Method for producing a zinc-metal oxide layer on metal components for corrosion resistance - Patent Application 20070122999

Abstract

To provide a method for producing a zinc-metal oxide layer on a metal component for corrosion resistance. The present invention provides a method for manufacturing a finished metal object or product having a corrosion-resistant layer integral with or within a top portion of at least one of its surfaces that may be exposed to a corrosive environment. In one embodiment, the manufacturing method is directed to a finished metal tubing product having a corrosion-resistant layer within its inner surface that is exposed to a fluid, the corrosion-resistant layer being a zinc-metal oxide layer, such as a zinc-chromium oxide layer or a zinc mixed metal oxide layer. In addition to the manufacturing method, the present invention provides a finished metal object or product having a corrosion-resistant layer integral with or within a top portion of at least one of its surfaces that may be exposed to a corrosive environment.

Description

[Technical Field]

[0001] Background of the Invention FIELD OF THE INVENTION In various embodiments, the present invention provides methods for producing a corrosion-resistant layer integral with the uppermost portion of a metal surface of a metal object or product to reduce corrosion of the metal surface and release of metal species. In particular, the present invention provides methods for producing a finished metal object or product having a stabilized zinc-metal oxide or zinc mixed-metal oxide layer integral with an exposed surface, such as the interior surface of a metal tube, to reduce corrosion of the metal and release of metal species from the metal. In various embodiments, the present invention also provides a finished metal object or product having an integral corrosion-resistant layer. [Background technology]

[0002] 2. Description of Related Art In the power generation industry, metal corrosion in water-cooled systems is a major reliability factor. Aqueous corrosion is driven by metal release from metal surfaces, which is controlled by the solubility of metal species under given conditions. Any metal, steel, or alloy is subject to such corrosion. The released material often deposits elsewhere in the system, such as in low-flow areas or on heat transfer surfaces, causing fouling and efficiency loss. Particularly in the nuclear industry, metal release is a source of radiation fields outside the reactor vessel.

[0003] Many techniques have been developed to reduce metal corrosion and metal release by protecting metal surfaces from corrosion. For example, in nuclear applications, in situ treatments are used to "precondition" metal surfaces during high-temperature functional testing (HFT). In modern pressurized water reactors (PWRs), steam generator tubing is treated with a given chemistry for a given period of time to precondition the interior surfaces of the tubing. In boiling water reactors (BWRs), feedwater tubing and other metal surfaces (e.g., feedwater heaters) that transport or may be exposed to reactor coolant during normal operation can also be preconditioned. The expectation of such "preconditioning" is to provide a stable corrosion film on the metal surface that limits subsequent corrosion and metal release during operation, thereby reducing the incorporation of radioactive species into the reactor water during plant operation. Unfortunately, such preconditioning does not provide a stable, long-lasting corrosion-resistant film that avoids metal release during normal operation. Furthermore, plants have only a limited amount of time available for such preconditioning, or otherwise cannot devote the extensive exposure times that may be necessary to establish stable films on most metal surfaces. Summary of the Invention [Means for solving the problem]

[0004] Thus, there is a need for more stable corrosion-resistant layers for exposed metal surfaces, particularly those used in PWR or BWR environments. In particular, there is a need for manufacturing methods for creating metal products having more stable corrosion-resistant layers integral with the uppermost portion of the exposed surface of the metal product, such as a zinc-metal oxide layer that is resistant to corrosion and corresponding metal release.

[0005] Brief summary of the invention The present invention provides a method for producing a finished metal object or product having a corrosion resistant layer in at least one uppermost portion of its surface, which when in use may be exposed to at least a potentially corrosive environment or a corrosive environment. Thus, the corrosion resistant oxide layer may be, for example, in Creating a protective layer after fabrication by creating a protective layer in situ (i.e., It should be understood that the corrosion-resistant layer is formed during the manufacturing process used to create the finished metal object or product, as opposed to a finished metal object or product that is fully manufactured, ready for its intended use, and deployed for such use, after which, but prior to use, the protective layer is created. In one embodiment, the corrosion-resistant layer is a zinc-metal oxide layer having zinc incorporated therein that combines with metals in the metal object or product to form the zinc-metal oxide layer. In one embodiment, the zinc-metal oxide layer is a zinc-chromium oxide layer. In another embodiment, the corrosion-resistant layer is a zinc-mixed metal oxide layer having zinc incorporated therein that combines with more than one metal in the metal object or product to form the zinc-mixed metal oxide layer. It should be understood that the zinc-metal oxide layer and zinc-mixed metal oxide layer are more stable than similar layers that are manufactured in situ.

[0006] In one embodiment, the present invention provides a method for making a finished metal product having a corrosion-resistant oxide layer, the method comprising the steps of: incorporating zinc into at least one surface of a semi-finished metal product that may be exposed during use of a finished metal product produced from the semi-finished metal product, the semi-finished metal product comprising a metal; forming the semi-finished metal product into a finished metal product of a predetermined shape after the incorporation of zinc; and heat-treating the semi-finished metal product in a controlled environment after formation to form a zinc-metal oxide layer within a top portion of the at least one surface. In one embodiment, the semi-finished metal product comprises a metal that is chromium, and the zinc-metal oxide formed is a zinc-chromium oxide layer. In one embodiment, the controlled environment comprises the presence of hydrogen and oxygen.

[0007] In another embodiment, the present invention provides a method for making a metal tube having a corrosion-resistant oxide layer within a portion of the exposed interior surface of the metal tube, the method comprising: incorporating zinc into at least one surface of a semi-finished metal product comprising a metal; forming a metal tube from the semi-finished metal product, the at least one surface being the interior surface of the metal tube; and heat-treating the metal tube after formation in a controlled environment to form a zinc-metal oxide layer within a top portion of the interior surface of the metal tube. In an additional embodiment, after heat treatment, the metal tube is further processed by cold pilgering or using a cold drawing process. In another embodiment, zinc is incorporated into an already formed metal tube, followed by heat treatment, and optionally further processed after heat treatment by cold pilgering or using a cold drawing process. In one embodiment, the semi-finished metal product comprises a metal that is chromium, and the zinc-metal oxide formed is a zinc-chromium oxide layer. In one embodiment, the controlled environment comprises the presence of hydrogen and oxygen.

[0008] In another embodiment, the present invention provides a metal product having a corrosion-resistant surface, including a finished metal product ready for use without further processing of the metal, having a corrosion-resistant layer within at least one surface of the finished metal product, the corrosion-resistant layer being produced by the method described herein. For example, in one embodiment, the process includes incorporating zinc into at least one surface of a semi-finished metal product that may be exposed during use of a finished metal product produced from the semi-finished metal product, the semi-finished metal product comprising the metal; forming the semi-finished metal product into a finished metal product of a predetermined shape after the incorporation of zinc; and heat-treating the semi-finished metal product in a controlled environment after formation to form a zinc-metal oxide layer within a top portion of the at least one surface. In one embodiment, the semi-finished metal product comprises a metal that is chromium, and the zinc-metal oxide formed is a zinc-chromium oxide layer. In one embodiment, the controlled environment includes the presence of hydrogen and oxygen. In an additional embodiment, the metal product includes a tube, the corrosion-resistant layer being within the interior surface of the tube, the tube and the corrosion-resistant layer being created by the method described herein.

[0009] The present invention is more stable and can be applied in situ after the fabrication of the finished metal object or product is complete. It should be appreciated that the corrosion-resistant layer formed during the manufacturing process of a given finished metal object or product (e.g., a finished metal tube that has been joined in a process in which it may be used) has a longer-term viability than the corrosion-resistant layer formed during the manufacturing process of the given finished metal object or product, which is ready for service or use, for example, by being introduced and joined for service. Thus, the present invention provides this more stable corrosion-resistant layer by incorporating zinc into the top or upper portion of a given surface of a semi-finished metal object or product (i.e., before the final component manufacturing step) to ensure that the subsequent final heat treatment used in the manufacturing process produces a zinc-metal oxide or zinc mixed metal oxide layer and a stable surface structure prior to use. As a result, corrosion of the underlying metal can be reduced, which can significantly increase asset reliability and plant availability and efficiency. This also reduces the impact of any metal release that may otherwise result from such corrosion. In nuclear applications, where the finished metal object or product may include tubes used as steam generator tubing associated with a pressurized water reactor (PWR), or feedwater tubing or other metal surfaces (e.g., feedwater heaters) that may be exposed to reactor coolant from a boiling water reactor (BWR), reducing the release of metal into the fluid can reduce radiation fields that may otherwise be generated elsewhere in these systems.

[0010] It should be appreciated, therefore, that in some embodiments, the results of the present invention provide component surfaces that do not require any lengthy preconditioning period to ensure minimal corrosion and metal release, as is done in situ. The present invention has broad applicability to the manufacture of coolant circulation components for power plants made of any chromium-containing steel or alloy, including chromium-free alloys, particularly for steam generators, heat exchangers, and moisture separators. However, the present invention can also be used for piping and other related components. While the emphasis is on new component manufacturing, these principles can be applied to chemically and / or physically cleaned component surfaces already in use, with the goal of extending their life and reducing metal release upon placement back in service.

[0011] The benefits of installing components manufactured using this process include reduced corrosion resulting in improved component performance, reliability and lifespan; improved plant hygiene, less fouling, less activation, less metal release resulting in improved core performance and fuel reliability in nuclear plants; and less metal release, particularly of nickel from high nickel alloys and cobalt from steel or any other alloy, which significantly reduces the out-of-core radiation field in nuclear plants, thereby reducing the accumulated radiation exposure (CRE) to plant personnel. In an embodiment of the present invention, for example, the following items are provided: (Item 1) 1. A method for making a metal object having a corrosion resistant layer, comprising: incorporating zinc into at least a portion of at least one metal surface of the metal object that may be exposed during use of the metal object; forming a corrosion resistant layer within said portion of said at least one metal surface; A method comprising: (Item 2) Item 10. The method of claim 1, wherein the corrosion resistant layer comprises a zinc-metal oxide layer. (Item 3) Item 10. The method of claim 1, wherein the metal surface comprises chromium and the zinc-metal oxide layer comprises a zinc-chromium oxide layer. (Item 4) Item 10. The method of claim 1, wherein the metal surface comprises an alloy containing chromium. (Item 5) Item 10. The method of claim 1, wherein the metal surface comprises nickel-iron-chromium containing steel. (Item 6) Item 10. The method of claim 1, further comprising incorporating a second metal into the portion of the at least one metal surface. (Item 7) 7. The method of claim 6, wherein the second metal comprises chromium. (Item 8) 1. A method for making a metal object having a corrosion resistant layer, comprising: incorporating zinc into at least a portion of at least one metal surface of the metal object that may be exposed during use of the metal object; forming a corrosion resistant layer comprising a zinc mixed metal oxide within said portion of said at least one metal surface; A method comprising: (Item 9) 9. The method of claim 8, wherein the metal surface comprises an alloy containing chromium. (Item 10) 9. The method of claim 8, wherein the metal surface comprises nickel-iron-chromium containing steel. (Item 11) 9. The method of claim 8, further comprising incorporating a second metal into the portion of the at least one metal surface. (Item 12) Item 12. The method of item 11, wherein the second metal comprises chromium. (Item 13) 1. A method for making a finished metal product having a corrosion resistant layer, comprising: incorporating zinc into at least one metal surface of the semi-finished metal product that may be exposed during use of a finished metal product produced from the semi-finished metal product; forming the semi-finished metal product into a finished metal product of a predetermined shape after said incorporating; heat treating said semi-finished metal product after said forming to form a zinc-metal oxide layer within a top portion of said at least one surface; A method comprising: (Item 14) Item 13. The method of item 12, wherein the metal surface comprises chromium and the zinc-metal oxide comprises zinc-chromium oxide. (Item 15) 13. The method of claim 12, wherein the zinc-metal oxide comprises a zinc mixed metal oxide. (Item 16) Item 13. The method of item 12, further comprising incorporating a second metal into the portion of the at least one metal surface. (Item 17) 1. A method for making a metal tube having a corrosion resistant layer integral with an exposed interior surface of the metal tube, comprising: incorporating zinc into at least one metal surface of the semi-finished metal product; forming a metal tube from the semi-finished metal product, before or after said incorporating, wherein said at least one metal surface is an interior surface of said metal tube; after said forming, heat treating said metal tube to form a zinc-metal oxide layer within a top portion of an interior surface of said metal tube; A method comprising: (Item 18) 18. The method of claim 17, further comprising processing the metal tube after the incorporation, after the forming, and before the heat treatment by cold pilger rolling or using a cold drawing process. (Item 19) Item 18. The method of item 17, wherein the at least one metal surface comprises chromium and the zinc-metal oxide comprises zinc-chromium oxide. (Item 20) Item 18. The method of item 17, further comprising incorporating a second metal into the at least one metal surface. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram illustrating a method for manufacturing a metal product having a corrosion-resistant layer according to one embodiment of the present invention.

[0013] [Figure 2] FIG. 2 illustrates a method for manufacturing a metal tube with a corrosion resistant layer according to one embodiment of the present invention.

[0014] [Figure 3] FIG. 3 shows the results of theoretical calculations showing the composition of the metal surface as a function of depth for an I600 base alloy.

[0015] [Figure 4] FIG. 4 shows the results of theoretical calculations showing the composition of the metal surface as a function of depth for SS304 base steel. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention is described below with reference to the accompanying drawings. While the present invention is described in conjunction with a specific embodiment, it should be understood that the present invention includes different embodiments and can be applied to a wide variety of applications. Therefore, the following description is exemplary and is intended to cover alternatives, modifications, and equivalents within the spirit and scope of the present invention. Furthermore, while various embodiments may be described through the use of the terms "preferably," "for example," or "in one embodiment," this characterization should not be construed as limiting or describing only the embodiments of the present invention, as the present invention encompasses other embodiments that may not be specifically recited in this description. Furthermore, the use of the terms "invention," "the present invention," "embodiments," and similar terms throughout this description is used broadly and is not intended to imply that the invention requires or is limited to any particular aspect described in connection with one embodiment, or that such description is the only way the invention can be made or used.

[0017] In general, the present invention is directed to a method for manufacturing a finished metal object or product having a corrosion-resistant layer within a top portion of the exposed surface of the metal object or product. In other words, the corrosion-resistant layer is integral with the metal object or product and extends from the surface of the metal object or product to the interior of the metal object or product. The corrosion-resistant layer is thus an integral top portion of the metal having a given depth, the surface of which is exposed or to be exposed to a potentially corrosive or corrosive environment. The composition of this top portion of the metal is therefore different from the initial composition of this portion of the metal object because it now has the composition of the corrosion-resistant layer.

[0018] It should be understood that the metal object or product manufactured with a corrosion-resistant layer can be composed of any metal or alloy. For example, Inconel, Incoloy, stainless steel, chrome-molybdenum steel, low-alloy steel, Stellite / Haynes alloy, Hastelloy, and Ultimet can be used. In some embodiments, the metal or alloy contains chromium. In other embodiments, the metal or alloy contains relatively low levels of chromium or is chromium-free.

[0019] A corrosion-resistant layer may be formed within a portion of a given metal or alloy such that the surface of the corrosion-resistant layer is a surface that may be exposed to a corrosive environment. It should be understood that a corrosion-resistant layer need not extend laterally along the entirety of a given surface of a metal object or product. However, in some embodiments, a corrosion-resistant layer may be formed such that its surface is co-extensive with the entirety of a particular surface of a metal object or product. In other embodiments, the surface of a corrosion-resistant layer may extend only laterally such that a portion of a given metal surface of a metal object or product does not have a corrosion-resistant layer, while another portion of the same surface of the metal object or product contains a corrosion-resistant layer.

[0020] In one embodiment, the present invention is directed to the production of a finished metal object or product having a corrosion-resistant layer composed of a zinc-metal oxide or a zinc mixed metal oxide that is stable enough to provide long-term resistance during use of the finished metal object or product or exposure to a fluid corrosive environment. In one embodiment, the present invention is used to create a corrosion-resistant layer composed of a stable zinc-metal oxide layer or a zinc mixed metal oxide layer within at least one surface of the finished metal object or product. It should be understood that the zinc-metal oxide layer comprises zinc combined with a single metal inherently present in the metal, such as chromium. It should be understood that a zinc mixed metal oxide layer comprises zinc combined with more than one metal inherently present in the metal. In one embodiment, the present invention is used to create a stable zinc-chromium oxide layer within the surface of a finished metal object or product, such as a steel or any alloy containing chromium, including nickel-iron-chromium. It should be understood that in some embodiments, there may be an insufficient amount of chromium present to create a stable zinc-chromium oxide layer. If so, additional chromium can be added as described below to create a stable zinc-chromium oxide layer.

[0021] It should be understood that in some embodiments, other metals can be added together with the metal used to create the corrosion-resistant layer. For example, in one embodiment in which zinc is used to create the corrosion-resistant layer, another metal, such as chromium, can be incorporated into the metal object or product along with the zinc. In this embodiment, even if chromium is inherently present in the metal object prior to such addition, or even if the metal object or product does not contain chromium, the chromium is added to the metal object or product or infused with the zinc. In this manner, for example, in cases where the metal object or product is considered to be deficient in a given metal species, the metal object or product can be enhanced by adding a metal in addition to the metal being added to create the corrosion-resistant layer. Thus, the addition of such a metal in combination with the metal used to create the corrosion-resistant layer can be used to ensure the desired composition of the top layer of the metal object or product. It should be understood that more than one metal can be added together with the metal used to create the corrosion-resistant layer. In some embodiments, additional metals that can be incorporated include aluminum, molybdenum, titanium, zirconium, platinum, and mixtures of the above.

[0022] More specifically, it should be understood that the incorporation or addition of one or more additional metals together with the metals used in combination to form the corrosion-resistant layer can result in the formation of a better corrosion-resistant layer. Adjusting the composition of the top layer of a metal object or product provides a composition that, in combination with the incorporation of a metal specifically used to form the corrosion-resistant layer, results in a more desirable composition for the corrosion-resistant layer itself. For example, if the metal object or product is chromium-deficient or has a relatively low chromium concentration, the incorporation or addition of chromium together with a metal specifically incorporated to form the corrosion-resistant layer, such as zinc, acts to increase the chromium concentration and result in the desired zinc-chromium oxide formation, which provides a better corrosion-resistant layer compared to when the chromium is absent. In some embodiments, additional metals that can be incorporated include aluminum, molybdenum, titanium, zirconium, platinum, and mixtures of the above.

[0023] In one embodiment, the present invention is directed to a method of manufacturing a metal tube having a corrosion-resistant layer within a top portion of its inner surface through which corrosive fluids may flow during use of the metal tube. Such metal tubes can be used, for example, as steam generator tubing associated with pressurized water reactors (PWRs) or as feedwater tubing and other metal surfaces (e.g., feedwater heaters) that transport or may be exposed to reactor coolant from boiling water reactors (BWRs).

[0024] The present invention is also generally directed to various finished metal objects or products having a corrosion-resistant layer created during the manufacturing process, as opposed to a corrosion-resistant layer created or deposited on the metal surface after the manufacturing of the finished metal object or product is complete. As noted above, the corrosion-resistant layer is formed within the upper portion or surface of a given metal or alloy, such that the surface of the corrosion-resistant layer is the surface that may be exposed to a corrosive environment. Again, it should be understood that the metal object or product manufactured with a corrosion-resistant layer can be composed of any metal or alloy.

[0025] As noted above, in one embodiment, the finished metal object or product may be a metal tube or tubing that carries corrosive fluids. For example, the metal tubing may be used as steam generator tubing associated with a PWR, or feedwater tubing and other metal surfaces that may be exposed to reactor coolant from a BWR. The use of such tubing in a nuclear environment is particularly important because, in addition to damage to the tubing itself, the Any corrosion of the tubing will cause metals to be released from the tubing into the fluid and often deposit elsewhere in the system, such as in low flow areas or on heat transfer surfaces, causing fouling and efficiency loss. In the nuclear industry in particular, these metal releases can be a source of radiation fields outside the reactor vessel that should otherwise be avoided.

[0026] In one embodiment, the present invention is used to make, and includes, finished metal objects or products having a zinc-metal oxide layer or a zinc mixed-metal oxide layer within the surface of the finished metal object or product that may be exposed to potentially corrosive or corrosive environments or fluids. In one embodiment, the present invention is used to make, and includes, finished metal objects or products having a zinc-metal oxide layer or a zinc mixed-metal oxide layer within the surface of the finished metal object or product that may be exposed to potentially corrosive or corrosive environments or fluids. In one embodiment, the present invention is used to make, and includes, finished metal objects or products created from any metal or alloy that has a stable zinc-chromium oxide layer within the surface of the finished metal object or product that may be exposed to potentially corrosive or corrosive environments or fluids, including any chromium-free metal or alloy that contains insufficient chromium to form a stable zinc-chromium oxide or sufficient corrosion-resistant layer.

[0027] Various embodiments of the methods and products of the present invention are described below in conjunction with the figures. The use of the term "metal" is intended to be generic and, therefore, should be understood to include any metal or alloy. Additionally, the use of the term "semi-finished metal product" should be understood to refer to any metal object that still requires further processing before becoming a finished metal object or product ready for use or for which there are still one or more steps or processes, such as annealing, that must be completed before becoming a finished metal object or product ready for use or sale. For example, semi-finished metal products include products produced from hot metal or products cast from hot metal or molten steel, including ingots, blooms, billets, slabs, rods, and tube rounds. When these semi-finished metal products are further processed, for example, by annealing or other steps commonly performed in metal manufacturing as known to those skilled in the art, they become finished metal products. Thus, a "finished" metal object or product is one that does not require further processing steps in standard metal manufacturing methods, and the metal object or product is ready for use as ultimately intended.

[0028] It should also be understood that the corrosion-resistant layer produced by the method of the present invention is not a physically separate layer, such as a cladding attached to the surface of a metal object or product, or an additional, separate coating applied over the top portion of an existing surface of a metal object or product. Rather, as noted above, the corrosion-resistant layer is produced within the top portion of the metal body of the object or product itself, such that the corrosion-resistant layer extends from the surface into the interior of the metal object or product. The depth of the corrosion-resistant layer can vary depending on the composition of the metal object or product, the metal used to form the corrosion-resistant layer, whether any additional metals are added, and the conditions under which the corrosion-resistant layer is formed. Thus, the composition of this layer can differ from the composition of the underlying portion of the metal that exists below the corrosion-resistant layer, which latter composition being the composition of the starting metal or alloy itself or the composition of the semi-finished metal product. For example, a corrosion resistant layer can be created by incorporating zinc into the metal and subsequently oxidizing it to create a zinc-metal oxide layer or a zinc mixed metal oxide layer that resides within the upper portion of the metal object and extends from the exposed outer surface of the metal to a given depth within the metal, as described above.

[0029] It should also be understood that the composition of the corrosion resistant layer may vary with depth in the metal itself. Using zinc as an example, the amount of zinc that diffuses into the metal can affect the overall depth or thickness of the corrosion-resistant layer. Furthermore, the concentration of zinc within the corrosion-resistant layer or within the top portion of the metal itself can vary as a function of depth within the metal. Thus, the overall composition of the corrosion-resistant layer may itself vary with depth within the metal itself, and the depth of the corrosion-resistant layer may differ, for example, depending on different methods and conditions used to create the corrosion-resistant layer during manufacturing (e.g., the type of method used to contact the metal with zinc and the operating conditions for such methods, such as the temperature during contact, the composition of the base metal or alloy used, etc.). However, in one embodiment, the depth or thickness of a corrosion-resistant layer can be defined as the depth within a metal for which its composition differs from the underlying composition of the metal itself, or the composition of the semi-finished metal product, prior to creation of the corrosion-resistant layer. In one embodiment, the depth or thickness of a corrosion-resistant layer can be defined as the depth within a metal having a composition that includes zinc, where zinc has been used to form the corrosion-resistant layer and the initial composition of the metal object does not contain zinc. In one embodiment, the depth or thickness of the corrosion resistant layer can be defined as the depth into a metal having a composition that includes zinc-metal (eg, zinc-chromium) or a zinc mixed metal oxide.

[0030] FIG. 1 illustrates a method for manufacturing a metal product having a corrosion-resistant layer according to one embodiment of the present invention. In this process 100, in a first step 102, zinc is incorporated into at least one surface of a semi-finished metal object or product. Depending on the specific process used to diffuse the zinc into the surface of the metal, the zinc can be incorporated into one or more surfaces exposed to the zinc. For example, zinc can be incorporated into any surface of the semi-finished metal object or product, including large metal components and tubing used in heat exchangers. However, at a minimum, zinc is incorporated into certain metal surfaces that may be exposed to a corrosive or potentially corrosive environment during use of the finished metal object or product. It should be understood that the process of the present invention can be applied to any finished metal object or product. In one embodiment, the semi-finished metal object or product is a nickel-iron-chromium steel or any alloy, including, for example, any alloy containing chromium. In some embodiments, the semi-finished metal object or product is composed of any metal or alloy, including metals or alloys that do not contain any chromium or that contain insufficient amounts of chromium necessary to form a stable zinc-chromium oxide or a sufficient corrosion-resistant layer.

[0031] The process for incorporating zinc into a metal surface can be carried out using any process known in the art for incorporating metal atoms or compounds into a metal surface. For example, diffusing pack diffusion, pack cementation, chemical deposition, or vapor deposition processes can be used. The form of zinc used can be any form of zinc that can diffuse or be incorporated into a metal surface and ultimately form a zinc-metal bond, such as zinc-chromium oxide. For example, in one embodiment, diethylzinc or dimethylzinc can be used. In one embodiment, highly reactive diethylzinc gas (or other zinc gas) can be diluted with an inert gas. The diluted diethylzinc gas (or other zinc gas) is then contacted with the desired metal surface into which zinc diffusion is desired. It should be understood that the incorporation of zinc into a metal surface is not limited to any particular chemical or physical mechanism, such as diffusion. In other words, the incorporation of metal atoms or compounds into a metal surface by diffusion should not be construed as limiting the present invention specifically or exclusively to the incorporation of metal atoms or compounds into a metal surface by the process of diffusion.

[0032] It should be understood that gaseous diethyl-zinc and dimethyl-zinc can be used to produce metal objects or products that have "depleted" zinc, or zinc that does not contain the zinc-64 isotope that can be activated and converted to zinc-65. The former zinc-65 should be minimized in nuclear applications. Therefore, it is not recommended for use in a nuclear environment. In such cases, it is preferred to produce finished metal objects or products that are depleted in zinc-64. However, it should be understood that finished metal objects or products used in non-nuclear applications can also be produced with any zinc isotope composition, including neutral zinc.

[0033] In step 104, the semi-finished metal object or product is formed into the shape desired for the finished metal object or product. Now that zinc is incorporated into the semi-finished metal object or product, it must then be formed into the shape desired for the finished metal object or product. Thus, step 104 is the forming of the semi-finished metal object or product into the shape desired for the finished metal product. For example, if the finished metal product is a flat metal panel, then in this step 104, whatever starting semi-finished metal object or product, such as a metal slab, may be used, and the metal slab may be formed into a flat metal panel of the desired shape. Step 104 can be performed using any process known in the art for taking a semi-finished metal object or product and forming it into the shape desired for the finished metal object or product, as is commonly done in metal manufacturing methods known to those skilled in the art.

[0034] In step 106, the semi-finished metal object or product having the desired shape for the finished metal object or product is heat treated. This heat treatment involves heating the semi-finished metal object or product, as is commonly done in annealing processes used in standard manufacturing of finished metal objects or products. This heat treatment is carried out in a controlled environment to promote the formation of a stable zinc-metal oxide layer, such as a zinc-chromium oxide layer or a zinc mixed metal oxide layer. For example, the controlled environment may include the presence of hydrogen and oxygen so that the incorporated zinc forms a stable zinc-metal oxide layer, such as a zinc-chromium oxide layer, within the uppermost portion of the surface of the semi-finished metal object or product. The creation of this oxide layer provides a stable oxide layer that is more stable than oxide layers created using a finished metal object or product, such as preconditioning, performed during operational chemical exposure after manufacturing. Without being bound by theory, it is believed that by incorporating zinc into a portion of the metal surface prior to heat treatment, the corresponding zinc-metal oxide layer extends deeper or is thicker and more stable and permanent than any zinc oxide layer formed in situ, which then has a shallower depth to where the zinc is incorporated.

[0035] As discussed above, it should be understood that one or more additional metals can be incorporated into the metal object or product along with the zinc. For example, in an embodiment in which zinc is used to create a corrosion-resistant layer, another metal, such as chromium, can be added to the metal object or product along with the zinc. In one embodiment, the chromium is incorporated into the metal object or product in a manner similar to the incorporation of the zinc. In one embodiment, the chromium is incorporated into the metal object or product at the same time as the incorporation of the zinc. It should be understood that any metal can be incorporated into the metal object or product along with the zinc to enable the creation of a tailored metal composition within the top portion of the metal object or product. For example, if the metal object or product is considered deficient in a given metal species, that metal species can be incorporated into the metal object or product, including within the top portion, along with the metal used to create the corrosion-resistant layer. It should be understood that more than one metal can be added along with the metal used to create the corrosion-resistant layer.

[0036] While the above has been described in relation to the manufacture of metal objects or products and the benefits of forming a corrosion-resistant layer during such manufacture, in other embodiments, a corrosion-resistant layer can still be formed for a metal object or product after manufacture and after such metal object or product has been used or put into service. In some embodiments, a corrosion-resistant layer can be formed in place or in situ with the metal object or product. In these situations, the metal used to form the corrosion-resistant layer can be incorporated into the metal object or product in a manner similar to that described above in relation to FIG. 1. The additional metal can be formed in the manner described above in relation to the metal object or product. It should be understood that the data may also be incorporated in this manner.

[0037] Figure 2 illustrates a method for manufacturing a metal tube having a corrosion-resistant layer according to one embodiment of the present invention. The manufacturing method 200 is similar to that shown in Figure 1, except that the process 200 shown in Figure 2 is specifically directed to the manufacture of metal tubing as a finished metal object or product.

[0038] In a first step 202, zinc is incorporated into at least one surface of a semi-finished metal product, which in this embodiment is a semi-finished metal product that may ultimately be manufactured into a metal tube, such as a flat strip. The incorporation of zinc can similarly be carried out using any of the same processes described above in connection with Figure 1. However, it should be understood that in this embodiment, the zinc needs to be incorporated into the top portion of the surface of the semi-finished metal product that may ultimately form the interior surface of the metal tubing.

[0039] In a second step 204, a semi-finished metal object or product, such as a flat strip, is formed into a tube (e.g., a welded tube). This step is performed as is commonly done in metal manufacturing methods known to those skilled in the art. However, it should be understood that, depending on the process used to form the metal tube, in another embodiment, zinc may not be incorporated into the metal surface until after the metal tube is formed. In that case, the first step 202 of incorporating zinc may occur after the second step 204 of forming the metal tube. In this embodiment, the metal tube (e.g., a seamless metal tube) may be formed from a semi-finished metal object or product, such as an ingot or bloom or a billet of metal. Once the metal tube is formed, zinc may be incorporated into the interior surface of the metal tube as described above in connection with FIG. 1 .

[0040] In the next step 206, the metal tube can optionally (as represented by the dashed lines) be further processed, for example, in the case of a metal tube, by cold pilger rolling or by using a cold drawing process, as is known in the art.

[0041] The next step, 208, is to heat treat the metal tube, which process can be carried out in a similar manner and for the same purposes as described in relation to FIG.

[0042] Figure 3 illustrates an example of theoretical calculations of the composition of a metal surface as a function of depth for an I600-based alloy. Figure 4 illustrates an example of theoretical calculations of the composition of a metal surface as a function of depth for an SS304-based steel. For clarity, each of these figures illustrates the concentration of various components of a metal from the exposed surface (left side of each graph) to a given depth within the metal (right side of each graph). Specifically, the relative concentrations of nickel, iron, chromium, and zinc are shown individually from the bottom to the top of each graph. As shown, the amount of zinc (the top component in each graph) decreases from left to right to a depth where no zinc is present and the composition of the metal is the same as the composition of the starting semi-finished metal object or product prior to zinc incorporation. While Figures 3 and 4 illustrate the results of theoretical calculations for these specific metals, it should be understood that similar metals are expected to behave similarly.

[0043] Various embodiments of the present invention have been described above. However, it should be understood that alternative embodiments are possible and that the present invention is not limited to the specific embodiments described above.

Claims

1. 1. A method for making a metal object having a corrosion resistant layer, comprising: incorporating zinc into at least a portion of a metal body of the metal object during manufacture of the metal object, wherein the at least a portion extends inward from a metal surface of the metal body toward an interior of the metal body, the metal surface of the metal body being exposed during use of the metal object; forming a corrosion-resistant layer comprising the zinc within the portion of the metal body prior to use; The zinc is incorporated into the metal surface by diffusion pack diffusion, penetration, chemical deposition, or vapor deposition. method.

2. The method of claim 1 , wherein the corrosion resistant layer comprises a zinc-metal oxide layer.

3. The method of claim 2, wherein the metal surface comprises chromium and the zinc-metal oxide layer comprises a zinc-chromium oxide layer.

4. The method of claim 1 , wherein the metal surface comprises an alloy containing chromium.

5. The method of claim 1 , wherein the metal surface comprises nickel-iron-chromium containing steel.

6. The method of claim 1 further comprising incorporating a second metal within said at least a portion of said metal body.

7. The method of claim 6 , wherein the second metal comprises chromium.

8. 1. A method for making a metal object having a corrosion resistant layer, comprising: incorporating zinc into at least a portion of a metal body of the metal object during manufacture of the metal object, wherein the portion extends inwardly from a metal surface of the metal body, thereby avoiding the formation of a zinc-containing layer on an outer surface of the metal body to which the metal surface of the metal body is exposed during use of the metal object; forming a corrosion resistant layer comprising the zinc within the portion of the metal body; The zinc is incorporated into the metal surface by diffusion pack diffusion, penetration, chemical deposition, or vapor deposition. method.

9. The method of claim 8 , wherein the metal surface comprises an alloy containing chromium.

10. The method of claim 8, wherein the metal surface comprises nickel-iron-chromium containing steel.

11. The method of claim 8 further comprising incorporating a second metal within said at least a portion of said metal body.

12. The method of claim 11 , wherein the second metal comprises chromium.

Images