The influence of the cathode shape on the phase composition and structure during oxidation

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The microstructure of the oxide layer and the density distribution of the phases in the volume of the film in two positions of the cathode — stationary and during its rotation — were investigated.

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Nội dung Text: The influence of the cathode shape on the phase composition and structure during oxidation

  1. International Journal of Mechanical Engineering and Technology (IJMET) Volume 10, Issue 03, March 2019, pp. 1210–1216, Article ID: IJMET_10_03_123 Available online at ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication Scopus Indexed THE INFLUENCE OF THE CATHODE SHAPE ON THE PHASE COMPOSITION AND STRUCTURE DURING OXIDATION N. F. Kolenchin Chief Researcher of Technopolis, Doctor of Engineering, Associate Professor, Industrial University of Tyumen, Volodarskogo, 38, Tyumen, Russia, 652000 ABSTRACT The process of anodizing aluminium alloys in an ozone-containing electrolyte when a flat cathode is replaced with a needle shape was studied. The microstructure of the oxide layer and the density distribution of the phases in the volume of the film in two positions of the cathode — stationary and during its rotation — were investigated. The mechanical properties of the surface layer were determined. Key words: anodizing, needle-shaped cathode, ozonation, pore microgeometry, phase distribution, surface hardness, activation. Cite this Article: N. F. Kolenchin, The Influence of the Cathode Shape on the Phase Composition and Structure During Oxidation, International Journal of Mechanical Engineering and Technology 10(3), 2019, pp. 1210–1216. 1. INTRODUCTION The search for ways to activate the interelectrode gap during oxidation is one of the promising areas in the technology of hardening the surface layer of aluminium alloys. For most researchers, the range of factorial variability is determined by the boundaries of the electrolytic cell and, as a rule, is associated with changes in the chemical composition of the electrolyte and the conditions of energy excitation. The performance is estimated by the degree of activity of the main participants of the process - aluminium and oxygen under the conditions of the necessary and sufficient influence of the working environment. Non-traditional is the technology of external activation of oxygen and its transfer into the interelectrode space. This is achieved through the introduction of ozone into the electrolyte [1]. Being the strongest oxidizer, ozone itself or atomic oxygen formed during its decomposition acts, contributing to the intensification of the process, changing the structure and phase composition of the oxide. Questions of the effectiveness of oxide formation in the ozonized electrolyte are the motive for finding ways to activation, including changing the shape of the cathode. 1210
  2. N. F. Kolenchin With traditional anodizing, a flat-shaped cathode is used [2,3]. This ensures a uniform distribution of the electric field strength in the electrolytic cell. To change the density of discharges on the surface of the anode, the amperage or voltage of forming increases. Study of the influence of the cathode shape on the near-anode space is prompted by the results of processes of the same nature in which a pointed cathode was used. According to the authors of [4], the scientific and practical aspects of electrochemical processes in process gases between the needle-shaped cathode and the liquid anode, when the pores of the oxide layer are filled with steam, confirm the identity of the processes for anodizing aluminium and its alloys. During plasma-electrolytic anodizing [5], passing a pulsed current of high density, a thin cathode is placed over the surface of the electrolyte, and the anode is immersed to a depth of 1 mm. At the time of the breakdown, a vapour-gas funnel with oxygen donors is formed. The rate of oxidation increases and the phase composition of alumina changes. Studying the effect of electrode geometry on the distribution of electric fields in a discharge of a high-current low-inductance vacuum spark type [6], when the cathode was the pointed tip and the anode was the plane, it was found that the field strength is maximum at the anode surface at a distance of approximately 1/4 of the radius from the centre, Figure 1. Figure 1 Distribution of the electric field in the "tip-plane" geometry During electrospark processing [7], to increase the magnitude of the electric field at one of the electrodes, the diameter of the second electrode is reduced or sharpened to a radius of curvature of about 1 micron. This significantly increases the emission of electrons due to the tunnel effect, which allows you to make high-current installations more reliable and compact, without the presence of hot electrodes in them. As a result of the use of filament-like inclusions of titanium and zirconium oriented along the axis of the needle and facing the emitting surface, the authors of [8] achieved stability of electron emission. Needle-shaped cathodes are also used in a vacuum-arc evaporator [9] for surface metallization due to the evaporation of droplets flying from the cathode, which contributes to improved adhesion and increased corrosion resistance of surface oxides. 1211
  3. The Influence of the Cathode Shape on the Phase Composition and Structure During Oxidation 2. MATERIALS AND METHODS The anodizing scheme is shown in Figure 2. Figure 2 Scheme of anodizing with a needle-shaped cathode: 1- bath tank; 2- compressor; 3- ozone dryer; 4- ozone generator; 5- rotameter; 6- bubbler; 7- current source; 8- refrigeration unit; 9- sliding current lead; 10- needle-shaped cathode; 11- electric motor; 12- storage tank; 13- pump The anodizing process was carried out in a bath tank (1), made of corrosion-resistant steel 12Kh18N10T with a capacity of 20 litres. Air was injected into the supply system by a SO-45A grade compressor (2), passing through an air absorption dryer (3) HLS-R012-HL0030 entered an ozoniser (4) "OZON-5PV1" with a power capacity not more than 150 W and a maximum productivity of 16g/m3. Ozone-resistant PVC hoses and glass tubes were used as a pipeline for transporting the ozone-air mixture. The ozone content in the air was determined using Medozon 254/5 with a measurement range from 0 to 150 mg/l. The ozone content in the liquid was measured by Medozon-254/5Zh with a range of measured concentrations from 0.1 to 25 mg/l. Adjustment of the flow rate of the gas-air mixture was carried out using a rotameter (5) Emis Meta 210-R-008V-G. As a current source (7), a VSA-5K selenium rectifier was used which allows adjusting the current in the range from 0 to 20 A at voltages up to 90 V. Contact with the cathode was carried out using a sliding contact (9). The current strength was monitored with a high-precision desktop digital multi-meter MS8050. Cooling and mixing were carried out in a storage tank (12) with the help of a refrigeration unit (8) VS 0.7-3 and a bubbler (6). The electrolyte with dissolved ozone was fed into the electrolytic bath by a pump (13). A needle-shaped cathode (10), which is a cylindrical structure with a 40- mm outer diameter and a 20-mm inner diameter, is made of thin, corrosion-resistant wire with a diameter of 0.1 mm. To ensure uniform contact with the anode plane, the working, end surface of the needle- shaped cathode was sanded. The rotation speed was provided by an adjustable electric motor (11). The distance between the electrodes varied in the interval of 0.1-0.5 mm. Tests were conducted on the D16T alloy. The size of the anode was 60x40x3 mm. The study of the structure of the samples was carried out using a JEOLJ5M-6150 scanning electron microscope with an attachment module for X-ray spectral analysis and an Integra Aura atomic- power probe microscope using the semi-contact method with scanning the sample previously purified from organic pollutants with ethyl alcohol. Samples were scanned with a resolution of 1024 points per side. When scanning, the relief of the sample and the distribution of the 1212
  4. N. F. Kolenchin amplitude and phase of the probe oscillations over the scan area were recorded. The lateral scanning resolution of the microscope is at least 3 nm, the height resolution is at least 0.5 nm. Hardness was measured using an ultrasonic contact thickness gauge "Konstanta-K5". This device allows measuring oxide coatings up to 2 mm thick, excluding preliminary sample preparation. Hardness was determined by a multifunctional ultrasonic device "Konstanta- K5U". Measurement limits were from 20 to 80 HRC, error was +/-2. The concentration of transmitted ozone in the air mixture corresponded to 3 mg/l. 3. RESULTS AND DISCUSSION The oxidation process was carried out in a 10% aqueous solution of sulfuric acid in the mode of falling power. Two options were considered — static, when the cathode is stationary, and mobile, when the cathode is rotating. When the cathode is stationary, the anode surface located inside the needle-shaped electrode is partially etched due to insufficient cooling in the domed zone and an increased etching rate due to the heating of the electrolyte. The surface of the anode located opposite the needles was formed with the original texture presented in Figure 3. a b Figure 3 The oxide surface formed with a stationary cathode. Voltage of forming - 25V. Electrolyte - a 10% solution of sulfuric acid cooled to 0 °C: а - microgeometry of the flat part of the surface; b - microgeometry of the surface in 3D The crater-shaped surface is the result of the concentration of current density on the edges of the cathode — the centre of elevated temperature and, accordingly, the zone of elevated 1213
  5. The Influence of the Cathode Shape on the Phase Composition and Structure During Oxidation etching rates. The film was formed diametrically unevenly in thickness with a decrease in the direction of the axis of the stationary electrode. Over 60 minutes, the average oxide thickness turned out to be insignificant and amounted to 15 µm. By imparting rotation to the cathode at a speed of 150 rpm, the electrolyte bubbling in the contact zone was improved. The maximum convergence of two electrodes, with intensive homogenization, provides an increase in current density at the tip, which contributes to the dissolution of ozone in the interelectrode gap and, accordingly, increases the likelihood of its participation in oxide formation. The results of measurements of the thickness and hardness of the oxide layer over 30 minutes of the process are shown in Table 1. The oxide formed on the inner surface of the sample has low hardness and thickness. Table 1 Properties of the formed layer with different modes of anodizing Initial current Hardness, НRC Thickness, µm density, Along the Inside the Outside the Along the Before the Outside the A/dm2 contact line contact line contact line contact line contact line contact line 1 62 41 60 54 32 51 5 66 43 63 57 36 53 10 72 48 68 64 39 57 A set of needles is a definite obstacle to the ozonized electrolyte in the inner zone. The strip under the needle electrode has some advantages in terms of the parameters under study over anodizing with a flat electrode. On average, the thickness and hardness of the oxide layer were 10% greater. а b Figure 4 Geometry of the pores formed during the rotation of the cathode at a speed of 150 rpm with a magnification: a-100 times; b-1000 times 1214
  6. N. F. Kolenchin The results of the electronic study of the oxide layer are presented in Figure 4. Stretched pores in one vector variant are the result of stretching the discharge spot in the course of the electrode rotation. The concentration of current density at the cathode tip during movement creates an elongated temperature zone into which dissolved gases are drawn. The unreacted part of ozone, in the form of large gas bubbles, is split into small fractions, increasing the solubility and intensification of the anodic process. The vector of oxidation takes a two-dimensional direction; one beam is directed perpendicular to the anodizing plane, and the other - towards the rotation of the cathode, which causes some curvature of the pores. This is confirmed by the results of studies of the formed structure in the mode of reflected electrons presented in Figure 5. The darker part of the image indicates the formation of phase structures with the highest density of atoms. The distribution of dark areas occurs both along the filiform channels and in the horizontal direction. Figure 4 Geometry of the pores formed during the rotation of the cathode at a speed of 150 rpm with a magnification: a-100 times; b-1000 times 4. CONCLUSIONS 1. The use of a needle-shaped cathode during oxidation in an ozone-containing medium increases the potential of the electric field in the reaction zone and contributes to the dissolution of unreacted ozone in the electrolyte as a result of grinding the macro-bubbles of the gas-air mixture; 2. An increase in the temperature gradient and an increase in the concentration of the oxidizing agent in the pore space intensifies the process of oxidation with an increase in the hardness and thickness of the oxide by 10% in comparison to anodizing with a flat-shaped anode; 3. The mobile version of the electrode changes the pore geometry in the direction of movement of the pointed cathode with the formation of phase structures in two directions — perpendicular to and along the anode surface. REFERENCES [1] Vigdorovich V.I., Kolenchin N.F. Anodizing of aluminum alloys in the ozone-containing sulfuric acid medium.// V.I. Vigdorovich, N.F. Kolenchin / Practice of anticorrosive protection. 2017. No. 2 (84). p. 38-50. [2] Averyanov E.E. Handbook on anodizing / E.E. Averyanov. - Moscow: Mashinostroyeniye, 1988. - 224 p. 1215
  7. The Influence of the Cathode Shape on the Phase Composition and Structure During Oxidation [3] Tomashov N.D. The influence of various factors on the growth of anodic oxide film on aluminum in a solution of sulfuric acid / N.D. Tomashov, A.V. Byalobzhesky // Research on the corrosion of metals: collection: works of the Institute of Physics and Chemistry of the Academy of Sciences of the USSR. - Moscow, 1955. - p. 109-116. [4] Borisenko A.V. Scientific bases and practical aspects of electrochemical processes in the gas phase in the zone of a dark electrical discharge between the needle cathode and the liquid anode / A. V. Borisenko. - Karaganda: KarSU, 2007. - 238 p. [5] Averyanov E.E. Questions of the theory of formation and formation of anodic oxides: dissertation for the degree Dr. Techn. Sciences / E.E. Averyanov. - Kazan, 2004.- 274 p. [6] Burkov V. М. Electrochemical forming with vibration of the tool electrode / V. M. Burkov. - Ivanovo: ISUCT, 2008. - 413 p. [7] Pyachin S. A. Regularities of the formation of oxides on the surface of metals when exposed to electric discharges / S. A. Pyachin, A. A. Burkov, M. A. Pugachevsky // Fizika i Khimiya Obrabotki Materialov. 2011. No. 2. p. 51-59. [8] Author's certificate 423198 USSR, IPC 6 H01J1 / 304. Autoemission needle cathode / R. I. G. Garber, B. G. Lazarev, L. Sh. Lazareva, Zh. I. Dranova, I. M. Mikhailovsky, V. B. Kulko. - No. 1776842; Appl. 04.24.72; Publ.05.04.74, Bull. No. 13 [9] Dubrovskaya (Pryadko), E.L. Evaporation of cathode material drops in a vacuum-arc reflection discharge plasma: author's abstract of a dissertation for the degree Cand. Phys.- Math. Sciences / E. L. Dubrovskaya (Pryadko). - Tomsk, 2012. - 29 p. [10] Spalart P. R., Allmaras S. R. “A one-equation turbulence model for aerodynamic flows”, AIAA Paper 1992-0439. 1216



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