1070-4272/03/7611-1744$25.00C2003 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 76, No. 11, 2003, pp. 1744!1746. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 11,
2003, pp. 1794!1796.
Original Russian Text Copyright + 2003 by Yurkinskii, Firsova, Afonicheva.
AND CORROSION PROTECTION OF METALS
Electrochemical Oxidation of Tantalum
V. P. Yurkinskii, E. G. Firsova, and E. V. Afonicheva
St. Petersburg State Polytechnic University, St. Petersburg, Russia
Received July 1, 2003
Abstract-Growth kinetics of anodic oxide films on tantalum in a molten eutectic mixture of lithium, sodium,
and potassium nitrates was studied and the influence exerted by introduction of hydroxide and chromate ions
into the nitrate mixture was considered. The basic kinetic parameters of anodic oxidation of tantalum under
galvanostatic conditions were determined.
Oxidation of tantalum is widely used in various
fields of technology. Use of molten salts and, in par-
ticular, nitrates makes it possible to intensify anodic
oxidation of Ta and to obtain oxide coatings with
better service characteristics, compared with the coat-
ings formed in aqueous electrolytes [1, 2]. It has been
established that the presence of oxygen-containing
ions in a melt strongly affects the kinetics of forma-
tion of oxide films on rectifying metals and their
electrical properties [3, 4].
In continuation of previous investigations [2, 5],
we studied the growth kinetics of anodic oxide layers
on tantalum in a molten eutectic mixture of lithium
(37.5 mol %), sodium (18 mol %), and potassium
nitrates and the influence exerted by introduction of
hydroxide and chromate ions into the nitrate melt.
The study was carried out in the temperature range
4133503 K in a nitrogen atmosphere. The design of
the electrochemical cell and methods used for pre-
treatment of nitrate salts and the electrode material
have been described previously [3, 5].
Hydroxide and chromate ions were introduced into
the melt in the form of KOH and Na
cally pure grade. Na
was preliminarily remelted
and KOH was dehydrated by prolonged vacuum treat-
ment in the course of heating (1.532.0 h) from room
temperature to 800 K, with subsequent keeping at this
temperature for about 334 h. The content of potassium
hydroxide in the nitrate melt was 2 wt %; sodium
chromate was introduced into the melt to saturation.
Tantalum was oxidized in the galvanostatic mode
at current densities of 2.5315 A m
. The procedure
for galvanostatic measurements was described in
The thickness of the oxide film was determined by
the capacitance technique [3, 6].
The dependences of the potential of the tantalum
anode on the oxidation time for all the melts under
study at different temperatures and current densities
were linear. Figure 1 shows as an example the E!i
curves obtained in the LiNO
As is known , the linear run of the E!i curves is
observed in oxidation of rectifying metals at high field
strengths in the oxide film. In this case, the anodic
process is limited by mass transfer of ions across the
oxide layer formed on the surface of tantalum. The
growth of oxide layers under conditions of high field
strength in the oxide film is characterized by the de-
Fig. 1. Chronopotentiometric dependence obtained on tan-
talum at 428 K in the LiNO
melt. (E) Potential and (t) time. Current density, A m
(1) 2.11, (2) 4.44, (3) 7.37, (4) 8.70, and (5) 11.81.