RU2199607C2 - Method of treatment of zirconium alloy - Google Patents

Abstract

FIELD: protection of articles used in nuclear power such as tubes of fuel channels and fuel-element jackets against thermal corrosion. SUBSTANCE: article made from zirconium alloy is subjected to heat treatment and film of zirconium dioxide free from through pores and microcracks is formed on surface by any method; then one of chemical elements beginning with fifth group and higher is implanted in film. Ion implantation is effected at energy ensuring length of ion beam path not exceeding thickness of oxide film. EFFECT: enhanced efficiency. 2 cl, 7 dwg

Description

The technical field to which this invention relates is nuclear energy. The following products are made of zirconium alloys [1, p. 38]: especially thin-walled pipes for cladding of fuel elements, thin-walled pipes for channels of water-water and boiling reactors; rods for fuel plug caps and rods of large diameters for massive end products of cassette assemblies; sheets for the manufacture of casings of cassettes and calender tubes; sheets and tapes for spacing grids and other parts of cassettes; a wire for the same purposes, rods and pipes of different sizes for mounting and suspension of fuel rods in assembly cassettes, as well as other parts located in the reactor core. All these elements are in contact either with an aqueous coolant at a temperature of 265-288 ^o C [2, p. 14.15], or with a gas - nitrogen-helium mixture with an admixture of CO, СО ₂ , Н ₂ О [1, p. 49], and they undergo high-temperature, later thermal corrosion.

Thermal corrosion of zirconium alloys is characterized by the following factors [1, p. 149]. On the surface of the oxide film, adsorption of oxidizing molecules (water, oxidizing components of the gas atmosphere, such as O ₂ , CO, CO ₂ , etc.) takes place, which, capturing electrons, form oxygen ions (and hydrogen in the case of interaction with water) . Oxygen ions diffuse under the action of the gradient of the electrochemical potential to the metal – oxide interface, and electrons in the opposite direction [3, p. 140], resulting in the formation of ZrO ₂ molecules at the metal-oxide interface and the completion of oxide layer.

The resulting oxide film consists mainly of monoclinic zirconium dioxide and is sufficiently dense and well adhered to the metal surface. The kinetics of the oxidation of zirconium and its alloys does not obey any one law, or rather, cannot be described by a single kinetic equation. The initial oxidation period is usually described by a parabolic equation expressing the fact that the film growth rate is inversely proportional to its thickness. Theoretically, this corresponds to the aforementioned [3] diffusion of oxygen through the lattice of zirconium dioxide over anionic vacancies. The mathematical expression of the parabolic law: Δm = c ₁ t ^0.5 , where Δm is the gain (increase in mass per unit area); c ₁ is the parabolic oxidation constant; t is time. Upon reaching a certain critical film thickness (depending on the type of alloy, heat treatment, conditions of the oxidizing medium, temperature, irradiation, etc.), the phenomenon of fracture or a transition to a linear law occurs in the kinetics of the corrosion process. The kinetics equation takes the form: Δm = c ₂ t, where c ₂ is the linear oxidation constant. In this case, the film detects micropores of various sizes, longitudinal and transverse microcracks. The linear nature of the kinetics is explained by the fact that the film retains a thin layer adjacent to the metal surface, not damaged by pores and cracks, of zirconium dioxide. This barrier layer is constant in thickness; therefore, the flow of oxygen ions arriving at the metal – oxide interface is constant, and, consequently, the oxidation rate during linear kinetics.

In the pre-fracture period of oxidation, the film is well adhered to the metal, black in color, shiny, smooth. She is characterized by high protective properties. Such a protective film is pre-stoichiometric. Its formula is ZrO _2-x , where x≤0.05. When a fracture occurs, the color of the film becomes gray, then with increasing thickness, white. The film of this type is stoichiometric, it becomes loose, crumbling. This stage of corrosion is called failure.

There is a traditional method of increasing the corrosion resistance of zirconium alloys [1, p. 84.85], an analogue consisting in the fact that products made of zirconium alloy (for example, pipes 82.7 • 6.5 mm from zirkaloy-2 alloy) are subjected to a certain technological processing - in this case, cold rolling, and then create an oxide film on the surface of the product, for example, by autoclaving. In this procedure, the formed oxide film corresponds to the pre-fracture stage of kinetics. This conclusion follows from the fact that after corrosion testing of pipe samples in pairs at 400 ^o C for 72 hours (standard autoclaving procedure), the sample has a uniform black shiny oxide film and a gain of not more than 22 mg / dm ² (film thickness corresponding to 1.5 μm) for caged pipes made of zircaloy-2 and alloy N-2.5 (Zr-2.5% Nb).

The disadvantage of this method is that when the protective oxide film is thus formed, the corrosion resistance of the heat-treated pipes deteriorates: the gain reaches 35, and not 22 mg / dm ² (film thickness, respectively, 2 μm) for the heat-treated alloy N-2.5 [1, p.90], and this accelerates the onset of a fracture in kinetics and thereby increases the oxidation rate, as shown in [4].

 The closest technical solution for the present invention, taken as a prototype, is a "method of manufacturing parts from a zirconium alloy for atomic reactors" [5]. The method is based on the well-known principle of surface chemical-thermal treatment, which in many cases improves the erosion and corrosion resistance of alloys. The method consists in the following: 1) the part is subjected to final plastic processing; 2) a coating with a thickness of 0.1-0.01 microns is formed on its surface, precipitating one or more elements of group 5 and / or 6 from the gas phase; 3) after the coating is formed on a substrate of a zirconium alloy, it is subjected to diffusion treatment in an inactive medium in order to diffuse the element forming the coating into a zirconium alloy; 4) the coated zirconium alloy is treated with high temperature water vapor to form an oxide coating.

 Firstly, the disadvantage of this method is that the procedure for implementing the method is quite complicated - it has four technological stages.

Secondly, the disadvantage of this method is that after the coating is formed on a zirconium alloy substrate, it is subjected to diffusion treatment in an inactive medium in order to diffuse the elements from which the coating is formed. In this case, heat treatment is implicitly assumed, which includes holding the product for a certain period of time at a certain temperature. However, with this kind of procedure, a change in the structure, phase composition of the alloy occurs, and both mechanical and corrosive properties may deteriorate. The heat treatment mode - the annealing temperature and the exposure time of zirconium products is strictly regulated by the technical conditions for the product and cannot be changed arbitrarily. So, for example, for channel pipes made of Zr-2.5% Nb alloy for RBMK reactors, only three heat treatment modes are permissible [6, p. 48]: annealing at 540 ^o С - 5 hours (annealed state, the so-called regular state); aging at 515 ^o C - 24 hours (TMO-1) and aging at 530-540 ^o C - 24 hours (TMO-2).

 However, it does not follow from the prototype that under these specific annealing conditions, diffusion dissolution of the coating occurs, which means that the technical result is an increase in the corrosion resistance of products under standard heat treatment conditions in this case is unattainable.

 The technical result obtained by the implementation of the invention consists, firstly, in simplifying the method — at least one technological stage is excluded, namely, diffusion treatment of the coating in an inactive medium in order to diffuse the element into a zirconium alloy, and secondly, in increasing the corrosion resistance of products made of zirconium alloy, already subjected to thermomechanical processing.

The specified technical result is achieved in that (1) the product from the zirconium alloy is subjected to thermomechanical treatment, an oxide film of the same alloy is formed on the surface of the product in any way, characterized in that ions of one of the elements of the fifth group or higher are introduced into the oxide film by ion implantation, moreover, the specified implementation is carried out at an energy providing the mean free path of the ion beam, preferably not more than the thickness of the oxide film; (2) the method according to claim 1, characterized in that nitrogen ions are used as interstitial ions starting from a fluence of 10 ¹⁵ cm ^-2 and higher.

 In the method described above, the ion implantation is not the metal itself, but an oxide film already formed on its surface. By ion doping of an oxide film already formed on the metal surface, the rate of the oxide formation reaction can be reduced, i.e. increase the corrosion resistance of the metal.

в случае диффузии реагентов через слой образующейся оксидной фазы константа скорости реакции имеет вид [7, с.293 (ф.9.83)]:

где R - газовая постоянная;
Т - температура;
b - стехиометрический коэффициент;
е - заряд электрона;
σ_s - суммарная ионная и электронная составляющие электропроводности оксидной фазы;
t_el - число переноса электронов и дырок оксидной фазы;
t_ion - число переноса анионов и катионов оксидной фазы;
Р_in - давление кислорода на границе оксид - металл ("внутреннее");
Р_ex - давление кислорода на границе оксид - газовая фаза ("наружное").In support of the foregoing, we cite well-known arguments. When oxidizing a metal by reaction:

in the case of diffusion of reagents through the layer of the formed oxide phase, the reaction rate constant has the form [7, p. 293 (f.9.83)]:

where R is the gas constant;
T is the temperature;
b is the stoichiometric coefficient;
e is the electron charge;
σ _s is the total ionic and electronic components of the electrical conductivity of the oxide phase;
t _el is the electron and hole transport number of the oxide phase;
t _ion is the transport number of anions and cations of the oxide phase;
P _in is the oxygen pressure at the oxide - metal interface ("internal");
P _ex is the oxygen pressure at the oxide - gas phase boundary ("external").

где: ν $\genfrac{}{}{0ex}{}{\text{0}}{\text{O}}$ - параметр, входящий в выражение для коэффициента диффузии кислорода;

константа равновесия квазиреакции образования кислородных вакансий;

концентрация примеси в оксидной фазе;

соответствующее давление кислорода соответственно в слое оксида с нарушением стехиометрии и в слое с посторонней примесью.The kinetics of the heterogeneous reaction (1) can be controlled by introducing impurities into the reaction product. Indeed, as shown [7, p. 313], the introduction of a cationic impurity with a valency lower than the valency of the base metal leads to a decrease in the rate constant of the oxidation reaction if the stoichiometry of the reaction product is violated in the direction of excess non-metal — in this case, oxygen. The introduction of foreign impurity ions into the reaction product with a valency greater than the valence of the metal ion also leads to a decrease in the rate constant of the oxidation reaction if the stoichiometry of the reaction product is violated in the direction of excess metal. In this case, the oxidation constant (2) is converted to the form [7, p.313 (f.9.165)]:

where: ν $\genfrac{}{}{0ex}{}{\text{0}}{\text{O}}$ - parameter included in the expression for the diffusion coefficient of oxygen;

the equilibrium constant of the quasi-reaction of the formation of oxygen vacancies;

impurity concentration in the oxide phase;

the corresponding oxygen pressure, respectively, in the oxide layer with violation of stoichiometry and in the layer with an extraneous impurity.

As we see, it is valid in the case of the mechanism of particle transfer through anionic vacancies, i.e. when the stoichiometry of the growing oxide phase is violated in the direction of excess cations, as is the case with a corrosion film on zirconium, the introduction of an impurity cation with a higher degree of oxidation into the oxide phase leads to a rapid attenuation of this mechanism - the value of k _{Δ is} inversely proportional to the square of the concentration of extraneous impurities.

 In order to achieve a more noticeable effect of reducing the oxidation rate, it is necessary to dope an oxide film corresponding to the stage of pre-fracture oxidation.

 In this case, as mentioned above, the film is dense, well adhered to the metal, does not contain pores and microcracks, and the diffusion of oxygen through it occurs exclusively in the form of ions. When a film corresponding to the post-critical oxidation stage is subjected to doping, the effect of reducing the oxidation rate will be less noticeable, which is associated with diffusion of oxygen not in the ionic, but mainly in the atomic form along the pores and microcracks in the film.

 A method of processing zirconium alloys is as follows. The product is annealed. Then, a zirconia film is formed on the article. After this, one of the elements starting from group 5 and above is introduced into the obtained film by ionic incorporation.

 An example implementation of the described method.

1. They took a pipe with serial number 1587 from Zr-2.5% Nb alloy after the final thermomechanical treatment 2 (ТМО-2), used in the manufacture of pipes of technological channels of the second unit of the Ignalina NPP. This treatment consists in the fact that the pipe is subjected to cooling from a high temperature region in a mixture of helium and argon, rolling to a size of 88x4 mm and aging at a temperature of 530-540 ^o C for 24 hours. After such treatment, the pipe is characterized by a lower creep rate than the standard treatment, but at the same time a higher oxidation rate.

2. Then, oxide films were formed on samples of given sizes cut from the pipe by thermal oxidation in air at a temperature of 400 ^° C, corresponding to various kinetic stages and correspondingly with different protective properties.

Corrosion tests were carried out in an electric resistance furnace in an air atmosphere at a temperature of 400 ^o C. The accuracy of maintaining the temperature was ± 2 ^o C and was carried out by the control unit Ш-4531. Temperature control was carried out by a chromel-alumel thermoelectric converter, thermo emf. which was measured with a universal voltmeter Щ-68003. An indicator of the corrosion rate was either a gain - an increase in the mass of the oxidized sample per unit area (mg / cm ² ), or the film thickness of the formed oxide, or the color of the oxide film. In the first case, the samples were weighed on a laboratory balance VLR-20 with a sensitivity of 0.005 mg. In the second case, a photographic image of a transverse metallographic section was studied, obtained using a metallographic microscope at a magnification of 160 times. In the third case, an FMN-2 type macro photography apparatus was used.

Вычисленные значения коэффициентов уравнений и показателя степени следующие: а= 3,1117 мг/дм•ч^-n; b=-28,6119 мг/дм²; К_л=0,573 мг/дм²•ч; n=0,599. Как видно из этих данных, кинетика окисления на допереломной стадии близка к параболической, а на послепереломной - к линейной. Координаты точки пересечения находятся из решения системы уравнений (4). При τ = τ_пер левые части уравнений системы равны, соответственно получаем следующее уравнение относительно τ_пер:
aτ $\genfrac{}{}{0ex}{}{\text{n}}{\text{пер}}$ -b-K_лτ_пер = 0 (5)
Решая данное уравнение численными методами, получаем: τ_пер = 164 ч, Z_пep=65,4 mr/дм². С учетом этих данных послепереломную стадию кинетики окисления можно описать следующим уравнением:
Z = Z_пер+K_л(τ-τ_пер) (6)
где параметр К_л - есть константа линейного окисления;
Z_nep - увеличение массы на единицу площади в момент перелома.The kinetic oxidation curve of the samples is presented in figure 1. As can be seen from the drawing, this is a typical oxidation curve of zirconium alloys and it is characterized by the presence of two sections. The first of them corresponds to the pre-fracture stage of oxidation, when the growth rate of the oxide film decreases over time. The second section corresponds to the post-critical stage, when the growth rate of the oxide film does not depend on time. The experimental points of mass increase per unit area (mg / dm ² ) on the duration of oxidation are well interpolated by the dependences:

The calculated values of the coefficients of the equations and the exponent are as follows: a = 3.1117 mg / dm • h ^-n ; b = -28.6119 mg / dm ² ; K _l = 0.573 mg / dm ² • h; n = 0.599. As can be seen from these data, the kinetics of oxidation at the pre-fracture stage is close to parabolic, and at the post-fracture stage, it is linear. The coordinates of the intersection point are found from the solution of the system of equations (4). When τ = τ left _lane of the system equations are respectively the following equation with respect to τ _lane:
aτ $\genfrac{}{}{0ex}{}{\text{n}}{\text{per}}$ -bK _l τ _lane = 0 (5)
Solving this equation by numerical methods, we obtain: τ _per = 164 h, Z _per = 65.4 mr / dm ² . Based on these data, the post-critical stage of oxidation kinetics can be described by the following equation:
Z = Z _per + K _l (τ-τ _per ) (6)
where the parameter K _l is the linear oxidation constant;
Z _nep is the increase in mass per unit area at the time of fracture.

 The appearance of oxide films obtained during oxidation is also typical.

 Black oxide films were formed at the pre-fracture stage (Fig. 2a), and light gray at the post-fracture stage (Fig. 2b).

3. Ion implantation was carried out into the surface layer of the test samples. Nitrogen - an element of group 5 was used as a dopant. Since the electronegativity of nitrogen is less than the electronegativity of oxygen (3 and 3.5 conventional units, respectively), the tendency to accept an electron by oxygen is higher than nitrogen, therefore, nitrogen can be considered as an electron donor. Samples in the amount of 3 pieces for each point corresponding to different stages of oxidation on the kinetic curve were subjected to ion implantation: τ _o = 0 - pure metal, τ ₁ = 42.5 h (weight gain 32 mg / dm ² - oxide film thickness 2.2 μm - pre-fracture stage, the film does not contain through pores or microcracks) and τ ₃ = 397 h (weight gain 200 mg / dm ² - oxide film thickness 13 μm - post-fracture stage, the film contains through pores and microcracks).

 Ion introduction was carried out at the R-7M cyclotron of the Ural State Technical University. The energy of nitrogen ions was chosen such that the mean free path of the ion beam did not exceed the thickness of the formed oxide film. In this case, all ions were localized in the formed oxide film. For these reasons, the ions were accelerated to an energy of 3.8 MeV; the beam current at the probe was about 15–20 μA. The dispersion of particles in the beam in energy was about 0.5. The design of the installation is shown in figure 3. The ion beam is scanned by a system of magnets (4) and enters the experimental chamber (1) on a sample (2) located on a copper water-cooled holder (3), and partially enters the Faraday cup (4). The distribution of the concentration of embedded ions over the depth of the oxide film is shown in Fig.4. As follows from the graph, the doping impurity - nitrogen is completely localized in the oxide phase. The profile is built on the basis of the calculation algorithm given in [8].

In FIG. Figure 5 shows the appearance of samples with oxide films with an initial thickness of 2.2 μm, then subjected to ion implantation to various fluences: a) 10 ¹⁴ , b) 10 ¹⁵ , c) 10 ¹⁶ cm ² and subsequent reoxidation to a resource of 400 hours (fluence is the ratio of the number of ionizing particles penetrating into the volume of an elementary sphere to the cross-sectional area of this sphere, see for example [9, p. 12]). As can be seen from figa, at the first value of the fluence, the implanted ions do not inhibit the oxidation kinetics: the entire surface of the film is uniform in color. At subsequent fluence values, a distinct effect is observed: the region subjected to ion implantation retains the black color characteristic of the pre-fracture stage of oxidation, sharply contrasting with the white reference surface characteristic of post-fracture oxidation (Fig. 5b, 5c). In FIG. 6 presents images of metallographic sections of the samples shown above in FIGS. 5b and 5c, respectively. The boundary clearly shows the interface between the ion implantation region (1) and the control region (2). The thickness of oxide films on the surface after ion implantation is at least three times less than on the control surface.

In FIG. Figure 7 shows the appearance of an oxide film of a sample oxidized to a resource of 400 hours under the same conditions, but on which ion implantation was carried out not in the oxide film, but directly into the alloy matrix before oxide formation. Outwardly, the area of implementation is not allocated. The weight gain during this time was 186 mg / dm ² , which is only 10% less than the weight gain of the control samples (see Fig. 1).

When ions were introduced into the oxide film formed at the post-fracture oxidation stage, the oxidation constant was 0.461 mg / dm ² (see Fig. 1, curve 3), which is only 20% less than the corresponding value for the control samples, as reported above.

Sources of information
1. 3aimovsky A.S. Nikulina A.V., Reshetnikov N.G. Zirconium alloys in nuclear energy. - M.: Energoizdat, 1981, 232s.

 2. Durability of pipes of technological channels. Rodchenkov B.A., Rybkin E.Yu., Vasnin A.M. et al. // Questions of Atomic Science and Technology, series: Materials Science and New Materials. 1990. issue 2 (36), p. 14-21.

 3. Kofstad P. High-temperature oxidation of metals. - M.: Mir, 1969, 392s.

 4. Corrosion of Zr-2.5% Nb alloy in a gaseous medium. Perekhozhev V.I., Polyakov P.I., Kalachikov V.E. and others // Questions of atomic science and technology. Series: Materials Science and New Materials. 1990, issue 2 (36), p. 33-38.

 5. 3 application 3-71507. Japan. Published: РЖ - Inventions of the countries of the world. 1993, 4., issue 49 MKI S 23, p. 27.

 6. The technology of manufacturing pipes from Zr-2.5% Nb alloy installed in RBMK reactors. A.V. Nikulina, N.G. Reshetnikov, T.B.Shebaldov and others // Questions of atomic science and technology, series: Materials science and new materials. 1990, no. 2 (36), p. 46-54.

 7. Kovtunenko P. V. Physical chemistry of a solid. - M: Higher school, 1993, 352s.

 8. Spatial distributions of energy released in a cascade of atomic collisions in solids // A.F. Burenkov, F.F. Komarov, M.A. Kumakhov, M.M. Temkin. - M.: Energoizdat, 1985, 248s.

 9. Pershenkov B.C., Popov V.D., Shalnov A.V. Surface radiation effects in elements of integrated circuits. - M.: Energoizdat, 1988, 255s.

Claims (2)

 1. The method of processing zirconium alloys, which consists in the fact that the product from zirconium alloy is subjected to thermomechanical treatment, form an oxide film of the same alloy by any method on the surface of the product, characterized in that ions of one of the elements of the fifth group are introduced into the oxide film by ion implantation or above, and the specified implementation is carried out at an energy providing the mean free path of the ion beam, preferably not more than the thickness of the oxide film. 2. The method according to claim 1, characterized in that nitrogen ions are used as interstitial ions, starting with a fluence of 10 ¹⁵ cm ^-2 and higher.

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