MXPA96003099A - Method for attenuating the corrosion cracking by metal efforts in high temperature water through the control of the gri punta - Google Patents

Abstract

A method for attenuating the initiation or propagation of a crack in a surface of a metal component in a boiling water reactor, the method includes the step of injecting a solution or suspension of a compound adjusted to pH within the reactor charge water The compound has the property of changing the pH of the water at high temperature within the crack from a value outside a predetermined pH range (ie, pH 6.0 to 8.0) to a value within the predetermined pH range without causing any significant change in the pH of the loading water, the rate of crack growth when the pH of the crack is outside the predetermined pH range is greater than the crack growth rate when the crack pH is within the range of Default pH

Description

METHOD FOR ATTENUATING CORROSION CRACKING BY METAL EFFORTS IN HIGH TEMPERATURE WATER THROUGH pH CONTROL OF THE CRACK TIP FIELD OF THE INVENTION This invention relates to methods for controlling stress corrosion cracking of metal components exposed to high temperature water. As used herein, the term "high temperature water" means that the water has a temperature of approximately 150 ° C or higher. High temperature water can be found in a variety of known apparatuses, such as water coils, nuclear reactors and steam powered power plants.

BACKGROUND OF THE INVENTION Nuclear reactors are used in the generation, research and propulsion of electrical energy. A reactor pressure vessel contains the reactor coolant, ie water, which removes the heat from the reactor core. The respective piping circuits transport the heated water or steam to the steam generators or turbines and transport the circulated water or feed water back to the container. The operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 2 & amp; C for a boiling water reactor (BWR), / about 15 MPa, and 320 ° C for a water reactor. under pressure (PWR). The materials used in both BWRs and PWRs must support various load, environmental and radio conditions. Some of the materials exposed to high temperature water include carbon steel, alloy steel, stainless steel and nickel-based, cobalt-based and zirconium-based alloys. Despite careful selection and treatment of these materials for use in water reactors, corrosion occurs on materials exposed to high temperature water. Such corrosion contributes to a variety of problems, for example stress corrosion stress, crevice corrosion, erosion corrosion, adhesion of pressure relief valves and accumulation of the gamma-emitting Ca-60 isotope. Stress corrosion cracking (SCC) is a known phenomenon that occurs in the components of a reactor, such as structural members, pipes, fasteners and welds, exposed to high temperature water. As used herein, SCC refers to aggradation propagated by static or dynamic stresses in combination with corrosion at the tip of the crack. The reactor components are subjected to a variety of stresses associated with for example, differences in thermal expansion, the operating pressure necessary for the containment of the cooling water of the reactor, and other sources such as the residual stress from the welding , cooling work and other asymmetric metal treatments. In addition, water chemistry, welding, slit geometry, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC. It is well known that SCC occurs at high speeds when oxygen is present in reactor water at concentrations of approximately 1 ppb or greater. SCC is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals, are produced from the radiolytic decomposition of reactor water. Such oxidizing species increase the electrochemical corrosion potential (ECP) of metals. Electrochemical corrosion is aided by a flow of electrons from anodic to cathodic areas on metal surfaces. The ECP is a measure of the thermodynamic tendency for the phenomenon of corrosion to occur, and is a fundamental parameter in the determination of speeds of, for example, SCC, corrosion fatigue, corrosion film thickening and general corrosion. In a BWR, the radiolysis of the primary water coolant in the reactor core causes the total decomposition of a small fraction of the water for Ha, H ^ O ^, Qa chemicals. and the oxidation and reduction radicals.

For steady-state operating conditions, the equilibrium concentrations of 0a, H-gO-j. , and H-j. , they are established both in the water that is recirculated and in the steam that goes to the turbine. This concentration of 0- ?, H.J.O.J., and Hj., Is oxidant and results in conditions that can promote cracking by corrosion stresses (I6SCC) of susceptible building materials. One method used to attenuate the (IGSCC) of the susceptible material is the application of hydrogen water chemistry (HWC), wherein the oxidizing nature of the BWR environment is modified to a less reducing condition. This effect is achieved by adding hydrogen gas to the feed water of the reactor. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species homogeneously and on the metal surfaces to reform the water, thus decreasing the concentration of the oxidizing species dissolved in the water in the vicinity of the metal surfaces. The speed of these recombination reactions depends on local radiation fields, water flow rates and other variables.

The injected hydrogen fl reduces the level of the oxidizing species in the water, so that such as dissolved oxygen, and as a reduces the FCP of the metals in the water. However, factors such as the variation in the water flow velocity and the time or intensity of exposure to neutron radiation or gamma radiation result in the production of oxidizing species at different levels in different reactors. Therefore, 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 the protection of IGSCC in high temperature water. As used herein, the term "critical potential" means a corrosion potential below a scale of values of * proximately -230 to 300 V based on the normal hydrogen electrode scale (EHN tSHEl). The IGSCC proceeds at an accelerated rate in systems where the FCP is above critical potential, and at a rate that is substantially low or zero in systems where the FCP is below the critical potential. Water-containing oxidizing species such as oxygen increase the ECP of the metals exposed to water over the critical potential, while water with few or no oxidizing species shows results in an ECP below the critical potential. Because of this, the success of SCC in the BWRs is highly influenced by the corrosion potential. Figure 1 shows the crack growth rate observed and predicted with or a function of the corrosion potential for Type 30 stainless steel < + sensitized to the oven at 27.5 to 30 pa / a 2 &. ° C on the solution conductivity scale from 0.1 to 0.5 j-iS / cm. The data points at high corrosion potentials and growth rates correspond to water chemistry conditions irradiated in either test or commercial reactors. The reduction of corrosion potential is the most widely distributed approach to attenuate SCC in existing plants. The fundamental importance of the corrosion potential against, for example, the dissolved oxygen concentration per se is known in Figure 2, wherein the crack growth rate of a Pd coated CT sample falls dramatically once the conditions are reached. of excess hydrogen, despite the presence of a relatively high oxygen concentration. Figure 2 is a graph of the crack length versus time for a Pd coated CT sample of sensitized Type 304 stainless steel showing crack velocity a = 0.1 μM HjeSOt * in water 2 ftß ° C, which contains approximately 100%. 400 ppb of oxygen. Because the CT sample was coated with Pd, the change to excess hydrogen caused the corrosion potential and the rate of crack growth to be reduced. However, in the evolution of an understanding of SCC, the emphasis has shifted from controlling dissolved oxygen, to corrosion potential (the most fundamental parameter), to crack tip chemistry (an even more fundamental parameter) . Dissolved oxygen (other oxidants and reducers as well as other factors, such as the flow rate) establishes the corrosion potential; the corrosion potential, in turn, alters the crack chemistry through the concentration of anions and the change in pH. For this, the pH control of the crack tip represents the fundamental method for controlling the environmental component of the SCC. If reasonable approaches are identified to limit the pH change in the crack, SCC could be greatly reduced and become completely independent of dissolved oxygen concentration or corrosion potential.

BRIEF DESCRIPTION OF THE INVENTION The present invention is a method for controlling stress corrosion cracking of metal components by identifying and acting on the root cause of accelerated environmental crack growth in high temperature water, i.e., the pH of the solution in the crack . The evolution of the understanding of SCC has shifted from an emphasis on dissolved oxygen per se to corrosion potential, which controls the suceptibility to SCC. At a given oxygen level, the high flow rate tends to increase, while noble metal coatings can decrease the corrosion potential. The high corrosion potential causes a concentration of anions as well as pH changes in the crack. However, experiments have shown that it is not the corrosion potential per se that increases the SCC, but the corrosion potential effect & about the pH of the tip of the crack. For this reason, methods to limit the pH change in the crack can greatly reduce the SCC independently of the concentration of dissolved oxygen or the corrosion potential. According to the invention, the propagation of a crack connected to the surface or the initiation of a crack in a groove immersed in high-temperature charging water is attenuated by handling the pH of the high-temperature water within the crack or crevice. . The method for attenuating the propagation of a crack in a surface of a metallic component submerged in high temperature charging water comprises the step of injecting a solution of pH adjusting compound into the charging water. The pH adjustment compound has the ability to change the pH of the high temperature water within the crack from a pH that produces a relatively high rate of crack growth at a pH that produces a relatively low growth rate of the crack, without causing any can. Significant pH of the loading water. Approaches to managing pH within a crack or slit in a boiling water reactor vary from the use of chemistry regulating agents to adapted water chemistry (ie, controlled additions of an acid or base, depending on the anionic species). in water). Ideally, the species should have a minimal impact on the low temperature conductivity (so that the radiation waste from increased demineralized charges will not be significantly increased) and should preferably be disassociated in high temperature water. The advantages of this general approach may include: 1) lack of intex of corrosion potential or associated oxidizing species 2) decreased emphasis of anionic impurities, since its effect on crack pH chemistry would be directly controlled; 3) the possible reduction in the flow through the reactor water cleaning system, which typically represents 1-3% of the reactor water flow and has an impact on the overall thermal efficiency. According to a preferred method of the invention, the special characteristics of the differential aeration slit, i.e., the anion concentration and the low internal corrosion potential, can be used as the solution bath for the problem of controlling the pH of ls. crack. As an example, a nitrate solution can be injected into the reactor water to obtain a desired pH in the high temperature water within a crack in a reactor component. The nitrate anion (NOg, -) will be driven over the potential gradient along the crack, causing an increase in its concentration in the crack. However, once it migrates into the cjrieta, where the potential is low and therefore the environment is more reducing, the nitrate anion is reduced to ammonia. Ammonia itself is a regulator whose pH in water at 250 to 300 ° C is appro priately or. for a concentration of 25 ppm. Assuming a typical increase of 30x (above the general nitrate level), by the potential gradient, an ammonium level of 25 ppm would be reached within the crack at approximately a nitrate level of 1 ppm in the loading water. Since some fraction of the ammonium formed in the crack is dissociated to ammonium cation and hydroxyl anion (NH + 0H = NH ^ 1 * + OH "), and since the ammonium cation will tend to be expelled from the low slit the action of the potential gradient, it is expected that the nitrate level will be increased, for example from 1 to 3-5 ppm, to obtain the desired pH as a function of the temperature, the presence of other impurities in the water, etc. , the nitrate concentration could be varied on a significant scale, for example from <; 0.1 a > 10 ppm. This would have a much lower effect than the proportional effect on the pH, because the ammonium is a pH regulator, (ie, any increase in the total ammonia concentration rises between the concentration of ammonium and hydroxyl cation by a much smaller amount, since not all the ammonia dissociates to form those ions).

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the rate of crack growth observed and predicted as a function of the corrosion potential for Type 304 stainless steel sensitized to the furnace in water at 2 & amp; C. Figure 2 is a graph of crack length versus time for a CT specimen coated with Pd of Type 304 stainless steel sensitized in water at 2AA ° C containing approximately 400 ppb of oxygen and 0.1 juM H_j.SC. , -,. Figure 3 is a plot of the crack length versus time for a CT sample coated with Pd of sensitized Type 304 stainless steel. Figure 4 shows the effect of additions of H ^ SO ^ on the rate of crack growth of sensitized Type 304 stainless steel that was tested in water at 2 &9 ° C desaeread. Figure 5 shows the influence of the molar ratio of borate: sulfate on the crack growth rate of sensitized Type 304 stainless steel which was tested in water at -9 ° C tested with H "S0t. Figure 6 shows the interrelationships between the corrosion potential and the conductivity of the solution to establish a crack tip anion activity by comparing the rate of crack growth in solutions aereated against solutions eroded.

FIGS. 7A and 7B are graphs respectively of the crack extension against the response in time of type 304L non-sensitized stainless steel and Alloy 62 weld metal to changes in water chemistry. Figure 7C is a graph of crack length versus time for a Pd-coated CT sample of sensitized Type 304 stainless steel showing accelerated crack growth that can be obtained at thermodynamically lower potencies provided sufficient Ha is added. S0it. The figure & is a graph of crack extension versus time for Type 304 stainless steel sensitized in water at 2AA ° C showing the effect of various concentrations of 0H ~ as NaOH. Figure 9A and 9B show respectively the effects of the sulfide concentration on the excess anodic current density (metal dissolution rate) and the slow strain rate behavior of AJloy O00. Figure 10 shows the effect of impurities on crack initiation as measured during the slow strain rate test of Type 304 stainless steel sensitized in water at 2AA ° C with 200 ppb 0 ». Figure 11 is a graph showing crack growth and crack chemistry for low alloy steel and external water having 1 ppb of sulfate. Figure J 2 is a graph showing the changes in the chemistry of the internal slit against the distance in a tube of 1 mm internal diameter of Allay 600 co or it is received filled nicely with a high concentration of (0.3 M) of Na_5.S0 and exposed to aerated water at 2AA ° C. The line denotes the predicted pH value. Figure 13 is a graph showing the changes in the chemistry of the inner groove versus the distance in 1-mm internal diameter tube of annealed solution Alloy 600 initially filled with a high concentration of (0.2 N) of NßN0a and exposed to pure water at 2ft & ° C (< 40 ppb oxygen). Figure 14 is a plot of potential against "Pourbaix" pH of the nitrogen-water system at? 50 ° C showing the thermodynamic stability of ammonium against nitrate at low potentials. Figure 15 is a graph showing the changes in the chemistry of the internal slit against the distance in a tube of 1-mm internal diameter of the initial type 304 stainless steel annealed solution filled with a high concentration of (0.2 N) of NaNO »and exposed to pure water desaereada at 2Sñ ° C (< 40 ppb of oxygen).

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Before describing the best fashions of practicing the invention, certain basic concepts will be explained in order to facilitate the understanding of the invention. The pH of the aqueous solution can change dramatically with temperature. In particular, the pH of the water at high temperature is generally very different from that of the low temperature. However, the pH of an aqueous solution is not only a function of temperature, but also other variables such as the composition of species dissolved in the solution. Even the direction of pH change varies from one species to another. For example, the neutral water pH (eg, ultra high purity) is 7.0 to 25 ° C, approximately 6.2 to 6 ° C and approximately 5.63 to 2ft ° C, a general change in fH * J of 25 times (pH is -log CH * 3. The variation of pH in pure water is not onotonic, although it reaches a minimum at 270 ° C and then begins to increase again.In contrast, a significant increase in pH against temperature occurs for the same species For example 0.01 M NaHS0"is approximately 2.4 ph at 25 ° C, although it increases up to approximately 6 to 300 ° C due to the low ionization of sulphate (HS0. + - = H * + SO ,, 3 * -) Note that the change in pH against the temperature for a NaHSO solution is opposite to that of pure water.Other species, including some pH regulators, change to acids with temperature.

For example, the pH of low concentrations of ammonium (for example, 40 ppm) at high temperatures is approximately 10.3, although it is approximately 6.2 a? ßß ° C. The concentration of pH species in the solution would be constant anywhere in the water other than for concentration mechanisms. For example, the interior pH of a crack tip can vary markedly from the pH in the charge reactor coolant. The simplest conceptor mechanism is located in the dissolution in an occluded region (girt / slit), which occurs in low alloy and carbon steels as a result of the dissolution of the MnS incJusiun. The slightly elevated gaseous / slit chemistry (up to several ppm S) is sufficient to create conditions which increase the corrosion crack growth rates by stresses up to several orders of magnitude (see Figure 11). Another common mechanism is boiling, which produces huge differences in chemistry in regions where mass transport is restricted. Thermal gradients (boiling conditions) can also produce very large changes in chemistry in occluded regions. Changes of several orders of magnitude are estimated under some conditions in light water reactors. Potential gradients are the most common mechanism of concentration in boiling water reactors. Because oxygen is consumed in the slit, the corrosion potential is much less than the outside of the slit. This creates a potential gradient that causes anions (eg, chloride, sulfate, or hydroxide) to move within the slit, and cations (eg, sodium) to move out of the slit. In general, if the anion other than hydroxide is present, allow acidification in the slit, as it can "support" (charge balance) the additional H * associated with acidification. Only the present anion is hydroxide (although cations other than H * are present), then only an upward change in pH (alkalization) may occur. Measurements made in slit tubes exposed to water at? & amp; C by D. F. Taylor showed that a significant different concentration and pH could persist in differential aeration cells. The solid state behavior was always obtained in less than 1 day, and their experiments were operated for a week or more, and showed an external pH of 5.5, with an internal pH of < 1.0 in some cases; in other cases, the internal pH was as high as 11 (see figures 2 and 13). At concentrations more representative of typical BWR operating conditions, An resen showed that the potential gradient typically caused a 30x increase factor in the anion concentration in an increased strain stress corrosion crack. [Reported in "Modeling of Water and Material Chemistry Effects on Crack Tip Chemistry and Result in Crack Trawth Kinetics", Proc. 3rd Int. Conf., Degradation of Materials i.n Nuclear Fo er Indust.ry, Traverse City, MI, Aug. 31-Se t. 4, 19 &7J. This was true for low alloy steels, where the anion results from the dissolution of internal Mn3 inclusions to the crack, as well as with stainless steel, where the anions are associated with impurities that are added intentionally to the water. load at very low concetrations. A factor of 30x is not huge, particularly when the relevant impurity levels are low to start with (for example, L-7, M), and can lead to pH changes within the crack that are lower than those of 1.5 units of essentially neutral charging water. However, this difference is sufficient to induce enormous changes in the rate of crack growth, for example, by a factor of more than 100-1000 times. This is the condition (pH) within the crack that the method of the present invention seeks to control (see figure 11). In accordance with the preferred embodiments of the invention, the propagation of a crack in a metal component immersed in water at high temperature can be attenuated by controlling the pH of the solution within the crack. In the case of a water-cooled nuclear reactor, for example, a BWR, the internal pH of the crack can be controlled by adding a pH adjusting agent to the circulating water, for example, by injecting a solution of a suspension of the agent into the water. the water of feeding. The pH adjusting agent can take the form of a chemical pH regulating agent or an acid or bae, depending on the magnitude and direction of the change required in the pH. As used hereinafter, the term "solution" means a solution or a suspension. The susceptibility of metal components in high temperature water to SCC is correlated with high corrosion potentials. The high corrosion potential causes the concentration of anions as well as pH changes in the crack. This is the effect of the corrosion potential on the pH of the crack tip that increases the SCC. The following logic establishes the fundamental importance of the pH of the crack tip. The potential for corrosion (on external surfaces) and the conductivity of the solution (charge) will interact to create a specific crack chemistry. By this, a given crack chemistry can be obtained by different combinations of potential and conductivity, ie, high potential, moderately low conductivity can give the same crack chemistry as the low theoretical potential, albeit with high conductivity. The formation of analytical and experimental model exists to demonstrate these basic interactions, although precise relationships on the full scale of potential and conductivity levels (and types) are not fully quantified / confirmed either analytically or experimentally. Figures 1 and 3 show examples of different sensitivity to, for example, conductivity as a function of corrosion potential. In particular, Figure 3 is a plot of the crack length versus time for a CT coated sample can Pd of sensitized Type 304 stainless steel showing increased tolerance at high levels of impurity (0.1 μM HaS0t,, or 0.663 μS / cm) at 33 MPaJ "m when a low corrosion potential (which results from the hydrogen of stoichiometric excess, ie 76 ppb Ha) remains on a catalytic surface, despite the presence of 400 ppb of oxygen. the excess of oxygen at 6124 hr, the corrosion potential and the growth rate increase dramatically, the excess of hydrogen at 6244 hr causes the corrosion potential and the growth rate to be reduced again. a low potential (generated under excess of hydrogen although moderately high oxygen, the conditions of a CT sample coated with P) the tolerance to impurities is dramatically higher than with poten high corrosion However, even at low potential, the addition of sufficient impurity causes an increase in the rate of crack growth (see Figures 4 to 6). Figure 4 shows indeed the HaS0t additions, on the crack growth rate of sensitized Type 304 stainless steel tested in de-aerated water at 269 ° C with 0O = -0.6 V-ßH ?: in experiments of slow forming velocities lx 10 -, * sec-1. Figure 5 shows the influence of the molar ratio of borate: sulfate on crack growth rate of sensitized Type 304 stainless steel tested in de-aerated water at 2A9 ° C with 0.3-10 ppm H_BSG- + in formation velocity experiments slow to 1? IO -. ^, Sec-i. Figure 6 shows the interrelationships between the corrosion potential and the conductivity of the solution to establish a crack tip anion activity by comparing crack growth velocities in airborne versus unleaded solutions for stainless steel and A.llsy 62 at 2A6 ° C. If the growth rates under the conditions are similar, then it is inferred that crack tip chemistries are similar. Each point represents a pair of data obtained at similar crack growth rates and loading conditions for stainless steel and Alloy 62 in water at 266 ° C. Since generally perfect comparisons, which require identical crack growth rates under aerated and desaereated conditions are not available, the labels (e.g., 2.9x) indicate the crack growth velocity ratio in airborne versus unleaded solutions. The arrows indicate where the points would change if the union of growth velocity were perfect. In a desired solution, the ppm of the anion is approximately equal in the charge solution and crack tip. The rate of crack growth is controlled by the crack chemistry, not, for example, by the external potential per se. While some crack growth models depend on the cathodic reduction of oxygen in the crack mouth, the corrosion potential eJeved is not necessary. The best datas to demonstrate that point come from the slow-speed velocity tests by Shack and others, reported in "Environmental Assisted Cracking in Light Water Reactor," Semiannua.l Report, Apr il-Septe ber 1965, NURFG / CR-4667 , Vol. 1 and Andreeen CT tests, reported in "Msdeling of Water and Material Chemistry E fe fects on Crack Tip Chemistry and Result of Crack Growth Kinetics", Proc. 3rd Int. Conf., Degradation of Materials in Nuclear Power Industry, Traverse City, MI, Aug. 31-Sept. 4, 1967. The data of Shack and others appear in Figures 4-6 of this; The Andresen data appear in figures 6 and 7 of this. The data conclusively show that high growth rates can be reached at -05 VßHe if an acid addition is made up to a level of ~ 10 μS / cm. Shack and others have shown this on sensitized stainless steel; Apdresen has shown it on sensitized and non-sensitized stainless steel and Alloy 62 welding metal. The improvement is not subtle: it may not obtain such high growth rates as aerated water. The cracking responds to changes in the pH of the solution at the tip of the crack, not to the activity of the sulfate per se. Increases in the rate of growth on sensitized stainless steel have been demonstrated by Andresen to occur for either lower changes (see data in Figures 4-6 and 7A-7C) or higher pH changes (see data in Figure 6, although more pH change is required for NaOH than for HaSOt,). Likewise, both Shack and others co-Apdresen (see data in Figures 4-6 and 7A-7C) have performed tests that show that, in completely de-aerated water, the increase in crack growth rate occurs for acid additions (H ^). SO-,), although not for most neutral additions (for example NaaSOt,, which changes in some way to basic in water at 266 ° C). Shack and others also tested with mixtures of borate and HßS0t + and showed that only when the buffering capacity of borate ee exceeds the growth rate increases (see data in Figures 7A-7C). Figures 7A and 7B are respectively graphical of crack extent versus time response of non-sensitized Type 304L stainless steel and Alloy 62 weld metal to changes in water chemistry showing high crack growth rates in the material not sensitized in disheveled water at high rates. This illustrates the interrelationship between the conductivity of the solution and the corrosion potential to create a specific crack tip solution chemistry and therefore a specific crack growth rate. The straight line segments represent predictions of crack growth velocity behavior for the different conditions shown. Figure 7C is a plot of J at crack length versus time for a Pd coated CT sample of sensitized Type 304 stainless steel showing that accelerated crack growth can be obtained at thermodynamically lower potentials (characteristic of completely de-aerated water) , or catalytic surfaces with excess hydrogen, say 76 ppb H__. with 400 ppb 0-j.) provided sufficient HaS0 ,, is added. This also shows that the low corrosion potentials provide a high tolerance to impurities since, at high corrosion potentials, the effects on crack growth of ~ 0.1 μM H __. S0t,. Figure 6 is a graph of crack extension versus time for Type 304 stainless steel sensitized in water at 266 ° C showing the effect of various concentrations of UH ~ to NaUH. Note that there is no acidification in the water present (there is no corrosion potential gradient in the crack), so that an acid impurity can be used. In aerated water, N ._5.SOi.,. acidified within the crack to H3.SO.,, co or shown directly by Taylor et al., reported in "High-Te perature Aqueous Crevice Corrosion in AJloy 600 and 304L Stainless Steel", Prac. Conf. On Localized Crack Chemistry and Mechanics of Environ entally Assisted Cracking, AIME, Philadelphia, Oct. 1963. Taylor and others also show that switching to high pH occurs in pure water, NaQH. While cracking may occur if the pH increases or decreases, increased rates of growth require a more basic than acidic change. This is based on the data collected by Andresen on stainless steel sensitized with additions of NaOH, reported in "Effe ts of Specific Anionic Impurities and Environmetal Cracking of Austenitic Materials in? 66 ° C Water". Note, however, that, at high pH, the reduction of sulfate, sulfite, etc. Sulfur is important. It has been shown that sulphite levels (Sa_) greater than ~ 100 ppm in the crack cause depassivation and accelerated crack growth rates. FIGS. 9A and 9B respectively show effects of the sulfide concentration on the excess anodic current density (velocity of metal dissolution) and the behavior of the slow deformation velocity of Alloy 600. A threshold step is shown. a ~ 100 ppm sulfur. The test conditions were as follows: 7500 ppm B, pH = 7, 290 ° C. For Figure 9A the strain rate was 2 x 10-3 sec-1. For Figure 9B, the applied potential was -720 mVßMK. Therefore, by controlling the pH of the crack, the lowest possible crack growth rates are obtained for a given material charge and temperature. The interrupted slow-speed crack initiation tests performed on smooth samples by Andresen and reported in "The effects of Aqueous Impurities on Intergranular Stress Corrosion Cracking of Sensitized Type-304 Stainless Steel", EPRI NP-3364, Final Report, l ^ iav. 1963, shows deformed higher ions for crack initiation for basic impurities and deforms lower ions for crack initiation for acid impurities. Figure 10 shows the effect of 10 μS / cm of impurities on crack initiation as measured by repeated interruption during the slow strain rate test (3.3 x 10-1 'sec-1) of type 304 stainless steel sensitized to water at? 66 ° C with 200 ppb Ojj,. The results correlated poorly with the level of impurities (or conductivity of the eoJution), although they showed good correlation with the pH (approximate) at the test temperature. Andresen and others also performed the work reported in "Eehavior of Short Cracks in Stainles Steel at 266 ° C", Paper No. 495, Corrosion 90, NACE, Las Vegas, Nevada, which shows that small cracks behave better than cracks large (ie they become de-aerated within and have a high concentration of the anion at the tip) once they reach a depth of 20 to 50 μm and have coalesced multiple cracks within a larger crack. The question of how cracks behave when they are very small and exposed to oxygen-containing water (a little closer to "initiation") is more complex. Because of this, the benefits of pH control apply to "soft" surfaces, very small cracks and large cracks. In accordance with the preferred methods of the present invention, the handling of the pH in the crack can be obtained by four related techniques (some of which depend on the presence of a gradient in the corrosion potential): I.- If there is the detailed cone of all the anions and cations in the water of BWR (some BWRs measure the species mediate ion chromatography in line,) then it is possible to "manually adjust" the pH of the tip of the crack by adding the precise concentrations of acidic, neutral or alkaline species so that, at the concentration within the crack or slit, the resulting pH falls within the desired range of 6.0 to 6.0. Such additions, in most cases, would cause the charging water to be outside this pH range. While it is obviously feasible in the concept, this "manual" approach would be difficult since it would depend on accurate knowledge of the "mixed" concentration of all species in the BWR water which may vary over relatively short periods (hours). . Minor variations in the concentration of some species could cause comparatively large change in the pH of the crack. Therefore, while it is conceptually viable, the use of some type of chemical regulator is preferred., since it provides a much wider tolerance to changes in impurity concentrations. II.- The use of a regulator to "knock down" the effects of pH change of other anionic impurities that are concentrated in the crack. Since only small changes in pH usually occur from anionic impurities such as sulfate and chloride (ie, <1 to 1.5 pH units), the pH regulator does not need to be a regular pH strong. For this, an ideal example would be a compound that only dissociates eemanally in water at high temperature. The advantage would be that, with the vast majority of those composed in non-ionized form, it would not concentrate too much in a crack under the action of the potential gradient. An example can be silicon hydroxide, which only dissociates by approximately 1% from (Si (0H) t, = H * + SiO (0H) af-> A small amount of ba? E (such as sodium hydroxide or lithium approximately 0.1% by weight of silicon hydroxide) would be added to "adjust" the initial pH in some way above the neutral point on the desired scale III.- Use of a more normal pH regulator ( stronger) to "knock down" the pH-changing effects of other anionic impurities that are concentrated in the crack.The higher ion dissociation of this pH regulator would promote the concentration in the crack of the anionic part of the pH regulator. is the boric acid (HaB0a), whose pH is often "adjusted" with lithium hydroxide, where the anion H BOjg- would concentrate in the crack, however, the charge balance cation formed in the crack is mainly H *, means that the balance of H * and H? - B0-j. ~), is similar both within out of the crack.

Because significant dissociation occurs (H.3BO3 =. + H3BO .__. ~), The borate anion migrates under the action of the potential gradient, and therefore the concentration of the pH regulator in the crack would be greater than in the load. Furthermore since only small changes usually occur in the pH in BWRs (ie <1 to 1.5 pH units), the required pH regulation capacity does not need to be high. Therefore, very low concentrations of boric acid could be used in the loading water (perhaps 0.1 to 10 ppm B, which is approximately 100 to 10.00 times lower than the level used in pressurized water reactors, where boron serves as a neutron absorber). Proportionally less lithium hydroxide would be used than that typically used in PWRs (which is approximately 2000 ppm B or boric acid and 1.2 ppm Li as LiOH), since the LiOH (Li * and 0H ~> completely ionized will concentrate in the crack more than the partially ionized boric acid. sufficient to control the pH in the crack within the specified scale, ie 6.0 to 6.0 Other species, such as ammonium hydroxides or phosphates, can also be used IV.- The fourth alternative is a method to control SCC through self-regulation Its purpose is to reduce cracking by a very low minimum value independent of the corrosion potential and, to a reasonable degree, the presence of aggressive impurities such as sulfate and chloride. If a salt, such as sodium, potassium, or lithium nitrate, is added to an NH03 acid, the addition of water will not significantly change the pH of the loading water, however, since the ambient Within the cracks in the de-aerated and at low potential, the nitrate diffusing into the cracks will reduce to ammonia, producing a localized increase in pH. In high-demand areas (ie, high corrosion potential), higher pH regulation action occurs because more nitrates are concentrated within the crack. This increase in the autoregulatory pH is due to the equilibrium between NHa i N, 0H, and NHt, * / 0H-. For this, nitrate (reduced to ammonia) provides protection (only) in the locations (cracks) where changes in pH would otherwise occur (from anion concentration the cell of "differential aeration" slits "avoiding in this way a significant increase in the rate of crack growth The capacity for "self-regulation of pH" is associated with several characteristics of the system ": 1.- The presence of almost neutral nitrate species in the cargo water that has They are shown as non-harmful for crack initiation 2.- Diffusion and ionic migration (driven by the "differential aeration" slit cell) of eeas eßpecieß within the incipient crack, large cracks and crevices causes an increase in its concentration on its load values Fl consumption of oxygen near the nozzle of the crack creates a low potential in the crack (this is the origin of the cell of "cleavage" of aereacióp differential), and also causes the reduction of nitrate to ammonia. For this reason, in the BWR regions where the corrosion potentials (and dissolved oxygen and hydrogen peroxide concentration) are higher and give rise to a greater susceptibility to SCC, the concentration of a ania in the crack is also higher . For this, the pH regulator is formed where it is necessary, and its concentration is automatically regulated by the same phenomenon that is most available for high susceptibility to SCC. A balance in mass transport kinetics occurs because as the nitrate is driven into the crack under the action of the potential gradient, then the cation of a opia eß i pulsed out of the crack. However, since the concentration of ammonium ion is lower than nitrate (ionic ammonium is also formed), the ammonium ion in excess (and l.a amopia) will exist in the crack. 3.- Unlike other species (for example NaOH), the pH in the crack will not rise in direct proportion to the concentration, since the balance between NHa, K + OH, and NHt, * / 0H ~ provides an action pH regulator that must maintain a pH at crack temperature between 6.0 and 6.5. The use of ammonia balance is also attractive because of its relatively strong temperature independence. In contrast, while traditional pH regulators such as boric acid and phosphate could achieve a similar objective, they have disadvantages such as higher concentrations for a given pH buffering capacity and undesirable nuclear consequences (i.e. boron). Direct additions of amopia have also been used (for example in PWRs) as a pH regulator; its desveptajaß, compared with the preferred embodiment described herein, includes in the need for a higher concentration ie, it would not derive benefit from (lack of) the concentration of this cation in the potential gradient, as opposed to the nitrate anion) and its volatility (large concentrations would have to be added continuously to compensate for its loss in the vapor phase). The addition of a nitrate salt to the BWR water would not significantly change the pH of the charge water, although it could be combined with small additions of a strong base (e.g., sodium hydroxide) or a weak base (e.g. manganese) to cause a slight alkaline change in pH. A slight increase in the pH in the loading water is desirable because the system changes to a minimum in the solubility for many oxides, which, in turn, can decrease the formation and release crude deposits on the fuel. The success of this approach depends on: (1) the reduction of nitrate to love in the crack; (2) the presence of a nitrate / ammonium concentration that is sufficiently in excess of other harmful impurities (eg, chlorine and sulfate) so that the pH of the crack remains in the desired region; (3) this ability of nitrate in BWR water oxygenated charge, so that relatively little ammonia is lost towards the vapor phase; (4) the ability to measure / regulate nitrate concentration, for example, using anion exchange resins that are partially converted to nitrate bake) this maintains the nitrate concentration in the reactor water cleaning system). The potential advantages of this general approach include: (1) lack of interest for the corrosion potential and associated oxidizing species; (2) decreased emphasis of the anionic impurities, since its effect on crack chemistry (pH) would be directly controlled (although it would not be allowed to rise to a level approaching that of nitrate, and (3) the possible reduction in the flow through the reactor water cleaning system, which typically represents 1 to 3% of the reactor water flow and has an impact on the overall thermal efficiency, as shown in Figure 14, below a oxide potential reduction of approximately -0.3 VßHß, the most stable ammonium species However, the reduction kinetics of nitrate to ammonia is not very fast, since it has been observed in an "open" autoclave which, following the moderate exposures "average residence time of 20 to 30 minutes) in water at high temperature (containing 95 ppb of hydrogen), very little ammonia is formed or formed, however, following a one-week exposure in a hend tube 1-mm-ID idura, Taylor observed that the almost complete reduction of nitrate to ammonia, with an associated increase in pH (see Figure 15). In summary, the concept of the present invention involves the control of SCC limiting the changes in the pH of the crack in water at high temperature, ie within the range of about 6.0 to 6.0. This seems to correspond to the minimum in the solubility of iron oxide and nickel against the pH in water at high temperature. The invention addresses a more fundamental issue in SCC than controlling dissolved oxygen or corrosion potential since the latter interact with the impurities in the charging solution to generate a changed pH crack chemistry. If the pH is controlled, the SCC can be greatly reduced completely independently of the dissolved oxygen concentration or the corrosion potential. The approaches to limit changes in the pH scale from chemical regulatory agents (such as NH., 0H, NasH, a_x> P0t, and HaB0a> for the adjusted water chemistry, ie, controlled additions and a acid (such as HaP0t,, Ca.Ht, 00H, CI-C-OH, HaB0a and HXCQ-?) or a base (such as ZnO), NaOH, LiOH and k'OH), depends on the anionic species in the water. While it would be desirable to identify species that would not be necessary in significant concentrations or that do not significantly dissociate in low temperature water (and therefore would not be efficiently removed by, or reduce the operational life of, demi-mineralizers), it is also possible to reduce the fraction of the reactor water that is purified in the water cleaning system of the reactor, since the impurities (as well as the corrosion potential) no longer influences the SCC if the pH of the crack is controlled. It is also possible to "convert" the demineralizer so that it becomes, for example, a bar Ha * instead of H * based on the cations. The above method has been described for illustration purposes. The variations and modifications of the method described for practicing experiments in the relevant techniques will be easily apparent. All these variations and modifications are intended to be covered by the claims that are established in the succeeding one.

Claims (7)

NOVELTY OF THE INVENTION! CLAIMS

1. - A method for attenuating the propagation of a crack connected to the surface in a metallic component immersed in the high temperature charge water of a boiling water reactor, comprising the step of injecting a solution of a compound into the charge water, said compounds having the property of changing the pH of the water at high temperature within the crack from an external value at a predetermined scale of pH to a value within said predetermined range of pH without causing any significant degradation of the pH of the water of loading, wherein the crack growth rate when the crack pH is outside said predetermined scale is greater than the crack growth rate when the pH of the crack is within said predetermined pH scale.

2. The method according to claim 1, characterized in that the predetermined pH scale is 6.0 to 6.0.

3. The method according to claim 1, further characterized in that said compound is a pH regulator.

4. The method according to claim 3, further characterized in that the pH regulator is boacid or silicon hydroxide.

5. The method according to claim 3, further characterized in that it comprises the step of adding a baße to the loading water to adjust the pH to poc above the neutral.

6. A method for attenuating the propagation of a crack connected to the surface in a metal component immersed in high temperature charge water of a boiling water reactor, comprising the step of injecting a solution of a compound into the charge water , said compound having the property of supporting the reduction within the crack to release a pH regulator.

7. The method according to claim 1 or 6, further characterized in that said pH regulator is ammonia. 6. The method according to claim 1 or 6, further characterized in that said compound is a species of nitrate. 9. A method for attenuating the initiation of a crack connected to the surface in a slit in a metal component submerged in the high temperature charge water of a boiling water reactor, comprising the step of injecting a solution of a compound into the reactor. of the charge water, said compound having the property of changing the pH of the water at high temperature within the slit from a value outside a predetermined pH scale to a value within said predetermined pH scale without causing any significant pH change of the charge water, wherein a crack is initiated in the slit, when the pH of the slit is outside said predetermined pH range and the crack is not initiated in the slit when the pH of the slit is within said slit of the slit. Default pH. 10. The method of compliance with claim 1 or 9, further characterized in that said compound dissociates into the slit to release a pH regulator.