13.9 Anodization of Alloys
503
13.9.2 Miscellaneous Other Alloys
Ternary alloys of nanopore-forming metals—The details of the anodization process
of alloys with complex composition are not fully clear from the scientific point
of view. The reason for the lack of detailed information is that the motivation of
the anodization of such alloys is rather practical, ranging from the decrease of the
corrosion resistance, the improvement of the biocompatibility of the metal surface or
the acceleration of the biomineralization process by providing a porous surface layer.
The examples discussed here refer to Ti–6Al–7Nb [249–251] and Ti–13Nb–13Zr
[252] alloys.
The nanopore or nanocolumn structure of the anodized ternary alloys is by far
not as regular as that of pure metals (although the regularity is not a pronounced
goal here). The disorder partly stems from the phase structure of these alloys. The
local crystallographic peculiarities impact the pore formation process, and hence,
various pore/tube domains may form or three-dimensional network-like structures
are produced. The anodization of the ternary alloys containing aluminium is often
performed in alkaline solution that leads to the leaching of aluminium. Therefore, it
is not a general rule that the pore/tube walls contain all components of the precursor
metal. The release of aluminium is beneficial for the biocompatibility of the base
metal.
Ti–Ni alloys—The alloys containing Ti, Ni and occasionally a third component in a
small concentration are important for biomedical applications. From electrochemical
point of view, the difference as compared to the above discussed other alloys is that
anodized Ni alone does not form a porous structure, while a porous surface layer
can be achieved with the anodization of Ti–Ni alloys [253–255]. Since there is no
recipe for Ni anodization, the baths used differ from those applied for Ti, mostly
because higher water content is required and chloride ions must be present for the
development of a porous structure. Since the etching of the remaining wall structure
composed of mostly TiO 2 does not take place in the absence of the fluoride ions, the
upper limit for the porous layer thickness originates from the delamination of the
layer from the substrate, not from the layer dissolution.
The chloride-containing solution leads to a materials loss during anodization, and
the preferred leaving component is nickel. For this reason, the Ti:Ni ratio in the
porous layer is likely to be larger than in the master alloy. Ni can be seen in the EDS
spectra of the porous layers, although exact composition is not given.
References
1. Keller F, Hunter MS, Robinson DL (1953) J Electrochem Soc 100:411–419
2. Hunter MS, Fowle P (1954) J Electrochem Soc 101:514–519
3. Diggle JW, Downie TC, Goulding CW (1969) Chem Rev 69:365–405
4. O’Sullivan JP, Wood GC (1970) Proc Roy Soc Lond A 317:511–543
5. Bailey G, Wood GC (1974) Trans IMF 52:187–199
503
13.9.2 Miscellaneous Other Alloys
Ternary alloys of nanopore-forming metals—The details of the anodization process
of alloys with complex composition are not fully clear from the scientific point
of view. The reason for the lack of detailed information is that the motivation of
the anodization of such alloys is rather practical, ranging from the decrease of the
corrosion resistance, the improvement of the biocompatibility of the metal surface or
the acceleration of the biomineralization process by providing a porous surface layer.
The examples discussed here refer to Ti–6Al–7Nb [249–251] and Ti–13Nb–13Zr
[252] alloys.
The nanopore or nanocolumn structure of the anodized ternary alloys is by far
not as regular as that of pure metals (although the regularity is not a pronounced
goal here). The disorder partly stems from the phase structure of these alloys. The
local crystallographic peculiarities impact the pore formation process, and hence,
various pore/tube domains may form or three-dimensional network-like structures
are produced. The anodization of the ternary alloys containing aluminium is often
performed in alkaline solution that leads to the leaching of aluminium. Therefore, it
is not a general rule that the pore/tube walls contain all components of the precursor
metal. The release of aluminium is beneficial for the biocompatibility of the base
metal.
Ti–Ni alloys—The alloys containing Ti, Ni and occasionally a third component in a
small concentration are important for biomedical applications. From electrochemical
point of view, the difference as compared to the above discussed other alloys is that
anodized Ni alone does not form a porous structure, while a porous surface layer
can be achieved with the anodization of Ti–Ni alloys [253–255]. Since there is no
recipe for Ni anodization, the baths used differ from those applied for Ti, mostly
because higher water content is required and chloride ions must be present for the
development of a porous structure. Since the etching of the remaining wall structure
composed of mostly TiO 2 does not take place in the absence of the fluoride ions, the
upper limit for the porous layer thickness originates from the delamination of the
layer from the substrate, not from the layer dissolution.
The chloride-containing solution leads to a materials loss during anodization, and
the preferred leaving component is nickel. For this reason, the Ti:Ni ratio in the
porous layer is likely to be larger than in the master alloy. Ni can be seen in the EDS
spectra of the porous layers, although exact composition is not given.
References
1. Keller F, Hunter MS, Robinson DL (1953) J Electrochem Soc 100:411–419
2. Hunter MS, Fowle P (1954) J Electrochem Soc 101:514–519
3. Diggle JW, Downie TC, Goulding CW (1969) Chem Rev 69:365–405
4. O’Sullivan JP, Wood GC (1970) Proc Roy Soc Lond A 317:511–543
5. Bailey G, Wood GC (1974) Trans IMF 52:187–199