Chromium-Aluminum Binary Alloy Having Excellent Corrosion Resistance and Method of Manufacturing Thereof

Abstract

The present disclosure relates to a chromium-aluminum binary alloy with excellent corrosion resistance and a method of producing the same, and more particularly to a chromium-aluminum binary alloy with excellent corrosion resistance. The chromium-aluminum binary alloy may be easily produced and has ductility, thus being highly applicable as a coating material for a material requiring high-temperature corrosion resistance and wear resistance.

Claims

1. A method of producing a chromium-aluminum binary alloy with corrosion resistance, the method comprising: (Step 1) mixing and melting a raw material comprising: 6 to 30% by weight of aluminum (Al), the balance of chromium (Cr), and other unavoidable impurities with respect to a total weight of the alloy; and (Step 2) solution treating the alloy melted in Step 1.

2. The method of claim 1, wherein the melting of Step 1 is performed at a temperature of 1400° C. to 1800° C.

3. The method of claim 1, wherein the solution treating of Step 2 is performed at a temperature of 950° C. to 1200 ° C.

4. The method according to claim 1, wherein the chromium-aluminum binary alloy has a hardness of 250-450 Hv.

5. The method of claim 1, wherein the raw material comprises 6 to 25 by weight of aluminum.

6. The method of claim 1, wherein the raw material comprises 6 to 20 by weight of aluminum.

7. The method of claim 1, wherein the raw material comprises 10 to 20 by weight of aluminum.

8. The method according to claim 1, wherein the chromium-aluminum binary alloy is used for a material for components of a nuclear plant, or structural material used in thermal power plant, air craft engine or a gas turbine.

9. The method according to claim 1, wherein the chromium-aluminum binary alloy is used for a surface coating material for a metal material.

10. The method of claim 1, wherein the solution treating of Step 2 is performed at a temperature of 1000° C. to 1200° C.

11. The method of claim 1, wherein the solution treating of Step 2 is performed at a temperature of 1050° C. to 1200° C.

Description

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The present invention provides a chromium-aluminum binary alloy with excellent corrosion resistance, the chromium-aluminum binary alloy including 1 to 40% by weight of aluminum, the balance of chromium, and other unavoidable impurities with respect to a total weight of the alloy.

[0038] The present invention also provides a chromium-aluminum binary alloy with excellent corrosion resistance, the chromium-aluminum binary alloy including 6 to 30% by weight of aluminum, 6 to 25% by weight of aluminum, 6 to 20% by weight of aluminum or 10 to 20% by weight of aluminum, the balance of chromium, and other unavoidable impurities with respect to a total weight of the alloy.

[0039] The present invention also provides a chromium-aluminum binary alloy with excellent corrosion resistance, the chromium-aluminum binary alloy including 10 to 25% by weight of aluminum, 10 to 20% by weight of aluminum or 10 to 18% by weight of aluminum, the balance of chromium, and other unavoidable impurities with respect to a total weight of the alloy.

[0040] In this invention, the high temperature is not limited as long as it is in the range of 280 360° C., which is the operating condition of the nuclear power plant or at least 950° C., 1000° C., 1100° C., and 1200° C., which are the temperatures in the case of accident of the nuclear power plant. However, it is preferably up to 2000° C. or 1800° C.

[0041] Hereinafter, the chromium-aluminum binary alloy with excellent corrosion resistance according to the present invention will be described in more detail.

[0042] Conventionally, in order to improve safety of a nuclear power plant, a SiC/SiC.sub.f material, a FeCrAl alloy, a Zr—Mo-coated cladding tube, a Zr-coated cladding tube and the like have been developed as advanced materials, but have drawbacks as described above. The SiC/SiC.sub.f material is fast melted in a steady-state but its production cost is high and it is highly reactive to the nuclear fuel pellet at a high temperature of at least 1400° C. The FeCrAl alloy has a melting point of 1500° C. or less, lacks high temperature stability, has a high neutron absorption cross-sectional area; and has a low tritium capturing capacity, indicating the alloy has not economic feasibility. The Zr—Mo-coated cladding tube asks high production costs for the three layered structure and is not economically feasible due to the high neutron absorption cross section of Mo. The Zr—FeCrAl cladding tube has problems in which a composition of the coating material is changed by interdiffusion of Zr and Fe at a temperature of 950° C. or higher and a base material of the Zr cladding tube form a Zr—Fe-based intermetallic compound to be weakened. The Zr-pure Cr coated cladding tube is weak to an impact due to low ductility of the Cr layer and has a relatively low high-temperature oxidation resistance compared to the FeCrAl alloy. Also, when pure Cr is coated on Zr, there is a problem that the coating layer peels off due to pitting corrosion of the Cr layer in the normal corrosion environment. Accordingly, combinations of materials and coating technologies reported so far have a difficulty in realizing both safety and economic feasibility in a steady-state and an accident-state of nuclear power.

[0043] However, the present invention provides a chromium-aluminum binary alloy in which a content of aluminum is 1 to 40% by weight with respect to a total weight of the alloy.

[0044] Cr forms a stable oxide of Cr.sub.2O.sub.3 by an oxidation reaction and Al forms a stable oxide Al.sub.2O.sub.3 by an oxidation reaction, thus increasing corrosion resistance of the Cr—Al binary alloy. In addition, a proper combination of Cr—Al composition suppresses pitting corrosion caused by pure Cr, and can achieve excellent performance in normal environment corrosion of a reactor. When applied to nuclear power, the chromium-aluminum binary alloy has excellent corrosion resistance in an accident-state as well as a steady-state operation, thus providing effects of being able to significantly increase economic feasibility and accident safety of nuclear power.

[0045] If the concentration of aluminum included in the binary alloy is less than or more than the range limit mentioned above, an Al.sub.8Cr.sub.5 intermetallic compound is produced so that corrosion resistance would be reduced and corrosion resistance would be comparatively poor. If the concentration of aluminum is more than the upper limit of the above range, the Al.sub.8Cr.sub.5 intermetallic compound production would be a problem. Due to the characteristics of the intermetallic compound, the brittleness is very strong, which results in lack of processability and difficulty in controlling the composition. In addition, since a melting point decreases as an added amount of aluminum increases, there is a problem in that it becomes impossible to use the binary alloy at high temperatures, such as a nuclear power plant accident environment.

[0046] The aluminum is preferably included in an amount of 6% to 30% by weight, 6% to 25% by weight, 6% to 20% by weight, 10% to 20% by weight, or 10% to 18% by weight.

[0047] Within the designated range of Al content, if the concentration of less than the lower limit of the range, dissolution would be observed during corrosion reaction in a steady-state, indicating that the material is hard to be applied as a nuclear power plant material. In the meantime, if the concentration is over the upper limit of the range above, Al is phase-decomposed in Cr—Al during long-time exposure under the normal operating condition of a nuclear power plant, so that the microstructure is formed in a region having a high Al content and a low region, resulting in the reduced corrosion resistance.

[0048] In another aspect, the aluminum is preferably included in an amount of 10% to 25% by weight, 10% to 20% by weight, or 10% to 18% by weight.

[0049] If the concentration of Al is out of the designated range above, the corrosion resistance is relatively poor under the high temperature operating condition of the nuclear power plant, for example, at the high temperature condition of 1200° C.

[0050] The present invention provides a method of producing a chromium-aluminum binary alloy with excellent corrosion resistance, the method including: mixing and melting raw materials including aluminum (Al), the balance of chromium (Cr), and other unavoidable impurities with respect to a total weight of the alloy (Step 1); and solution treating the alloy melted during Step 1 (Step 2).

[0051] The content of aluminum is put into the range of weight % within the predetermined range.

[0052] Hereinafter, a method of producing a chromium-aluminum binary alloy with excellent corrosion resistance according to the present invention will be described for each step in more detail.

[0053] In the method of producing a chromium-aluminum binary alloy with excellent corrosion resistance according to the present invention, Step 1 is a step of mixing and melting raw materials including a proper weight % of aluminum (Al), the balance of chromium (Cr), and other unavoidable impurities with respect to a total weight of the alloy.

[0054] In Step 1, the raw materials are mixed and melted in a molten metal bath to produce an alloy in which the raw materials are homogeneously mixed.

[0055] Conventionally, in order to improve safety of a nuclear power plant, a SiC/SiC.sub.f material, a FeCrAl alloy, a Zr—Mo-coated cladding tube, a Zr-coated cladding tube and the like have been developed as advanced materials, but have drawbacks as described above. Accordingly, combinations of materials and coating technologies reported so far have a difficulty in realizing both safety and economic feasibility in a steady-state and an accident-state of nuclear power.

[0056] However, the present invention provides a chromium-aluminum binary alloy in which an amount of aluminum is 1% to 40% by weight.

[0057] Compared to oxide-based (SiO.sub.2, Cr.sub.2O.sub.3, Al.sub.2O.sub.3, ZrO.sub.2), carbide-based (Cr.sub.3C.sub.2, SiC, ZrC), nitride-based (ZrN) intermetallic compounds, and a MAX phase (C or N-added compound), the chromium-aluminum binary alloy is easy to produce. Also, the ductility of the chromium-aluminum binary alloy not only makes it easy to produce a product but also improves applicability as a coating material. In addition, the chromium-aluminum binary alloy has excellent corrosion resistance to significantly reduce a hydrogen explosion phenomenon caused by an excessive oxidation reaction when used as a component and a coating material of a nuclear power plant.

[0058] In another aspect, the aluminum is preferably included in an amount of 10% to 25% by weight, 10% to 20% by weight, or 10% to 18% by weight.

[0059] In another aspect, the aluminum is preferably included in an amount of 15% to 20% by weight.

[0060] The corrosion resistance is relatively excellent within the predetermined range of the Al content at a high temperature condition of a nuclear power plant, for example, at the high temperature condition of 1200° C.

[0061] Meanwhile, the melting in Step 1 may be performed at a temperature of 1400° C. to 1800° C. When the melting of Step 1 is performed less than 1400° C., there may be a problem in which a liquid molten state is not maintained and thus an alloy is not properly formed, and when the melting of Step 1 is performed more than 1800° C., there may be caused problems in which reactivity of molten metal is increased to include a large amount of impurities, and Al having a low melting point is evaporated to have a difficulty in controlling the composition, and costs increase.

[0062] In the method of producing a chromium-aluminum binary alloy with excellent corrosion resistance according to the present invention, Step 2 is a step of solution treating the alloy melted during Step 1.

[0063] In Step 2, the alloy melted in Step 1 is heated up to a range in which the melted alloy becomes a solid solution, and is quenched to maintain the solid solution state, and through this step, the alloy elements may readily form the solid solution.

[0064] The solution treating of Step 2 may be performed at a temperature of 950° C. to 1200 ° C., 1000° C. to 1200 ° C., or 1050° C. to 1200° C. When the temperature is lower than 950° C. in the solution treating of Step 2, there is a problem in which the precipitate AlCr.sub.2 is not completely melted and thus a desired property is not obtained, and when the temperature is higher than 1200° C., a production cost is increased so that the solution treating of Step 2 is economically infeasible.

[0065] The present invention provides a chromium-aluminum binary alloy with excellent corrosion resistance, which is produced according to the above-described method.

[0066] The present invention relates to a chromium-aluminum binary alloy, the binary alloy being able to have excellent mechanical property and corrosion resistance at room temperature as well as at high temperatures. In particular, the chromium-aluminum binary alloy may have hardness of 250 to 450 Hv, and superior high-temperature oxidation resistance to a zircaloy-4 alloy, pure chromium, and a FeCrAl alloy.

[0067] The present invention provides a high-temperature environment structural material including the chromium-aluminum binary alloy with excellent corrosion resistance.

[0068] Since the chromium-aluminum binary alloy according to the present invention has excellent corrosion resistance even at high temperature as well as at room temperatures, the chromium-aluminum binary alloy may be not only used as a material for components of a nuclear power plant but also be applied to a structural material used in a high temperature environment, such as thermal power generation and an aircraft engine, and a gas turbine.

[0069] The present invention provides a surface coating material including the chromium-aluminum binary alloy with excellent corrosion resistance.

[0070] According to the present invention, since the chromium-aluminum binary alloy has superior corrosion resistance, is easy to produce, and has ductility, the chromium-aluminum binary alloy may be applied as a coating material. In addition, when the alloy of the present invention is used as a coating material, the second phase must not be formed even after being coated.

[0071] The chromium-aluminum binary alloy may be utilized as a zirconium coating material used in a nuclear power plant, and as a coating material of a metal structural material used at a high temperature in addition to the nuclear power plant.

[0072] In the case, the metal material may be stainless steel or inconel and has advantages of reducing a cost and a term for technology development compared to an advanced anti-oxidation material, by coating the alloy of the present invention on such a metal material.

[0073] Hereinafter, the present invention will be described below in detail with reference to the following examples. However, the following examples are provided for illustrative purposes only, and the scope of the present invention should not be limited thereto in any manner.

<Example 1>Production of a Cr—1Al Alloy

[0074] Step 1: a melting temperature was set to 1600° C., and through a vacuum arc melting, an alloy having a composition including 1% by weight of aluminum, the balance of chromium and other unavoidable impurities was produced.

[0075] Step 2: the alloy undergone Step 1 was solution treated at 1100° C. for 20 minutes to produce a chromium-aluminum binary alloy.

<Example 2>Production of a Cr-2Al Alloy

[0076] In step 1 of Example 1, except that the amount of aluminum was changed to 2% by weight, a chromium-aluminum binary alloy was produced by performing the same procedure as Example 1.

<Example 3>Production of a Cr-4Al Alloy

[0077] In step 1 of Example 1, except that the amount of aluminum was changed to 4% by weight, a chromium-aluminum binary alloy was produced by performing the same procedure as Example 1.

<Example 4>Production of a Cr-6Al Alloy

[0078] In step 1 of Example 1, except that the amount of aluminum was changed to 6% by weight, a chromium-aluminum binary alloy was produced by performing the same procedure as Example 1.

<Example 5>Production of a Cr-10Al Alloy

[0079] In step 1 of Example 1, except that the amount of aluminum was changed to 10% by weight, a chromium-aluminum binary alloy was produced by performing the same procedure as Example 1.

<Example 6>Production of a Cr-15Al Alloy

[0080] In step 1 of Example 1, except that the amount of aluminum was changed to 15% by weight, a chromium-aluminum binary alloy was produced by performing the same procedure as Example 1.

<Example 7>Production of a Cr-20Al alloy

[0081] In step 1 of Example 1, except that the amount of aluminum was changed to 20% by weight, a chromium-aluminum binary alloy was produced by performing the same procedure as Example 1.

<Example 8>Production of a Cr-25Al alloy

[0082] In step 1 of Example 1, except that the amount of aluminum was changed to 25% by weight, a chromium-aluminum binary alloy was produced by performing the same procedure as Example 1.

<Example 9>Production of a Cr-30Al alloy

[0083] In step 1 of Example 1, except that the amount of aluminum was changed to 30% by weight, a chromium-aluminum binary alloy was produced by performing the same procedure as Example 1.

<Comparative Example 1>Pure chromium

[0084] A commercial high purity chromium for a coating raw material (purity of 99.9% or more) was prepared as Comparative Example 1.

<Comparative Example 2>FeCrAl

[0085] A commercial FeCrAl alloy (product name: Kantal APMT) was prepared as Comparative Example 2. The alloy is composed of 21 wt % Cr, 5 wt % Al, 3 wt % Mo, and the balance of Fe.

<Comparative Example 3>zircaloy-4

[0086] A commercial zircaloy-4 (product name: zircaloy-4) was prepared as Comparative Example 3. The alloy is composed of 1.5 wt % Sn, 0.2 wt % Fe, 0.1 wt % Cr, and the balance of Zr. In the figure, it is indicated as Z4.

<Experimental Example 1>Hardness Measurement

[0087] To investigate a mechanical property of the chromium-aluminum alloys produced in Examples 2, 3, 4, 6, and 9 and metal materials of Comparative Examples 1 to 3, hardness was measured in a condition of maintaining a load of 98 mN for 10 seconds at room temperature by a micro Vickers hardness tester and the result is shown in FIG. 1. At this time, the hardness value was measured 10 times for each sample and an average was taken.

[0088] As shown in FIG. 1, it can be seen that Example has a high hardness of about 260 to 410 Hv. On the other hand, the pure chromium in Comparative Example 1 had hardness of about 290 Hv, and the FeCrAl-alloy in Comparative Example 2 had hardness of about 260 Hv, and the zircaloy-4 in Comparative Example 3 had hardness of about 240 Hv, but it can be seen that hardness of these Comparative Examples does not exceed about 300 Hv.

[0089] As a result of observing indentation after the hardness measurement, since a hardness value was high, but a crack around the indentation was not observed in the alloys of Examples of the present invention, it was confirmed that there is no brittleness appearing in an oxide material and an intermetallic compound.

[0090] From these results, it can be seen that the hardness of the chromium-aluminum binary alloys according to the present invention is excellent compared to that of the metal materials of Comparative Examples. In addition, since the alloys of Examples of the present invention have higher hardness than zircaloy-4, the alloys of Examples of the present invention will have high wear resistance compared to zircaloy-4 when applied to a cladding tube.

<Experimental Example 2>Measurement of Oxidation Resistance Under Normal Operating Conditions

[0091] To investigate high-temperature oxidation resistance of the chromium-aluminum alloys produced in Examples 1 to 9 and metal materials of Comparative Examples 1 to 3, the samples were prepared in 50 mm length, which were dipped in the solution composed of water:nitric acid:hydrofluoric acid(HF) at the ratio of 50:40:10 to eliminate impurities on the surface and any minute defect on the surface. The surface area and the primary weight of the surface-treated samples were measured before autoclaving. Then, the degree of corrosion was evaluated quantitatively by calculating the weight increase relative to the surface area by measuring the weight increase of the sample after corrosion for 120 days in 360° C. coolant. The results of the corrosion test are shown in FIG. 2.

[0092] As shown in FIG. 2, when the amount of Al was included 6 to 30% by weight, particularly 6 to 20% by weight, corrosion resistance was excellent.

[0093] FIG. 3 shows the result of the experiment which was performed by the same manner as the above except that the corrosion was induced for 300 days. The result was consistent with the result shown in FIG. 2. The most important point in the normal corrosion in a nuclear power plant is that it must show the weight increasing behavior during the corrosion reaction. The decrease of the weight during the corrosion test indicates that corrosion product (oxide) could be separated off from the cladding tube or melted so that the concentration of impurities in the coolant is increased and at the same time the radiation contained in the impurities could contaminate the inside of the power plant.

[0094] FIG. 6 shows the result of the observation of oxide film microstructure in the sample. As shown in FIG. 6, the sample was corroded for 240 days, followed by observation of the formed oxide film. The oxide was formed with the composition of CrAl.sub.2O.sub.3 and in the thickness of 300 nm, however it was not separated in two phases of Cr.sub.2O.sub.3 and Al.sub.2O.sub.3.

[0095] FIG. 7 shows the result of the investigation of phase changes of mother materials after 240 days of corrosion of the sample of Example 7(Cr-20Al) and the sample of Example 9 (Cr-30Al).

[0096] As shown in Example 7, the composition was all the same in almost every point of observation, indicating that the phase change was not occurred.

[0097] However, as shown in Example 9, the intermetallic compound such as AlCr.sub.2 was generated, and accordingly phase change was observed, precisely two different phases such as high Al content region (40 wt %) and low Al concentration region (25 w %) were observed. This phase change above seemed to result in the decrease of corrosion resistance.

<Experimental Example 3>High-Temperature Oxidation Resistance Measurement

[0098] To investigate high-temperature oxidation resistance of the chromium-aluminum alloys produced in Examples 1 to 9 and metal materials of Comparative Examples 1 to 3, a temperature was raised to 1200° C. at a heating rate of 50° C./min and was maintained for 7200 seconds, and air-cooled to perform an experiment on high temperature steam oxidation with a thermogravimetric analyzer (TGA-51-SHIMADZU) shown in FIG. 4, and the result is shown in FIG. 5. In addition, after the experiment on the high temperature steam oxidation, cross-sections of Examples 1 to 9 were observed by a scanning electron microscope and the results are shown in FIG. 6.

[0099] As shown in FIG. 5, it can be seen that an oxidation amount of Examples 1 to 9 is significantly less than that of the zirconium alloy of Comparative Example 3.

[0100] According to the present invention, a chromium-aluminum binary alloy is easy to produce, has a high melting point of 1600° C. or more, and has ductility, thus being highly applicable to a material requiring high-temperature corrosion resistance and wear resistance, as a coating material. In addition, the chromium-aluminum binary alloy has excellent corrosion resistance in an accident-state of nuclear power as well as a steady-state operation, thus providing effects capable of significantly increasing economic feasibility and accident safety of nuclear power.

[0101] The chromium-aluminum binary alloy according to the present invention has high applicability as a coating material of a material which is easy to manufacture, has a high melting point of 1600° C. or more, has ductility, and is required to have high temperature corrosion resistance and abrasion resistance.

[0102] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.