KR20210060152A - Carbon steel tube by plating melted aluminium having excellent corrosion resistance - Google Patents

Abstract

The present invention relates to a molten aluminum plated carbon steel pipe with excellent corrosion resistance, which removes non-plating by controlling an interface structure of a plated layer and significantly improves corrosion resistance and plating adhesion.

Description

{Carbon steel tube by plating melted aluminum having excellent corrosion resistance}

The present invention relates to a hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance, and to a hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance that can dramatically improve corrosion resistance and plating adhesion without causing unplated by controlling the interface structure of the plating layer. will be.

Corrosion resistance of steel when it comes into contact with sea water has emerged as a very important problem. To meet these demands, many types of seawater rivers have been developed. Most of the steel materials show corrosion when they come into contact with seawater, and ship owners are deepening their concerns due to the limited lifespan of various parts used in ships. In order to solve this problem, research has been conducted on improving the corrosion resistance of seawater steel, and seawater steel having improved corrosion resistance by adding an element for improving corrosion resistance to carbon steel is being developed.

However, the seawater steel itself still has a limit on corrosion when it comes into contact with seawater, so seawater pipes currently used in ships are used by coating the surface of carbon steel pipes with zinc plating, paint, or the like. If the seawater pipe is used for a long period of time by hypochlorous acid or other seawater components, if extreme corrosion proceeds, corrosion is rapidly accelerated at the defect site, and there is a problem in that water leakage occurs due to sudden corrosion breakthrough at an unexpected time.

In order to solve the above problem, a method of coating GRE (Glass Reinforecd Epoxy) on the surface of a carbon steel pipe is being reviewed, but there is a problem that the cost is expensive, and there is a problem that difficulties arise in the process of connecting with other pipes when branching a pipe. .

KR 101045389 B1

The present invention was devised to overcome the corrosive limit of existing hot-dip aluminum-plated products, and is to provide a hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance even in an extreme corrosive environment by introducing a new plating layer structure.

In order to achieve such an object, the hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention is formed on the surface of the carbon steel pipe base material and the carbon steel tube base material made to flow a fluid therein, and weight% Furnace, Si: 3 to 15%, including a molten aluminum-based plating layer containing 3 to 15% of the balance Al and inevitable impurities, and an aluminum oxide layer formed on the molten aluminum-based plating layer, and the interface between the carbon steel tube base material and the molten aluminum-based plating layer It includes a Fe-Al alloy phase formed by diffusion and penetration of Al into the surface of the carbon steel tube base material, and includes a composite alloy layer having a thickness of 100 to 200 μm, and the composite alloy layer is in weight %, Al: 10-40%, Si: 0.05-1%, Mn: 0.1-1.5%, balance Fe and inevitable impurities are included.

In the hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention, the molten aluminum-based plating layer may further include 0.1 to 10% Mg by weight.

In the hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention, the hot-dip aluminum-based plating layer is in weight %, Ca: 0.001 to 5%, Sr: 0.005 to 2%, Mn: 0.01 to 2%, Cr: 0.01 ~ 2%, Mo: 0.01 ~ 2%, and Sn: may further include at least one selected from the group consisting of 0.1 ~ 10%.

The molten aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention includes a degreasing step and a washing step for the surface of the carbon steel pipe base material, a pickling step for activating the surface of the carbon steel pipe base material, and a washing step. Pretreatment process; And, after performing a water-soluble flux treatment on the surface of the carbon steel tube base material that has undergone the pretreatment process, the carbon steel tube base material is immersed in a plating solution to which a melting flux is added to melt the surface of the carbon steel tube base material. Plating step of forming the composite alloy layer and the molten aluminum-based plating layer by plating molten aluminum at the same time; And, the molten aluminum-based plating layer by immersing the carbon steel pipe on which the composite alloy layer and the molten aluminum-based plating layer are formed in an aqueous oxalic acid solution. It may be formed through an oxalic acid treatment process of forming an aluminum oxide layer thereon; and a post-treatment process including a chemical cleaning step and a water washing step of surface treatment of the carbon steel pipe on which the aluminum oxide layer is formed.

According to an embodiment of the present invention, in the water-soluble flux treatment, the pretreated carbon steel pipe is prepared in a range of 30 to 60 wt% of potassium chloride (KCl), 20 to 40 wt% of potassium fluoride (KF), and _{15 ammonium fluoride (NH 4} F). It may be immersed in a water-soluble flux containing ~40% by weight.

According to an embodiment of the present invention, in the melt flux treatment, the carbon steel pipe subjected to the water-soluble flux treatment is cryolite 10 to 30 wt%, aluminum fluoride (AlF ₃ ) 10 to 40 wt%, potassium chloride (KCl) 35 It may be immersed in a melt flux containing ~65% by weight and 10 to 25% by weight of sodium chloride (NaCl).

The hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention has an advantage of excellent corrosion resistance and plating adhesion.

In addition, the hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention has an effect of providing a plated product capable of substantially zeroing the non-plating rate and the occurrence rate of pin holes on the surface of the base material. This has the effect of maximizing the corrosion resistance, weather resistance, and heat resistance of the plated product.

1 is a SEM photograph of a cut surface of a molten aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail. The following embodiments and drawings are provided as examples in order to sufficiently convey the spirit of the present invention to those skilled in the art. In addition, unless there are other definitions in the technical and scientific terms used in the present invention, the present invention has the meaning commonly understood by those of ordinary skill in the art to which this invention belongs, and the present invention in the following description and accompanying drawings Descriptions of known functions and configurations that may unnecessarily obscure the gist of are omitted.

In the present invention, unless otherwise specified,% means% by weight.

FIG. 1 is a photograph of a hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention, and then observing the cross-section with an SEM.

Referring to FIG. 1, a hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention includes a carbon steel tube base material 10, a composite alloy layer 20, a hot-dip aluminum-based plating layer 30, and an aluminum oxide layer 40. ) In sequence.

When the present inventors use a conventional carbon steel pipe for hot-dip aluminum plating treatment, the produced hot-dip aluminum-plated carbon steel pipe has a problem that the corrosion resistance is not excellent, and according to the results of the interface analysis, the carbon steel tube base material 10 and the molten aluminum plating layer ( 30) It was confirmed that there was a problem of plating adhesion between.

Accordingly, as a result of a long study of the present inventors, in order to improve corrosion resistance and plating adhesion, a composite having a thickness of 100 to 200 μm while including an Fe-Al alloy phase between the carbon steel tube base material 10 and the hot-dip aluminum-based plating layer 30 By including the alloy layer 20, the composite alloy layer 20 by weight, Al: 10 to 40%, Si: 0.05 to 1%, Mn: 0.1 to 1.5%, by including the balance Fe and inevitable impurities , Solved the above problems.

That is, the molten aluminum-plated carbon steel pipe having excellent corrosion resistance according to the present invention is formed on the surface of the carbon steel tube base material 10 and the carbon steel tube base material 10 made to flow a fluid therein, in weight %, Si: A molten aluminum-based plating layer 30 containing 3 to 15%, remaining Al and inevitable impurities, and an aluminum oxide layer 40 formed on the molten aluminum-based plating layer 30, and the carbon steel tube base material 10 A composite alloy layer having a thickness of 100 to 200 μm, including an Fe-Al alloy phase formed by diffusion and penetration of Al from the interface between the molten aluminum-based plating layer 30 into the surface of the carbon steel tube base material 10 ( 20), and the composite alloy layer 20 contains, in weight%, Al: 10 to 40%, Si: 0.05 to 1%, Mn: 0.1 to 1.5%, the balance Fe and unavoidable impurities.

The carbon steel tube base material 10 is for carbon steel for securing high strength and high ductility, and any one of low carbon steel, medium carbon steel and high carbon steel is sufficient, and it is preferable to contain Si, Mn, and the like.

The molten aluminum-based plating layer 30 is formed on the surface of the carbon steel tube base material 10 to improve corrosion resistance. The molten aluminum-based plating layer 30 contains 3 to 15% of Si by weight, the balance of Al, and unavoidable impurities. At this time, the Si is added to impart excellent processability or oxidation resistance, and the Si: 3 to 15% is preferably added. However, the addition of excessive Si exceeding 15% is not preferable because there is a concern that the plating bath temperature is excessively increased, coarse Si primary crystals are crystallized, and corrosion resistance and workability may be deteriorated.

In addition, the molten aluminum-based plating layer 30 may further include Mg, which may impart excellent corrosion resistance. At this time, the content of Mg is preferably 0.1 to 10% by weight, more preferably 4 to 8% by weight. If the above criteria are satisfied, it helps to reduce the occurrence of non-plating. However, when the Mg content is an excessive content of more than 10%, the effect of improving corrosion resistance and plating property is saturated, causing an increase in cost.

In addition, the molten aluminum-based plating layer 30 is in weight%, Ca: 0.001 to 5%, Sr: 0.005 to 2%, Mn: 0.01 to 2%, Cr: 0.01 to 2%, Mo: 0.01 to 2% and Sn: It may further include one or more selected from the group consisting of 0.1 to 10%. These elements form a thin passivation film on the surface of the plating layer, thereby suppressing the surface reaction and suppressing the reaction with the surrounding corrosion-causing electrolytic material, thereby further improving the corrosion resistance of the hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to the present invention. I can. However, if the content is excessive, it is advantageous in terms of corrosion resistance, but during plating, it may cause a large amount of dross in the plating bath to cause plating defects, so the Ca: 0.001 to 5%, Sr: 0.005 to 2%, Mn : 0.01~2%, Cr: 0.01~2%, Mo: 0.01~2%, and Sn: 0.1~10% It is good to satisfy the categories.

Next, the composite alloy layer 20 is a layer formed through a plating process that satisfies the following relational equation 1, and Al among the molten aluminum composition diffuses and penetrates into the surface of the carbon steel tube base material 10 and is present inside the base material. It combines with the Fe element to form an Fe-Al alloy phase and has a thickness of 100 to 200 μm. Accordingly, the present invention provides a molten aluminum-plated carbon steel pipe having excellent corrosion resistance by including the composite alloy layer 20. In addition, by including the composite alloy layer 20, the present invention can prevent corrosion, cracks, etc. that extend into the carbon steel pipe due to external impacts or pinholes generated under severe corrosive environments.

[Relationship 1]

18,770 <LMP <20,000

(In Equation 1, LMP = T(logt _r + C), where T is the temperature of the molten _{plating solution (K), t r} is the immersion time (hr), and C is the constant 20.)

In addition, since the composition of the composite alloy layer 20 is mainly Fe and Al, Fe-Al alloy phases are largely distributed, but as described above, the carbon steel tube base material 10 contains Si, Mg, etc., so that Al-Si A based alloy phase, an Al-Mn based alloy phase, and the like may be further formed.

In particular, 10 to 40% of Al is included in the composition of the composite alloy layer 20, which contributes to the improvement of corrosion resistance and plating adhesion of the hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to the present invention.

If Si: 0.05 to 1% and Mn: 0.1 to 1.5% are included in the composition of the composite alloy layer 20, the corrosion resistance of the molten aluminum-plated carbon steel pipe having excellent corrosion resistance according to the present invention is further improved, and the carbon steel pipe base material (10) It is possible to further improve the plating adhesion between the molten aluminum-based plating layer 30.

Meanwhile, the composite alloy layer 20 contains inevitable impurities, wherein the impurities are all prescribed components included in the carbon steel tube base material 10 other than Si and Mn, that is, inevitable impurities included in the carbon steel tube base material 10 It may include. In other words, the term "inevitable impurities" used in the present invention may include inevitable impurities (eg, P, S, etc.) included in the carbon steel pipe base material 10 and inevitable impurities included in the plating bath.

Next, the hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance according to an embodiment of the present invention, A pretreatment step including a degreasing step and a washing step for the surface of the carbon steel pipe base material 10, a pickling step for activating the surface of the carbon steel pipe base material 10, and a washing step; And, the carbon subjected to the pretreatment step After performing a water-soluble flux treatment on the surface of the steel pipe base material 10, the carbon steel tube base material 10 is immersed in a plating solution to which a melting flux is added to melt the surface of the carbon steel tube base material 10 and melt at the same time. Plating process of forming a molten aluminum-based plating layer by plating aluminum; And, forming an aluminum oxide layer 40 on the molten aluminum-based plating layer 30 by immersing the carbon steel pipe on which the molten aluminum-based plating layer 30 is formed in an aqueous oxalic acid solution. It may be formed through an oxalic acid treatment process; and a post-treatment process including a chemical cleaning step and a water washing step of surface treatment of the carbon steel pipe on which the aluminum oxide layer 40 is formed.

In detail, in the degreasing step, the carbon steel pipe base material 10 is immersed in a degreasing tank in which an alkali solvent or an organic solvent is stored, and cutting oil or press oil adhered to the surface of the carbon steel tube base material 10 due to various mechanical processing, It corresponds to the process step of removing various foreign substances such as gamma agent according to drawing processing and polishing oil or metal powder according to polishing processing.

The alkaline solvent used in the degreasing step is sodium carbonate (Na ₂ CO ₃ ), sodium phosphate (Na ₃ PO ₄ ), sodium silicate (= sodium metasilicate; Na ₂ SiO ₃ ), as in most plating methods. One or several types are selected and dissolved in water. In some cases, sodium hydroxide (NaOH) is used in combination for the purpose of improving the cleaning effect by increasing the alkalinity.

However, it is most preferable to remove foreign substances adhering to the surface of the base material as much as possible by appropriately selecting and applying the type of alkali solvent, its pH and use temperature according to the material and size of the base material to be plated. As the organic solvent, it is common to use a chlorine-based solvent such as trichloroethylene (C ₂ HCl ₂ ) and methylchloroform (CHCl _{2 ).}

After going through the degreasing step as described above, by immersing the degreasing-treated carbon steel tube base material 10 in a washing tank in which water at room temperature is stored, the alkali solvent and residual foreign matter adhered to the surface of the carbon steel tube base material 10 are removed. It goes through the washing step.

The flushing tank used in the flushing step is preferably formed in a structure in which normal temperature water is continuously supplied from one side to the inside of the flushing tank, and the treated water inside the flushing tank is discharged naturally from the other side, and a carbon steel pipe In order to more effectively remove the alkali solvent adhering to the surface of the base material 10, in some cases, two or more washing tanks may be installed and the washing step of one amount may be divided into several times.

After the degreasing step and the water washing step as described above, a pickling step of immersing the base material into the interior of the pickling tank in which an aqueous solution of acid is stored is performed.This pickling step is performed with a trace amount of remaining on the surface of the carbon steel pipe base material 10. At the same time as neutralizing the alkali solvent, removing the scale or oxide film formed on the surface of the carbon steel tube base material 10, and giving the surface of the base material an etching effect by acid corrosion to give a molten aluminum-based plating layer ( It becomes the activation step of the base material surface to improve the adhesion of 30).

The aqueous solution of acid used in the pickling step can also be selectively applied to aqueous solutions of various acids such as _{hydrochloric acid (HCl), sulfuric acid (H 2} SO ₄ ) or nitric acid (HNO _{3 ), or a mixed acid solution thereof, depending on the material of the base material.} However, as an example of the carbon steel pipe material being used in the present invention, an aqueous solution having a concentration of hydrochloric acid (HCl) of about 10 to 20% may be mentioned, and the pickling treatment contributes to the plating rate of the surface of the carbon steel pipe base material 10. Since the bar is large, it is desirable to ensure that the aqueous solution of the acid selected according to the material of the base material is always maintained at a constant concentration inside the pickling tank.

After the above-described pickling step, the pickling-treated carbon steel pipe base material 10 is immersed in a water washing tank in which water at room temperature is stored to remove the acid solution attached to the surface of the base material. In the case of the water washing step performed after the pretreatment process is completed and after the pickling step, the same technical matters as the washing step performed after the degreasing step are applied.

After passing through the pretreatment process as described above, the plating process is performed. In the plating process, after performing a water-soluble flux treatment on the surface of the carbon steel tube base material 10 that has undergone the pre-treatment process, the carbon steel tube base material 10 is immersed in a plating solution to which a melting flux is added to the carbon steel tube base material 10. ), a plating process of plating molten aluminum at the same time as the molten flux treatment is performed sequentially.

In detail, in the water-soluble flux treatment, the carbon steel tube base material 10, which has undergone a pre-treatment process, is immersed in a flux treatment tank in which the water-soluble flux is dissolved, and oxidation on the surface of the carbon steel tube base material 10 before aluminum hot dip plating. This is a step for forming a chemical film or the like on the surface of the base material so that a good plating layer is formed by suppressing the formation of a film or an oxide scale, while allowing the diffusion and penetration of aluminum components to be smoothly performed during plating.

범위내로 유지시킨 상태에서, 전처리공정을 거친 상기 탄소강관 모재(10)를 플럭스 용제에 약 1 ~ 10분간 침지시키는 공정과정으로 진행된다.The water-soluble flux used in the water-soluble flux treatment may include 30 to 60% by weight of potassium chloride (KCl), 20 to 40% by weight of potassium fluoride (KF), and 15 to 40% by weight of _{ammonium fluoride (NH 4 F).} A flux solvent was prepared by adding and dissolving a water-soluble flux having the above composition category in 10 to 30% by weight based on 100% by weight of water, and then storing the thus prepared flux solvent in a flux treatment tank to control the temperature of the flux solvent. 40 to 90

In a state maintained within the range, the carbon steel pipe base material 10, which has undergone a pretreatment process, is immersed in a flux solvent for about 1 to 10 minutes.

Each of the components used in the water-soluble flux and the mixing ratio thereof, together with the molten flux added to the plating bath in the plating process performed after the water-soluble flux treatment, substantially reduce the subplating rate of aluminum to the surface of the carbon steel tube base material 10. In order to achieve zero, the optimum component ratio for forming a chemical film by a water-soluble flux over the entire surface of the carbon steel tube base material 10 to a certain thickness is limited.

On the other hand, when the water-soluble flux is solventized with water at a concentration of less than 10% by weight, the thickness of the chemical film formed on the surface of the carbon steel tube base material 10 becomes relatively thin, as well as a long time for drying the chemical film. It is unsuitable to immediately apply the flux-treated carbon steel tube base material 10 to the next processing step, and even if the water-soluble flux is dissolved with water at a concentration exceeding 30% by weight, the thickness of the chemical film layer is over a certain range. Since it is not raised to, the concentration range exceeding 30% by weight becomes uneconomical.

이하로 유지시키게 되면, 상기 탄소강관 모재(10)의 표면에 약품피막층을 형성시키는 데에 10분 이상의 과도한 시간이 소요되고, 플럭스 용제를 90

이상의 온도로 유지시키게 되면, 상기 탄소강관 모재(10)의 표면에 약품피막층을 형성시키는 시간을 1분 정도로 단축시킬 수는 있으나, 플럭스 용제로부터 수분의 증발이 촉진되어 플럭스 용제의 농도를 적정 수준으로 유지하기가 어렵게 된다.In addition, the temperature of the flux solvent is 40

If maintained below, it takes an excessive time of 10 minutes or more to form the chemical film layer on the surface of the carbon steel pipe base material 10, and the flux solvent is 90

If maintained at the above temperature, the time for forming the chemical film layer on the surface of the carbon steel pipe base material 10 can be shortened to about 1 minute, but the evaporation of moisture from the flux solvent is promoted to reduce the concentration of the flux solvent to an appropriate level. It becomes difficult to maintain.

After the water-soluble flux treatment as described above, the carbon steel tube base material 10 subjected to the water-soluble flux treatment is immersed into the inside of the aluminum-silicon plating solution to which the melting flux is added, so that Al into the inside of the surface of the carbon steel tube base material 10 A plating process is performed in which the Fe-Al alloy phase formed by diffusion penetration is formed, and a molten aluminum-based plating layer 30 having an aluminum-silicon composition is formed thereon.

의 온도로 가열 및 용해시킴으로서 용융도금액을 조성하는 한편, 이와 같이 조성된 용융도금액의 표면에 용융플럭스를 뿌려 녹인 상태에서, 수용성 플럭스 처리된 모재를 상술한 관계식 1의 조건으로 용융도금액속에 침지시켰다가 꺼내는 과정을 거쳐서 이루어진다.In the plating process, 3 to 15% by weight of silicon is added based on 100% by weight of aluminum, and 680 to 750

While heating and dissolving at a temperature of to form a molten plating solution, in a state in which the molten plating solution was melted by spraying a melting flux on the surface of the molten plating solution, the water-soluble flux-treated base material was placed in the molten plating solution under the conditions of the above-described relational formula 1. It is done through the process of immersing and taking out.

The reason for adding silicon to the molten aluminum in the plating process as described above is to suppress excessive diffusion of the alloy layer formed when the silicon component is plated, so that plating of a uniform thickness along the surface of the carbon steel tube base material 10 is possible. This is because corrosion-resistant aluminum plating can be applied not only to seawater pipes of medium and large structures, but also to precision parts such as small drainage pipes.

Therefore, if less than 3% by weight of silicon is added based on 100% by weight of molten aluminum, the function of suppressing excessive diffusion of the aluminum alloy layer is not properly expressed, and the silicon is 15% by weight based on 100% by weight of molten aluminum. If it is added in excess of %, it is not preferable because the management of the plating bath becomes difficult due to the high melting point of silicon.

정도로 상승시킨 상태에서 용탕을 충분히 교반하여 조성토록 하는 것이다. In addition, various methods can be applied to the method of forming a molten aluminum plating bath containing silicon, but as a representative example, after first dissolving an aluminum alloy ingot containing 25% silicon, the purity is 99.9%. Pure aluminum ingot (ingot) is added at a weight-to-weight ratio to create a bath of 3 to 15% by weight of silicon, and then adjust the temperature of the bath to 680 to 750 according to the silicon content.

It is to make the composition by sufficiently stirring the molten metal in a state raised to the degree.

On the other hand, the melting flux used in the plating process is cryolite 10 to 30% by weight, aluminum fluoride (AlF ₃ ) 10 to 40% by weight, potassium chloride (KCl) 35 to 65% by weight, and sodium chloride (NaCl) 10 to It contains 25% by weight, and is sprayed onto the surface of the molten plating solution before immersion of the carbon steel tube base material 10 treated with a water-soluble flux in a state in which the powder particles of the melting flux are evenly mixed.

More specifically, based on 100% by weight of the molten plating solution composed of an aluminum-silicon alloy, the molten flux is 5 to 15% by weight and added to the surface of the plating solution. The flux layer is formed in a floating state on the top.

The components of the molten flux and the mixing ratio thereof are formed by forming a flux layer of a constant thickness over the entire upper surface of the molten plating solution, thereby blocking contact between the molten aluminum-silicon component and external air, thereby preventing various impurities or aluminum oxide (Al ₂ O ₃ ) is effectively prevented, and the subplating rate of aluminum on the surface of the base material is virtually zero due to the interaction with the chemical film layer coated on the surface of the carbon steel tube base material 10 by water-soluble flux treatment. It is the optimal range that can be made to be made.

In addition, according to the component of the molten flux and the mixing ratio thereof, when the carbon steel tube base material 10 is immersed into the plating bath by securing excellent fluidity of the flux layer, resistance to the base material and contact with external air The resulting aluminum oxide can be minimized, and when the base material is taken out of the plating bath, the amount of flux adhering to the surface of the base material can be minimized.

Therefore, if the melting flux is added in less than 5% by weight based on 100% by weight of the molten aluminum-silicon melt plating solution, the thickness of the flux layer formed on the top of the molten plating solution becomes relatively thin. In the process of immersing or taking out the steel pipe base material 10 in the plating bath, there is a high concern that a contact between the molten plating solution and external air may occur.

On the contrary, if the addition amount of the melting flux exceeds 15% by weight based on 100% by weight of the molten aluminum-silicon melt plating solution, contact with external air and heat dissipation from the molten plating solution can be effectively blocked. However, the amount of molten flux adhering to the plated surface of the carbon steel pipe base material 10 increases somewhat, which adversely affects the plating process.

As described above, the state of the plating bath is set so that a flux layer is formed on the upper surface of the molten aluminum-silicon molten plating solution using a molten flux, and then the base material treated with water-soluble flux is immersed in the plating bath. When the plating operation is performed on the surface of the base material for 20 minutes, it is possible to form a very excellent aluminum alloy layer and an aluminum-silicon plating layer over the surface of the carbon steel tube base material 10.

In other words, while the flux layer provided by the melting flux _{suppresses the generation of various impurities and aluminum oxide (Al 2} O ₃ ), trace amounts of impurities or aluminum oxide are almost always due to the water-soluble flux component coated on the surface of the base material. By this dissolution and removal, the aluminum component is very easily fused to the surface of the base material.

Due to this, it is possible to form a very excellent aluminum alloy layer in the state that there is no non-plated surface or pin hole on the surface of the base material, and the flux layer existing on the upper surface of the molten plating solution is By blocking heat dissipation, it is possible to save fuel and energy by preventing heat loss from the plating bath to the outside.

Due to this, it is possible to impart sufficient corrosion resistance to the surface of the carbon steel tube base material 10 while securing excellent dimensional accuracy that is difficult to achieve by the existing zinc plating method or aluminum plating method, and at the same time, it is imparted by aluminum plating. Since heat resistance, weather resistance, and abrasion resistance expressed by the aluminum alloy layer can also be improved along with this, it is optimal for offshore plants and parts for ships that are susceptible to seawater, and parts such as power generation pipes and steel towers used in harsh corrosive environments. Corrosion-resistant plating products that can be applied can be manufactured.

In addition, compared to coating products using expensive stainless steel materials or corrosion-resistant materials used for the purpose of replacing existing galvanized products, and aluminum powder, which was entirely dependent on import, its price is very low, yet sufficient service life and reusability. By ensuring this, it is possible to provide a more economical and practical corrosion-resistant plating product.

After the plating process corresponding to the main part of the present invention as described above, an oxalic acid treatment process of oxidizing the surface of the plated carbon steel tube base material 10 is performed.

이하로 공랭시키는 공랭단계와, 상기 공랭단계를 거친 탄소강관를 수냉시켜 잔여열을 제거하는 수냉단계를 포함할 수 있다. In detail, the oxalic acid treatment process may be performed after sufficiently cooling the plated carbon steel tube base material 10, but is not limited thereto. For example, 100 carbon steel pipes that have gone through the plating process

Hereinafter, an air cooling step of air cooling and a water cooling step of removing residual heat by water cooling the carbon steel pipe that has been subjected to the air cooling step may be included.

The oxalic acid treatment step is a step of selectively eluting the surface of the molten aluminum-based plating layer 30, after the plating process

이하의 온도조건하에서 5 ~ 20분간 침지시켜 용융 알루미늄계 도금층(30)의 표면에 부착된 용융플럭스 분말을 1차적으로 제거하는 동시에 용융 알루미늄계 도금층(30)의 표면을 선택적으로 용출시키게 된다.The carbon steel pipe on which the composite alloy layer 20 and the molten aluminum-based plating layer 30 are formed is added to an oxalic acid aqueous solution to which 0.5 to 10% by weight of _{oxalic acid (C 2} H ₂ O _{4) is added based on 100% by weight of water.} ~ 50

The molten flux powder adhered to the surface of the molten aluminum-based plating layer 30 is primarily removed by immersion for 5 to 20 minutes under the following temperature conditions, and the surface of the molten aluminum-based plating layer 30 is selectively eluted.

The reason why the addition amount of oxalic acid added to the oxalic acid aqueous solution is limited to 0.5 to 10 wt% is that when the addition amount of oxalic acid is less than 0.5 wt%, the surface elution of the molten aluminum-based plating layer 30 hardly occurs, and the addition amount of oxalic acid is 10 wt% This is because when the amount is exceeded, a phenomenon in which the molten aluminum-based plating layer 30 itself is rapidly melted occurs.

로 한정한 이유는, 옥살산 수용액의 온도가 20

이하일 경우 용융 알루미늄계 도금층(30)의 표면이 선택적으로 용출되는 시간이 많이 소요되고, 옥살산 수용액의 온도가 50

이상일 경우 용융 알루미늄계 도금층(30)의 표면이 선택적으로 용출되는 시간은 단축시킬 수는 있으나, 용융 알루미늄계 도금층(30) 자체가 급격히 녹아나오는 현상이 발생하기 때문이다. In addition, the temperature of the oxalic acid aqueous solution is 20 ~ 50

The reason for limitation is that the temperature of the oxalic acid aqueous solution is 20

If it is less than or equal to, it takes a lot of time for the surface of the molten aluminum-based plating layer 30 to be selectively eluted, and the temperature of the oxalic acid aqueous solution is 50

If this is the case, the time for selectively eluting the surface of the molten aluminum-based plating layer 30 can be shortened, but this is because a phenomenon in which the molten aluminum-based plating layer 30 itself is rapidly melted occurs.

When the surface of the molten aluminum-based plating layer 30 is selectively eluted in the oxalic acid treatment process, the surface of the molten aluminum-based plating layer 30 becomes porous and an aluminum oxide layer formed on the surface of the molten aluminum-based plating layer 30 ( The thickness of 40) can be further expanded.If the thickness of the aluminum oxide layer 40 formed on the surface of the aluminum plating layer is further expanded to about 1 to 10 µm, the hardness, corrosion resistance, and abrasion resistance of the plating surface can be significantly increased. Not only is it improved, but also the reaction with the fluid flowing in the carbon steel pipe can be suppressed. Here, the fluid may be a liquid that corrodes the metal surface, such as seawater.

After passing through the oxalic acid treatment process of the present invention as described above, A post-treatment process including a chemical cleaning step and a water washing step of surface treatment of the carbon steel pipe on which the aluminum oxide layer 40 is formed; is performed.

In the chemical cleaning step, the carbon steel pipe on which the aluminum oxide layer 40 is formed is _{immersed in a 5 to 15% aqueous solution of nitric acid (HNO 3} ) for about 3 to 10 minutes, so that residual flux powder that may remain attached to the plating surface is removed by a separate machine. It is a process step that makes the plating surface of the base material smooth and smooth by using the corrosion action of acid while making it easy and complete removal within a very quick time without processing.

After going through the chemical cleaning step as described above, the carbon steel pipe on which the aluminum oxide layer 40 is formed is immersed in a water washing tank in which water at room temperature is stored, and the plating surface is washed with water. When finally dried, the method for manufacturing a molten aluminum-plated carbon steel pipe having excellent corrosion resistance according to the present invention is completed.

Hereinafter, for a detailed description of the present invention, it will be described in detail with reference to the following examples, but the present invention is not limited to the following examples.

(Example)

A carbon steel pipe having a low carbon steel composition of C: 0.08% by weight, Si: 0.5% by weight, and Mn: 1% by weight was sequentially immersed in a degreasing tank, a pickling tank, and a water washing tank to pretreat the surface.

의 온도조건하에서 약 2분간 침지 후 건조하였다. Thereafter, the carbon steel pipe is immersed in a water-soluble flux bath, but a water-soluble flux consisting of a component ratio of 40% by weight of potassium chloride (KCl), 25% by weight of potassium fluoride (KF) and _{35% by weight of ammonium fluoride (NH 4 F) is 15% by weight.} The carbon steel pipe was added to the flux bath dissolved at a concentration of 80

It was immersed for about 2 minutes under the temperature condition of and then dried.

Thereafter, a melting flux consisting of a component ratio of 30% by weight of cryolite _{, 10% by weight of aluminum fluoride (AlF 3} ), 40% by weight of potassium chloride (KCl) and 20% by weight of sodium chloride (NaCl) is based on 100% by weight of molten aluminum. The carbon steel pipe treated with a water-soluble flux in the molten plating solution added in an amount of 10% by weight was plated under the conditions of the plating bath shown in Table 1 below.

이하가 될 때까지 공랭시키고, 잔여열을 제거하기 위해 수냉시켰다. The carbon steel pipe that has been plated in this way is lifted to 100 in the air.

It was air-cooled until it became the following, and water-cooled in order to remove residual heat.

의 온도조건하에서 10분간 침지시켜 알루미늄 산화층을 형성시켰다. Next, the water-cooled carbon steel pipe was 30 in an oxalic acid aqueous solution to which 5% by weight of _{oxalic acid (C 2} H ₂ O _{4) was added based on 100% by weight of water.}

It was immersed for 10 minutes under the temperature condition of to form an aluminum oxide layer.

Finally, the carbon steel pipe on which the aluminum oxide layer was formed _{was immersed in a 10% aqueous solution of nitric acid (HNO 3} ) for about 5 minutes and then taken out and washed with water and dried to prepare a molten aluminum-plated carbon steel pipe having excellent corrosion resistance according to the present invention. .

Thereafter, physical property evaluation was performed on each of the manufactured hot-dip aluminum-plated carbon steel pipes. For each physical property evaluation, the composition and thickness of the composite alloy layer were measured on a test piece obtained by cutting a hot-dip aluminum-plated carbon steel pipe into a size of 50 X 150 mm, and the results are shown in Table 2 below. The composition of the composite alloy layer was calculated through SEM-EDS point analysis three times, and then calculated through their average value, and the thickness of the composite alloy layer was measured three times in an optical microscope X1000 times field of view, and then calculated through their average value. I did.

In addition, in order to evaluate the plating adhesion of each of the manufactured hot-dip aluminum-plated carbon steel pipes, plating adhesion evaluation was performed on the above test pieces, and the results are shown together in Table 1 below. After the 0T-bending test, the plating adhesion was tested by taping the outer winding of the bend, and the degree of peeling of the plating layer was evaluated based on the following criteria.

◎: No peeling of plating on the tape, no peeling observed on the curved surface

○: No plating peeling operation was observed on the tape, fine peeling was observed on the curved surface

△: No plating peeling operation was observed on the tape, partly peeling was observed on the curved surface

Х: Plating peeling operation is stained on the tape, observed with the naked eye

In addition, corrosion resistance is the time that has elapsed until the area of red rust generated on the surface of the hot-dip aluminum-plated carbon steel pipe becomes 5% after performing the corrosion promotion test with the salt spray test (salt spray standard test in accordance with KS-C-0223). It was measured.

◎: When it exceeds 1000 hours.

○: When it exceeds 500 hours.

△: In the case of 200 to 500 hours.

Х: For less than 200 hours.

Remark Melt plating solution conditions Composite alloy layer composition complex
Alloy layer
thickness
(㎛) Plated
Adhesion Corrosion resistance Si content
(weight%) Temperature
(℃) LMP Al Si Mn O Comparative Example 1 4 670 18576 5 0.53 1.05 50 X △ Invention Example 1 4 680 18773 10 0.55 0.99 105 ◎ ○ Inventive Example 2 5 700 19167 20 0.45 0.91 120 ◎ ◎ Inventive Example 3 4 730 19535 30 0.41 0.92 150 ◎ ◎ Invention Example 4 5 750 19925 40 0.39 0.88 180 ◎ ◎ Comparative Example 2 5 750 20152 45 0.35 0.85 0.45 250 X X

However, in Table 1, LMP is calculated by the above-described relational equation 1.

As shown in Table 1, the molten aluminum-plated carbon steel pipe having excellent corrosion resistance according to the present invention has a composition of 10 to 40% by weight of Al while diffusion and penetration of Al into the surface of the base material of the carbon steel pipe, and a composite having a thickness of 105 to 180 µm. By including the alloy layer, it was confirmed that there was no occurrence of non-plating, no peeling of the plated layer during processing, and a hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance even under an extreme corrosive environment was manufactured.

Although a preferred embodiment of the present invention has been described above, it is clear that various changes, modifications, and equivalents can be used in the present invention, and the same can be applied by appropriately modifying the above embodiment. Therefore, the above description does not define the scope of the present invention determined by the limits of the following claims.

10: carbon steel pipe base material
20: composite alloy layer
30: molten aluminum-based plating layer
40: aluminum oxide layer

Claims (6)

A carbon steel tube base material made to allow fluid to flow therein, and a molten aluminum-based plating layer formed on the surface of the carbon steel tube base material and including, by weight %, Si: 3 to 15%, the balance Al and inevitable impurities, and the molten aluminum Including an aluminum oxide layer formed on the system plating layer,
It includes a Fe-Al alloy phase formed by diffusion and penetration of Al from the interface between the carbon steel tube base material and the molten aluminum plating layer into the surface of the carbon steel tube base material, and includes a composite alloy layer having a thickness of 100 to 200 μm, ,
The composite alloy layer is a molten aluminum-plated carbon steel pipe having excellent corrosion resistance, including by weight %, Al: 10 to 40%, Si: 0.05 to 1%, Mn: 0.1 to 1.5%, the balance Fe and inevitable impurities.
The method of claim 1,
The molten aluminum-based plating layer is a hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance further including, by weight %, Mg: 0.1-10%.
The method of claim 1,
The molten aluminum-based plating layer is in wt%, Ca: 0.001 to 5%, Sr: 0.005 to 2%, Mn: 0.01 to 2%, Cr: 0.01 to 2%, Mo: 0.01 to 2%, and Sn: 0.1 to 10 Hot-dip aluminum-plated carbon steel pipe with excellent corrosion resistance further comprising at least one selected from the group consisting of %.
The method of claim 1,
The hot-dip aluminum-plated carbon steel pipe having excellent corrosion resistance,
A pretreatment process including a degreasing step and a washing step for the surface of the carbon steel pipe base material, a pickling step for activating the surface of the carbon steel pipe base material, and a washing step; And,
After performing a water-soluble flux treatment on the surface of the carbon steel tube base material that has undergone the pre-treatment process, the carbon steel tube base material is immersed in a plating solution to which a melting flux is added to melt the surface of the carbon steel tube base material and melt aluminum at the same time. Plating step of forming the composite alloy layer and the molten aluminum-based plating layer by plating; And,
An oxalic acid treatment step of forming an aluminum oxide layer on the molten aluminum-based plating layer by immersing the carbon steel pipe on which the composite alloy layer and the molten aluminum-based plating layer are formed in an aqueous oxalic acid solution; and,
A hot-dip aluminum-plated carbon steel pipe with excellent corrosion resistance formed through a post-treatment process including a chemical cleaning step and a water washing step of surface treatment of a carbon steel pipe having an aluminum oxide layer formed thereon.
The method of claim 4,
The water-soluble flux treatment is
Excellent corrosion resistance by immersing the pretreated carbon steel pipe in a water-soluble flux containing 30 to 60% by weight of potassium chloride (KCl), 20 to 40% by weight of potassium fluoride (KF) and 15 to 40% by weight of _{ammonium fluoride (NH 4 F).} Hot-dip aluminum-plated carbon steel pipe.
The method of claim 4,
The melt flux treatment is
The carbon steel pipe treated with a water-soluble flux was treated with cryolite 10 to 30% by weight, aluminum fluoride (AlF ₃ ) 10 to 40% by weight, potassium chloride (KCl) 35 to 65% by weight, and sodium chloride (NaCl) 10 to 25% by weight. Hot-dip aluminum-plated carbon steel pipe with excellent corrosion resistance that is deposited in the containing melt flux.

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