International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 04, April 2019, pp. 315–321, Article ID: IJMET_10_04_031
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=4
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(4), 2019, pp. 315–321.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=4
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.
http://www.iaeme.com/IJMET/index.asp
315
editor@iaeme.com
The Influence of the Cathode Shape on the Phase Composition and Structure During Oxidation
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.
http://www.iaeme.com/IJMET/index.asp
316
editor@iaeme.com
N. F. Kolenchin
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; 7current source; 8- refrigeration unit; 9- sliding current lead; 10- needle-shaped cathode; 11electric 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 SO45A 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
http://www.iaeme.com/IJMET/index.asp
317
editor@iaeme.com
The Influence of the Cathode Shape on the Phase Composition and Structure During Oxidation
with a resolution of 1024 points per side. When scanning, the relief of the sample and the
distribution of the 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 "KonstantaK5U". 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
http://www.iaeme.com/IJMET/index.asp
318
editor@iaeme.com
N. F. Kolenchin
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
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
density,
Along the
Inside the
Outside the
A/dm2
contact line contact line contact line
1
62
41
60
5
66
43
63
10
72
48
68
Along the
contact line
54
57
64
Thickness, µm
Before the
Outside the
contact line
contact line
32
51
36
53
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
http://www.iaeme.com/IJMET/index.asp
319
editor@iaeme.com
The Influence of the Cathode Shape on the Phase Composition and Structure During Oxidation
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 twodimensional 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
Mashinostroyeniye, 1988. - 224 p.
http://www.iaeme.com/IJMET/index.asp
anodizing
320
/
E.E.
Averyanov.
-
Moscow:
editor@iaeme.com
N. F. Kolenchin
[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.
Spalart P. R., Allmaras S. R. ―A one-equation turbulence model for aerodynamic flows‖,
AIAA Paper 1992-0439.
[10]
http://www.iaeme.com/IJMET/index.asp
321
editor@iaeme.com