INVESTIGATION OF THE INITIATION OF PITS
IN MAGNETIC HARD DISKS BY
ELECTROCHEMICAL IMPEDANCE
SPECTROSCOPY
Soyoung Jung and T.M.Devine
University of California, Berkeley
Department of Materials Science and Engineering
577 Evans Hall, MC 1760, Berkeley,CA 94720
Electrochemical impedance spectroscopy was used to
investigate the initiation of pitting corrosion in magnetic
hard disks. The disks are composites consisting of an
aluminum alloy substrate that is coated successively with
layers of nickel-phosphorus, a chromium seed-layer, a
cobalt-chromium alloy, in which data is magnetically
stored, and, finally, a diamond-like carbon (DLC) layer.
The DLC is intended to protect the disk against wear and
corrosion.1-3
Corrosion of hard disks occurs by pitting corrosion of the
metallic layers located below the DLC. The objective of
the current research is to determine the role of the DLC
in the initiation of pitting corrosion. The DLC is thought
4
to contain pores that are
The
diameter of a water molecule is 0.32 nm. With time of
exposure to an aqueous solution, water and selected ions
are thought to permeate through the pores in the DLC
and to come into contact with and corrode the Co-Cr
magnetic layer.
The electrochemical properties of the DLC are not well
understood. It is unclear, for example, if the DLC
galvanically couples to the underlying metal and
anodically polarizes the Co-Cr alloy, which eventually
pits.
In the present research, the influence of thickness of the
DLC (1nm, 2nm, 3nm and 5nm) and of dopant
(hydrogen and nitrogen) of the DLC on the
protectiveness of the DLC are investigated.
In
particular, electrochemical impedance spectra (from 0.01
Hz to 60 KHz) are reported as a function of thickness
and dopant of the DLC and as a function of applied
potential (from –1.0V to +1.0V vs SCE in increments of
0.2V) in borate buffer (pH 8.4) containing 0.1 M NaCl.
The results indicate that at the corrosion potential (
vs SCE) and at applied cathodic potentials within
approximately 400 mV of the corrosion potential, the
DLC behaves as an electrical insulator. However, at
potentials less than -400 mV vs SCE, the impedance at
0.01 Hz of the hard disks decreases by several orders of
magnitude. At anodic potentials, the impedance at low
frequencies of the disk decreases by several orders of
magnitude at potentials
related to the formation of corrosion pits.
The potential dependence of the impedance is thought to
provide important insight into the mechanism of pitting
corrosion of the hard disk.
At this time, there are at least three possible explanations
for the potential dependence of the impedance at low
frequencies. First, the potential dependence of the
impedance at low frequencies might be caused by the
electric field enhanced electrical conductivity of the
DLC. That is, above a critical value of the voltage
gradient across the DLC of approximately 106 V/cm the
Frankel-Poole conduction mechanism dramatically
increases the electrical conductivity of the DLC, which
results in a significant decrease in the impedance of the
hard disk at low frequencies. If the Frankel-Poole
conduction mechanism is operative, then the pitting
corrosion of the Co-Cr alloy will be caused by the highly
!!#"$&%')(+*-,/.01#2'
conducting DLC anodically polarizing the Co-Cr
alloy to potentials high enough to cause pitting.
A second possible cause of the potential-induced
decrease in impedance of the disk is the formation of
surface states that increase the conductivity of the
thin film of DLC.
Field-assisted migration of water and ions through
pores in the DLC is a third, but less likely,
explanation for the potential dependency of the low
frequency impedance of the DLC. It seems that field
assisted migration of ions and water through pores in
the DLC should increase continuously with voltage
rather then increase abruptly at a critical value of
voltage. Experiments currently underway are seeking
to distinguish between the Frankel-Poole conduction
mechanism, the formation of surface states, and fieldassisted migration of water and ions through pores in
the DLC. Specifically, the impedance of the hard
disk is being measured as a function of anodic and
cathodic polarization in aqueous solutions containing
Cu++ and Fe+++ ions. The presence of Cu++ and Fe+++
allow reduction/oxidation reactions to occur on the
DLC at potentials in the range of 0V to +0.8V. These
experiments will better define the critical value of the
anodic potential at which the impedance of the DLC
dramatically decreases. Presumably, if the FrankelPoole conduction mechanism is operative, the
impedance of the disk will be lowered when the
absolute value of the voltage gradient across the DLC
exceeds a critical value. The polarity of the voltage
gradient will not matter. In addition, the impedance
of the hard disk is being measured in different
aqueous solutions, in which the sizes of the anions
and cations are systematically varied. Since the size
of the pores in the DLC are thought to be
diameter, the rate of field-assisted migration of ions
through the DLC should depend on the size of the
ions. The results of these tests will be presented at
the meeting.
In summary, polarization of disks in borate buffer
(pH 8.4) + 0.1M NaCl to potentials below –0.4V vs
SCE causes a significant decrease in the impedance
of the disks at low frequencies. Experiments are
underway to determine if the change in impedance is
caused by changes in the electrical conductivity of
the DLC or by field assisted permeation of the DLC
by water and ions. In either case, the results indicate
that the initiation of pits in Co-Cr covered by
nanoporous, 1-4 nanometer thick layer of DLC is
strongly dependent on potential.
Acknowledgements
We wish to thank Drs. Jing Gui and Rak Thangaraj of
Seagate for technical and financial support and Dr.
Chris Kumai of U.C., Berkeley, for technical support.
References
1. V. Novotny and N. Staud, J. Electrochem. Soc.,
135, 2931 (1988).
2. G.S. Frankel, in Corrosion Mechanisms in
Theory and Practice, P. Marcus and J. Oudar,
Editors, p. 547, Marcel Dekker, Inc., New York,
NY (1995).
3. A. D. Hodges, Soyoung Jung and T.M. Devine,
“Pitting Corrosion of Magnetic Hard Disks,”
Proceedings of the Symposium on Pits and
Pores, ed. P. Schmuki, Fall Meeting ECS (2000).
4. A. Zeng, E. Liu, I.F. Annergren, S.N. Tan, S.
Zang, P. Hing and J. Gao, Diamond and Related
Materials, 11, 160 (2002).
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