5 Metallic Technetium, Corrosion, Technetium Alloys …
113
is observed at potentials higher than 0.1 V. The metallic technetium stability region
is separated from the passivation region (TcO 2 ) by a line parallel to the hydrogen
evolution line and shifted by about 0.3 V in respect to the latter. An intensive TcO 2
dissolution is observed in the entire pH range and starts above 0.76 V for pH of 0
and above −0.36 V for pH equal to 14.
The iron protection due to inhibiting action of the pertechnetates has been
described in the literature (Sympson and Cartledge 1956; Cartledge 1957; de Zoubov
and Pourbaix 1966; Cartledge 1971). Cartledge (1955) assumed that the addition of
TcO
−
4 ions at a level of 1 mM (KTcO 4 ) has a significant effect on the corrosion rate
of Fe. He pointed out that the passivation action is related to the feeble reversible
adsorption of TcO
−
4 ions to the iron. The inhibitory properties are also attributed to
TcO 2 , which is formed as a result of the reduction of TcO
−
4 on the surface of the metal.
Cartledge also indicated that mild carbon steel can be protected against corrosion in
aerated distilled water even up to 250 °C when the latter contains 5 ÷ 50 ppm of Tc.
His later work (Cartledge 1971) concerned on analysis of the corrosion process of
metallic Tc and Tc (0.1% w/o)-Fe alloy in H 2 SO 4 /Na 2 SO 4 solutions (pH of 0.35 or
2.7). For the Tc–Fe alloy Cartledge obtained the Tafel slope of 110 mV/decade. The
rate of the oxidation of the pure metal or its oxides was estimated at a level of μA
cm
−2 . Surface of the fresh Tc–Fe electrode exhibits uniform Tc distribution with the
average concentration of 10
12 atom cm
−2 while the anodic polarization at 4.6 × 10
−2
A cm
−2 leads to the surface enrichment with Tc up to 10
16
÷ 10
17 atom cm
−2 . Such
surface enrichment results in a decrease in the hydrogen evolution overpotential and
ennobles the open circuit potential.
Cartledge (1971) analyzed hydrogen evolution on a metallic technetium electrode
in a 0.5 M H 2 SO 4 solution and determined the exchange current density of 9.5 ×
10
−5 A cm
−2 . Trasatti (1972b) suggested a slightly higher value of this parameter
(1.6 × 10
−4 A cm
−2 ) and this range of 10
−4 A cm
−2 was recalled in a more recent
work of Jaksic (Jaksic 2000). Other than activity toward the hydrogen evolution,
fundamental electrochemical properties of the metallic technetium are discussed
below. The experimental value of the work function (φ) of polycrystalline metallic
technetium is equal to 4.58 eV. Theoretical calculations for hcp (001) and fcc (111)
surfaces of Tc yield the φ values of 5.36 eV and 5.42 eV, respectively (Skriver 1992).
Other Tc surfaces for which the work function has been reported include (0001) Tc
(4.66 eV), Tc (10-10) prismatic (4.48 eV) and Tc (10-10) basal (4.19 eV) (Taylor
2011).
Taylor (2011) reported the results of computational studies on H and O adsorption
at the surface of Tc and a Tc–Fe alloy. The density functional theory was used to
calculate its adsorption energies. For the oxygen, this parameter was found to depend
on the technetium surface geometry and equals −3.67 eV for Tc (basal), −3.47 eV
for Tc (prismatic) −3.48 eV for Tc (pyramidal). The author assumed that these values
are higher than those found for the same processes taking place on a TcO 2 surface. An
E–pH diagram illustrating O, OH and H adsorption regions is presented in Fig. 5.3.
Zhuz and Wang (2016) studied halides adsorption on a Tc surface (Zhuz and Wang
2016). This process leads to a change in the work function (φ) of hcp Tc surfaces
with the φ increasing in the order F < Cl < Br < I and varying from 0.15 eV for
113
is observed at potentials higher than 0.1 V. The metallic technetium stability region
is separated from the passivation region (TcO 2 ) by a line parallel to the hydrogen
evolution line and shifted by about 0.3 V in respect to the latter. An intensive TcO 2
dissolution is observed in the entire pH range and starts above 0.76 V for pH of 0
and above −0.36 V for pH equal to 14.
The iron protection due to inhibiting action of the pertechnetates has been
described in the literature (Sympson and Cartledge 1956; Cartledge 1957; de Zoubov
and Pourbaix 1966; Cartledge 1971). Cartledge (1955) assumed that the addition of
TcO
−
4 ions at a level of 1 mM (KTcO 4 ) has a significant effect on the corrosion rate
of Fe. He pointed out that the passivation action is related to the feeble reversible
adsorption of TcO
−
4 ions to the iron. The inhibitory properties are also attributed to
TcO 2 , which is formed as a result of the reduction of TcO
−
4 on the surface of the metal.
Cartledge also indicated that mild carbon steel can be protected against corrosion in
aerated distilled water even up to 250 °C when the latter contains 5 ÷ 50 ppm of Tc.
His later work (Cartledge 1971) concerned on analysis of the corrosion process of
metallic Tc and Tc (0.1% w/o)-Fe alloy in H 2 SO 4 /Na 2 SO 4 solutions (pH of 0.35 or
2.7). For the Tc–Fe alloy Cartledge obtained the Tafel slope of 110 mV/decade. The
rate of the oxidation of the pure metal or its oxides was estimated at a level of μA
cm
−2 . Surface of the fresh Tc–Fe electrode exhibits uniform Tc distribution with the
average concentration of 10
12 atom cm
−2 while the anodic polarization at 4.6 × 10
−2
A cm
−2 leads to the surface enrichment with Tc up to 10
16
÷ 10
17 atom cm
−2 . Such
surface enrichment results in a decrease in the hydrogen evolution overpotential and
ennobles the open circuit potential.
Cartledge (1971) analyzed hydrogen evolution on a metallic technetium electrode
in a 0.5 M H 2 SO 4 solution and determined the exchange current density of 9.5 ×
10
−5 A cm
−2 . Trasatti (1972b) suggested a slightly higher value of this parameter
(1.6 × 10
−4 A cm
−2 ) and this range of 10
−4 A cm
−2 was recalled in a more recent
work of Jaksic (Jaksic 2000). Other than activity toward the hydrogen evolution,
fundamental electrochemical properties of the metallic technetium are discussed
below. The experimental value of the work function (φ) of polycrystalline metallic
technetium is equal to 4.58 eV. Theoretical calculations for hcp (001) and fcc (111)
surfaces of Tc yield the φ values of 5.36 eV and 5.42 eV, respectively (Skriver 1992).
Other Tc surfaces for which the work function has been reported include (0001) Tc
(4.66 eV), Tc (10-10) prismatic (4.48 eV) and Tc (10-10) basal (4.19 eV) (Taylor
2011).
Taylor (2011) reported the results of computational studies on H and O adsorption
at the surface of Tc and a Tc–Fe alloy. The density functional theory was used to
calculate its adsorption energies. For the oxygen, this parameter was found to depend
on the technetium surface geometry and equals −3.67 eV for Tc (basal), −3.47 eV
for Tc (prismatic) −3.48 eV for Tc (pyramidal). The author assumed that these values
are higher than those found for the same processes taking place on a TcO 2 surface. An
E–pH diagram illustrating O, OH and H adsorption regions is presented in Fig. 5.3.
Zhuz and Wang (2016) studied halides adsorption on a Tc surface (Zhuz and Wang
2016). This process leads to a change in the work function (φ) of hcp Tc surfaces
with the φ increasing in the order F < Cl < Br < I and varying from 0.15 eV for