RU2633688C1 - Method for treating surface of zirconium alloy plate - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method for treating zirconium alloy plate surface includes application of magnesium oxide powder onto the plate surface and laser treatment, which is carried out in 1-10 passes at average laser radiation power of 10-60 W, pulse frequency of 20-100 kHz, laser beam scanning rate of 100-1000 mm/sec. Then annealing is carried out in muffle furnace at 700-1100°C for 5-25 minutes.

EFFECT: formation of transparent oxide film of mixed composition with high hardness, adhesion strength and corrosion resistance.

9 dwg, 1 tbl

Description

The invention relates to the field of materials science and surface treatment of metals, namely to reduce the corrosion rate of a zirconium alloy as a result of laser processing, i.e. increase the corrosion resistance of its surface. The invention can be used in those areas of industry, in particular in nuclear energy, where communications and other technological elements made of zirconium alloys are used. In addition, zirconium and its alloys are also used as implants in reconstructive surgery [1].

It is known from the literature that the corrosion resistance of zirconium alloys is due to the spontaneous creation of zirconia films on their surface [2]. However, this film has structural defects, so there is reason to believe that measures can be taken to improve this film and improve its quality. To this end, the task was to increase the corrosion resistance of the zirconium alloy by improving the protective properties of the surface film by pulsed laser treatment followed by doping of the surface layer with magnesium oxide and annealing.

The anticorrosive properties of the surface of zirconium alloys, in particular the protective properties of the oxide film, are significantly affected by the final processing method. Machining usually leads to the emergence of micro and macro stresses, which negatively affects the corrosion resistance. A common technique for surface treatment of zirconium alloy is surface etching with hydrofluoric acid. Depending on the duration, this process is called etching, lightening or chemical polishing. A significant drawback in this operation is the inevitable contamination of the surface with fluorides, which leads to structural changes in the crystal lattices during irradiation, which is why such products do not withstand tests in emergency conditions of overheating with the release of coolant.

Many features of the corrosion kinetics of zirconium materials are explained by the changes occurring in oxide films during their growth and degradation. Since the corrosion behavior of zirconium alloys is often associated with the electrophysical properties of a barrier oxide film, it is important to study the effect of various surface treatment methods on the properties of anticorrosive oxide films on zirconium alloys. To improve the properties, a modification of the composition of zirconium alloys by alloying with various elements is also used, as well as a change in the microstructure due to optimization of heat treatment. It is known that the structure no less determines the properties of the material than the composition. So, for example, there is a known method of protecting a zirconium alloy from corrosion by the electric spark treatment of its surface with a rotating working tool, consisting of a wire of various alloying metals, with the formation of complex layers, which significantly increase the corrosion resistance of zirconium alloys. The disadvantage of the method of spark processing is the high consumption of alloying materials, low controllability of the process [3]. In [4], methods for increasing the operational properties of zirconium components of the active zones of reactors by modifying the structural phase state and surface composition were investigated and described. It is shown that through the use of various methods it is possible to obtain composite zirconium materials with increased strength, high creep resistance and the development of fatigue cracks. Using liquid metal reinforcement, the strength of zirconium components can be increased; solid-phase doping with carbon nanotubes can reduce the likelihood of hydrogen embrittlement due to the ability of carbon nanotubes to irreversibly sorb hydrogen absorbed by zirconium components; as a result of surface modification and creation of protective coatings on the surface of zirconium components, it is also possible to reduce the level of hydrogenation of zirconium and its alloys during operation and increase corrosion resistance, thereby improving performance and prolonging the life of products. All these modification methods can be used either individually or in different combinations, depending on what set of properties you need to get. It was shown in [5] that the anticorrosion properties of zirconium alloys also depend on the density of the surface oxide layer: the more loose and thicker oxide layers, the lower the resistance to electrochemical corrosion.

It is known that zirconium oxides, which are modified by oxides of other metals, can improve functional properties: hardness, wear resistance, etc. [6]. A known method of increasing the corrosion resistance of zirconium alloys by the method of short-pulse laser treatment in an atmosphere of air and argon [7] (prototype). In this case, protective oxide layers are formed that more effectively protect the alloy from corrosion than spontaneously formed oxide film. In order to increase the efficiency of the formed oxide film on zirconium alloys, it is proposed to modify zirconium oxide with oxides of other metals. In our work, magnesium oxide was investigated. Thus, the goal that we set is to improve the protective properties of zirconium oxide obtained by short-pulse laser treatment, as described in [7], which we selected as a prototype. During the experiment, it was found that the purpose of the invention, namely the synthesis of mixed oxides, is achieved by laser processing in the following modes: the number of passes - 1 ... 10; average laser radiation power 10 ... 60 W; pulse frequency 20 ... 100 kHz; laser beam scanning speed 100 ... 1000 mm / s.

A specific example of the invention

Sample preparation

The initial samples were samples of the claddings of fuel elements made of alloy E110 (Zr - 1 wt.% Nb) in the delivery state. Samples with dimensions of 10 × 10 × 1 mm were pretreated with hydrofluoric acid.

For laser processing, we used an LDesigner F1 ytterbium fiber laser operating in a pulsed mode of radiation generation and controlled by a personal computer. The samples were processed by the method of short-pulse laser exposure. To create a protective layer of mixed oxides on the surface of the zirconium alloy, on which the naturally formed oxide foam is already located, subsequent processing was carried out in three stages. The first step was the laser dispersion of a finely divided powder of magnesium oxide into a zirconium matrix in an inert atmosphere of argon. Thus, magnesium oxide in the form of a powder with a layer thickness of up to 100 μm was deposited on the initial untreated surface of the zirconium alloy sample. Further, processing was carried out using two laser passages: the average laser radiation power for the 1st pass was 39.3 W, the second pass was 22.5 W; pulse frequency of two passes - 80 kHz; the scanning speed of the laser beam of the 1st pass is 500 mm / s, of the 2nd - 100 mm / s. In this treatment mode, the presence of magnesium oxide dissolved in the specified surface layer is observed on the surface of the zirconium alloy coated with an oxide film. To obtain a composite coating, and not a mixture of the starting components, after applying magnesium oxide, laser treatment was performed in an air atmosphere: the number of passes was 10; average laser power - 50 W; pulse frequency - 100 kHz; laser beam scanning speed - 700 mm / s. This treatment was the second stage to obtain a protective layer of mixed oxides on the surface of the zirconium alloy. The third final stage was annealing in a muffle furnace, which is necessary for the formation of a dense oxide film based on composite material. Annealing in a muffle furnace was carried out at a temperature of 900 ° C for 10 minutes. With such annealing parameters, a dense transparent oxide film of mixed composition with high hardness and adhesive strength is formed on the sample. For various tests under the same conditions, 3-5 samples were prepared.

Corrosion-electrochemical studies

Polarization measurements of the studied samples were carried out in the potentiodynamic mode at a potential sweep speed of 2 mV / s. For electrochemical studies, Ipc-Pro L model potentiostats and a YSE-2 cell were used at an ambient temperature of (20 ± 2) ° C. As a model corrosion medium, a 1% (mass.) Solution of sodium chloride in water at pH 7.0 was selected. Chloride anions are depassivators and can enhance the corrosion of passive metals [8, 9]. The second model of corrosive medium was chosen Ringer's solution [8] at a pH of 6.2, which corresponds in composition and properties to the biological environments of the human body.

Samples for electrochemical studies were plates with an area of 1 cm ² . The synthesized surface layer served as the working surface of the electrode. Non-working electrode surfaces were insulated (varnished). Before electrochemical tests, the samples were washed with distilled water and their entire surface was degreased with ethanol. Next, the samples were placed in an electrochemical cell with the appropriate solution, kept until a stationary potential was established (10-15 min), and the anode polarization was turned on at a speed of 2 mV / s. Potentials were measured relative to a saturated silver chloride electrode. For comparison, the polarization curves of the initial zirconium alloy sample were taken under the same conditions.

Thus, the following samples were subjected to corrosion-electrochemical tests:

samples of a series No. 1 - initial, a sample in a condition of delivery;

samples of series No. 2 - laser dispersion of a fine powder of magnesium oxide into a zirconium matrix in an inert atmosphere of argon (stage 1 in the formation of a protective layer of mixed oxides on the surface of the zirconium alloy);

samples of series No. 3 - laser processing in an atmosphere of air in order to synthesize surface layers and obtain a composite coating with a metal matrix (stage 2 of the formation of a protective layer of mixed oxides on the surface of a zirconium alloy);

samples of series No. 4 - laser treatment of the surface of a zirconium alloy in argon atmosphere with the application of magnesium oxide powder, followed by laser surface treatment in an atmosphere of air with further annealing in a muffle furnace (stage 3 of the formation of a protective layer of mixed oxides on the surface of the zirconium alloy).

The current-voltage characteristics of the test samples in a 1% NaCl solution and Ringer's solution are presented in FIG. 1 and 2, respectively. From the presented curves it follows that all the samples studied do not have a so-called active dissolution site. This is due to the fact that all the studied samples have a protective oxide layer, i.e. initially passivated. For such samples, the anodic polarization in a wide range of potentials is not accompanied by an increase in the anode current. Characterization of the protective properties of the oxide layer, i.e. corrosion resistance of the samples will be the value of the anode potential at which the speed of the anode process increases sharply, i.e. the passivation potential or, in the case of rather thick films, the breakdown potential [9]. Obviously, the more positive the breakdown potential, the more effectively the protective layer increases the corrosion resistance of the samples.

In FIG. 1 and 2, the following designations of the curves obtained in the study of samples with different processing conditions are adopted. Numbering 1-4 corresponds to the numbering of the processing modes indicated above. As can be seen from the figures, each sample has its own value of the potential for the onset of active corrosion damage, which indicates surface depassivation. The higher the value of the anode potential, the better the anticorrosion properties and operational characteristics of the material. With an increase in potential, dissolution as a result of passivation occurs first in samples of series No. 1 and No. 2. Then, dissolution occurs first in sample No. 3, and last, at the deepest anode potentials, in sample No. 4. As follows from the figures, the potential for re-passivation of the processed samples in each medium is higher than that of the initial samples. The samples with a protective layer of mixed oxides on the surface of the zirconium alloy possess the most increased passivation potential (series No. 4). For clarity, the degree of increase in anticorrosion properties, breakdown potentials for samples of each series in the studied electrolytes are presented in table 1.

In order to characterize the obtained protective layers, additional physical studies were performed. The chemical composition of the surface layers of zirconium alloy samples was studied by X-ray photoelectron spectroscopy (XPS). The tests were carried out on a SPECS spectrometer using MgKα radiation (1253.6 eV) in combination with layer-by-layer etching of the surface with argon ions with an energy of 4 keV and a current density of 30 μA / cm ² (etching rate ~ 1 nm / min). The experimental data were processed using the CasaXPS software package. The relative error in determining the concentration of elements is ± 3% of the measured value.

We studied samples with laser dispersion of a fine powder of magnesium oxide into a zirconium matrix in an inert argon medium (samples of series No. 2) and samples after laser treatment in an atmosphere of air to synthesize surface layers and obtain a composite coating with a metal matrix (samples of series No. 3). Their RE spectra are shown in FIG. 3-5 and 6-8, respectively.

From the analysis of the RE spectra of Zr3d surface layers of the sample of series No. 2 (figure 3), it follows that on the surface of the zirconium alloy, zirconium is oxidized to ZrO ₂ . It is seen that the oxidation of the zirconium alloy is shallow, at a depth of 5 nm the zirconium alloy is predominantly not oxidized. The oxygen signal intensity decreases from the surface deeper into the substrate.

Magnesium is present in the surface layers of a sample treated in an inert medium. Its RE spectrum of Mg2s is shown in FIG. 5. After laser treatment at a depth of up to 3 nm, magnesium is present in the state oxidized to Mg ²⁺ , then it is mainly not oxidized.

On the RE spectrum of Zr3d sample series No. 3 (Fig. 6) throughout the depth of analysis of zirconium in the alloy is oxidized to ZrO ₂ . The RE spectra of Mg2p are shown in FIG. 8. It can be seen that in the surface layers of the coating obtained by treating the zirconium alloy plate in air, there is trace magnesium in trace amounts, its concentration does not exceed 1 at%. In this case, magnesium oxide facilitated the modification of the crystal lattice, and after the interaction, part of it was sublimated as oxide, and the other was previously reduced by the reaction 2MgO + Zr = 2Mg + ZrO ₂ . As a result, null-valent magnesium was introduced into the crystal lattice of the zirconium substrate.

A review of the RE spectra of oxygen (O1s) for samples of series 2 and 3 (Figs. 4 and 7, respectively) indicates that as the depth increases, i.e. as it penetrates the thickness of the formed layer, a gradual decrease in the oxygen concentration is observed, in other words, a gradient of its concentration is observed. This contributes to a smooth, gradual transition from oxide to metal phases, which ultimately ensures good adhesion of the formed layers.

, с шагом по углу 0,02° и временем экспозиции в каждой точке 5 с. Дифрактограмма веществ представляла собой кривую зависимости интенсивности отраженных рентгеновских лучей от угла Вульфа-Брэгга 2θ. Результаты рентгеноструктурного анализа исследуемых образцов представлены на фиг. 9. В исходных образцах регистрируется одна фаза α-Zr (гексагональная решетка). На дифрактограмме образца с диспергированным в поверхностном слое оксидом магния, также обнаружена только фаза α-Zr. Оксид магния не был обнаружен вследствие высокой дисперсности и аморфного состояния данного оксида. После обработки на воздухе обнаружены фаза α-Zr и две фазы диоксида циркония в двух модификациях: моноклинная и тетрагональная. На дифрактограмме после отжига в муфельной печи не обнаружена фаза α-Zr: присутствуют только фазы диоксидов циркония с моноклинной и тетрагональной решеткой, что говорит о значительной толщине полученного защитного слоя смешанных оксидов на поверхности циркониевого сплава.X-ray diffraction studies were performed using a DRON-6M automated diffractometer. The survey was carried out using the constant time method in monochromatic Co-Kα radiation with a wavelength of λ = 1.7902

, in increments of 0.02 ° angle and exposure time at each point 5 s. The diffractogram of the substances was a curve of the intensity of the reflected x-rays from the Wulf-Bragg angle 2θ. The results of X-ray diffraction analysis of the test samples are presented in FIG. 9. In the initial samples, one phase α-Zr (hexagonal lattice) is recorded. In the diffraction pattern of the sample with magnesium oxide dispersed in the surface layer, only the α-Zr phase was also detected. Magnesium oxide was not detected due to the high dispersion and amorphous state of this oxide. After treatment in air, the α-Zr phase and two phases of zirconium dioxide were detected in two modifications: monoclinic and tetragonal. The α-Zr phase was not detected in the diffractogram after annealing in a muffle furnace: only zirconia phases with a monoclinic and tetragonal lattice are present, which indicates a significant thickness of the obtained protective layer of mixed oxides on the surface of the zirconium alloy.

Hardness studies were carried out according to the Vickers method using a PMT-3 microhardness tester with a load of 100 g. The microhardness of the sample surface was measured with a protective layer of mixed oxides formed in three stages on the surface of the zirconium alloy. The microhardness value of this sample is 9.3 GPa. For comparison, the microhardness of the surface of the initial untreated zirconium alloy sample was calculated, which amounted to 1.8 GPa. Thus, according to the analysis of the data obtained, it follows that the resulting protective layer of mixed oxides on the surface of the zirconium alloy (samples of series No. 4) is 5 times higher than the microhardness of the initial sample.

Studies on the adhesion strength of the sample surface were carried out in accordance with GOST R 54563-2011. "Paintwork materials. Determination of adhesion by the lattice notch method." The tests were performed on a sample with a protective layer of mixed oxides formed in three stages on the surface of the zirconium alloy and the initial untreated sample. Analyzing the results of the obtained data, we can conclude that the initial sample has scratches on the substrate, which is a relatively soft material, i.e. in this case there is no peeling, because the natural oxide film is thin and it is dissolved in the metal matrix of the zirconium alloy. A sample with a protective layer of mixed oxides formed in three stages on the surface of a zirconium alloy has high adhesive strength with a substrate, since when scratching only in several places, peeling of the coating occurs, which corresponds to 1 point of adhesive strength according to GOST. This corresponds to the maximum adhesion of the coating to the substrate. High adhesive strength is achieved in this case due to the gradient layers obtained by laser treatment. On the coatings obtained in this way, a gradual transition in phase composition from substrate to coating is carried out. The surface of the coating is very hard and in order to violate its integrity, continuity, the force required is much greater than for coatings on samples of series 1-3.

List of sources

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Claims (1)

A method of surface treatment of a zirconium alloy plate, including short-pulse laser treatment, characterized in that before the laser treatment, magnesium oxide powder is applied to the surface of the plate, laser processing is carried out in 1-10 passes with an average laser power of 10-60 W, pulse frequency 20- 100 kHz, the laser beam scanning speed of 100-1000 mm / s, and then annealing in a muffle furnace at a temperature of 700-1100 ° C for 5-25 minutes.

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