element and Mg element source for providing diffusion. Therefore, the above-mentioned Zn-containing metal or alloy powder may be one or more of various powders containing Zn in a metallic state, for example, Zn powder, Zn-Mn alloy powder, Zn-Ti alloy powder, Zn-Ni alloy powder, Zn-rare earth alloy powder, etc. The above-mentioned Mg-containing metal or alloy powder may contain one or more of a plurality of powders containing Mg in a metallic state, for example, Mg powder, Mg-Zn metal powder, Mg-Sn alloy powder, Mg-Mn alloy powder, Mg-Ni alloy powder, Mg-rare earth alloy powder, and the like. The above-mentioned Zn-containing metal or alloy powder and Mg-containing metal or alloy powder may be in the form of spheres, flakes, diamonds or the like, and have an average particle diameter of 150 μm or less, preferably 75 μm or less. Although other elements are contained in the Zn-containing alloy powder and the Mg-containing alloy powder described above, in the method proposed in the example of the present invention, it was found that the other elements hardly permeate into the protective layer proposed in the present invention, or the amount permeating into the protective layer is so low as to be negligible.

The above-described Zn-containing metal or alloy powder and Mg-containing metal or alloy powder are combined to form the metal powder in the embodiment of the present invention. The proportion of Zn element and Mg element in the metal powder plays an important role in regulating and controlling the chemical composition and performance of the protective layer provided by the invention. The protective layer proposed in the embodiment of the present invention is a Zn-based alloy protective layer, and therefore, the Zn element in the metal powder occupies a dominant position by weight, and the Mg element in the metal powder occupies a subordinate position by weight. Through a large number of experiments, when the Zn element in the metal powder accounts for 70-95 wt% and the Mg element in the metal powder accounts for 5-30 wt%, the obtained protective layer has relatively excellent corrosion resistance, wear resistance and proper thickness. When the amount of Mg element in the metal powder is less than 5% by weight, the corrosion resistance in the protective layer is not sufficiently improved. When the amount of Mg element in the metal powder exceeds 30% by weight, the growth rate of the protective layer is slow and the corrosion resistance is rather lowered.

The relative proportions of the Mg element and the Zn element in the metal powder are different, and the contents of the Mg element and the Zn element in the protective layer are affected. Generally, the element content change in the protective layer and the element content change in the metal powder occur simultaneously, but do not show the same proportional linear relationship. For example, when the metal powder contains 30 wt% of Mg and 70 wt% of Zn, the protective layer contains 34 wt% of Mg, 64.7 wt% of Zn, and 1.3 wt% of Fe by the method proposed in the present invention. The metal powder contained 15 wt% of Mg and 85 wt% of Zn, and the obtained protective layer contained 15.92 wt% of Mg, 65.61 wt% of Zn, and 18.47 wt% of Fe. The metal powder contained 5 wt% of Mg and 95 wt% of Zn, and the obtained protective layer contained 1.93 wt% of Mg, 80.43 wt% of Zn and 16.43 wt% of Fe.

In the thermal diffusion method for preparing the protective layer, a dispersing agent is also put into a closed container together with the metal powder. When rolling at high temperatures in a closed container, interdiffusion occurs between the metal powder and the metal powder, so that there is a risk of the powders sticking together. The dispersant plays a main role: after the dispersing agent is added, the metal powder is isolated, and the bonding risk of the metal powder is reduced.

According to the method provided by the invention, the dispersing agent comprises one or more of sodium carbonate, potassium carbonate, calcium carbonate, magnesium carbonate, barium carbonate, calcium oxide, magnesium oxide, barium oxide, sodium oxide and potassium oxide.

The dispersant powder in the examples of the present invention has a particle size of generally 0.3mm or less, preferably 0.125mm or less. The ratio of the dispersing agent to the metal powder is 10: 90-50: 50 by weight. With a dispersant ratio below 10:90, there is a risk of metal powder sticking. When the proportion of the dispersing agent is higher than 50:50, the metal powder is excessively diluted, the contact probability of the metal powder and the surface of a steel part is reduced, the thermal diffusion process is slowed down, the thickness of a protective layer is not enough, and even a complete protective layer cannot be formed. Further, through systematic research, the content of the dispersing agent and the thickness of the protective layer show an exponential function relationship at the same thermal diffusion temperature.

In order to reduce the possibility that the content of the dispersing agent in a local space in the closed container is too high so as to cause the slowing of the thermal diffusion process, the dispersing agent and the metal powder can be premixed firstly and then placed into the closed container in the embodiment of the invention. The premixing process is a physical stirring process, namely the dispersing agent and the metal powder are stirred in a container by external force to achieve the purpose of uniform dispersion.

In the diffusion process of Zn and Mg, gas-solid diffusion exists besides solid-solid diffusion generated in the collision and friction processes of metal powder and the surface part of the steel part. The gas-solid diffusion means that a small amount of metallic Zn and Mg elements are converted into Zn and Mg steam at high temperature, and the Zn and Mg atoms are transported by the transfer and diffusion of the Zn and Mg steam. And the Zn and Mg vapor reaches the surface of the steel product, and Zn and Mg atoms generate vapor deposition behavior and diffusion behavior to form the protective layer.

In order to promote the generation, transfer and transportation of Zn and Mg vapor in the thermal diffusion process, a certain amount of catalyst can play an important role. Small amounts of chloride can act as a catalyst to perform the catalytic function described above. For divalent metals such as Zn and Mg, chlorine atoms can react with the divalent metals to generate metastable chlorides M₂Cl₂Namely:

MCl₂(gas) + M (solid) → M₂Cl₂(gas);

wherein M represents a metal atom.

The Fe atom has catalytic adsorption effect on the metastable-state chloride at high temperature, namely:

M₂Cl₂(gas) + Fe (solid) → M-Fe (solid) + MCl₂(gas);

the effect of the above action is that the chlorides increase the incidence of vaporisation of the metallic elements. These chlorides include ammonium chloride, sodium chloride, potassium chloride, ferrous chloride, magnesium chloride, zinc chloride, and the like, as well as mixtures of two or more thereof. When the addition amount of the chloride is 0.02-0.2 wt% of the weight of the metal powder, the requirement of improving the diffusion transfer of the metallic elements can be met.

According to the method of the embodiment of the invention, the thermal diffusion process is carried out in a closed container. The container is filled with steel products after surface cleaning (oil and rust removal), metal powder for providing Zn atoms and Mg atoms, a dispersing agent and a catalyst. As proposed in the examples of the present invention, the transfer and diffusion of atoms from the metal powder to the surface of the steel article is governed by two mechanisms. One is the physical collision and friction between the metal powder and the steel part, during which the atoms in the metal powder are transferred to the steel surface, thus forming the protective layer. Another mechanism is that the metastable chloride formed by the metal powder and the catalyst can effect atomic transfer and diffusion. If the oxygen content in the closed space exceeds a certain limit, two diffusion transfer mechanisms are affected. Firstly, a thin layer of oxide film is formed on the surface of the steel product to prevent atoms from diffusing into the matrix. In addition, the affinity of oxygen and metal atoms is strong, and the chemical balance formed by metastable state chloride is destroyed.

The inner space of the sealed container may be filled with an inert gas, a reducing gas, or a mixture thereof to remove oxygen and reduce oxidation of the metal powder. The inert gas may include one or more of nitrogen, helium, neon, argon, krypton, and xenon, and the reducing gas includes one or both of hydrogen and ammonia.

In addition to the above-described method of filling the atmosphere, the method of removing oxygen from the internal space of the closed container may be a method of removing oxygen from the internal space of the container by evacuation. According to the method provided by the invention, the required vacuum degree reaches below 100Pa on the premise of not influencing the transfer and diffusion of the metal state atoms to the surface of the steel.

The closed container is placed in a thermal field with the temperature of 390-450 ℃, and the preferred temperature is 400-420 ℃. Through the action of heat conduction and heat radiation, the temperature of the steel part after the surface cleaning (degreasing, descaling) treatment inside the container and the substances providing Zn atoms and Mg atoms reaches the same temperature as that of the thermal field. The closed container may be stationary or rotating at this temperature. Under the action of the thermal field and the action of rolling (under the condition of container rotation), the substances providing Zn atoms and Mg atoms are contacted with the steel product, and the released Zn atoms and Mg atoms are diffused into the surface of the steel product. The Zn atoms and Mg atoms diffused into the surface of the steel product have metallurgical reaction with Fe atoms to form the protective layer provided by the embodiment of the invention.

The time the closed container is in the thermal field determines the thickness of the protective layer. In general, the diffusion of Zn and Mg atoms into a ferrous part follows a parabolic law, i.e. the increase in thickness of the protective layer is parabolic in relation to time. At the beginning of thermal diffusion, the thickness of the protective layer increases more rapidly, and at the later stage of diffusion, no matter how long the diffusion time is, the thickness of the protective layer increases more slowly. Therefore, from the point of view of cost-effectiveness, the thermal diffusion method proposed in the present invention produces a protective layer on the surface of the steel article, the thickness of which is generally not more than 100 μm. If the thickness of the protective layer is too thin, a continuous and complete protective layer cannot be formed, and therefore the thickness of the protective layer is generally more than 5 μm, preferably more than 15 μm. The time for thermal diffusion is generally between 1 hour and 5 hours.

After the set required protective layer thickness is reached, the temperature of the thermal field can be reduced to room temperature by natural cooling or forced cooling. After the temperature of the steel part with the protective layer and the substances providing Zn atoms and Mg atoms are balanced with the room temperature, the closed container is opened, and the steel part with the protective layer is taken out.

After the surface of the steel product with the protective layer is taken out from the closed container, a small amount of metal powder, dispersant and other powder adheres to the surface. Therefore, the purpose of separating surface powder can be achieved by blowing the protective layer on the surface of the steel part or vibrating, rolling and the like the steel part, thereby completing the whole process of coating the protective layer.

The following describes another method of thermal diffusion process for preparing a protective layer, the "indirect method".

In the indirect method, first, a steel product subjected to surface cleaning (degreasing and descaling) and a substance that provides metallic Zn powder are placed in a closed container. The surface cleaning (degreasing, descaling) treatment process here is the same as the surface cleaning treatment of the above-described "direct method" process.

In the "indirect method" implementation, the internal space of the closed container may be in a normal pressure state, a special atmosphere state, or a vacuum state. The normal pressure state means that the atmosphere in the closed container is the same as the external atmospheric environment; the special atmosphere state is that inert gas or reducing gas is filled in the closed container, and the inert gas comprises one or more of nitrogen, helium, neon, argon, krypton and xenon. The reducing gas comprises one or two of hydrogen and ammonia. The vacuum state is a vacuum degree of 100Pa or less.

The closed container is placed in a thermal field with the temperature of 390-450 ℃, preferably 400-420 ℃. The closed container can be static or rolling at the temperature. The powder of Zn in metallic state is provided to contact with the steel product to form a Zn-Fe intermetallic compound layer. The internal space of the closed container may be in a normal pressure state, a special atmosphere state, or a vacuum state. After thermal diffusion is carried out for 1-5 hours, preferably 2-4 hours, at the temperature of a thermal field, the steel part with the Zn-Fe protective layer and the powder providing the metallic Zn are cooled to be balanced with the room temperature, and then the steel part with the Zn-Fe intermetallic compound protective layer is taken out. The thickness of the Zn-Fe intermetallic compound protective layer is controlled to be 5-100 mu m.

The steel part with the Zn-Fe intermetallic compound protective layer and the powder for providing metallic Zn and metallic Mg are placed in a closed container. The state of the atmosphere in the internal space of the closed vessel is the same as in the "direct method". The proportions of Zn atoms and Mg atoms in the metal powder, the components and proportions of the dispersant, and the kind of the catalyst are the same as those in the plating process by the "direct method" described above.

And after the protective layer reaches the set protective layer thickness, reducing the temperature of the steel part with the protective layer and the substances providing Zn atoms and Mg atoms to be balanced with the room temperature, and taking out the steel part with the protective layer of the Zn-Fe-Mg intermetallic compound. Separating the powder on the surface of the protective layer of the steel product, thereby completing the whole process of the 'indirect' protective layer plating.

In the examples of the present invention, the protective layer prepared according to the "direct method" or the "indirect method" had no significant difference in performance. Factors that are relevant to the performance of the protective layer are the chemical composition, structure and thickness of the protective layer.

In the method provided by the embodiment of the invention, the main thermal diffusion process is carried out in a closed container, and the material type and the shape of the steel product are not limited at all. Steel articles include, but are not limited to, carbon steels, low alloy steels, high strength steels, stainless steels, cast irons, ferrous powder metallurgy, and the like. The shape of the steel product is not particularly limited, and includes, but is not limited to, steel products having regular shapes such as plates, rods, belts, wires, etc., and steel products having special shapes (irregular shapes) such as semi-blind holes, threads, internal grooves, etc.

More specific embodiments of the present invention are provided below.

Example 1

And (3) putting the penetrating agent into a closed container, and putting the steel part subjected to surface cleaning treatment into the container. The penetrating agent here includes metal powder, dispersant, catalyst. In the metal powder, the metallic Mg atoms are calculated by weight: metallic Zn atoms are 30:70 by weight. The dispersant is calcium oxide, and the dispersant comprises the following components in percentage by weight: the metal powder is 10:90 by weight. The catalyst is ammonium chloride, and the dosage of the ammonium chloride is 0.02 percent of the weight of the metal powder. The inside of the closed container is filled with nitrogen. And slowly rotating the closed container in an external heating field at 400 ℃, and fully and uniformly mixing the raw material penetrating agent and the steel workpiece. At 400 ℃, a protective layer is formed on the surface of the steel product after 4 hours of diffusion. Cooling to room temperature, and taking out the steel product with the protective layer.

The chemical element composition of the protective layer prepared by the above manner was 1.27 wt% Fe, 34.06 wt% Mg, 64.67 wt% Zn as shown in fig. 1B by Scanning Electron Microscope (SEM) physicochemical analysis.

As shown in FIG. 1A, the microstructure of the protective layer prepared as above contains a large amount of Zn-Mg binary alloy phase, and Zn-Fe-Mg ternary alloy phase is also distributed and intercalated in the protective layer. The chemical element composition of the Zn-Mg binary alloy phase matrix contains 48.4 wt% of Zn and 51.6 wt% of Mg. The chemical element composition of the Zn-Fe-Mg ternary alloy phase contains 2.13 weight percent of Fe, 18.15 weight percent of Mg and 79.72 weight percent of Zn.

Example 2

And (3) putting the penetrating agent into a closed container, and putting the steel part subjected to surface cleaning treatment into the container. The penetrating agent here includes metal powder, dispersant, catalyst. In the metal powder, the metallic Mg atoms are calculated by weight: metallic Zn atoms are 30:70 by weight. The dispersant is magnesium oxide, and the dispersant comprises the following components in percentage by weight: the metal powder is 10:90 by weight. The catalyst was ammonium chloride in an amount of 0.05% by weight of the metal powder. The inside of the closed container is filled with helium gas. And slowly rotating the closed container in an external heating field at 400 ℃, and fully and uniformly mixing the raw material penetrating agent and the steel workpiece. At 400 ℃, a protective layer is formed on the surface of the steel product after 5 hours of diffusion. Cooling to room temperature, and taking out the steel product with the protective layer.

The chemical element composition of the protective layer prepared in the above manner was analyzed to be 2.69 wt% Fe, 31.1 wt% Mg, 66.21 wt% Zn, as shown in fig. 2B.

As shown in fig. 2A, the microstructure of the resultant protective layer is divided into two layers. At the interface between the protective layer and the steel product, the Zn-Mg binary alloy phase is completely converted into the Zn-Fe-Mg ternary alloy phase, and a compact and complete Zn-Fe-Mg ternary alloy phase layer is formed. The compact and complete Zn-Fe-Mg ternary alloy phase layer is a mixed structure of a Zn-Mg binary alloy phase and a Zn-Fe-Mg ternary alloy phase with a polygonal columnar structure.

Example 3

And (3) putting the penetrating agent into a closed container, and putting the steel part subjected to surface cleaning treatment into the container. The penetrating agent comprises metal powder, a dispersing agent and a catalyst. In the metal powder, the metallic Mg atoms are calculated by weight: metallic Zn atoms are 25:75 by weight. The dispersant is barium oxide, and the dispersant comprises the following components in percentage by weight: the metal powder is 10:90 by weight. The catalyst is potassium chloride, and the dosage of the potassium chloride is 0.05 percent of the weight of the metal powder. The closed container was filled with hydrogen gas. And slowly rotating the closed container in an external heating field at 450 ℃, and fully and uniformly mixing the raw material penetrating agent and the steel workpiece. At 450 deg.c, after 1 hr diffusion, one protecting layer is formed on the surface of the steel product. Cooling to room temperature, and taking out the steel product with the protective layer.

Through analysis, the chemical element composition of the protective layer prepared in the above manner is 3.88 wt% Fe, 22.26 wt% Mg, 73.86 wt% Zn, as shown in fig. 3A and 3B, the protective layer is transformed into a dense and complete Zn-Fe-Mg ternary alloy phase layer. The Fe element diffused into the protective layer from the steel product in the protective layer is enough in quantity, and all Zn-Mg binary alloy phases can be converted into Zn-Fe-Mg ternary alloy phases to form a compact and complete structure.

Example 4

And (3) putting the penetrating agent into a closed container, and putting the steel part subjected to surface cleaning treatment into the container. The penetrating agent here includes metal powder, dispersant, catalyst. In the metal powder, the metallic Mg atoms are calculated by weight: metallic Zn atoms are 20:80 by weight. The dispersant is sodium carbonate, and the dispersant comprises the following components in parts by weight: the metal powder is 10:90 by weight. The catalyst is sodium chloride, and the dosage of the sodium chloride is 0.05 percent of the weight of the metal powder. The closed container was filled with ammonia gas. And slowly rotating the closed container in an external heating field at the temperature of 420 ℃, and fully and uniformly mixing the raw material penetrating agent and the steel workpiece. At 420 ℃, a protective layer is formed on the surface of the steel product after 2 hours of diffusion. Cooling to room temperature, and taking out the steel product with the protective layer.

Through analysis, the chemical element composition of the protective layer prepared in the above way is 8.62 wt% of Fe, 18.49 wt% of Mg and 72.89 wt% of Zn, and as shown in FIGS. 4A and 4B, the protective layer still maintains a compact and complete structure and consists of a compact Zn-Fe-Mg ternary alloy phase.

Example 5

The steel part with the protective layer is prepared in a closed container by an indirect method.

And putting the steel part subjected to surface cleaning treatment, Zn powder and alumina powder into a closed container. The weight ratio of alumina powder to Zn powder is 50: 50. The closed container is under normal pressure. And (3) slowly rotating the closed container in an external heating field at 390 ℃, and thermally diffusing for 1 hour to obtain a Zn-Fe alloy layer with the thickness of about 10 microns on the surface of the steel workpiece. Cooling the closed container to a proper temperature, and taking out the steel product with the Zn-Fe alloy layer.

Next, in a closed container, a penetrant is put in, and at the same time, a steel product with a Zn-Fe alloy layer is put in. The penetrating agent comprises metal powder, a dispersing agent and a catalyst. In the metal powder, the metallic Mg atoms are calculated by weight: metallic Zn atoms are 15:85 by weight. The dispersant is potassium carbonate, and the dispersant comprises the following components in parts by weight: the metal powder was 30:70 by weight. The catalyst is magnesium chloride, and the dosage of the magnesium chloride is 0.02 percent of the weight of the metal powder. The sealed container was in a vacuum state with a degree of vacuum of 50 Pa. And slowly rotating the closed container in an external heating field at 400 ℃, and fully and uniformly mixing the raw material penetrating agent and the steel workpiece. At 400 ℃, a protective layer is formed on the surface of the steel product after 2 hours of diffusion. Cooling to room temperature, and taking out the steel product with the protective layer.

Through analysis, the chemical element composition of the protective layer prepared in the above way is 18.47 wt% of Fe, 15.92 wt% of Mg and 65.61 wt% of Zn, and as shown in FIGS. 5A and 5B, the protective layer still maintains a compact and complete structure and consists of a compact Zn-Fe-Mg ternary alloy phase.

Example 6

And (3) putting the penetrating agent into a closed container, and putting the steel part subjected to surface cleaning treatment into the container. The penetrating agent here includes metal powder, dispersant, catalyst. In the metal powder, the metallic Mg atoms are calculated by weight: metallic Zn atoms are 25:75 by weight. The dispersant is calcium carbonate, and the dispersant comprises the following components in parts by weight: the metal powder was 45:55 by weight. The catalyst is sodium chloride, and the dosage of the sodium chloride is 0.02 percent of the weight of the metal powder. The closed vessel was filled with argon. And slowly rotating the closed container in an external heating field at 400 ℃, and fully and uniformly mixing the raw material penetrating agent and the steel workpiece. At 400 ℃, a protective layer is formed on the surface of the steel product after 2 hours of diffusion. Cooling to room temperature, and taking out the steel product with the protective layer.

The protective layer prepared in the above manner was analyzed to have a chemical element composition of 22.02 wt% Fe, 26.57 wt% Mg, and 51.41 wt% Zn, as shown in fig. 6A and 6B, and still maintained a dense and integral structure consisting of a dense Zn-Fe-Mg ternary alloy phase.

Example 7

And (3) putting the penetrating agent into a closed container, and putting the steel part subjected to surface cleaning treatment into the container. The penetrating agent here includes metal powder, dispersant, catalyst. In the metal powder, the metallic Mg atoms are calculated by weight: metallic Zn atoms are 10:90 by weight. The dispersant is magnesium carbonate, and the dispersant comprises the following components in parts by weight: the metal powder was 30:70 by weight. The catalyst is zinc chloride, and the dosage of the zinc chloride is 0.02 percent of the weight of the metal powder. The sealed container is filled with neon gas. And slowly rotating the closed container in an external heating field at 400 ℃, and fully and uniformly mixing the raw material penetrating agent and the steel workpiece. At 400 ℃, a protective layer is formed on the surface of the steel product after 2 hours of diffusion. Cooling to room temperature, and taking out the steel product with the protective layer.

The protective layer prepared in the above manner was analyzed to have a chemical element composition of 19.92 wt% Fe, 8.89 wt% Mg, and 71.19 wt% Zn, as shown in fig. 7A and 7B, and still maintained a dense and integral structure consisting of a dense Zn-Fe-Mg ternary alloy phase.

Example 8

And (3) putting the penetrating agent into a closed container, and putting the steel part subjected to surface cleaning treatment into the container. The penetrating agent here includes metal powder, dispersant, catalyst. In the metal powder, the metallic Mg atoms are calculated by weight: metallic Zn atoms are 5:95 by weight. The dispersant is sodium oxide, and the dispersant comprises the following components in percentage by weight: the metal powder was 30:70 by weight. The catalyst is ammonium chloride, and the dosage of the ammonium chloride is 0.02 percent of the weight of the metal powder. The interior of the closed container is filled with neon. And slowly rotating the closed container in an external heating field at 400 ℃, and fully and uniformly mixing the raw material penetrating agent and the steel workpiece. At 400 ℃, a protective layer is formed on the surface of the steel product after 2 hours of diffusion. Cooling to room temperature, and taking out the steel product with the protective layer.

The protective layer prepared in the above manner was analyzed to have a chemical element composition of 16.33 wt% Fe, 1.93 wt% Mg, 1.32 wt% Al, and 80.43 wt% Zn, as shown in fig. 8B, and a small amount of Al element was present in the protective layer due to diffusion of Al contained in the steel article into the protective layer under the action of the thermal field. As shown in FIG. 8A, the protective layer still maintains a compact and complete structure, and is composed of a compact Zn-Fe-Mg ternary alloy phase.

Example 9

And (3) putting the penetrating agent into a closed container, and putting the steel part subjected to surface cleaning treatment into the container. The penetrating agent here includes metal powder, dispersant, catalyst. In the metal powder, the metallic Mg atoms are calculated by weight: metallic Zn atoms are 15:85 by weight. The dispersant is potassium oxide, and the dispersant comprises the following components in percentage by weight: the metal powder is 50:50 by weight. The catalyst is ammonium chloride, and the dosage of the ammonium chloride is 0.02 percent of the weight of the metal powder. The interior of the closed container is filled with neon. And slowly rotating the closed container in an external heating field at 390 ℃, and fully and uniformly mixing the raw material penetrating agent and the steel workpiece. At 390 ℃, a protective layer is formed on the surface of the steel product after 1 hour of diffusion. Cooling to room temperature, and taking out the steel product with the protective layer.

The chemical element composition of the protective layer prepared in the above manner was analyzed to be 41.32 wt% Fe, 16.12 wt% Mg, and 42.56 wt% Zn, as shown in fig. 9B.

According to the embodiments, the microstructure of the protective layer of the steel product prepared by the invention has various variable structures according to the content ratio and distribution state of the three elements of Zn, Fe and Mg in the protective layer. The protective layer has a sacrificial anode protection effect on steel products, has extremely excellent corrosion resistance, and can endow the steel products with better corrosion resistance. The method can uniformly cover a protective layer on the surface of a steel product, particularly on the surface of steel with a complex shape.

The above embodiments are merely examples, and the present invention is not limited thereto. The inventors have also conducted a number of experiments, all of which show that the protective layer has excellent corrosion resistance.

The protective layer prepared by the method of the invention is characterized by being directly bonded with the surface of a steel product, belongs to metallurgical bonding and has firm bonding force. The composition change of the protective layer and its corrosion resistance obtained under different process parameters are shown in table 1 below.

TABLE 1 compositional changes of the protective layer obtained under different process parameters and its corrosion resistance

Table 2 shows the corrosion resistance of the protective layer obtained by the process of the present invention compared to the conventional process.

TABLE 2 comparison of corrosion resistance between protective layers obtained by different processes

As shown in table 2, the corrosion resistance of the protective layer provided by the present invention was greatly improved.

The protective layer prepared by the method has the main alloy elements consisting of three elements of Zn, Fe and Mg, and the sum of the three elements exceeds 90 weight percent of the chemical element components of the protective layer. The protective layer is an intermetallic compound protective layer, and the structure consists of a Zn-Mg binary alloy phase and a Zn-Fe-Mg ternary alloy phase or only consists of the Zn-Fe-Mg ternary alloy phase according to different components. The protective layer can uniformly cover the surface of a steel product with a complex shape, even if the steel product has complex shapes such as threads, groove teeth, semi-blind holes and the like, and can completely and uniformly cover the steel product.

In addition, according to the protective layer provided by the invention, the protective layer contains 1.93-34.05 wt% of Mg, and the Mg content of the hot galvanizing aluminum-magnesium coating is exceeded, so that the protective layer provides more excellent corrosion resistance. Furthermore, the corrosion electrode potential (-2.37V) of Mg is far lower than that of Fe (-0.037V), the magnesium content in the protective layer is high, the protective layer can provide excellent sacrificial anode protection effect, and the protective layer still has the anti-corrosion protection effect on steel parts under the protective layer even if the protective layer is scratched or scratched by external force.

Furthermore, under the action of thermal diffusion of the protective layer, Fe element in the steel product gradually diffuses into the protective layer. The proportion of Fe in the chemical element composition of the protective layer significantly contributes to the variable microstructure of the protective layer.

When the content of the Fe element in the protective layer is not more than 1.3 wt%, the microstructure of the protective layer takes a Zn-Mg binary alloy phase as a matrix, and a Zn-Fe-Mg ternary alloy phase (shown in figure 1A) is also distributed and intercalated in the matrix, namely the whole protective layer comprises a mixed microstructure of the Zn-Mg binary alloy phase and the Zn-Fe-Mg ternary alloy phase. The Zn-Fe-Mg ternary alloy phase presents an irregular polygonal columnar structure and belongs to a typical intermetallic compound structure. The number of the Zn-Fe-Mg ternary alloy phases with the polygonal columnar structure is closely related to the content of Fe element, and the higher the content of the Fe element is, the more the number of the Zn-Fe-Mg ternary alloy phases with the polygonal columnar structure is in the protective layer. Fe element diffuses into the protective layer from the steel product, dissolves in the Zn-Mg binary alloy phase, and converts the Zn-Mg alloy phase into a Zn-Fe-Mg ternary alloy phase with a polygonal columnar structure.

When the content of Fe element in the protective layer exceeds 1.3 wt% and does not exceed 3.2 wt%, the protective layer is converted into a two-layer structure form (as shown in fig. 2A). As Fe element diffuses into the protective layer from the steel part, at the interface between the protective layer and the steel part, the Zn-Mg binary alloy phase close to the interface is completely converted into the Zn-Fe-Mg ternary alloy phase, and a compact and complete Zn-Fe-Mg ternary alloy phase layer is formed. On the compact and complete Zn-Fe-Mg ternary alloy phase layer, because the compact and complete Zn-Fe-Mg ternary alloy phase layer is far away from the interface, enough Fe element is not diffused to the compact and complete Zn-Fe-Mg ternary alloy phase layer, and the compact and complete Zn-Fe-Mg ternary alloy phase layer has a mixed structure of a Zn-Mg binary alloy phase and a Zn-Fe-Mg ternary alloy phase with a polygonal columnar structure. The reason is that the amount of the Fe element diffused into the alloy can not completely convert all the Zn-Mg binary alloy phase into the Zn-Fe-Mg ternary alloy phase with a columnar structure, so that a mixed structure with the Zn-Mg binary alloy phase and the Zn-Fe-Mg ternary alloy phase coexisting is formed. The content of the Fe element in the protective layer is continuously increased, and when the content of the Fe element in the protective layer is close to 3.2 wt%, most of the Zn-Mg binary alloy phase in the protective layer is converted into a Zn-Fe-Mg ternary alloy phase with a columnar structure, so that the Zn-Fe-Mg ternary alloy phase is in a structure with absolute advantage in quantity.

When the content of the Fe element in the protective layer exceeds 3.2 wt%, the protective layer is transformed into a compact and complete Zn-Fe-Mg ternary alloy phase layer (as shown in FIG. 3A). The amount of Fe element diffused into the protective layer from the steel part is enough, and all Zn-Mg binary alloy phases can be converted into Zn-Fe-Mg ternary alloy phases to form a compact and complete structure. The content of Fe element in the protective layer is continuously increased, and when the content of Fe element in the protective layer does not exceed 40 wt%, the protective layer still keeps a compact and complete structure and consists of a compact Zn-Fe-Mg ternary alloy phase.

According to the method proposed in the present invention, Fe can affect the structure, composition and corresponding phase transformation of the coating. When the content of the Fe element is less than 3.2 wt%, a Zn-Mg binary phase and a Zn-Fe-Mg ternary phase exist in the protective layer. Two phases of different structure and composition are present in the protective layer, and due to the difference in corrosion potential of the two phases, selective corrosion sites may be formed on the surface of the protective layer in the presence of some corrosive environments. Therefore, according to the method proposed by the present invention, it is preferable that the content of Fe element in the protective layer exceeds 3.2 wt%, and it is more preferable that the content of Fe element exceeds 4 wt%.

When the content of Fe element in the protective layer exceeds 41.32 wt%, the protective layer starts to be cracked. The Fe element in the protective layer is too high, and the protective layer has a hard and brittle property, so that a chapping phenomenon occurs.

The protective layer provided by the invention has excellent corrosion resistance. The protective layer provided by the invention can provide corrosion resistance exceeding 1000 hours in neutral salt fog (ASTM B117). Numerous experiments have shown that the Mg element in the protective layer plays a decisive role in corrosion resistance. Within a certain range, the higher the content of Mg element, the better the corrosion resistance of the protective layer. According to the protective layer provided by the invention, experimental analysis shows that the corrosion resistance of the protective layer is greatly improved when the content of Mg element is within the range of 5-30 wt%, and the corrosion resistance of the protective layer exceeds that of neutral salt spray (ASTM B117) for 1000 hours. The content of Mg element exceeds 30 wt%, and the protective layer shows that blackening phenomenon is easily generated due to easy oxidation of Mg. Further, the content of Mg element exceeding 34.05 wt% may inhibit the growth rate of the thickness of the protective layer during the preparation process, and is not economically feasible.

According to the method provided by the invention, the prepared protective layer forms an intermetallic compound protective layer with excellent corrosion resistance on the surface of the steel product. Further, the microstructure of the protective layer of the intermetallic compound produced has a variable structure, very significantly differing under a microscope from protective layers produced by electroplating, hot-dipping or other means.

According to the method provided by the invention, when the intermetallic compound protective layer is prepared by a direct method or an indirect method, because the intermetallic compound protective layer is inevitably contacted with oxygen in the atmospheric environment, the oxygen-philic elements such as Zn, Mg and the like are oxidized, and an oxide film layer with the thickness of 0.2-3 mu m is formed on the surface of the prepared intermetallic compound protective layer.

In addition, according to the method provided by the invention, the surface of the protective layer has a micro-concave-convex structure, so that the protective layer can be endowed with good bonding force with organic matters or inorganic matters, and is suitable for secondary coating treatment, such as surface chromate treatment coating or sealant treatment coating. One or more inorganic layers, organic layers or inorganic-organic mixture layers can be coated on the surface of the protective layer, so that the protective layer provided by the invention has special functions such as insulation, heat insulation and aesthetic property.

In accordance with the method of the present invention, there is also provided a protective layer for coating a surface of a ferrous article prepared by the above method, the protective layer comprising: 1.27-41.32 wt% Fe, 1.93-34.05 wt% Mg, and the remainder Zn and unavoidable impurities. The inevitable impurities may be, for example, a trace amount of Al present in the steel product or oxygen from an oxide film formed on the surface of the protective layer or the layer.

Corresponding to the method provided by the invention, the invention also provides a steel product with the protective layer prepared by the method.

It is to be understood that the invention is not limited to the specific arrangements and instrumentality described above and shown in the drawings. A detailed description of known methods is omitted herein for the sake of brevity. In the above embodiments, several specific steps are described and shown as examples. However, the method processes of the present invention are not limited to the specific steps described and illustrated, and those skilled in the art can make various changes, modifications and additions or change the order between the steps after comprehending the spirit of the present invention.

It should also be noted that the exemplary embodiments mentioned in this patent describe some methods or systems based on a series of steps or devices. However, the present invention is not limited to the order of the above-described steps, that is, the steps may be performed in the order mentioned in the embodiments, may be performed in an order different from the order in the embodiments, or may be performed simultaneously.

Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments in the present invention.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and various modifications and changes may be made to the embodiment of the present invention by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A protective layer for coating a surface of a ferrous object, said protective layer comprising:

1.27-41.32 wt% Fe,

1.93-34.05 wt.% Mg, and

the remainder being Zn and unavoidable impurities;

wherein the protective layer contains a Zn-Fe-Mg ternary alloy phase.

2. The protective layer of claim 1, wherein the protective layer is formed by infiltrating metallic Mg atoms and metallic Zn atoms in an infiltrant into the surface of the steel article by means of thermal diffusion in a closed vessel.

3. The protective layer of claim 1,

when the content of Fe element in the protective layer is not more than 1.3 wt%, the whole protective layer comprises a mixed structure in which a Zn-Mg binary alloy phase and a Zn-Fe-Mg ternary alloy phase coexist;

when the content of the Fe element in the protective layer exceeds 1.3 wt% and does not exceed 3.2 wt%, a Zn-Fe-Mg ternary alloy phase layer is formed at the interface between the protective layer and the steel part, and a mixed structure comprising a Zn-Mg binary alloy phase and a Zn-Fe-Mg ternary alloy phase is formed on the ternary alloy phase layer;

when the content of Fe element in the protective layer exceeds 3.2 wt% and does not exceed 41.32 wt%, the protective layer includes a dense and complete Zn-Fe-Mg ternary alloy phase layer.

4. The protective layer of claim 1, wherein the protective layer comprises:

4-41.32% by weight of Fe,

5-30% by weight of Mg, and

zn and inevitable impurities remain.

5. The protective layer of any of claims 1 to 4, wherein the protective layer has a thickness greater than 5 μm and not more than 100 μm.

6. The protective layer of claim 5, wherein the protective layer has a thickness greater than 15 μm and no more than 100 μm.

7. The protective layer for steel products according to claims 1 to 4, characterized in that the surface of the protective layer has an oxide layer with a thickness of 0.2-3 μm.

8. The ironwork protective layer of claims 1-4, wherein the proportion of Fe, Mg, and Zn in the protective layer is greater than 90 wt.%;

the inevitable impurities include oxygen.

9. The ironwork protective layer of claim 2, wherein the infiltrant comprises a metal powder, a dispersant, and/or a catalyst; the metal powder includes a powder containing an element Mg in a metallic state and a powder containing an element Zn in a metallic state.

10. The protective layer for steel products according to claim 9, wherein said metallic powder contains Mg element in metallic state 5-30 wt% and Zn element in metallic state 70-95 wt% based on the weight of the metallic powder.

11. A method of making a protective layer for a ferrous object, the method comprising:

1.27-41.32 wt% Fe,

1.93-34.05 wt.% Mg, and

the remainder being Zn and unavoidable impurities;

wherein the protective layer contains a Zn-Fe-Mg ternary alloy phase.

12. The method of claim 11, wherein the infiltrant includes a metal powder, a dispersant and/or a catalyst.

13. The method according to claim 12, wherein the metal powder comprises a powder containing an Mg element in a metallic state and a powder containing a Zn element in a metallic state.

14. The method according to claim 13, wherein the metallic powder contains Mg element in metallic state in an amount of 5 to 30 wt% and Zn element in metallic state in an amount of 70 to 95 wt% based on the weight of the metallic powder, and the particle size of the metallic powder is not more than 150 μm.

15. The method of claim 12, wherein the dispersant comprises one or more of sodium carbonate, potassium carbonate, calcium carbonate, magnesium carbonate, barium carbonate, calcium oxide, magnesium oxide, barium oxide, sodium oxide, and potassium oxide.

16. The method according to claim 12, characterized in that the proportions by weight of the dispersant are:

the dispersing agent is metal powder which is 10: 90-50: 50; and is

The particle size of the dispersing agent is not more than 0.3 mm.

17. The method of claim 16, wherein the dispersant particle size is no more than 0.125 mm.

18. The method of claim 12, wherein the catalyst is chloride, and the chloride is present in an amount of 0.02 to 0.2 wt% based on the weight of the metal powder.

19. The method according to claim 11, wherein the inner space of the closed vessel is in a vacuum state or an atmosphere state.

20. The method of claim 19,

when the internal space state of the closed container is a vacuum state, the vacuum degree is not more than 100 Pa;

when the internal space state of the closed container is an atmosphere state, the atmosphere is a protective atmosphere or a reducing atmosphere;

the protective atmosphere comprises one or more than two of nitrogen, helium, neon, argon, krypton and xenon;

the reducing atmosphere comprises one or both of hydrogen and ammonia.

21. The method as claimed in claim 11, wherein the temperature of the thermal diffusion is between 390-450 ℃ and the time of the thermal diffusion is between 1-5 hours.

22. The method of claim 11, further comprising:

and coating a coating on the surface of the generated protective layer.

23. The method according to claim 11, characterized in that it comprises the steps of:

putting a steel product, metal powder consisting of Zn-containing metal or alloy powder and Mg-containing metal or alloy powder, a dispersing agent and a penetrating agent consisting of a catalyst into a closed container;

heating the closed container in which the steel part and the penetrant are put, and rotating the closed container;

heating the sealed container for a predetermined time and then cooling the container.

24. A ferrous object comprising a protective layer as claimed in any one of claims 1 to 10, said ferrous object comprising a ferrous object having a regular shape and a ferrous object having an irregular shape.