Corrosion and electrochemical behavior of aluminium treated with high-temperature pulsed plasma in CsCl–NaCl–NaNO3 melt

Corrosion Science 53 (2011) 2015–2026

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Corrosion and electrochemical behavior of aluminium treated with high-temperature pulsed plasma in CsCl–NaCl–NaNO3 melt L.A. Yolshina ⇑, V.Ya. Kudyakov, V.B. Malkov, N.G. Molchanova Institute of High-Temperature Electrochemistry, Ural Division RAS, Ekaterinburg, Russia

a r t i c l e

i n f o

Article history: Received 31 May 2010 Accepted 18 February 2011 Available online 26 February 2011 Keywords: A. Aluminium A. Molten salts B. Polarization C. Oxidation

a b s t r a c t Dense protective layers of aluminium corrosion products, whose composition depends on the oxidation temperature, are formed on the surface of aluminium treated with high-temperature pulsed plasma (HTPP) without visible remelting and then held in a chloride–nitrate melt in conditions of anodic polarization. Modification of aluminium treated with HTPP changes the properties of 20 lm layer under its surface and the oxide layer formed by such treatment has different morphology: it consists of smaller crystals and so has the other protective properties as compared with aluminium untreated by plasma. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The possibility of changing surface properties rather than affecting the volume properties of metal materials, which may lead to the improvement of such important properties as corrosion resistance, hardness and wear resistance, is very promising. Treatment with high-temperature pulsed plasma (HTPP) has been recently used for this purpose [1–4]. Pulsed high-energy plasma which initially was used for limited thermonuclear fusion, has found application for surface modification in recent years. Initially pulsed high-energy plasma was used for limited thermonuclear fusion. In recent years it has found application for surface modification. Considering that plasma disruption to the first wall of a reactor caused considerable changes in the surface morphology, the investigations were undertaken to elucidate the effect of plasma on the corrosion and mechanical properties of materials. In the case of pulsed plasma treatment the surface is very quickly heated to 1600–1800 K so that a high energy is converted to heat energy of the surface. Then the surface is cooled quickly if the material has good thermal diffusivity (like in steel or aluminium); the cooling rate can be as high as 108 K/s. Upon exposure to an ion beam with a current density of 10–1000 A cm2, a pulse length of 108 to 109 s, and an ion energy of 100–1000 keV, the metal surface heats to temperatures corresponding to any aggregate state, even to boiling. The HTPP hardening process is accompanied by implantation of active particles of plasma whose composition can be different depending on the task posed [5]. The impact of bunches leads to an intensive warming up of the ⇑ Corresponding author. Tel./fax: +7 3433477431. E-mail addresses: [email protected], [email protected] (L.A. Yolshina). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.02.029

surface, its fusion on the depth of an order of particles running with the subsequent fast cooling. Thus sometimes the hydrodynamic instability takes place which leads to formation of a special micro relief on the surface. Temperature-phase changes are defined by kinetics of melt cooling near to a point of phase transition. This treatment is followed by generation of defects (point, twoand three-dimensional ones) in the surface layer, and these defects have a considerable effect practically on all the properties of solids and largely determine specific features of the surface modification during plasma treatment and a gas exchange between plasma and the solid. Particles implanted deep in the solid change its phase composition, and, in the case of gas forming ions, determine the gas release from the plasma facing electrodes and the permeability of the gas through the walls. The additional stresses and defects arising on the surface cause a rearrangement of the surface itself; that is, they lead to a considerable change in its topology. Generally, a near-surface layer is modified to a considerable depth of up to 30 lm upon exposure to chemically active ions or plasma even if the energy of bombarding particles is no higher than several electron volts [6]. For this reason, the formation of an etching relief can be accompanied by changes in the surface topography, which are caused by ion-stimulated structural and chemical transformations in near-surface layers. The HTPP treatment of metallic materials results in structural changes of the surface layer. Depending on material and its original structure, HTPP treatment fuses and smoothens the surface, decreases the sizes of metal grains; and, as a consequence, results in the formation of fine-crystalline, amorphous, or amorphous-crystalline structures [7]. It was found [8] that such treatment of aluminium reduces its corrosion rate by one order of magnitude in molten alkali chlorides at temperatures not higher than the aluminium melting point. A

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study of the interaction between aluminium and a chloride melt containing 1–30 wt.% sodium nitrate showed the possibility of forming either dense oxide layers on the aluminium surface or powders of aluminium oxide on the surface of the aluminium anode or in the bulk of the electrolyte [9]. The researches of corrosion-electrochemical behavior of titanium processed by HTPP in the fused mix of alkali chlorides and nitrates, have shown that the extremely densely packed layers of titanium dioxide particles up to 100 nm are formed on the surface of titanium modified by a plasma shot [10]. The size and morphology of these oxide crystals essentially differ from ones formed on untreated titanium. These oxide layers are protective for titanium in a wide interval of temperatures and concentration of sodium nitrate. The aim of this study was to analyze in detail the effect of a high-temperature pulsed plasma treatment of aluminium on its corrosion-electrochemical behavior in a molten eutectic mixture of caesium and sodium chlorides containing 1–30 wt.% sodium nitrate in the temperature range of 790–900 K under argon atmosphere. 2. Experimental The aluminium samples were treated at the department of magnetic systems, TRINITI State Research Centre (Troitsk, Moscow region, Russian Federation), on an MKT-4 coaxial plasma accelerator in hydrogen and a Desna-250 cylindrical accelerator in helium and oxygen. The samples were ‘‘flags’’ with a submerged surface area of 2 cm2, which were cut out of aluminium foil (99.99 wt.% Al) 200 lm thick. The samples were exposed to HTPP in the following conditions: (1) Hydrogen plasma at an accelerating voltage of 12.5 kV. One ‘‘shot’’ was taken on each side of the sample in an MKT-4 (H–Ti) accelerator. (2) Helium plasma at an accelerating voltage of 10 kV (He–Ti), one ‘‘shot’’. (3) Oxygen plasma at an accelerating voltage of 12.5 kV (O–Ti), one ‘‘shot’’.

The pulsed treatment regimes were chosen such that regions of visible remelting (defects) were not formed on the surface exposed to a plasma pulse. Our previous experiments on the effect of the HTPP treatment showed that corrosion begins indeed in remelting regions, and the average rate of corrosion of samples with an obviously remelted surface can be even higher than that of the initial metal without pre-treatment [8]. The method of the electrochemical experiment was described in detail in [8]. The electrolyte was based on a eutectic mixture of caesium and sodium chlorides, in which 1–30 wt.% sodium nitrate was added. The main interest of present research was to investigate the influence of NaNO3 concentration on the corrosion process. The eutectic mixture of caesium and sodium chlorides was a basis of molten electrolyte, therefore NaCl or CsCl concentration in this electrolyte was not considered. The total mass of the salts in the single experiment was always constant (50 g). A remelted and ground salt mixture was placed in an electrochemical three-electrode cell (Fig. 1), the cell was evacuated, the gas space was filled with argon, and the cell was heated. The sample was degreased in ethyl alcohol, dried, and placed in the salt electrolyte when an experiment temperature was reached. The aluminium corrosion potentials were measured immediately in the melt relative to a reference chlorine electrode. The chlorine reference electrode was a graphite tube immersed into the CsCl–NaCl melt. The tip of the tube was made from pure spectral carbon

Fig. 1. Electrochemical cell: 1– quartz test-tube; 2– auxiliary electrode; 3 – vacuum rubber tight fuse; 4 – thermocouple; 5 – molybdenum wire; 6 – chlorine reference electrode; 7 – alumina tube; 8 – asbestoses diaphragms; 9 – working aluminium electrode.

[11]. Purified chlorine gas was flown through the graphite tube under atmospheric pressure. The auxiliary electrode consisted of a molybdenum wire immersed in molten CsCl–NaCl. The reference and auxiliary electrodes were placed in the quartz tubes and connected with the molten salt by a porous asbestos diaphragm. All potential values are given respectively chlorine reference electrode. As soon as a corrosion potential was established (in 3–4 h), the anodic polarization was performed in a potentiostatic regime. The anodic polarization curves, which were measured in analogous conditions, were reproducible within 1–2%. The aluminium corrosion rate was determined also by the gravimetric and chemical–analytical methods (in a single experiment). After the experiment was complete, the electrode was withdrawn from the cell and washed in distilled water. The chilled salt cake was taken out of the cell and was dissolved in distilled water. If a powder was present in the bulk of the electrolyte, it was filtered, and the solution was analyzed for the concentration of aluminium ions and nitrite ions. Considering that after the interaction of aluminium with the chloride–nitrate melt the surface of the samples was coated with layers of hard corrosion products, which were not necessarily dense, loosely adhered to the metal base, and occasionally peeled off, the gravimetric data, could not be always used to make unambiguous conclusions as to the aluminium corrosion rate. For this reason, only those gravimetric data were taken into account which was obtained when the aluminium surface was covered with the dense strong oxide layers or the layers, which could be easily removed from the surface of the corroded metal by careful washing

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in distilled water. Also, the corrosion rate was calculated from the anodic polarization curves and was evaluated from the passivation current densities. The surface of the HTPP-treated samples was photographed, before and after their staying in the chloride–nitrate melt, using a GSM-5900 LV scanning electron microscope in secondary electron radiation. Also, ED spectra were measured on the surface of the aluminium samples, and these spectra were used to determine the concentration of each element in the surface layer. The phase composition of the surface was analyzed in a RIGAKU DNAX 2200PC X-ray diffractometer. The concentration of aluminium ions which passed to the chloride–nitrate melt during the exposure of the aluminium samples was determined by a spectrometric analysis in an Optima 4300 DV spectrometer with inductively coupled plasma. The concentration of nitrite-ions after a hot-temperature exposure of the salt electrolyte was measured by the volumetric method with oxidation by potassium permanganate in an acid medium. The microphotographs of the filtered and dried nanopowder of aluminium oxide were also taken under a GSM-5900 LV scanning electron microscope. All the experimental data were processed with a confidence probability of 0.95. The error of the gravimetric measurements was not over 30% with respect to electrochemical measurements 12%. 3. Results and discussion 3.1. Mechanism of anodic oxidation of aluminium in chloride–nitrate melts Anodic oxidation of metals in molten oxygen-containing salts (nitrates, carbonates, sulphates, borates and phosphates) and alkalis allows producing oxide coatings of different structures and applications depending on the salt bath composition and the temperature conditions it makes possible to use a wide range of temperatures and a diversity of chemical reactions including complexation, oxidation–reduction, and acid–base equilibrium. The corrosion products in these media under polarization can be both N2, NO, NO2, H2 and CO gases and solids such as carbon and carbides. Oxide ions O2 are released and form low-soluble compounds with metal cations. The accumulation of oxide ions O2 in the melt retards the process of corrosion through the formation of low-soluble compounds protecting the surface of the corroded metal [12]. We analyzed the corrosion-electrochemical interaction of aluminium with chloride–nitrate melts containing 1–30 wt.% sodium nitrate. In steady-state conditions, the following reactions can take place on aluminium without a current applied:

able is an aluminium interaction reaction leading to the formation of caesium or sodium aluminates by the scheme (4). The above-mentioned chemical equations of reactions essentially reflect the final result of the processes as regards the chemical composition of corrosion products. A detailed mechanism of their implementation is much more complex, and the reactions include many stages. The kinetics of individual stages of the overall reactions (1)–(4) has been beyond the scope of this study. Thus, an interaction of metals with sodium or caesium nitrate can lead to reduction of nitrate-ions to different degrees of oxidation (0, +2 and +4) and formation of the corresponding oxides. However, according to the thermodynamic data, the most probable are the reactions (1) and (2) since when aluminium is kept under open circuit conditions in a chloride–nitrate melt, nitrogen dioxide is not released almost up to the oxidation temperature, which is as high as 870 K. These reactions are accompanied by the process of thermal decomposition of sodium nitrate with the formation of sodium nitrite and oxygen. This process takes place at a temperature above 700 K according to the equation

2NaNO3 ! 2NaNO2 þ O2 :

ð5Þ

Nitrates of heavy alkali metals (with a low ionic potential of cations, e/r), which include caesium, decompose at higher temperatures (850 K) according to the equation (6) [14]:

4CsNO3 ! 2Cs2 O þ 4NO2 þ O2 :

ð6Þ

Thermal decomposition of nitrates progresses at a considerable rate only at temperatures above 823 K, ensuring a wide temperature range of their use in electrochemical reactions. Nitrite-ions accumulating in the melt during thermal decomposition of sodium nitrate, can have both the oxidation and reduction properties. However, in the presence of a stronger oxidant (nitrateions), nitrite ions display their reduction properties and probably produce an adverse effect on the formation of a dense oxide film on aluminium by impairing the adhesion of the oxide to the metal surface and increasing the defect content of the anode oxide film. Our investigations [9] showed that such concentration of nitriteions actually decreases in chloride–nitrate melts with the same concentration of alkali metal nitrate as the temperature rises. This decrease can be explained by a competitive reaction of thermal decomposition according to (6) and also is confirmed by visually observable allocation of gaseous NO2 at experiment temperature above 870 K. Oxide films formed on the metal surface at an anodic current are stronger and denser. Also they adhere well to the base for a long time, and do not peel off. A probable reason of it is that anodic oxidation occurs on the surface, which was partially oxidized upon immersion in the oxygen-containing melt. Anodic dissolution of the metal does not take place during the electrochemical treatment, but nitrite ions discharge by the reaction

6MeNO3 þ 10Al ! 3Me2 O þ 5Al2 O3 þ 3N2 ;

ð1Þ

 ! NO2 ; NO2  e

2MeNO3 þ 2Al ! Me2 O þ Al2 O3 þ 2NO;

ð2Þ

and then nitrate ions discharge in 0.6 V:

6MeNO3 þ 2Al ! 3Me2 O þ Al2 O3 þ 6NO2 ;

ð3Þ

6MeNO3 þ 10Al ! 6MeAlO2 þ 2Al2 O3 þ 3N2 ;

ð4Þ

where Me is Na or Cs. A thermodynamic evaluation of these reactions in the temperature range of 800–900 K gives highly negative Gibbs energies of all the above reactions. The values of Gibbs energies, which were calculated for the interaction of aluminium with sodium or caesium nitrate using the thermodynamic data in [13], suggest an irreversible character of a reaction with a large equilibrium constant. It is obvious from the thermodynamic calculations that the most prob-

2017

 ! NO2 þ 1=2O2 NO3  e

ð7Þ

ð8Þ

with liberation of a large quantity of a red gas (nitrogen dioxide) visible to the human eye. Obviously, all the aforementioned reactions of the interaction between aluminium and molten nitrates of alkali metals could be easily effected in interactions with individual nitrates if it were not for considerable thermal decomposition of the nitrates at the given temperatures. Addition of a small quantity of nitrates (up to 30 wt.%) to a molten eutectic mixture of caesium and sodium chlorides ensures that aluminium is oxidized without considerable decomposition of sodium nitrate. For example, when a chloride– nitrate melt containing 15 g of sodium nitrate was held for 10 h

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at 810 K, only 0.67 g of sodium nitrite was detected analytically in the salt cake. An increase in the aluminium oxidation temperature above the decomposition temperature of sodium nitrate up to 900 K was justified because it was possible to produce crystalline oxide films and nanopowders in distinction to amorphous films and nanopowders, which are usually formed at lower temperatures. 3.2. Analyzing the surface morphology of aluminium samples after HTPP treatment and after high-temperature interaction with chloride–nitrate melt After treatment in hydrogen, helium or oxygen plasma, aluminium had an almost smooth surface with foil rolling streaks (Fig. 2). However, separate spherical formations about 1 lm in diameter were observed on the H–Al surface. Their composition could not be determined, and they could be assigned to hydrogen aggregates under the modified layer. Even the most careful examination of the He–Al surface at a considerable magnification did not reveal gas bubbles under the aluminium surface. We related their absence to the fact that in the case of ion bombardment, helium can be trapped in lattice defects and, also, accumulate at the metal grain boundaries, but generally it does not form blisters under the sur-

face of the treated metal [6]. One can clearly see gas bubbles in a long scratch on the O–Al surface. These gas bubbles are embedded during the plasma treatment, and later they can present the weakest points, at which corrosion begins after the oxygen bubbles collapse (Fig. 2c). At ionic introduction all the inert gases are grasped by defects of a crystal lattice [15]. Even if speed of generation of radiating defects is low, heavy inert gases are capable to replace atom of metal in the knot of crystal structure therefore the atom of metal moves to an interstice, and vacancy is occupied by atom of inert gas. Helium is not so aggressive, as heavier argon and krypton, but also it is mainly connected with defects. However it possesses extremely high diffusion coefficient on interstices, so it is capable to move for the long distances and desorbs to vacuum or is grasped by radiating defects or leaves on borders of grains, phases and so forth. Atoms of molecular gases leave defects and dissolve again in a lattice much easier. Molecular gases, in comparison with inert gases, much easier dissolve from a pore in solid substance bulk, and it leads to that the three-dimensional defects containing molecular gases, are usually gas bubbles rather than vacancy pores, as in a case of inert gases. That’s why one can see gas bubbles after treatment by hydrogen and oxygen and can not see any bubbles after helium treatment. However some amount of gases dissolved in

Fig. 2. Aluminium surface after its treatment in: a – hydrogen plasma; b – helium plasma; c – oxygen plasma.

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metal bulk presents in both cases. So as it was shown by previous investigations [16], both atomic and molecular gases can escape from spongy structure and deep channels, which were formed under high irradiation doses on aluminium, under heating more than 473 K with the formation of blisters and cracking. The surface structure of HTPP-treated aluminium after its interaction with a chloride–nitrate melt containing 1–30 wt.% sodium nitrate depends on the concentration of sodium nitrate in the salt electrolyte, the oxidation temperature, and the anodic current density. Not only the external appearance, but also the composition of the oxide film on aluminium considerably depends on the concentration of sodium nitrate in the melt, the oxidation current, and the anodic current applied. For example, an oxygen-deficient thin oxide layer is formed on the aluminium surface (Fig. 3) during anodic polarization of H–Al in a CsCl–NaCl–1 wt.% NaNO3 melt at 790 K. An EDS spectra analysis of the surface shows that the oxide film contains 31.33 wt.% oxygen and 68.67 wt.% aluminium (a stoichiometric Al:O ratio in Al2O3 is 52.9:48.1). Therefore, anion-deficient aluminium oxide uniformly distributed over the whole surface of the electrode is formed on the aluminium surface during its anodic polarization in a melt with a low concentration of sodium nitrate. In this case, the salt electrolyte accumulates a considerable quantity (more than 0.3 g) of an aluminium oxide nanopowder. As the temperature of H–Al oxidation in a chloride–nitrate melt containing 1% sodium nitrate rises further to 900 K, not only the surface morphology of aluminium modified by oxygen plasma, but also the composition of the corrosion products precipitated on the metal surface is altered. For example, the oxide film on alu-

Fig. 3. SEM image of the surface of H–Al after anodic polarization at 790 R in chloride melt containing 1 wt.% of sodium nitrate.

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minium includes 3 wt.% sodium and 1.7 wt.% caesium, adducing evidence that a film comprised of double sodium–caesium aluminates is formed as the oxidation temperature increases to 900 K. In the microphotographs of the surface of an aluminium anode taken after its 5 h exposure at 900 K one can see three-dimensional concentric circles of white color against a darker surface (Fig. 4a). It could be expected that in this way the white layer of aluminium oxide is distributed on the unoxidized aluminium surface. However, elemental maps showed that all the elements – aluminium, oxygen, sodium and caesium – were uniformly distributed over the whole surface of the sample. Then the observed inhomogeneity of the surface is not related to different phase compositions at different points, but is due to the formation of a rough loose oxide coating, which brought some portions of the anode surface out of focus. An examination of the bulging oxide formations on the aluminium surface clearly showed that they consist of a number of ultra fine particles in the form of plates and rods about 20 nm thick (Fig. 5). A similar nanopowders of aluminium oxide and sodium aluminate was found in the bulk of the electrolyte after modified aluminium was held in a chloride–nitrate melt with anodic polarization (Fig. 6). Their chemical composition was defined by means of X-rays and chemical analysis of these powders. An anodic interaction of H–Al with a CsCl–NaCl–10 wt.% NaNO3 melt at 900 K also leads to the formation of layers of hard corrosion products on the aluminium surface, which is followed by a collapse of the blisters formed upon embedding of hydrogen during a plasma shot. As it is known [17], hydrogen, as well as the other molecular gases, is much easier dissolved from a pore in volume of a firm body. As a result the three-dimensional defects containing the molecular gases are usually gas bubbles, rather than vacancy pores, as in a case with the inert gases. Obviously, possible corrosion destruction can be connected with destruction of a hydrogen bubble under the aluminium surface treated by HTPP. As it can be seen from Fig. 7, there are bubble craters of an irregular spherical shape on the oxidized surface of aluminium, which probably are traces of bubble formations. There is a cellular structure on the bottom of a bubble crater, which is characteristic for the surfaces treated by a high-temperature plasma shot [18]. The cells are large, up to 5 lm in diameter, and are distributed uniformly over the whole surface of a crater. It could be expected that the composition of the surface layers is considerably different on the smooth surface and in the bubble craters. Element maps showed that all the elements on the surface – aluminium, oxygen, sodium and caesium – are distributed uniformly both on the exposed surface in collapsed bubbles and on the whole surface of the samples, although the surface in a crater differs in its external appearance from the usual surface oxidized in a salt melt. This uniform distribution of the elements, which does not depend on the surface morphology of the oxidized sample, is similar to the distribution formed during anodic oxidation of O–Al in a melt containing 1 wt.% NaNO3. The morphology of the oxidized surface in the blisters correlates most with the morphology of aluminium oxide formed on the titanium surface by the method of plasma electrolytic oxidation in aqueous electrolyte solutions [19]. This method does not provide healing of pores less than 1 lm in size. In our case, the cellular structure is not present on the entire surface, but occurs on sites of hydrogen bubble breaking. The chemical composition of the oxide coating deposited in a chloride melt with addition of 10 wt.% sodium nitrate during anodic polarization of H–Al at 900 K almost fully coincides with that of a coating deposited on the electrode in the same conditions, but at the sodium nitrate concentration being ten times lower. The oxide film contains 43.2% aluminium, 50.7% oxygen, 2.9% sodium and 2.6% caesium (wt.%). In other words, the oxide layer is a mixed sodium–caesium aluminates, too. As the concentration of sodium nitrate increases from 1 to 10 wt.% in the salt cake, the concentration

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Fig. 4. SEM image of the surface of H–Al after anodic polarization at 900 R in chloride melt containing 1 wt.% of sodium nitrate.

Fig. 5. The increased image of surface roughness of H–Al after anodic polarization at 900 R in CsCl–NaCl–1 wt.% NaNO3.

of sodium in the oxide film remains unchanged, while the caesium concentration increases nearly by 1 wt.%. The formed oxide film is

Fig. 6. Nanopowder of Al2O3 allocated from the melt containing 1 wt.% of sodium nitrate.

more uniform and homogeneous in its thickness and composition, and the amount of the aluminium oxide nanopowders accumulated in the bulk of the electrolyte is considerably less (0.02 g).

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Fig. 7. SEM image and elemental maps of H–Al after anodic polarization at 900 R in chloride melt containing 10 wt.% of sodium nitrate.

As it can be seen from the photograph of the surface of H–Al held in a CsCl–NaCl–10 wt.% NaNO3 melt for 5 h at 900 K without anodic polarization (Fig. 8), the coating, formed during oxidation, is loose, adheres poorly to the aluminium base, and consists of numerous fine crystals. Their composition is independent from the external appearance of the surface: a layer comprised of 54 wt.% aluminium and 46 wt.% oxygen is formed both at blister sites and on the smoother surface of aluminium. That is, a loose layer of aluminium oxide is formed on the surface at different con-

centrations of sodium nitrate in the chloride–nitrate melt without anodic polarization at all the temperatures studied. In the presence of anodic polarization, the composition of the corrosion products considerably depends on the temperature: a layer of aluminium oxide is formed at 790 and 810 K, and a layer of double sodium– caesium aluminates appears at 900 K. Fig. 9 depicts the surface of H–Al oxidized at 810 K in a CsCl– NaCl–30 wt.% NaNO3 melt. Depending on the preliminary treatment of the surface and the oxidation regime, the surface layers

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shot on the aluminium surface does not impede the formation of nanoscale whiskers of aluminium oxide whose thickness is about 10 nm; length is up to 300 nm but precludes their free transfer to the salt melt. Thus, it may be taken as proved that similarly to the case of anodic oxidation of titanium modified by a plasma shot, nanocrystalline oxide powders are formed immediately at the metal– electrolyte interface [10]. However, if anodic polarization is applied to metals, which do not have a layer modified by a plasma pulse, the synthesized nanopowders easily separate from the metal surface and accumulate in the bulk of the melt. A modified surface, which retains its protective properties at temperatures of up to 900 K, prevents the release of aluminium oxide crystals to the salt bulk, and, for this reason, we can see the site of their true nucleation. Moreover, the gas bubbles do not break under the plasmatreated surface layer as in the case of the open-circuit exposure of H–Al and its anodic polarization in a CsCl–NaCl–30 wt.% NaNO3 melt at a temperature of 790 K. This is probably because the plasma-modified layer is more stable at 790 K than at 900 K. 3.3. Electrochemical experiment

Fig. 8. H–Al surface after 5 h exposure in molten CsCl–NaCl–10 wt.% NaNO3 at 900 R.

have considerably different morphologies. For example, an oxide layer with a developed surface morphology is formed upon anodic polarization of the initial aluminium foil. The coating is uniformly distributed on the metal surface and consists of plate nanocrystals of aluminium oxide whose thickness is about 50 nm; length is 5– 20 lm. The deposited coating adheres well to the base, does not crack nor or decomposes upon heat hardening and washing in distilled water. The coating formed after holding of H–Al in a CsCl–NaCl– 30 wt.% NaNO3 melt at a temperature of 810 K for 5 h, is loose, multilayered, and cracked all over. The oxide layers at the base of the coating are very thin; the thickness of the layer is no more than 100 nm. However, the layers are packed loosely, and, therefore, the outlook for high protective properties of such coatings are even worse than those of the coating deposited on initial aluminium during anodic polarization in the same conditions. The coating deposited upon anodic polarization of H–Al in a CsCl–NaCl–30% NaNO3 melt at a temperature of 810 K, considerably differs from the coatings deposited at different concentrations of sodium nitrate and temperatures. The coating consists of closely packed plates of aluminium oxide more than 50 lm in size. Cracks can be observed between the plates, too. At a larger magnification of the H–Al surface oxidized in the anodic direction in a CsCl– NaCl–30 wt.% NaNO3 melt at a temperature of 810 K it can be seen that the deposited oxide coating is nanostructured and has clearly defined nanoscale whiskers similar to those synthesized in the bulk of a salt cake by anodic oxidation of aluminium, which was treated by a plasma shot. Obviously, the layer modified by plasma

When aluminium is kept under open-circuit conditions in a melt of caesium and sodium chlorides containing up to 10 wt.% sodium nitrate, the aluminium surface is covered mostly with loose oxide layers, which adhere poorly to the base and, therefore, fall off as a nanopowder to the salt electrolyte. Individual particles of this nanopowder are flat plates of an irregular hexagonal shape besides the edge thickness is about 20 nm (Fig. 6). The length of the plates varies widely from 50 nm to several microns depending on the interaction temperature and the concentration of nitrate-ions in the melt. The HTPP treatment decreases slightly the rate of the interaction between aluminium and the chloride–nitrate melt even if the concentration of nitrate-ions is low (Table 1); hence, the quantity of the oxide nanopowder crumbling to the melt decreases. A modified layer, which is formed by a plasma shot, prevents separation of oxide particles and their crumbling to the melt. The formed oxide particles either stay under the modified layer or can be easily detected at blistering sites where gas bubbles collapse under the modified layer. The corrosion potential of aluminium in the melt is established the faster, the higher is the concentration of sodium nitrate. We also related this fact to the formation of harder oxide layers with increasing oxygen-containing addition in the salt electrolyte. When aluminium is kept under open-circuit conditions in a melt

Table 1 Chemical analysis of salt electrolyte after 20 h exposure of aluminium in chloride– nitrate melts. NaNO3 (wt.%)

Concentration of Al3+, (wt.%)

Concentration of NO 2, (wt.%)

790 R Al He–Al O–Al Al O–Al Al O–Al

1 1 1 10 10 30 30

0.052 0.014 6.8  103 0.012 1.7  103 4  103 7.6  104

60.1 60.1 60.1 0.3 0.2 0.93 0.58

900 R Al O–Al Al O–Al Al O–Al

1 1 10 10 30 30

0.017 1.7  104 6.7  104 3.2  104 4  104 2  105

0.2 0.2 0.24 60.1 0.3 0.27

Treatment

Al means untreated Al.

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Fig. 9. SEM image of aluminium oxidized in molten CsCl–NaCl–30 wt.% NaNO3 at 790 R: (a) untreated aluminium after anodic polarization; (b) H–Al without polarization, (c) H–Al after anodic polarization.

containing 1–10 wt.% sodium nitrate, the corrosion potential is established in 3–4 h in the whole range of temperatures studied. In this case, some refinement of the surface is observed first and the potential shifts to the positive side, then the oxide powder falls off to the salt electrolyte, and the corrosion potential shifts to the negative side. The potential changes in this way two to three times, and then the corrosion potential is established and remains constant for 5–8 h. Therewith, a considerable quantity of an aluminium oxide nanopowder accumulates in the bulk of the electrolyte melt. The corrosion potential values established at H–Al, He–Al, and O–Al in a chloride–nitrate melt containing 1 wt.% sodium nitrate are even slightly more negative than the potential at the initial aluminium both at 790 K and 900 K. This can be related with the activation of the aluminium surface at sites of the gas bubble collapse or helium escape from vacancy pores. These sites of the blister collapse can further be active spots of the surface, which are free of the ‘‘natural’’ oxide film always covering aluminium in nature. The increase of concentration of sodium nitrate to

10 wt.% leads to a certain shift of O–Al corrosion potential to the positive side as compared with the potential of untreated aluminium. The corrosion potential of aluminium in a CsCl–NaCl–30 wt.% NaNO3 melt is 200–370 mV more positive than the potential of initial aluminium in the same conditions. As well as with untreated aluminium, at all the concentrations of sodium nitrate, except 1 wt.%, the corrosion potential shifts to the positive side with increasing temperature. This change of the corrosion potential with the increasing temperature is characteristic of indifferent electrodes. The anodic polarization curves in potentiostatic mode were measured only after establishment of the corrosion potential and keeping at this potential for at least 1 h so that anodizing of aluminium was effected on an oxidized surface. In order to assess whether there are changes in the morphology and the protective properties of the synthesized oxide coatings formed before and after anodic polarization, the potential Eest was also measured. The potential Eest was established at the aluminium electrode after

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anodic polarization and the measurement procedure of this potential took 0.5 h. The corresponding values are also given in Table 2.

25

Al H-Al O-Al He-Al

i, mAcm-2

20

15

10

5

0

0

50

100

150

200

250

300

350

400

450

Е-Ecor, V Fig. 10. Anodic polarization curves of untreated aluminium in molten CsCl–NaCl– 1 wt.% NaNO3 at 790 K.

Table 2 Corrosion-electrochemical characteristics of aluminium in chloride–nitrate melt. Treatment

790 R Al H–Al He–Al O–Al Al O–Al Al H–Al O–Al 900 R Al O–Al Al O–Al Al H–Al O–Al

NaNO3 (wt.%)

Ecor, V(Cl2/ 2Cl)

Eest, V(Cl2/ 2Cl)

Vcor, (g cm2 h1)

ipass, (mA cm

1 1 1 1 10 10 30 30 30

2.279 2.685 2.674 2.707 2.572 2.376 1.904 1.574 1.707

2.251 2.616 2.501 2.693 2.517 2.438 1.700 1.746 1.674

1.5  102 2.2  103 2.0  103 1.0  103 2.1  103 0.8  103 5.7  104 2.0  104 3.3  104

27 2.2 3.1 5.4 7.0 5.7 0.58 0.18 0.26

1 1 10 10 30 30 30

2.307 2.695 2.279 2.219 1.439 1.180 1.078

2.212 2.647 2.253 2.221 1.226 1.149 1.095

8.5  103 3.6  104 8.4  104 2.7  104 4.3  104 1.1  105 4.3  105

22.5 – 2.0 0.43 0.4 0.27 0.16

2

)

6

6

5

5

4

4

i, mAcm-2

i, mАсm-2

Where Ecor – corrosion potential, Eest – potential which established after anodic polarization, Vcor – corrosion rate from gravimetric data, ipass – passivation current density from anodic polarization curves. Al means untreated Al.

The anodic polarization curves measured in a CsCl–NaCl–1 wt.% NaNO3 melt at 790 K for initial aluminium and aluminium treated with plasma of different types are shown in Fig. 10. All the anodic polarization dependences are typical passivation curves. Obviously, even with 1% concentration of sodium nitrate in the salt electrolyte, the passivation currents are considerably reduced (by a factor of 4–10) as compared to those of untreated aluminium. This fact is due to the protective properties of the modified surface layer of aluminium. Moreover they appear almost identically with all types of plasma: the passivation currents are minimum for H–Al and maximum for O–Al (the difference of the absolute values of the passivation current density is within the experimental error). The protective properties of the modified layer are also related to the fact that the formed oxide does not fall off to the melt, as is the case with untreated aluminium, but accumulates immediately under the modified layer or on its surface, probably inhibiting further dissolution of the metal. It should be noted however that both at 790 and 900 K the oxide film formed on aluminium in a CsCl– NaCl–1 wt.% NaNO3 melt, is loose, and it includes numerous defects, and cannot fully protect aluminium from further dissolution. As can be seen from Fig. 11, it is only the anodic polarization curves, which were measured for O–Al in a melt with 1% sodium nitrate, that change their trend as the oxidation temperature rises from 790 to 900 K. The anodic polarization curve of aluminium treated with oxygen plasma in a CsCl–NaCl–1 wt.% NaNO3 melt at 900 K has not a passivation section at all and represents a typical curve of anodic dissolution with a corrosion current of 3.2103 A cm2. The corrosion rate, which was measured by the gravimetric method for O–Al in CsCl–NaCl–1 wt.% NaNO3 at 900 K, is one of the largest among the values obtained for all the plasma-treated aluminium electrodes, but it is still one order of magnitude lower than the corrosion rate of initial aluminium. The magnitudes of aluminium ions escape into the melt are very low too. The possible reason of such behavior of O–Al in the melt with 1 wt.% sodium nitrate is poor adhesion of forming oxide scales to aluminium surface that leads to formation of large amount of nanoscale oxide powder in the melt bulk. Obviously oxide film formed on aluminium surface in CsCl–NaCl–1 wt.% NaNO3 at 900 K can’t provide protective properties in this melt. The results of the chemical–analytical determination of the concentration of aluminium ions Al3+ and nitrite-ions NO 2 after anodic polarization of aluminium in a chloride–nitrate melt are summarized in Table 1. As it can be seen, even if the chloride–nitrate melt contains 1 wt.% sodium nitrate, the yield of aluminium ions to the salt cake considerably increases by a factor of 5–100 depending on the treatment and on the oxidation temperature. The concentration of nitrite ions in the salt electrolyte after anodic polarization

3 2

3 2

1 0

10% 1% 30%

1

900 К 793 К 0

100

200

300

400

500

600

700

800

Е-Ecor, V Fig. 11. Anodic polarization curves of O–Al in CsCl–NaCl–1 wt.% NaNO3 at different temperatures.

0

0

100

200

300

400

500

600

700

800

Е-Ecor, V Fig. 12. Anodic polarization curves of O–Al at 790 R in CsCl–NaCl with the different concentrations of sodium nitrate.

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i, mAcm-2

1

0.8

0.6

0.4

0.2

0

0

100

200

300

400

500

600

700

800

Е-Ecor, V Fig. 13. Anodic polarization curves of aluminium depending of plasma type treatment in CsCl–NaCl–30 wt.% NaNO3 melt at 790 R.

aluminium in a CsCl–NaCl–30 wt.% NaNO3 melt at 790 K. The curve of initial aluminium includes a small section of anodic dissolution, while the curves of O–Al and H–Al look like passivation curves without an anodic dissolution section. In passing from initial aluminium to O–Al and H–Al, the passivation currents are reduced three times; the corrosion rate, which was calculated by the gravimetric method, decreases by a factor of two to three; and the yield of aluminium ions to the salt melt decreases fivefold. With such small values of the aluminium corrosion rate, one may think the corrosion rates of aluminium in a chloride–nitrate melt, which were determined by the three methods, to be in a fairly good agreement. As the oxidation temperature rises to 900 K, the behavior of the anodic polarization curves of H–Al in a CsCl–NaCl– 30 wt.% NaNO3 melt changes a little (Fig. 14). A relatively extended passivation section (400 mV) with a passivation current density of 0.05 mA cm2 is followed by a short section of anodic dissolution, which ends in the second passivation plateau with a passivation current of about 0.3 mA cm2. The last value is even larger than the value determined from the plot at 790 K. However, the corrosion rate of aluminium in a CsCl–NaCl–30 wt.% NaNO3 melt at 900 K, which was determined by the gravimetric method, decreases more than one order of magnitude in passing from initial aluminium to H–Al and O–Al. The concentration of aluminium ions in the salt cake is reduced analogously. The absolute values of the corrosion rate of O–Al decrease eight times and, according to the chemical analysis data, by a factor of 40 when, e.g., the temperature rises to 900 K. It follows that the nanostructured layer of caesium and sodium aluminates, which is formed on the aluminium surface under a layer modified by a plasma shot, has a higher cor-

0.35

790K 900 K

0.3 0.25

i, mAcm-2

is practically the same (0.1–0.2 wt.%) at both temperatures studied. As it was shown in Tables 1 and 2 concentration of NO 2 -ions diminishes with the increasing temperature as well as the values of passivation current densities and corrosion rates diminish with the increasing temperature. So we can make a conclusion that nitrite ions can deteriorate the formation of a dense oxide film on aluminium. The interaction of initial aluminium and aluminium exposed to oxygen plasma in a chloride melt containing 10 wt.% of sodium nitrate was also studied at different temperatures. The anodic polarization curves (Fig. 12) of O–Al in melts containing 1 and 10 wt.% sodium nitrate at 790 K have nearly equal passivation currents, but the trends of these curves are considerably different. For example, the curve, which was measured in a CsCl–NaCl–1 wt.% NaNO3 melt, first has a section of active dissolution, and only then a high constant value of the current density is established. The curve, which was measured in a CsCl–NaCl–10 wt.% NaNO3 melt, does not exhibit a section of anodic dissolution during the first 200 mV of anodic polarization and is characterized by extremely small values (0.4 mA cm-2) of the passivation currents; then a section of anodic dissolution of aluminium begins, and finally the currents acquire a constant value, which is independent of the applied potential and practically equals the corresponding value in the CsCl–NaCl–1 wt.% NaNO3 melt. The absolute values of the passivation current density on O–Al in a melt with 10 wt.% sodium nitrate at 790 K are slightly smaller than those on initial aluminium. At 900 K they decrease by a factor of five, and this fact may be an indication that the aluminates coating formed at a higher temperature possesses better protective properties as compared to those of a purely oxide coating deposited at a lower temperature. The corrosion potentials of O–Al in CsCl–NaCl–10 wt.% NaNO3 approach those of aluminium, which was not exposed to the plasma treatment. It is only at 790 K that the corrosion potential of aluminium, which was treated in oxygen plasma, is 200 mV more positive than the corrosion potential of initial aluminium. In the other cases, the difference of the corrosion potentials is within the measurement error. The potentials established at the electrodes after polarization are practically equal to the corrosion potentials. Therefore, anodic polarization does not cause considerable changes in the morphology of the oxide layer and its adhesion to the metal base. The yield of aluminium ions to the salt electrolyte also decreases by a factor of 2–10, with the difference being more pronounced at a lower temperature, at which the corrosion rate is much higher. The absolute values of the yield of aluminium ions to the CsCl–NaCl–10 wt.% NaNO3 melt at 900 K are in good agreement with the weight analysis data. The composition of the oxide film formed on the aluminium surface depends on the interaction temperature and, in the first place, on the anodic current applied. For example, only aluminium oxide Al2O3 is formed in the open-circuit conditions irrespective of the concentration of sodium nitrate in the melt at all the temperatures studied; in the presence of anodic polarization at 790 K aluminium oxide is formed, too, but as the temperature rises to 900 K, a layer of mixed aluminates of sodium and caesium is deposited. It is only when the sodium nitrate concentration is 30 wt.% in molten mixture that the corrosion potentials of initial and plasma-treated aluminium correspond to the oxide potential at both 790 and 900 K. The corrosion potentials of H–Al and O–Al are 200–300 mV more positive than those of initial aluminium, while the values of established potential are almost coincident. This fact probably suggests that the compositions of the oxide layers formed on all the three anodes are fully identical. In all the cases, dense protective oxide layers are deposited on the aluminium surface; still, these layers are nanostructured and are reinforced with thin fibres of the nanopowder, as is shown in Fig. 9, only on plasmatreated aluminium. Fig. 13 depicts anodic polarization curves of

0.2 0.15 0.1 0.05 0

0

200

400

600

800

1000

1200

E-Ecor, V

Fig. 14. Anodic polarization curves of H–Al in molten CsCl–NaCl–30 wt.% NaNO3 at different temperatures.

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4.5 10% 1% 30%

4 3.5

i, mАсm-2

3 2.5 2 1.5 1 0.5 0

0

100

200

300

400

500

600

700

Е-Ecor, V Fig. 15. Anodic polarization curves of O–Al in CsCl–NaCl with the different additives of NaNO3 at 900 R.

rosion resistance than the oxide layer formed on initial aluminium. An analogous conclusion was made from a consideration of the corrosion-electrochemical behavior of titanium exposed to hydrogen and helium plasma in chloride–nitrate melts containing 10 and 30 wt.% sodium nitrate [10]. The effect of the concentration of sodium nitrate in the salt melt on the corrosion characteristics of aluminium is prominent in Fig. 15. For example, the polarization curve of O–Al in a melt with 1 wt.% sodium nitrate does not have the section of anodic dissolution at all. When the concentration of sodium nitrate increases to 10 wt.%, it acquires the shape of a passivation curve, which is characterized by an extended passivation plateau with a passivation current density of 0.5 mAcm2. When the sodium nitrate concentration is 30 wt.%, the passivation current density decreases to 0.2 mA cm2. Thus, four parameters determine the corrosion resistance of aluminium exposed to a plasma pulse in different gas media in a chloride–nitrate melt. They are the concentration of sodium nitrate in the melt, the oxidation temperature, anodic polarization, and formation of a layer modified by a plasma pulse. This layer can considerably reduce the corrosion rate and decrease the yield of the aluminium oxide nanopowder, which is formed on the aluminium surface, to the salt electrolyte. It can be stated that there is a relationship between the morphology of the oxide film and its protective properties. For example, multilayered loose films crack as the layers are building up, and they cannot prevent further dissolution of aluminium under the oxide film. Thin films, which consist of closely packed nanocrystals of aluminium oxide or sodium–caesium aluminates, protect the metal base from dissolution the better, the finer the crystals comprising the oxide layer. 4. Conclusions The corrosion-electrochemical behavior of aluminium in a molten eutectic mixture of caesium and sodium chlorides containing 1 to 30 wt.% sodium nitrate at temperatures of 790–900 K in argon atmosphere was studied. It was shown that treatment with high-temperature pulsed plasma in hydrogen, helium and oxygen has little effect on the corrosion potential of aluminium in a chloride–nitrate melt containing from 1 to 10 wt.% of sodium nitrate. The morphology and the properties of the oxide films formed on the aluminium surface during a high-temperature interaction with a chloride–nitrate melt change significantly. A thick loose layer of aluminium corrosion products (anion-deficient aluminium oxide, which does not prevent further corrosion of aluminium) is deposited on the metal surface in open-circuit conditions at all the temperatures studied.

Anodic oxidation in melts with a low concentration of sodium nitrate (1 and 10 wt.%) does not provide a protective oxide coating because of poor adhesion of the synthesized oxide particles, which accumulate as a nanopowder in the course of anodic polarization in the bulk of the electrolyte. It is only anodic oxidation of aluminium containing 30 wt.% of sodium nitrate that allows producing thin protective films of aluminium oxide at 790 K and mixed caesium–sodium aluminates at 900 K on the aluminium surface, with long nanofibres in the layer modified by plasma shot. A relationship between the morphology of the surface oxide film and its protective properties was established. A nanostructured film of caesium–sodium aluminates, which was deposited in a melt with 30% sodium nitrate at 900 K, has the highest protective properties. Modification of aluminium by HTPP changes properties of 20 lm layer under its surface and the oxide layer formed after such treatment has different morphology consists of smaller crystals and so has better protective properties than the oxide layer formed under the same conditions on untreated aluminum. References [1] A. Markwitz, P.B. Jonson, P.W. Gilberd, G.A. Collins, D.D. Cohen, N. Dytlewski, Ion beam analysis of nanoporous surfaces produced by He-implantation and oxidized by plasma immersion ion-implantation, Nuclear Instruments and Methods in Physics Research B 161–163 (2000) 1048–1053. [2] B.A. Kalin, V.L. Yakushin, V.I. Vasiliev, S.S. Tserevitinov, Use of high temperature pulsed plasma fluxes in modification of metal materials, Surface and Coating Technology 96 (1997) 110–116. [3] E. Ingenbergs, J. Spörer, R. Heß, F. Hepp, Production of very hard surface layers by repetitive use of high energy plasma pulses, Surface and Coating Technology 86–87 (1996) 592–597. [4] J. Spörer, E. Ingenbergs, Treatment of surface layer of steel with high energy plasma pulses, Surface and Coating Technology 76–77 (1995) 589–594. [5] V.N. Pimenov, E.V. Dyomina, L.I. Ivanov, S.A. Maslayev, V.A. Gribkov, R. Miklaszewski, M. Scholz, A.V. Dubrovsky, I.V. Volobuev, Yu.E. Ugaste, F. Mezzetti, P. De Chiara, L. Pizzo, B. Kolman, A. Szydowski, Damage of structural materials for fusion devices under pulse ion and high-temperature plasma beams, Journal of Nuclear Materials 307–311 (2002) 95–99. [6] V.I. Polsky, B.A. Kalin, I.I. Kartsev, V.L. Yakushin, Damage of a surface of constructional metals at influence of plasma clots, Atomic energy 56 (1984) 83–88. [7] N. Yoshida, T. Hirai, K. Tokunaga, S. Itoh, The TRIAM Group, Plasma-surface interaction effects during high ion temperature long pulse experiments in TRIAM-1M, Journal of Nuclear Materials 290–293 (2001) 1030–1035. [8] L.A. Yolshina, V.Y. Kudyakov, V.G. Zyryanov, Corrosion electrochemical behavior of modified by plasma pulse shot aluminum in molten alkali chlorides, Protection of metals 34 (1998) 632–637. [9] L.A. Yolshina, V.Y. Kudyakov, V.B. Malkov, A.N. Yolshin, High- temperature electrochemical synthesis of thin oxide layers and nanopowders of some metal oxides, Physics and chemistry of glass 34 (2008) 808–817. [10] L.A. Elshina, V.Y. Kudyakov, V.B. Malkov, S.V. Plaksin, The influence of plasma treatment on the corrosion electrochemical interaction of titanium with the chloride–nitrate melt, Protection of metals and physical chemistry of the surfaces 46 (2010) 588–593. [11] H. Ikeuchi, C. Krohn, Thermodynamic properties of binary liquid aluminium chloride-alkali chloride mixtures, Acta Chemica Scandinavica, A 28 (1974) 48– 54. [12] P. Afanasiev, C. Geantet, Synthesis of solid materials in molten nitrates, Coordination Chemistry Reviews 178–180 (1998) 1725–1752. [13] V.P. Glushko, Thermodynamic Properties of Individual Substances, Science, Moscow, 1979. [14] K. Jones, The Chemistry of Nitrogen, Pergamon, Oxford, 1975. [15] K.N. Jumangulova, A.M. Jukeshov, T.S. Ramazanov, The interaction of plasma streams with the construction materials of equipment, Kasakh State University, Almaaty, 2007. [16] A. Hofman, A.Yu. Diduk, V.K. Semina, W. Szteke, Simulation of influence of uranium fission fragments on materials on fuel elements of reactors by high energy heavy ions, Questions of atomic energy and techniques, Series: Physics of radiation damages and radiation materials technology 88 (2005) 16–21. [17] I.A. Kursina, E.V. Kozlov, Yu.P. Sharkeev, S.V. Fortuna, N.A. Koneva, I.A. Bozhko, M.P. Kalashnikov, Nanocrystalline intermetallic and nitride structures formed under ion-beam action, NTL edition, Tomsk, 2008. [18] L. Bradley, L. Li, F.H. Stott, Characteristics of the microstructures of aluminabased refractory materials treated with CO2 and diode lasers, Applied Surface Science 138–139 (1999) 233–239. [19] S. Gnedenkov, S. Sinebryukhov, Composite polymer containing coatings on the surface of metals and alloys, Composite interfaces 16 (2009) 387–405.