7.4 Nano-Mechanical Properties of Passive Metal Surfaces Obtained …
205
slope of the dashed line is about 2.5 nm V
−1 . The values of H for bare Fe (110)
and Fe (100) surfaces without passive film may be estimated to be 3.0 and 2.7 GPa,
respectively, from the potential dependences of hardness (0.2 GPa V
−1 ) and film
thickness (2.5 nm V
−1 ).
Metals with a body-centered cubic structure (bcc) such as α-Fe have slip planes of
{110} and {112} with a slip direction of 111 [15]. The difference in H between bare
Fe (100) and Fe (110) surfaces may result from the difference in resolved shear stress
on the slip system {110}}111 or {112}}111 at the initial yielding point for plastic
deformation. The contact depth h c is in the range of 65–90 nm at L max = 400 µN.
Consequently, the ratio of d f to h c is only in the range from 0.03 to 0.05. The linear
increase in H with potential for both passive Fe (100) and Fe (110) surfaces in
response to the linear increase in d f with potential is ascribed to the contribution
of the passive film with a high value of H despite very small values of
d f
h c
. It is
known that the structures of passive films on iron single crystals (001) and (110) are
similar to that of magnetite [55, 56]. The value of H obtained by nano-indentation
for the magnetite single crystal (100) surface [24] (see Fig. 7.12) is 10.2 GPa which
is about 3.4 times as much as those of the passive Fe (100) and (110) surfaces. The
value of E s (Young’s modulus) obtained for the passive Fe (100) and (110) surfaces
is 175 ± 30 GPa [24], which is close to that (E s = 174 GPa) obtained by nanoindentation for the magnetite single crystal (100) surface, but it is lower than that
(E s = 208.2 GPa) for bulk α-Fe [15].
7.4.2 Effect of Chromate Treatment on Nano-Mechanical
Properties of Passive Fe Surfaces
It is well known [58, 59] that the corrosion resistivities of metals such as Al, Fe,
and steels are significantly improved by a chromate treatment (i.e. by immersion
of metals into chromic acid or dichromate solution). The chromate treatment is not
used nowadays because hexavalent chromium is toxic, and thus, highly regulated.
Nevertheless, it is worthy from the scientific viewpoint to investigate the effect of
the chromate on nano-mechanical properties of the passive metals which are directly
associated with the corrosion resistivities. For this purpose, electrochemical nanoindentation experiments have been performed to the passive Fe (100) and (110)
surfaces without and with chromate treatment [24, 52]. The chromate treatment was
made with natural immersion of the Fe specimens in 5 × 10
–2 M K 2 Cr 2 O 7 solution
for 24 h.
The solid lines in Fig. 7.21 represent the averaged load-depth curves for the Fe
(100) surface passivated at 0.25 V (SHE) for 1 h in pH 8.4 borate solution after the
chromate treatment [24]. For comparison, the averaged load-depth curves for the
Fe (100) surface passivated without the chromate treatment are represented by the
dashed lines in Fig. 7.21. It is clear that the hardness of the passive Fe (100) surface
is increased by the chromate treatment. Furthermore, the slope of the unloading
205
slope of the dashed line is about 2.5 nm V
−1 . The values of H for bare Fe (110)
and Fe (100) surfaces without passive film may be estimated to be 3.0 and 2.7 GPa,
respectively, from the potential dependences of hardness (0.2 GPa V
−1 ) and film
thickness (2.5 nm V
−1 ).
Metals with a body-centered cubic structure (bcc) such as α-Fe have slip planes of
{110} and {112} with a slip direction of 111 [15]. The difference in H between bare
Fe (100) and Fe (110) surfaces may result from the difference in resolved shear stress
on the slip system {110}}111 or {112}}111 at the initial yielding point for plastic
deformation. The contact depth h c is in the range of 65–90 nm at L max = 400 µN.
Consequently, the ratio of d f to h c is only in the range from 0.03 to 0.05. The linear
increase in H with potential for both passive Fe (100) and Fe (110) surfaces in
response to the linear increase in d f with potential is ascribed to the contribution
of the passive film with a high value of H despite very small values of
d f
h c
. It is
known that the structures of passive films on iron single crystals (001) and (110) are
similar to that of magnetite [55, 56]. The value of H obtained by nano-indentation
for the magnetite single crystal (100) surface [24] (see Fig. 7.12) is 10.2 GPa which
is about 3.4 times as much as those of the passive Fe (100) and (110) surfaces. The
value of E s (Young’s modulus) obtained for the passive Fe (100) and (110) surfaces
is 175 ± 30 GPa [24], which is close to that (E s = 174 GPa) obtained by nanoindentation for the magnetite single crystal (100) surface, but it is lower than that
(E s = 208.2 GPa) for bulk α-Fe [15].
7.4.2 Effect of Chromate Treatment on Nano-Mechanical
Properties of Passive Fe Surfaces
It is well known [58, 59] that the corrosion resistivities of metals such as Al, Fe,
and steels are significantly improved by a chromate treatment (i.e. by immersion
of metals into chromic acid or dichromate solution). The chromate treatment is not
used nowadays because hexavalent chromium is toxic, and thus, highly regulated.
Nevertheless, it is worthy from the scientific viewpoint to investigate the effect of
the chromate on nano-mechanical properties of the passive metals which are directly
associated with the corrosion resistivities. For this purpose, electrochemical nanoindentation experiments have been performed to the passive Fe (100) and (110)
surfaces without and with chromate treatment [24, 52]. The chromate treatment was
made with natural immersion of the Fe specimens in 5 × 10
–2 M K 2 Cr 2 O 7 solution
for 24 h.
The solid lines in Fig. 7.21 represent the averaged load-depth curves for the Fe
(100) surface passivated at 0.25 V (SHE) for 1 h in pH 8.4 borate solution after the
chromate treatment [24]. For comparison, the averaged load-depth curves for the
Fe (100) surface passivated without the chromate treatment are represented by the
dashed lines in Fig. 7.21. It is clear that the hardness of the passive Fe (100) surface
is increased by the chromate treatment. Furthermore, the slope of the unloading