52
2 Electrochemistry and Electrodeposition
to a threshold value as long as the electrical field within the passive layer is capable
of leading to ionic motion within the passive layer. As the layer grows to such a
thickness at which the electrical field falls below this limit, the growth stops. The
critical electrical field of provoking ionic mobility within the passive layer is in the
range of 10
7 V/cm. In experiments of electrochemical growth of metal oxides, the
total cell voltage can be in the 15–200 V range, where the potential drop on the
counter electrode is negligible as compared to the total cell voltage. For this reason,
in such experiments it is customary to report the total cell voltage and no reference
electrode is used.
For oxide formation, various potential regimes can be identified for a number of
metals (such as Al). For relatively small anodization voltage, a compact oxide layer
can be produced. At higher voltages, an oxide with large porosity and regular pore size
can take shape. At extremely large voltages, the stress in the oxide becomes so large
that the oxide layer is delaminated from the surface (burn-down of the oxide), and
hence, no compact layer can form. The potential intervals where these phenomena
take place usually depend on the solutes of the anodization media.
The anodic behaviour of metal alloys is usually much more complicated that that of
metallic elements. There are several alloys that dissolve actively altogether as if they
were of a metal of single component. This is common, for instance, for the Fe–Co–Zn
and Cu–Zn alloy families. Even for these alloys, the composition, and consequently,
the specific crystallographic form of the actual alloy strongly determines the electrode
potential at which the dissolution starts. This makes the possibility of an indirect phase
analysis of thin metallic layers (typically deposits), being often termed as anodic
linear sweep voltammetry (ALSV). In ALSV experiments, a positive-going sweep is
run with the alloy as working electrode, and the dissolution of the grains of various
alloy phases takes place in different potential regimes and manifests itself as peaks
of waves in the voltammograms. The charge ratio corresponding to the dissolution of
various alloy phases corresponds to the phase composition of the metal layer. Since
ALSV is a transient method, it can be used for thin layers only but not for bulk metals
since it requires the complete dissolution of the specimen.
Another type of anodic behaviour can be obtained if the dissolution of a real alloy
cannot take place as if it were a single component. If the potential regimes corresponding to the anodic dissolution of the element of an alloy differ very much and
the mobility of the atoms of the more noble element is high enough, anodic dealloying may happen. During dealloying, the atoms of the less noble alloy component
are oxidized and hence leave the metal, while the atoms of the more noble element
remain in elemental form. As a result of dealloying, the remaining metallic structure does not correspond to the atomic arrangement of the more noble element in
the parent alloy. Rather, the atoms of the remaining metal are rearranged within a
distance determined by the mobility of these atoms. This rearrangement also diminishes the excess energy of the system due to the newly produced large metal surface.
The resulting structure is highly porous. A typical alloy family leading to dealloying
upon positive polarization is Au–Cu alloys.
Finaly, it must also be mentioned that passivity of alloys also differs from the
passivity of metallic elements in the sense that the ratio of the constituents of the
2 Electrochemistry and Electrodeposition
to a threshold value as long as the electrical field within the passive layer is capable
of leading to ionic motion within the passive layer. As the layer grows to such a
thickness at which the electrical field falls below this limit, the growth stops. The
critical electrical field of provoking ionic mobility within the passive layer is in the
range of 10
7 V/cm. In experiments of electrochemical growth of metal oxides, the
total cell voltage can be in the 15–200 V range, where the potential drop on the
counter electrode is negligible as compared to the total cell voltage. For this reason,
in such experiments it is customary to report the total cell voltage and no reference
electrode is used.
For oxide formation, various potential regimes can be identified for a number of
metals (such as Al). For relatively small anodization voltage, a compact oxide layer
can be produced. At higher voltages, an oxide with large porosity and regular pore size
can take shape. At extremely large voltages, the stress in the oxide becomes so large
that the oxide layer is delaminated from the surface (burn-down of the oxide), and
hence, no compact layer can form. The potential intervals where these phenomena
take place usually depend on the solutes of the anodization media.
The anodic behaviour of metal alloys is usually much more complicated that that of
metallic elements. There are several alloys that dissolve actively altogether as if they
were of a metal of single component. This is common, for instance, for the Fe–Co–Zn
and Cu–Zn alloy families. Even for these alloys, the composition, and consequently,
the specific crystallographic form of the actual alloy strongly determines the electrode
potential at which the dissolution starts. This makes the possibility of an indirect phase
analysis of thin metallic layers (typically deposits), being often termed as anodic
linear sweep voltammetry (ALSV). In ALSV experiments, a positive-going sweep is
run with the alloy as working electrode, and the dissolution of the grains of various
alloy phases takes place in different potential regimes and manifests itself as peaks
of waves in the voltammograms. The charge ratio corresponding to the dissolution of
various alloy phases corresponds to the phase composition of the metal layer. Since
ALSV is a transient method, it can be used for thin layers only but not for bulk metals
since it requires the complete dissolution of the specimen.
Another type of anodic behaviour can be obtained if the dissolution of a real alloy
cannot take place as if it were a single component. If the potential regimes corresponding to the anodic dissolution of the element of an alloy differ very much and
the mobility of the atoms of the more noble element is high enough, anodic dealloying may happen. During dealloying, the atoms of the less noble alloy component
are oxidized and hence leave the metal, while the atoms of the more noble element
remain in elemental form. As a result of dealloying, the remaining metallic structure does not correspond to the atomic arrangement of the more noble element in
the parent alloy. Rather, the atoms of the remaining metal are rearranged within a
distance determined by the mobility of these atoms. This rearrangement also diminishes the excess energy of the system due to the newly produced large metal surface.
The resulting structure is highly porous. A typical alloy family leading to dealloying
upon positive polarization is Au–Cu alloys.
Finaly, it must also be mentioned that passivity of alloys also differs from the
passivity of metallic elements in the sense that the ratio of the constituents of the