RU2298049C2 - Zirconium alloys treatment process - Google Patents

Abstract

FIELD: nuclear power engineering, namely anticorrosion treatment of tubes of technological lines, envelopes of fuel elements and spacing grids.

SUBSTANCE: process comprises steps of forming on surface of article oxide film of the same alloy and implanting nitrogen ions to article surface. Nitrogen ion implantation to article surface is realized by means of nitrogen ion beam at irradiation dose 5 x 10¹⁷ - 10¹⁸ cm^-2 and article temperature 350 - 400°C provided due to dissipation of beam energy.

EFFECT: prevention of local corrosion of treated products, lowered rate of uniform corrosion.

11 dwg, 1 tbl, 1 ex

Description

The technical field to which this invention relates is nuclear energy. At the present stage of the development of nuclear energy, the important problem is to increase the degree of fuel burnup to increase the efficiency of existing and designed reactors. This requires a further increase in the corrosion resistance of zirconium alloys. Potentially promising, but not sufficiently developed as applied to zirconium-based materials, remains the method of modifying near-surface layers using radiation and plasma technologies.

A known method of processing zirconium alloys [1] to slow down uniform corrosion, which consists in the fact that element ions are implanted into the oxide passivating film previously created on the surface of the products, starting from group 5. In addition to corrosion uniformly distributed over the surface of the product, the kinetics of which can be slowed down by the above method, ulcer corrosion is characteristic of zirconium, the rate of which is significantly higher than the rate of uniform corrosion. It is ulcer corrosion that determines the service life of products made of zirconium alloys. The main cause of ulcer corrosion is the heterogeneity of the physicochemical properties of the surface of the material, namely: the presence of intermetallic inclusions [2] or the release of excess zirconium in the ZrO _2-x film in the form of metallic colloidal inclusions [3], cracks and pores in the film [4] . Since the volume of the growing oxide film is 1.5 times larger than the volume of the metal being oxidized, this leads to the appearance of stresses in the film, which are partially removed by the formation of cracks in the film. Microcracks are pathways for the rapid diffusion of oxygen to a metal base. Metal inclusions can also "bypass" the protective film and cause local - faster than uniform corrosion. Heat treatment to homogenize the films of the ZrO _2-x Zr heterostructure is not possible. In addition, the increased tendency of zirconium to adsorb various gases during thermal treatments can lead to its contamination. Annealing in vacuum provides higher resistance to continuous corrosion, but does not eliminate local corrosion [2]. Arbitraryly selected implantation modes described in the prototype make it possible to create an inner getter layer, but do not allow homogenizing the initial film and making it more dense and uniform in thickness, which is required to prevent local corrosion. The processing technology by high-intensity ion beams combines at the same time two beneficial actions: introducing the desired impurity and radiation annealing as a result of dissipation of the beam energy. The task of the invention is the selection of optimal implantation modes to prevent local corrosion. This problem is solved by the fact that the product is subjected to final annealing, then a zirconia film is formed on the product, after that one of the elements is introduced into the resulting film by ion implantation, starting from group 5 and above, and the beam current density should ensure that the product is heated during implantation to temperatures of 350-400 ° C, the ion fluence should exceed 10 ¹⁷ cm ^-2 .

An example implementation of the described method.

Standard spacer grids made of alloy E110 (Zr - 1% Nb), used in RBMK reactors, which underwent all standard processing, including annealing and oxidation, were cut into samples, as shown in Fig. 1.

Radiation processing of fragments of spacing gratings was carried out using a pulse-periodic ion source [5]. Nitrogen was used as the working gas, the accelerating voltage was 30 kV, the beam current density was 3 mA / cm ² , the pulse duration was 1 ms, and the repetition rate was 3–25 Hz. The working pressure in the vacuum chamber was 4-6 · 10 ^-2 Pa. The temperature of the samples, determined by the power of the ion beam, varied in the range of 50-400 ° C.

Samples treated with N ⁺ beams in a dose range of 10 ¹⁵ -10 ¹⁸ cm ^-2 were subjected to corrosion tests: the samples were “oxidized” in an electric resistance furnace in air at a temperature of 400 ° C. An indicator of the corrosion rate was the gain - an increase in the mass of the sample per unit area (mg / cm ² ). For susceptibility to pitting corrosion, the samples were tested in a solid electrolyte (Ni + Li ₂ O powder) for 50 hours at 520 ° C.

The chemical state of elements in the surface layers of zirconium dioxide before and after implantation of nitrogen ions was studied by X-ray photoelectron spectroscopy (XPS), the analysis depth of which is 3-5 nm. To study the phase composition variation in depth up to 120 nm, we used etching with an argon ion beam, which allowed us to remove the surface layers of the sample at a rate of ~ 2.5 nm / min.

The surface morphology of the samples before and after implantation was studied by scanning electron microscopy.

Figure 2 shows photographs of samples after testing for the tendency to pitting corrosion. Black areas of the images correspond to dense anion-deficient zirconium dioxide, which has good adhesive and protective properties; the white areas of the images correspond to stoichiometric zirconia, which is characterized by a loose structure, poor adhesion and, as a result, easily crumbles, leading to acceleration of corrosion of zirconium alloys. Plots affected by local corrosion are detected on control samples and samples after implantation at temperatures less than 300 ° C and doses less than 10 ¹⁷ cm ^-2 . On samples No. 62 and No. 58, implanted with doses of 10 ¹⁷ and 10 ¹⁸ cm ^-2 at 400 ° C, respectively, local corrosion could not be provoked.

Table 1 shows the data on the specific surface affected by local corrosion, based on the results of image processing by the SIAMS program. With an increase in implantation temperature up to 400 ° C and doses up to 10 ¹⁷ cm ^-2 and more, the area of local corrosion damage decreases from 20-30% to 0.1-02%. Such a significant improvement in the corrosion properties of objects is due to structural-phase changes caused by ion implantation.

Table 1 Arr. Implantation Options The specific surface occupied by local corrosion,% T, ° C Fluence, cm ^-2 12 fifty 10 ¹⁵ 33.3 four 10 ¹⁶ 4.8 26 200 10 ¹⁵ 13.9 eighteen 10 ¹⁶ 9.25 16 10 ¹⁷ 11.8 70 10 ¹⁸ 2.45 38 300 10 ¹⁶ 12.4 59 10 ¹⁸ 7.95 62 400 10 ¹⁷ 0.25 58 10 ¹⁸ 0.15 97 Control 20.25 104 30.55

The XPS method revealed additional phases that appeared after implantation. Figure 3 shows the XPS spectra, from which it follows that after implantation with a dose starting from 10 ¹⁷ cm ^-2 , inclusions of oxynitride phases ZrN _x O _y (line I) and lower metastable oxides Zr ₂ O ₃ (line II) appear. The matrix of ZrO ₂ corresponds to line III. The maximum of line I corresponding to the ZrN _x O _y phase is located at a depth approximately equal to the projective range of nitrogen ions with an energy of 30 keV. The forming layer of precipitates of the oxynitride phase serves as a getter for structural defects and impurities. At doses less than 10 ¹⁷ cm ^-2 no precipitation of the oxynitride phase is observed (the implanted nitrogen remains in the form of a solution in ZrO _2-x ), when a dose of 10 ¹⁸ cm ^-2 is exceeded, a dynamic equilibrium occurs between the introduction and sputtering processes with the same beam [6].

Figure 4 shows the electron microscopic images of the surface of the samples in the initial state and after implantation of nitrogen ions with a dose of 10 ¹⁸ cm ^-2 at an average implantation temperature of 400 and 500 ° C. The film on the original sample (figa) has a variable thickness (increase of 50,000), loose structure and traces of machining (increase of 5,000). Our previous studies [7], which showed the presence of H ₂ O and nitrogen in an unbound state, also indicate the friability of the initial film and the presence of pores in it. Implantation leads to the homogenization of the film (Fig. 4b): its surface is leveled, the texture of crystallites about 20-30 nm in size is clearly visible, small defects from mechanical processing are smoothed out. This effect cannot be achieved by conventional heat treatment.

Beam energy dissipation processes have a more effective effect on materials as a result of the initiation of radiation-stimulated processes of structural transformation, which, if possible for thermal heating, are possible at significantly higher temperatures and processing times. The latter circumstance makes the proposed method of processing high-intensity ion beams more technologically advanced and cost-effective. A further increase in the temperature of the samples during implantation causes undesirable changes both in the oxide film and in the metal base. On figs presents an electron microscopic image of the surface of the sample after implantation at 500 ° C. In the zirconium dioxide film, polymorphic transformations of the structure occurred (increase of 5000), the texture was preserved (increase of 50,000), but became more loose - defects appeared between the elements of the texture. The metal zirconium under the film acquired the structure of rack martensite, which is less corrosion resistant (Fig. 5 is an optical image of the Zr - 1% Nb alloy after etching of the oxide film) [8].

Thus, treatment with high-intensity beams of nitrogen ions leads to a structural-phase rearrangement of ZrO _2-x surface layers, which increases the corrosion resistance of ZrO _2-x / Zr - 1% Nb heterostructures. Optimal for use in the technology of passivation of Zr-Nb alloys are implantation modes at a product temperature of 300-400 ° C and ion irradiation fluences of 10 ¹⁷ -10 ¹⁸ cm ^-2 . It is these implantation modes that prevent local corrosion, while the rate of continuous corrosion is also reduced, the slowing of the kinetics of which is shown in Fig. 6, which shows the behavior of the samples during the oxidation process: Fig. 6a is the oxidation kinetics of samples implanted with nitrogen ions at 50 ° C; fig.6b - kinetics of oxidation of implanted samples at 200 ° C; figs - kinetics of oxidation of implanted samples at 300 ° C; fig.6d - kinetics of oxidation of implanted samples at 400 ° C. The maximum retardation of continuous corrosion is observed at 400 ° C and is about 20%.

Information sources

1. Matveev A.V., Belykh T.A., Perekhozhev V.I. and others. A method of processing zirconium alloys. Patent for invention No. 2199607 dated February 27, 2003.

2. Parfenov B.G., Gerasimov V.V., Benediktova G.I. Corrosion of zirconium and its alloys. M .: Atomizdat, 1967, 258 p.

3. Matveev A.V., Belykh T.A., Perekhozhev V.I. et al. Degradation of the protective properties of oxide films of zirconium-niobium alloys under neutron and ion irradiation / Physics and Chemistry of Materials Processing, 2000, No. 1, pp. 37-43.

4. Alekseev O.A. Destructive oxidation of zirconium at 573 and 623K. Nuclear Power Abroad, 1981, No. 2, pp. 30-35.

5. Gavrilov N.V., Nikulin S.P., Radkovsky G.V. A source of intense wide beams of gas ions based on a discharge with a hollow cathode in a magnetic field. Instruments and experimental technique, 1996, No. 1, pp. 93-98.

6. Miyagawa Y., Nakao S., Ikeyama M. et. al. High fluence implantation of nitrogen into titanium: Fluence dependence of sputtering yield, retained fluence and nitrogen depth profile / Nuclear Instruments and Method in Physics Research B 121, 1997, p. 340-344.

7. Belykh T.A., Gavrilov A.V., Golosov O.A. et al. Modification of oxidized Zr-Nb alloys by high-intensity ion beams. Physics and Chemistry of Materials Processing, 2003, No. 6, pp. 14-20.

8. Dobromyslov A.V., Taluts N.I. The structure of zirconium and its alloys. Yekaterinburg: Ural Branch of the Russian Academy of Sciences, 1997, 228 p.

Claims (1)

A method of processing zirconium alloys, comprising forming an oxide film of the same alloy on the surface of the product and implanting nitrogen ions into the surface of the product by treating it with a beam of nitrogen ions at an irradiation dose of 5 · 10 ¹⁷ -10 ¹⁸ cm ^-2 , characterized in that the implantation is carried out at a temperature products 350-400 ° C, provided due to the dissipation of beam energy.

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