Patent Publication Number: US-2012033779-A1

Title: Methods of determining in-reactor susceptibility of a zirconium-based alloy to shadow corrosion

Description:
BACKGROUND 
     1. Field 
     The present disclosure relates to methods for determining the occurrence of corrosion on materials within a reactor core during the operation of a nuclear reactor. 
     2. Description of Related Art 
     A form of radiation enhanced corrosion known as “shadow corrosion” has been known to adversely affect zirconium-based alloys within a reactor core during the operation of a nuclear reactor. The exact mechanism for shadow corrosion is not known, but the corrosion has been observed when a zirconium-based alloy is proximate to or in direct contact with a dissimilar metal in a reactor. 
     Shadow corrosion is a performance and reliability concern in the nuclear industry. However, shadow corrosion has only been observed during in-reactor operation and has not been demonstrated to occur in laboratory environments. As a result, the development of mitigating designs is cumbersome, time consuming, and expensive, because such development necessarily involves in-reactor testing. 
     SUMMARY 
     A method of determining in-reactor susceptibility of a zirconium-based alloy to shadow corrosion according to a non-limiting embodiment of the present invention may include immersing a first electrode and a second electrode in an electrolytic solution. The first electrode may be formed of the zirconium-based alloy, while the second electrode may be formed of a metallic material suitable for use in a nuclear reactor and having a higher electrochemical corrosion potential than the zirconium-based alloy. The method may additionally include irradiating the immersed first and second electrodes with electromagnetic radiation; and measuring a galvanic current between the first electrode and the second electrode to ascertain the relative in-reactor susceptibility of the zirconium-based alloy to shadow corrosion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the non-limiting embodiments herein may become more apparent upon review of the detailed description in conjunction with the accompanying drawings. The accompanying drawings are merely provided for illustrative purposes and should not be interpreted to limit the scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. For purposes of clarity, various dimensions of the drawings may have been exaggerated. 
         FIG. 1A  illustrates a stainless steel control blade handle shadow on an outer surface of a Zircaloy channel, wherein the dark lines depict the approximate dimension of the control blade handle. 
         FIG. 1B  illustrates the oxide thickness within a control blade handle shadow region. 
         FIG. 1C  illustrates the oxide thickness away from a control blade handle shadow region. 
         FIG. 2  illustrates a method of determining in-reactor susceptibility of a zirconium-based alloy to shadow corrosion according to a non-limiting embodiment of the present invention. 
         FIG. 3  illustrates the effect of UV illumination on corrosion potential behavior of NSF, Zircaloy-4, Ziron, and X750 electrodes in 0.01 M Na 2 SO 4  solution at 25° C., wherein test electrodes have been pre-immersed for 4 weeks in 300° C. water containing 1.1 ppm O 2 , according to a non-limiting embodiment of the present invention. 
         FIG. 4  illustrates the galvanic current response of NSF and X750 coupling in 0.01 M Na 2 SO 4  at 25° C. (with and without UV illumination) according to a non-limiting embodiment of the present invention. 
         FIG. 5  illustrates the galvanic current of NSF or Ziron coupled with Alloy X-750 in 300° C. water containing 1.1 ppm O 2  (with and without UV illumination) according to a non-limiting embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood that when an element or layer is referred to as being “on,” “connected to,” “coupled to,” or “covering” another element or layer, it may be directly on, connected to, coupled to, or covering the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout the specification. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It should be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of example embodiments. 
     Spatially relative terms (e.g., “beneath,” “below,” “lower,” “above,” “upper,” and the like) may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It should be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” may encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes,” “including,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Example embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, including those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Example embodiments of the present invention relate to methods of determining in-reactor susceptibility of a zirconium-based alloy to shadow corrosion. The shadow corrosion of a zirconium-based alloy is illustrated, for instance, in  FIGS. 1A-1C . 
     A method of determining in-reactor susceptibility of a zirconium-based alloy to shadow corrosion may include immersing a first electrode and a second electrode in an electrolytic solution. The first electrode may be formed of the zirconium-based alloy, while the second electrode may be formed of a metallic material suitable for use in a nuclear reactor and having a higher electrochemical corrosion potential than the zirconium-based alloy. The method may additionally include irradiating the immersed first and second electrodes with electromagnetic radiation; and measuring a galvanic current between the first electrode and the second electrode to ascertain the relative in-reactor susceptibility of the zirconium-based alloy to shadow corrosion. 
       FIG. 2  illustrates a method of determining in-reactor susceptibility of a zirconium-based alloy to shadow corrosion according to a non-limiting embodiment of the present invention. Referring to  FIG. 2 , a first electrode  201  and a second electrode  203  are immersed in an electrolytic solution  205 . After immersion, the first and second electrodes  201  and  203  are irradiated with electromagnetic radiation  207 . A galvanic current is then measured between the irradiated first and second electrodes  201  and  203  to ascertain the relative in-reactor susceptibility of the zirconium-based alloy to shadow corrosion. 
     The first electrode  201  may be formed of the zirconium-based alloy. The zirconium-based alloy may contain at least 95 percent zirconium by weight. The zirconium-based alloy may also include niobium. For instance, the zirconium-based alloy may be Zircaloy-2 or Zircaloy-4, although example embodiments are not limited thereto. 
     The second electrode  203  may be formed of a metallic material suitable for use in a nuclear reactor and having a higher electrochemical corrosion potential than the zirconium-based alloy. The second electrode  203  may be an iron-based alloy. For instance, the iron-based alloy may be stainless steel. Alternatively, the second electrode  203  may be a nickel-based alloy. The nickel-based alloy may include more than about 50 percent nickel by weight. For instance, the nickel-based alloy may be Inconel (e.g., X-750). In yet another non-limiting embodiment, the second electrode  203  may be formed of platinum. Although various materials have been identified above for illustrative purposes, it should be understood that other materials that are suitable for use in a nuclear reactor and that have a higher electrochemical corrosion potential than the zirconium-based alloy may also be used to form the second electrode  203 . 
     The first electrode  201  and the second electrode  203  are arranged within an autoclave  209 . The first electrode  201  may be arranged within a distance of about 0 to 100 mm to the second electrode  203 . Also, each of the first and second electrodes  201  and  203  may have an oxide layer formed thereon. 
     The electrolytic solution  205  may be an ionic solution. Without being limited by the following examples, the electrolytic solution  205  may be at least one of a salt solution, deionized water, distilled water, and water with a resistance greater than 10 Mohm. When the electrolytic solution  205  is a salt solution, the salt may be sodium sulfate or sodium chloride. The electrolytic solution  205  may be at a temperature between about 20 and 400 degrees Celsius. Furthermore, the pressure within the autoclave  209  may be within a range of about 0 to 2000 psig. 
     The electromagnetic radiation  207  may be irradiated into the autoclave  209  through a sapphire window  211 , although example embodiments are not limited thereto. The electromagnetic radiation  207  should be at a level sufficient to excite electrons in the oxide layer of the first and second electrodes  201  and  203  to the conduction band. The electromagnetic radiation  207  may be ultraviolet light (UV). Thus, in a non-limiting embodiment, the wavelength of the electromagnetic radiation  207  is shorter than that of visible light, in the range of about 10 nm to 400 nm. In particular, the electromagnetic radiation  207  may have a wavelength between about 200 to 400 nm, although example embodiments are not limited thereto. The ultraviolet light may be irradiated at an intensity of about 1 mW/cm 2  to 50 W/cm 2 . The irradiation period is not particularly limited thereto as long as a galvanic current between the first electrode  201  and the second electrode  203  can be adequately measured. 
     Without being bound by theory, the UV radiation appears to be sufficient to simulate the broad radiation spectrum that exists in-reactor but that is absent in normal laboratory corrosion tests. Consequently, the UV radiation appears to promote an enhanced electrochemical potential difference between the first and second electrodes  201  and  203  that is substantially lower in the absence of the radiation. 
     To ascertain the relative in-reactor susceptibility of the zirconium-based alloy to shadow corrosion, the measured galvanic current between the first electrode  201  and the second electrode  203  is compared with a reference value derived from measurements of a reference set of materials. With regard to structure and arrangement, the reference set of materials may be analogous to the first and second electrodes  201  and  203 . With regard to material, the reference set of materials may be a pairing of identical zirconium-based materials. For instance, the identical zirconium-based reference materials may be Zircaloy materials, although example embodiments are not limited thereto. Alternatively, the reference set of materials may be dissimilar materials with a known susceptibility to shadow corrosion. 
     When the reference set of materials are formed of identical materials, the zirconium-based alloy may be deemed to be susceptible to in-reactor shadow corrosion if the measured galvanic current between the first electrode  201  and the second electrode  203  exceeds the reference value provided by the reference set of materials. More particularly, the zirconium-based alloy may be deemed to be less favored for in-reactor use if the measured galvanic current exceeds a threshold value. For instance, the threshold value may be closer to measurements based on a Zircaloy/stainless steel pairing or a Zircaloy/Inconel pairing than measurements based on a Zircaloy/Zircaloy pairing. Stated more clearly, a Zircaloy/stainless steel pairing or a Zircaloy/Inconel pairing is known to result in the in-reactor shadow corrosion of the Zircaloy. That being said, the galvanic current measured between the first electrode  201  and the second electrode  203  may help predict the susceptibility of the zirconium-based alloy to shadow corrosion based on its relative magnitude to the reference value. 
     In sum, based on information obtained from pairings known to result in the in-reactor shadow corrosion, new zirconium-based alloys and/or coatings that reduce or prevent the occurrence of shadow corrosion may be developed and tested with greater ease. In sum, the present invention allows a simplified and more rapid method of developing solutions that mitigate shadow corrosion, thereby potentially saving years of expensive in-reactor testing. 
     To enhance the comprehension and appreciation of the present invention by those ordinarily-skilled in the art, the following description has been provided to detail the photoelectrochemical investigation of radiation enhanced shadow corrosion phenomenon conducted by the inventors. 
     The present invention is based on photoelectrochemical investigation of various alloys such as Zircaloy 4, NSF, Ziron, 304 stainless steel (SS), and Alloy X-750 in 0.01 M Na 2 SO 4  at 25° C. or in high purity water at 300° C. under intense ultraviolet (UV) illumination. UV was selected because its photon energy (˜5 eV) is similar to the energy gap of the electron-hole pairs in zirconium oxide. The data show that the photoexcitation of ZrO 2  caused the corrosion potential to shift in the anodic direction and produced anodic photocurrents under the oxidizing water chemistry condition when a zirconium alloy electrode was galvanically coupled with dissimilar electrodes, such as Alloy X-750, 304 SS, or Pt, causing accelerated corrosion of the zirconium alloy. Without being bound by theory, it is thus postulated that photoelectrochemical enhancement of surface reaction kinetics at the ZrO 2  surface may be responsible for the radiation enhanced corrosion on Zircaloy (i.e., shadow corrosion). 
     In operating BWRs, accelerated growth of ZrO 2  has been observed on a Zr alloy when it is in close proximity to a dissimilar material (e.g., Alloy X-750 and stainless steel). This anomalous oxide growth is called “shadow corrosion” since the pattern of the enhanced corrosion resembles the shape of the adjacent metallic components. 
     An example of shadow corrosion on a Zircaloy channel due to an adjacent control blade handle is shown in  FIG. 1A . Metallographic examination of shadow corrosion reveals a uniformly enhanced corrosion of the Zr alloys, as shown in  FIG. 1B .  FIG. 1B  illustrates the oxide thickness within a region of Zircaloy channel affected by control blade handle shadow.  FIG. 1C  illustrates the oxide thickness away from such a region. 
     Although shadow corrosion has been observed after in-reactor operation, the demonstration of shadow corrosion in the laboratory environments has not been readily achievable. The susceptibility of zirconium alloys to shadow corrosion was investigated by evaluating the photoelectrochemical corrosion characteristics of Zr and other alloys under UV irradiation. 
       FIG. 2  illustrates a method of determining in-reactor susceptibility of a zirconium-based alloy to shadow corrosion according to a non-limiting embodiment of the present invention. 
     Test Procedures 
     304 SS, Alloy X-750, and Zr alloys (Zircaloy 4, Ziron, and NSF) in sheet form were used. Specimens were polished using a wet 600-grit emery paper. The nominal chemical composition of Zr alloys is listed in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Nominal chemical composition of Zr alloys (in weight %). 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Test Alloys 
                 Sn 
                 Fe 
                 Cr 
                 Ni 
                 Nb 
                 Zr 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Zircaloy 4 
                 1.3 
                 0.2 
                 0.1 
                   
                   
                 Balance 
               
               
                   
                 GNF-Ziron 
                 1.3 
                 0.25 
                 0.1 
                 0.07 
                   
                 Balance 
               
               
                   
                 GNF-NSF 
                 1.0 
                 0.35 
                   
                   
                 1.0 
                 Balance 
               
               
                   
                   
               
            
           
         
       
     
     Photoelectrochemical response of oxidized surface was evaluated by introducing the UV light to the specimen through optical fiber glass. A schematic view of an autoclave system for electrochemical measurement under UV illumination is shown in  FIG. 2 . 
     Electrochemical Corrosion Potential Behavior 
     For a given water chemistry, the electrochemical corrosion potential (ECP) or corrosion potential is dependent on the oxide surface nature, such as oxide thickness, composition, conductivity, structure, etc. 
     The ECP of three zirconium alloys (Zircaloy-4, Ziron, and NSF) and Alloy X-750 was measured in 0.01 M Na 2 SO 4  at 25° C. as shown in  FIG. 3 . Test specimens were preoxidized for 4 weeks in 300° C. water containing 1.1 ppm O 2 . As shown in  FIG. 3 , when the UV light was turned on, the corrosion potential of zirconium alloys immediately decreased and then increased when the UV illumination stopped. In contrast, the corrosion potential of Alloy X-750 increased when the UV light was turned on. It is known that the oxides become more conductive in the radiation field, since photons at UV and higher energies excite electrons from the valence band to the conduction band where they are free to move. 
     Therefore, without being bound by theory, it may be postulated that the difference of corrosion potential between zirconium alloys and Alloy X-750 becomes larger in the presence of UV light and consequently enhances the susceptibility of galvanic corrosion between the zirconium alloy and X-750. 
     Galvanic Corrosion Behavior 
     Galvanic corrosion may occur when two different metals in contact (or connected by an electrical conductor) are exposed to a conductive solution. A difference in electrical potential exists between different metals, and serves as the driving force to pass current through the metals. This current flow results in increased corrosion of one of the metals in the couple. The larger the potential difference between two metals, the greater the possibility of galvanic corrosion. Note that galvanic corrosion only causes increased deterioration of one of two metals. Galvanic corrosion can often be recognized by the increased amount of corrosion close to the junction of two metals.) 
       FIG. 4  shows the galvanic current response of NSF coupled with X-750 in 0.01 M Na 2 SO 4  at 25° C. All specimens were fully pre-oxidized in 300° C. water as described previously. The anodic electrode is NSF, and the cathodic electrode is X-750. When the coupled specimens were illuminated by the UV light, the galvanic current immediately increased. The positive galvanic current indicates the galvanic current flow from NSF to the X-750 electrode, indicating the anodic corrosion of NSF. 
     The galvanic current of coupled electrodes was also measured in 300° C. water containing 1.1 ppm O 2  and is shown in  FIG. 5 . This galvanic current behavior is very similar to one measured at ambient temperature. 
       FIG. 5  shows the high temperature measurement of galvanic current on two different Zr alloys (Ziron and NSF) coupled with Alloy X-750. Test specimens have been preoxidized under the same water chemistry condition before measuring the galvanic current. It is clearly seen that both Ziron and NSF alloys showed low increases in galvanic current when both alloys were exposed to UV light. These data suggest that both Ziron and NSF alloys may be not strongly sensitive to photoelectrochemical response. 
     While a number of example embodiments have been disclosed herein, it should be understood that other variations may be possible. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.