Insulated Protective Coating Doped With a Noble Metal for Mitigation of Stress Corrosion Cracking

This invention relates to reducing the corrosion potential of components exposed to high-temperature water. This invention relates to a method for reducing the corrosion potential of components exposed to high-temperature water by the use of protective, catalytic, insulating coatings. This invention is particularly related to the use of insulating coatings that are doped with noble metals and that contain restricted mass transport crevices that lower the corrosion potential of a coated metal component when the coating is in contact with high temperature water.

As used herein, the term "high-temperature water" means water having a temperature of about 100.degree. C. or greater, steam, or the condensate thereof. High-temperature water is found in a variety of known apparatus, such as water deaerators, nuclear reactors, and steam-driven power plants. High temperature water may have elevated concentration of oxidizing species such as hydrogen peroxide and oxygen.

Nuclear reactors are used in electric power generation, research and propulsion. A typical nuclear reactor comprises a reactor pressure vessel contains the reactor coolant, i.e. high temperature water, which removes heat from the nuclear core. Respective piping circuits carry heated water or steam to the steam generators or turbines and carry circulated water or feedwater back to the vessel. Operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 288.degree. C. for a boiling water reactor (BWR), and about 15 MPa and 320.degree. C. for a pressurized water reactor (PWR). Much of a nuclear reactor is fabricated from metal components comprising various materials. The materials used in both BWRs and PWRs must withstand various loading, environmental and radiation conditions, including exposure to high temperature water.

Some of the materials exposed to high-temperature water include carbon steel, alloy steel, stainless steel and other iron-base alloys, as well as nickel-base, cobalt-base and zirconium-base alloys. Despite careful selection and treatment of these materials for use in water reactors, corrosion occurs on these materials when exposed to the high-temperature water. Such corrosion contributes to a variety of problems, for example, stress corrosion cracking, crevice corrosion, erosion corrosion, sticking of pressure relief valves and buildup of the gamma radiation-emitting Co-60 isotope.

Stress corrosion cracking (SCC) is a known phenomenon occurring in metal reactor components, such as structural members, piping, fasteners and welds that are exposed to high-temperature water. As used herein, SCC refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. The metal components of a reactor are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other asymmetric metal treatments. In addition, water chemistry, welding, crevice geometry, heat treatment, and radiation can increase the susceptibility of a metal component to SCC.

It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 1 to 5 parts per billion (ppb) or greater. SCC is further increased in components exposed to a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals, are produced from radiolytic decomposition of the reactor cooling water. Such oxidizing species increase the electrochemical corrosion potential (ECP) of metals. Electrochemical corrosion is caused by a flow of electrons from anodic to cathodic areas on metallic surfaces. The ECP is a measure of the thermodynamic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of various corrosion phenomena, including SCC, corrosion fatigue, corrosion film thickening, and general corrosion.

The ECP has been clearly shown to be a primary variable in controlling the susceptibility to SCC in BWR environments. FIG. 1 shows the observed (data points) and predicted (curves) crack growth rate as a function of corrosion potential for 25 mm CT specimens of furnace-sensitized Type 304 stainless steel at 27.5 to 30 MPa.sqroot.m constant load in 288.degree. C. water over the range of solution conductivities from 0.1 to 0.5 .mu.S/cm. Data points at elevated corrosion potentials and growth rates correspond to actual irradiated water chemistry conditions in test or commercial reactors.

Corrosion (or mixed) potential represents a kinetic balance of various oxidation and reduction reactions on a metal surface placed in an electrolyte, and can be decreased by reducing the concentration of oxidants such as dissolved oxygen. FIG. 2 is a schematic of E (potential) vs. log .vertline.i.vertline. (absolute value of current density) curves showing the interaction of H.sub.2 and O.sub.2 on a catalytically active surface such as platinum or palladium. The terms i.sub.0 represents the exchange current densities, which are a measure of the reversibility of the reactions. Above i.sub.0, activation polarization (Tafel behavior) is shown in the sloped, linear regions. The terms i.sub.L represent the limited current densities for oxygen diffusion to the metal surface, which vary with mass transport rate (e.g., oxygen concentration, temperature, and convection). The corrosion potential in high-temperature water containing oxygen and hydrogen is usually controlled by the intersection of the O.sub.2 reduction curve (O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.-) with the H.sub.2 oxidation curve (H.sub.2 .fwdarw.2H.sup.+ +2e.sup.-), with the low kinetics of metal dissolution generally having only a small role.

The fundamental importance of corrosion potential versus, for example, the dissolved oxygen concentration per se, is shown in FIG. 3, where the crack growth rate of a Pd-coated CT specimen drops dramatically once excess hydrogen conditions are achieved, despite the presence of a relatively high oxygen concentration. FIG. 3 is a plot of crack length vs. time for a Pd-coated CT specimen of sensitized Type 304 stainless steel showing accelerated crack growth at .apprxeq.0.1 .mu.M H.sub.2 SO.sub.4 in 288.degree. C. water containing about 400 ppb oxygen. Because the CT specimen was Pd-coated, the change to excess hydrogen caused the corrosion potential and crack growth rate to drop.

In a BWR, the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2, O.sub.2 and other oxidizing and reducing radicals. For steady-state operating conditions, approximately equilibrium concentrations are established for O.sub.2, H.sub.2 O.sub.2, and H.sub.2 in the water which is recirculated and for O.sub.2 and H.sub.2 in the steam going to the turbine. The resultant concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 produce an oxidizing environment and result in conditions that can promote intergranular stress corrosion cracking (IGSCC) of susceptible materials of construction.

One well-known method employed to mitigate IGSCC of susceptible material is the application of hydrogen water chemistry (HWC), whereby the oxidizing nature of the BWR environment is modified to a more reducing condition. This effect is achieved by adding hydrogen gas to the reactor feedwater. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species homogeneously and on metal surfaces to re-form water, thereby lowering the concentration of dissolved oxidizing species in the bulk water, including that portion of the water that is adjacent to metal surfaces. The rate of these recombination reactions is dependent on local radiation fields, water flow rates and other variables.

In HWC, the injected hydrogen reduces the level of oxidizing species in the water, such as dissolved oxygen, and as a result lowers the ECP of metals in the water. However, factors such as variations in water flow rates and the time or intensity of exposure to neutron or gamma radiation result in different concentrations of the stated oxidizing species in different reactors, and different concentrations at different locations within the same reactor. Thus, varying amounts of hydrogen have been required to reduce the level of oxidizing species sufficiently to maintain the ECP below a critical potential required for protection of metal reactor components from IGSCC in high-temperature water. As used herein, the term "critical potential" means a corrosion potential at or below a range of values of about -0.230 to -0.300 V based on the standard hydrogen electrode (SHE) scale. IGSCC proceeds at an accelerated rate in systems in which the ECP is above the critical potential, and at a substantially lower rate, or effectively at a zero rate, in systems in which the ECP is below the critical potential (see FIG. 1 ). Water containing oxidizing species such as oxygen increases the ECP of metals exposed to the water above the critical potential, whereas water with little or no oxidizing species present results in an ECP below the critical potential. Initial use of HWC focused on relatively large additions of dissolved hydrogen, which proved capable of reducing the dissolved oxygen concentration in the water outside of the core from .apprxeq.200 ppb to <5 ppb, with a resulting change in corrosion potential from .apprxeq.+0.05 V.sub.SHE to .ltoreq.-0.25 V.sub.SHE. This approach is in commercial use in both domestic and foreign BWRs. Corrosion potentials of stainless steels and other structural materials in contact with reactor water containing oxidizing species can usually be reduced below the critical potential by the use of HWC through injection of hydrogen into the reactor feedwater. For adequate feedwater hydrogen addition rates, conditions necessary to inhibit IGSCC can be established in certain locations of the reactor. Different locations in the reactor system require different levels of hydrogen addition. Much higher hydrogen injection levels are necessary to reduce the ECP within the high radiation flux of the reactor core, or when oxidizing cationic impurities, for example, cupric ion, are present.

It has been shown that IGSCC of Type 304 stainless steel (containing 18-20% Cr, 8-10.5% Ni and 2% Mn) and all other structural materials commonly used in BWRs can be mitigated effectively by reducing the ECP of the material to values below -0.230 V.sub.SHE. An effective method of achieving this objective is to use HWC. However, high hydrogen additions, for example, of about 200 ppb or greater in the water of the reactor core, that may be required to reduce the ECP below the critical potential, can result in a higher radiation level in the steam-driven turbine section from incorporation of the short-lived N.sup.16 species in the steam. For most BWRs, the amount of hydrogen addition required to provide mitigation of IGSCC of pressure vessel internal components results in an increase in the main steam line radiation monitor by a factor of five to eight. This increase in main steam line radiation can cause high, even unacceptable, environmental dose rates that can require expensive investments in shielding and radiation exposure control. Thus, recent investigations have focused on using minimum levels of hydrogen to achieve the benefits of HWC with minimum increase in the main steam radiation dose rates. In this context, it is important to recognize that significant mitigation of IGSCC can also occur when the corrosion potential is greater than -0.230 V.sub.SHE, such as when the corrosion potential is lowered by as little as 0.050 V.sub.SHE. Referring to FIG. 1, a reduction of 0.050 V.sub.SHE, for example, from -0.100 V.sub.SHE to -0.150 V.sub.SHE results in a reduction of the crack growth rate, at solution conductivities of 0.1-0.5 .mu.S/cm, by a factor of approximately two.

Another effective approach used to reduce the ECP is to either coat or alloy the stainless steel surface with palladium or other noble metals. The presence of palladium on the stainless steel surface reduces the amount of hydrogen required to reach the required IGSCC critical potential of -0.230 V.sub.SHE. The use of alloys or metal coatings containing noble metals permits lower corrosion potentials (e.g., .apprxeq.-0.5 V.sub.SHE) to be achieved at much lower hydrogen addition rates. For example, U.S. Pat. No. 5,135,709 (Andresen et al.) discloses a method for lowering the ECP on components formed from carbon steel, alloy steel, stainless steel and other iron-base alloys, as well as nickel-base alloys or cobalt-base alloys which are exposed to high-temperature water by forming the component to have a catalytic layer of a noble metal. Such approaches rely on the very efficient recombination kinetics of dissolved oxygen and hydrogen on catalytic surfaces (see the high i.sub.0 for H.sub.2 oxidation in FIG. 2, which causes most O.sub.2 reduction curves to intersect at -0.5 V.sub.SHE). This was demonstrated not only for pure noble metals and coatings, but also for very dilute alloys or metal coatings containing, for example, <0.1 wt. % Pt or Pd (see FIGS. 3 to 5). FIG. 4 shows corrosion potential measurements on pure platinum, Type 304 stainless steel and Type 304 stainless steel thermally sprayed by the hyper-velocity oxy-fuel (HVOF) technique with a powder of Type 308L stainless steel containing 0.1 wt. % palladium. Data were obtained in 285.degree. C. water containing 200 ppb oxygen and varying amounts of hydrogen. The potential drops dramatically to its thermodynamic limit of .apprxeq.-0.5 V.sub.SHE once the hydrogen is near or above the stoichiometric value associated with recombination with oxygen to form water (2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O). FIG. 5 shows corrosion potentials of Type 304 stainless steel doped with 0.35 wt. % palladium at a flow rate of 200 cc/min. in 288.degree. C. water containing up to 5000 ppb oxygen and various amounts of hydrogen.

If the surface recombination rate is much higher than the rate of supply of oxidants to the metal surface (through the stagnant, near-surface boundary layer of water), then the concentration of oxidants (at the surface) becomes very low and the corrosion potential drops to its thermodynamic limit of .apprxeq.-0.5 V.sub.SHE in 288.degree. C. water, even though the bulk concentration of dissolved oxygen remains high (FIGS. 3 to 5). Further, the somewhat higher diffusion rate of dissolved hydrogen versus dissolved oxygen through the boundary layer of water permits somewhat substoichiometric bulk concentrations of hydrogen to support full recombination of the oxidant which arrives at the metal surface. While some hydrogen addition to BWRs will still be necessary with this approach, the addition can be vastly lower, as low as .ltoreq.1% of that required for the initial HWC concept. Hydrogen additions remain necessary since, while oxidants (primarily oxygen and hydrogen peroxide) and reductants (primarily hydrogen) are produced by radiolysis in stoichiometric balance, hydrogen preferentially partitions to the steam phase in a BWR. Also, no hydrogen peroxide goes into the steam. Thus, in BWR recirculation water there is some excess of oxygen relative to hydrogen, and then, in addition, a fairly large concentration of hydrogen peroxide (e.g., .apprxeq.200 ppb). Approaches designed to catalytically decompose the hydrogen peroxide before or during steam separation (above the core) have also been identified.

While the noble metal approach works very well under many conditions, both laboratory data and in-core measurements on noble metals show that it is possible for the oxidant supply rate to the metal surface to approach and/or exceed the recombination rate (see FIGS. 6 and 7). FIG. 6 shows the effect of feedwater hydrogen addition on the corrosion potential of stainless steel and platinum at several locations at the Duane Arnold BWR located in Palo, Iowa. At .apprxeq.2 SCFM of feedwater hydrogen addition, the corrosion potentials in the recirculation piping drop below .apprxeq.-0.25 V.sub.SHE, However, in the high flux (top of core) regions, even for pure Pt, the corrosion potential remains above .apprxeq.-0.25 V.sub.SHE at feedwater hydrogen levels of .gtoreq.15 SCFM, where long-term operation is very unattractive due to the cost of hydrogen and the increase in volatile N.sup.16 (turbine shine). FIG. 7 shows corrosion potential vs. hydrogen addition for Pd-coated Type 316 stainless steel in 288.degree. C. water in a rotating cylinder specimen, which simulates high fluid flow rate conditions. The water contained 1.0 part per million (ppm) O.sub.2. As the hydrogen level was increased above stoichiometry, the potential decreased, but only to about -0.20 V.sub.SHE. The oxygen supply rate in these tests had exceeded the exchange current density (i.sub.0) of the hydrogen reaction (see FIG. 2), and activation polarization (Tafel response) of the hydrogen reaction began to occur, causing a shift to a mixed (or corrosion) potential which is in between the potentials measured in normal and extreme hydrogen water chemistry on non-catalytic surfaces.

At the point where the oxidant supply rate to the metal surface approaches and/or exceeds the recombination rate, the corrosion potential will rapidly increase by several hundred millivolts (e.g., to .gtoreq.-0.2 V.sub.SHE). Indeed, even under (relatively small) excess hydrogen conditions, pure platinum electrodes in the core of BWRs exhibit corrosion potentials which are quite high, although still somewhat lower than (non-catalytic) stainless steel (see FIG. 6). At very high hydrogen levels (well above those typically used in the original hydrogen water chemistry concept), the corrosion potential on noble metal surfaces will drop to <-0.3 V.sub.SHE (see FIG. 6). However, the huge cost of the hydrogen additions combined with large observed increase in volatile radioactive nitrogen in the steam (i.e., N.sup.16, which can raise the radiation levels in the turbine building) make the use of very high hydrogen addition rates unpalatable.

Therefore, it is desirable to develop other means for lowering the ECP of metal components in high temperature water in addition to HWC and catalytic coatings or alloys, particularly means that may overcome some or all of the limitations of these methods of lowering the ECP.

The present invention is an alternative method for achieving the objective of low ECPs which result in slow or no crack growth in stainless steel and other metals in high temperature water containing oxidant species such as hydrogen peroxide and dissolved oxygen. This is accomplished by coating the surfaces of IGSCC-susceptible reactor components with an electrically insulating material that is doped with a noble metal, such as zirconia. In accordance with the present invention, the metal corrosion potential is shifted in the negative direction without the addition of hydrogen.

This invention may be briefly described as a method for mitigating growth of a crack in a surface of a metal component adapted for use in high-temperature water, an uncoated surface of the metal component being susceptible to stress corrosion cracking in high-temperature water, comprising the step of applying a coating on the surface of the metal component, the coating comprising an electrically insulating material that is doped with a noble metal, the coating having restricted mass transport crevices which penetrate to the surface of the metal component and which restrict the flow of oxidants to the surface, the electrically insulating material doped to a concentration of the noble metal that is sufficiently small to avoid the establishment of conductive paths through the coating from the surface of the metal component to an outer surface of the coating, the crevices also exposing the noble metal to the oxidants thereby promoting reduction of the oxidants with available reductants, whereby the corrosion potential of the surface of the metal component is decreased by at least 0.050 V by application of the coating.

This invention is particularly advantageous in that these catalytic, insulating coatings lower the ECP both by limiting the transport of oxidant species to the surface of a metal component exposed to high temperature water and by the catalytic reduction of these species during the transport process.

FIG. 1 shows the observed and predicted crack growth rate as a function of corrosion potential for furnace-sensitized Type 304 stainless steel in 288.degree. C. water.

FIG. 2 is a schematic of E (potential) vs. log .vertline.i.vertline. (absolute value of current density) curves showing the interaction of H.sub.2 and O.sub.2 on a catalytically active surface such as platinum or palladium.

FIG. 3 is a plot of crack length vs. time for a Pd-coated CT specimen of sensitized Type 304 stainless steel in 288.degree. C. water containing about 400 ppb oxygen and 0.1 .mu.M H.sub.2 SO.sub.4.

FIG. 4 is a graph showing corrosion potentials of pure platinum (.quadrature.), Type 304 stainless steel (.smallcircle.) and Type 304 stainless steel thermally sprayed by the oxy-fuel (HVOF) technique with a powder of Type 308L stainless steel containing 0.1 wt. % palladium (.circle-solid.).

FIG. 5 is a graph showing corrosion potentials of Type 304 stainless steel doped with 0.35 wt. % palladium at a flow rate of 200 cc/min. in 288.degree. C. water containing various amounts of hydrogen and the following amounts of oxygen: (.circle-solid.) 350 ppb; (.tangle-solidup.) 2.5 ppm; and (.quadrature.) 5.0 ppm.

FIG. 6 is a graph showing the effect of feedwater hydrogen addition on the corrosion potential of Type 304 stainless steel at the top of the core (.circle-solid.), at the bottom of the core (.box-solid.), and in the recirculation piping (.tangle-solidup.); and of platinum at the top (.smallcircle.) and bottom (.quadrature.) of the core.

FIG. 7 is a graph showing corrosion potential vs. hydrogen addition for Pd-coated Type 316 stainless steel in 288.degree. C. water in a rotating cylinder specimen, which simulates high fluid flow rate conditions of 0.3 (.circle-solid.), 1.5 (.quadrature.) and 3.0 (.tangle-solidup.) m/sec.

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.

FIGS. 9A to 9E provide a schematic comparison of the corrosion potentials .o slashed..sub.c which form under high radiation flux on various coated and uncoated components.

FIG. 10 is a schematic of an insulated protective coating, depicted as thermally sprayed zirconia powder doped with a noble metal.

FIGS. 11 and 12 are plots showing the corrosion potential of Type 304 stainless steel uncoated (.circle-solid.) and coated (.quadrature.) with yttria-stabilized zirconia by air plasma spraying versus oxygen and hydrogen peroxide concentration, respectively.

FIGS. 13 and 14 are plots showing the corrosion potential versus oxygen concentration for uncoated Type 304 stainless steel (.circle-solid.); Type 304 stainless steel coated with yttria-stabilized zirconia with thicknesses of 3 mils (.quadrature.), 5 mils (.tangle-solidup.) and 10 mils (.smallcircle.); and pure zirconium (.box-solid.) after being immersed in pure water for 2 days and in water containing various water chemistry conditions for 3 months, respectively.

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 (<-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 (<-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.