Patent Number: 055815880
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a technique for solving the problem of achieving low corrosion potentials in the high-flux, in-core region (or in other regions which may have very high oxidant supply rates from high concentrations and/or high fluid flow rates/convection). The technique entails the formation of an electrically insulating protective coating that is doped with a noble metal on SCC-susceptible surfaces of metal components of a water-cooled nuclear reactor. The insulated protective coating is designed to alter the balance between the rate of supply of oxidants to the surface and the rate of recombination on the surface by limiting the supply kinetics (by restricting the mass transport of reactants through a porous, insulated layer), while at the same time providing for the catalytic reduction of the oxidants within the layer by a noble metal contained within the coating. This invention is related to allowed co-pending patent application Ser. No. 08/226,153 filed on Apr. 11, 1994 [Attorney Docket No. GENE-24-BR-05538], which is herein incorporated by reference. The technique of the present invention is based on the following fundamental considerations. The first consideration is that corrosion potentials are created only at metal-water interfaces. Thus, while on a metal coating the corrosion potential is formed at the interface of the metal coating with the bulk water, on a porous insulated coating, the corrosion potential is formed at the interface of the substrate metal and the water with which it is in contact (i.e., the water in the pores). The influence of corrosion potential on stress corrosion cracking results from the difference in corrosion potential at the generally high potential crack mouth/free surface versus the always low potential (e.g., -0.5 V.sub.SHE) within the crack/crevice tip. This potential difference causes electron flow in the metal and ionic flow in the solution, which induces an increase in the anion concentration in the crack, as in a classical crevice. FIG. 8 is a schematic of electrochemical processes which generally lead to elevated corrosion potentials on the outside (mouth) of a crack and low corrosion potentials in the inside (tip) of the crack. The potential difference .DELTA..o slashed..sub.c causes anions A.sup.- (e.g., Cl.sup.-) to concentrate in the crack, but only if there is both an ionic path and an electron path. FIGS. 9A to 9E provide a schematic comparison of the corrosion potentials .o slashed..sub.c which form under high radiation flux: (A) on an uncoated (e.g., stainless steel) component (high .o slashed..sub.c); (B) on a component coated with a catalytic metal coating where the rate of supply of reactants to the surface is not too rapid (low .o slashed..sub.c); (C) on a component coated with a catalytic metal coating where the rate of supply of reactants to the surface approaches or exceeds the recombination kinetics for H.sub.2 and O.sub.2 (moderate .o slashed..sub.c); (D) on a component coated with an insulated protective coating (at a low corrosion potential provided that oxidant concentrations do not get too high, see FIG. 11); and (E) on a component coated with an insulated protective coating that is doped with a noble metal (always at a low corrosion potential). Thus, to influence stress corrosion cracking, the elevated crack mouth corrosion potential must form on a surface that is in electrical contact with the component of interest. If an insulating coating (see FIGS. 9 and 10) were applied to a metal component and some porosity or cracking in the coating is assumed to exist, the corrosion potential would be formed only at the metal component-water interface. Thus, a crevice would be formed by the coating, but since it is electrically insulating, the crevice cannot represent an "electrochemical" crevice, but only a "restricted mass transport" geometry. The critical ingredient in "electrochemical" crevices is the presence of a conducting material in simultaneous contact with regions of high potential (e.g., a crack mouth) and regions of low potential (e.g., a crack tip). Thus, it would not help to have a component covered by an insulating layer, which layer is in turn covered by a metal layer (or interconnected metal particles) within which exists a crevice or crack. Under these conditions, the aggressive crevice chemistry could form in the outer metal layer, which in turn would be in contact with the component. Therefore, the amount of noble metal that is used as a dopant should be limited so that a conductive pathway cannot be formed through the thickness of the coating, such as by forming a series of interconnected noble metal particles. While the maximum amount of noble metal depends on many factors, including the particle size of the noble metals used to form the coating (in the case where the noble metal is added as a powder), the insulating material used, the existence of a removable phase as discussed elsewhere herein, the morphology of the coating and others. However, based on results reported with Au--Cu alloys, the amount of the noble metal in the coating should be about 20 atomic percent, or less, based on the concepts of percolation theory, see for example, K Sieradzki, "Atomistic and Micromechanical Aspects of Environment-Induced Cracking of Metals", Proceedings of the First International Conference on Environment Induced Cracking of Metals, NACE, 1989. The second consideration is that if the insulating coating is impermeable to water, then obviously there can be neither a corrosion potential formed on the underlying metal, nor concern for stress corrosion cracking. Any pores or fine cracks in an insulating layer provide highly restricted mass transport and thus are equivalent to a very thick near-surface boundary layer of stagnant water. Since oxidants are always being consumed at metal surfaces, this very restricted mass transport (reduced rate of oxidant supply) causes the arrival rate of oxidants through the insulated coating to the substrate to decrease below the rate of their consumption. Under these mass transport limiting circumstances, the corrosion potential rapidly decreases to values .ltoreq.-0.5 V.sub.SHE, even for high bulk oxidant concentrations, and even in the absence of stoichiometric excess hydrogen (or any hydrogen). Numerous observations consistent with this have been made, including low potentials on stainless steel surfaces at low oxygen levels (e.g., 1 to 10 ppb), as well as in (just inside) crevices/cracks, even at very high bulk oxygen levels. Thus, corrosion potentials .ltoreq.-0.5 V.sub.SHE can be achieved even at high bulk oxidant concentrations and, not only in the absence of stoichiometric excess hydrogen, but also in the absence of any hydrogen. This may prove to be a critical invention for BWR plants which are unable (because of cost or because of the high N.sup.16 radiation levels from hydrogen addition) to add sufficient hydrogen to guarantee stoichiometric excess hydrogen conditions at all locations in their plant. While the present invention provides protection from SCC in the absence of stoichiometric excess hydrogen (or any hydrogen), it should be noted that in order for the noble metal to catalyze oxidants in the crevices (restricted mass transport regions), that a reductant species such as hydrogen must be present, although the reductant need not be in stoichiometric quantities. Where the reductant species is present, the presence of the noble metal would allow the use of insulating coatings of the present invention to be used at oxidant concentrations that would be too large (i.e. that would produce a corrosion potential that is too large (e.g., greater than the critical potential) for insulating coatings that do not contain a noble metal dopant, such as disclosed in the reference described above. While various non-conducting materials could be used, zirconia (ZrO.sub.2) is a good initial choice because it can be thermally sprayed and is very stable in high-temperature water, both structurally (e.g., it is not prone to spalling and is not susceptible to environmentally assisted cracking) and chemically (e.g., it does not dissolve or react). Zirconia can be obtained in various particle sizes, so that there is flexibility in adjusting the thermal spray parameters. Alumina is also an option. The dissolution rate of alumina in 288.degree. C. water is higher than that for zirconia, but is still very low. Various other metal oxides, carbides, nitrides or carbides may also be suitable, so long as they are mechanically and chemically stable in a high temperature water environment, including not being subject to dissolution in high temperature water and not being subject to spalling under reactor operating conditions. The noble metals that may be used include any metals that are not subject to dissolution in high temperature water, and that will act as a catalyst for the reduction of oxidizing species such as oxygen and hydrogen peroxide that exist in high temperature water. Based on results with noble metal coatings in high temperature water described herein and well-known catalytic characteristics, it is believed that the metals iridium, palladium, platinum, osmium, rhodium or ruthenium will provide suitable noble metal dopants, and that the use of the noble metals palladium or platinum will be preferred, largely for cost considerations. Also, the noble metal may comprise an alloy of noble metals. The noble metal may be doped into the zirconia by any one of a number of known methods, including thermal spraying using a powder feed where the feed powder is a mixture of the insulating material and the noble metal, and others. FIG. 10 is a schematic illustration of an insulated protective coating doped with a noble metal of the present invention, depicted as particles 4 of zirconia powder and particles 6 of a noble metal powder which have been thermally sprayed onto a metal component surface 2. Due to the insulating nature of zirconia, there is no electrical connection between external (high oxidant) water and the metal component substrate. Thus, the insulated protective coating containing the noble metal prevents an electrochemical crevice cell from being formed (see FIG. 8), while restricting mass transport of oxidants to the underlying metal substrate (see FIGS. 2 and 7) to sufficiently low rates such that the corrosion potential of the metal component is always low (i.e., -0.5 V.sub.SHE). At the same time, the coating is also catalyzing the reduction of the oxidants in the crevices, which also further serves to reduce the corrosion potential, or provide tolerance to higher bulk oxidant concentrations. Preliminary experimental data (shown in FIG. 11 ) were obtained in 288.degree. C. pure water on a cylindrical stainless steel electrode coated with yttria-stabilized zirconia (YSZ) by air plasma spraying. A Cu/Cu.sub.2 O membrane reference electrode was used to measure the corrosion potentials of the stainless steel autoclave, a platinum wire and the YSZ-coated stainless steel specimen. At oxygen concentrations up to .apprxeq.1 ppm (during BWR operation, the equivalent oxygen concentration (O.sub.2 +0.5.times.H.sub.2 O.sub.2) is about 100 to 600 ppb), the corrosion potential of the YSZ-coated specimen remained at .ltoreq.-0.5 V.sub.SHE despite the high potentials registered on the stainless steel autoclave (+0.20 V.sub.SHE) and the platinum electrode (+0.275 V.sub.SHE). This is consistent with numerous observations of low potentials on stainless steel surfaces at low oxygen levels (e.g., 1 to 10 ppb) as well as inside crevices/cracks, even at very high oxygen levels. Similar observations were obtained in hydrogen peroxide, where low potentials were observed on the YSZ-coated specimen at concentrations above 1 ppm (see FIG. 12). By contrast, uncoated stainless steel exhibited a high corrosion potential of .apprxeq.+0.150 V.sub.SHE. Low potentials were also observed on the YSZ-coated specimen in water containing 1 ppm O.sub.2 when the specimen was rotated at 500 rpm, corresponding to 0.7 m/sec linear flow rate. This is not surprising, since the higher flow rates merely act to reduce the thickness of the stagnant boundary layer of liquid, a layer whose thickness is small relative to the zirconia coating. The success in maintaining low corrosion potentials under these conditions shows that the electrically insulating zirconia layer greatly reduces mass transport to the underlying metal surface such that, even in the absence of catalytic agents such as palladium, the cathodic (oxygen reduction) reaction is mass transport limited just as in uncoated specimens in solutions of very low dissolved oxygen content. Further corroboration exists in the corrosion potential measurements on Zircaloy in 288.degree. C. water, which apparently are always lower than -0.5 V.sub.SHE, even in aerated solutions. The relatively highly electrically insulating nature of the zirconia film causes the corrosion potential to be formed at the metal surface where the oxidant concentration is very low due to its restricted transport through the zirconia film. Additional experimental data is presented in FIGS. 13 and 14. A coating made of yttria-stabilized zirconia powder was deposited in three different thicknesses (3, 5 and 10 mils) on the fresh metal surface of Type 304 stainless steel (0.25 inches in diameter and 1 inch long) by air plasma spraying. The corrosion potentials of the zirconia-coated electrodes, a pure zirconium electrode and uncoated Type 304 stainless steel were measured against a Cu/Cu.sub.2 O/ZrO.sub.2 reference electrode in 288.degree. C. water containing various amounts of oxygen. After the corrosion potential measurement, test specimens were immersed in 288.degree. C. water containing various water chemistry conditions for 3 months at open circuit. In the initial tests, YSZ-coated stainless steel electrodes were mounted in the autoclave along with a zirconium electrode, an uncoated Type 304 stainless steel electrode and the reference electrode. All specimens were immersed in pure 288.degree. C. water at a flow rate of 200 cc/min for 2 days. The corrosion potential was measured sequentially with incremental addition of oxygen, as shown in FIG. 13. At given oxygen levels up to 200-300 ppb, the YSZ-coated electrodes showed low potentials (&lt;-0.5 V.sub.SHE) essentially equivalent to those of the pure zirconium electrode, compared to the Type 304 stainless steel corrosion potential values measured at the same level of oxygen. Further increase of the oxygen concentration increased the corrosion potential of the YSZ-coated electrodes. After the system was left in 288.degree. C. water containing various water chemistry conditions for 3 months, the corrosion potential was again measured by increasing the oxygen concentration (see FIG. 14). This data indicates that the corrosion potential behavior of the YSZ-coated electrodes was retained for extended periods. From the foregoing data, it is apparent that the application of a YSZ coating on the surface of Type 304 stainless steel appears is advantageous in maintaining a low corrosion potential (&lt;-0.5 V.sub.SHE) at high oxygen levels (up to about 300 ppb), even in the absence of hydrogen, by reducing mass transfer of oxygen to the metal surface and thereby mitigating SCC of the structural material. Since the oxygen concentration during operation of a BWR is about 200 ppb, SCC in BWR structural components could be mitigated by the application of a YSZ coating or any other electrically insulating protective coating on the surfaces of the structural material. The present invention is particularly suited for use in water-cooled nuclear reactors that contain high-temperature water, however, the invention may also be utilized in any other systems that utilize high-temperature water where SCC is a consideration, such as conventional turbines and generators. The foregoing method has been disclosed for the purpose of illustration. Variations and modifications of the disclosed method will be readily apparent to practitioners skilled in the art of water chemistry. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.