Improving corrosion resistance of metallic materials by electrical discharges in impulses.
Barhalescu, Mihaela ; Dumitrache, Constantin ; Oanta, Emil 等
1. INTRODUCTION
A problem of present interest in most factories is the rational and
efficient use of metallic materials. The superior capitalization of
metallic materials into products is obtained by applying the most
efficient bulk and surface thermal treatments. For the same purpose of
increasing the endurance of pieces (machine parts, tools) intensely
subject to wear, in the lately time a series of unconventional
superficial treatments were necessary.
The superficial layers obtained by electrical discharges in
impulses corrosion, were studied since 1999, at the Technical University
of Iasi, Science and Engineering of Materials Faculty (Pop et al. 1999).
The experiments were dedicated to the superficial treatment using
impulse electrical discharges using a Cu electrode on steel carbon OL 37
sample. The examination of the importance in the formation of the
superficial layer and its qualities thought potential in open circuit
was done by analyzing the corrosion potential and marking the
polarization curves (linear and cyclic), using the sea water as a
corrosion environment. The results of the study concerning the new
mechanical parts covered with a superficial layer were compared with the
results of the original parts.
The AFM images of the samples offer the opportunity to analyse the
layer after the corrosion (Barhalescu et al. 2007).
Copper sparking process, regardless of the work regime, leads to an
improvement of the material qualities in comparison with the corrosion
of the support.
Some future research topics are: the automation of the deposition
process in order to have a better quality of the layer and the
optimization of the process costs. In further researches is necessary to
study the oxides obtained during the sparking process and their
influence in the layer properties. Another research direction is the
study of the apparition of the Cl ions and their effect regarding the
corrosion speed.
2. EXPERIMENTAL RESEARCH
The sparking samples for experiments regarding the corrosion
resistance of the superficial layers obtained by electrical discharge
impulses were done with the ELITRON 22A equipment, using a Cu electrode.
The initial samples have a circular plane surface and they are made of
OL37 steel carbon. Before the experiment the surfaces were subjected to
a special treatment and degrease with a powerful solvent (Barhalescu
2007).
The sparking process using electrical discharges were done
manually, the active electrode having a 60[degrees] angle with respect
to the basic surface.
A significant importance regarding the formation of the superficial
layer and its qualities has the electrode section surface, influence
which is noticed at the work regime temperature variation and with
respect to the current density which passes through the electrode.
Table 1 presents the recommended values of the electrode cross
section for all the work regimes of the ELITRON--22A equipment and the
according current values (*** 1991).
The OL37 steel carbon sample was inserted in a Teflon holder so it
can be mounted into the electrode equipment. The corrosion testing was
done on the samples sparking at three work regimes of the ELITRON 22A
equipment: R1-0.5 A, R2-1.3 A and R3-2.3 A.
The corrosion resistance was studied through electrochemical
methods determining the potential in open circuit, the corrosion
potential and marking the polarization curves (linear and cyclic) with
VOLTALAB 32 equipment. The experimental data acquisition and processing
were done by the use of VOLTMASTER 2, specialized software.
Experimental results were compared for both the surface of the
initial sample material (OL 37) and the surface of the sample covered
with a Cu layer. The corrosion trends were made measuring the potential
in open circuit (Barhalescu 2007) (figure 1).
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
The corrosion potential was determined by tracing the curves of
linear polarization, recorded in sea water, using the Evans coordinates:
E = f(log I) (Marcus 2002) (figure 2).
The corrosion process analysis was made also based on the cyclic
polarization curves (figure 3).
The "wave-mode" images from the AFM microscope
(Barhalescu et al. 2007) of the samples superficial treated with the Cu
electrode, presents the surface relief using colour shades (figure 4).
The light colour zones are the highest ones.
In figure 5 can be noticed initial cracks and real cracks developed
in the Cu layer of the samples.
The AFM analysis done on the samples superficial treated with
copper electrode, show a non uniform layer. This layer has
discontinuities where the corrosive agent might have access to the
sample basic material (Barhalescu 2007).
3. CONCLUSIONS
Copper sparking, regardless of the work regime, leads to an
improvement of the qualities material in comparison with the corrosion
thermodynamic probability. In any case, the open circuit potential is
superior to the basic material. This may be explained by the great
malleability of the copper layer.
Both the support and the sparking samples, present the same type of
corrosion which is a general corrosion type, in the initial stage being
represented as corrosion dots.
Regarding the quality of the Cu sparking OL-37 samples, can be
emphasized the same conclusions as in the case of the open circuit
potential: the copper has a good protection; taking into account the way
how the sparking regime influences the protection it results that the
copper residue is better in the R1 and R2 regime (at low currents)
(Marcus 2002 & Barhalescu 2007).
The cyclic polarization curves obtained, in the case of the OL 37
support and the sparking samples are typical for the generalized
corrosion (uniform corrosion on the entire surface). This aspect is more
evident for the steel supports, where the cathode branch (return) is
overlapped on the anode branch (positive polarization).
Analyzing the cyclic polarization curves it can be noticed that for
the sparking samples the iron basic material is subjected to the
corrosion process, this certifies that the deposition electric sparking
process is not uniform.
4. ACKNOWLEDGEMENT
Some of the ideas presented in the paper are based on the results
of the "Computer Aided Advanced Studies in Applied Elasticity from
an Interdisciplinary Perspective" ID1223 project, under the
supervision of the National University Research Council (CNCSIS),
Romania, (Oanta et al., 2007).
5. REFERENCES
Barhalescu, M. (2007). Researches regarding the obtaining and
structural analysis on corrosion resistant thin superficial layers, Ph.
D Thesis, Jassy, Romania
Barhalescu, M.; Pascu, R.; & Dinescu, M. (2007).Corrosion
Protection of Metallic Surfaces by Processing with Impulse Electrical
Discharges, Protective coatings and thin films 07 of E-MRS 2007 Spring
Meeting, 28 May-1 June, Strasbourg
Marcus, P. (2002). Corrosion mechanisms in theory and practice,
Marcel Dekker Inc. ISBN 0-8247-0666-8, New York
Oanta, E.; Panait, C.; Nicolescu, B.; Dinu, S.; Hnatiuc, M.;
Pescaru, A.; Nita, A.; Gavrila, G. (2007-2010). Computer Aided Advanced
Studies in Applied Elasticity from an Interdisciplinary Perspective,
ID1223 Scientific Research Project, under the supervision of the
National University Research Council (CNCSIS), Romania;
Pop, D.; Petreus, I.; Pop, F. (1999) Corrosion Resistance Layers
Manufactured by Electrical Discharges, Buletinul Institutului
Politehnic, Tomul XLV, Ed. Gh. Asachi Technical University of Jassy, p.
95-99, ISSN 1453-1690.
*** (1991) Ustanovska ELITRON 22, Academia Nauk, Chisinau
Tab. 1. Values of the electrode cross section
Electric work Electrode
regime cross section Work current
ELITRON--22A value [mm] [A]
1 4 0.5
2 5 0.8
3 4 / 6 1.3
4 5 / 6 1.8
5 6 / 9 2.3
