272
8 Porous Nanostructured Materials
of about 10 nm provide a sufficiently hydrophobic character. The details of the
plasma polymerization are not given in detail; therefore, it is not clear by what kind
of chemical force the coatings are attached to the metal surface. Another surface
treatment process based on the reaction with a gas is the carbon fibre growth from
acetylene gas [95]. In this process, the carbon fibres are typically 1 µm thick; i.e.,
much thicker than the dendrite of the porous metal layer.
An ionically bonded hydrophobic protecting layer can be produced on the deposit
surface when it is immersed into the solution of an organic (mostly stearic) acid.
Mostly, ethanol is the solvent in this process (or, occasionally, methanol or acetone),
and the monovalent organic acid has a long apolar chain. It is not clear if the bond
to the surface is produced by the reduction of the acidic hydrogen atom of the acid
(Me + 2R–COOH Me(R–COO) 2 + H 2 ) or the reaction of the acid with the
already existing thin surface oxide (MeO + 2 R–COOH Me(R–COO) 2 + H 2 O).
The latter reaction is more likely when the electroplated porous metal was oxidized
prior to the organic acid treatment [96, 97]. Whichever reaction pathway is valid,
however, the result is that the apolar chains are ordered and point away from the
surface, hence inhibiting the penetration of a corrosive hydrophilic liquid to the
metal surface. It was generally assumed, although not evidenced directly, that the
immersion and spontaneous adsorption method leads to a monomolecular coverage.
In order to enhance the hydrophobic character of the organic layer, highly fluorinated
organic acids were also tested as adsorbates and yielded good results [89, 96–98].
The acid chemisorption method was shown to work for a variety of electroplated
porous layers composed of either metallic elements such as Zn [98–101], Ni [87,
89, 90, 102], Co [88], Cr [103] and Cu [104–106], or alloys such as Cu–Zn [96,
97]. An aluminium nanoparticle layer attached to steel surface by electrophoretic
deposition was also suitable for the same treatment [107]. The surface modification
of metals with organic acids works in the same manner in the case when the coating
contains codeposited non-metallic particles [108, 109]. The immersion time applied
to accomplish the formation of the hydrophobic coatings varies from minutes to a few
days. Electrochemical tests of the thus obtained specimens in laboratory environment
indicate a reduction of the corrosion rate by a factor between 2 and 30. The stability
of the self-assembled fatty acid layer is sufficient at pH > 3, but acidic solutions lead
to a loss of the hydrophobic character, indicating the degradation of the chemical
bond between the adlayer and the surface. The removal of the organic acids by heat
treatment also leads to the loss of hydrophobicity, which can be recovered by a
repeated immersion treatment into the organic acid solution [78, 99].
Attachment of thiols to porous metal surfaces can take place under similar conditions than the chemisorption of organic acids. However, thiols are known to form
self-assembled monolayers and the bond between the thiol group and the metal
surface is rather covalent. The surface modification with thiols was elaborated for
Zn [110] and Cu [111–113] porous layers. When alkyl trialkoxy silane compounds
with a long alkyl chain are used for achieving the hydrophobic character [114, 115],
the bond to the surface is assumed to form with the condensation of the surface –OH
groups of the metal. The chemical bond to the surface with siloxanes is assumed
8 Porous Nanostructured Materials
of about 10 nm provide a sufficiently hydrophobic character. The details of the
plasma polymerization are not given in detail; therefore, it is not clear by what kind
of chemical force the coatings are attached to the metal surface. Another surface
treatment process based on the reaction with a gas is the carbon fibre growth from
acetylene gas [95]. In this process, the carbon fibres are typically 1 µm thick; i.e.,
much thicker than the dendrite of the porous metal layer.
An ionically bonded hydrophobic protecting layer can be produced on the deposit
surface when it is immersed into the solution of an organic (mostly stearic) acid.
Mostly, ethanol is the solvent in this process (or, occasionally, methanol or acetone),
and the monovalent organic acid has a long apolar chain. It is not clear if the bond
to the surface is produced by the reduction of the acidic hydrogen atom of the acid
(Me + 2R–COOH Me(R–COO) 2 + H 2 ) or the reaction of the acid with the
already existing thin surface oxide (MeO + 2 R–COOH Me(R–COO) 2 + H 2 O).
The latter reaction is more likely when the electroplated porous metal was oxidized
prior to the organic acid treatment [96, 97]. Whichever reaction pathway is valid,
however, the result is that the apolar chains are ordered and point away from the
surface, hence inhibiting the penetration of a corrosive hydrophilic liquid to the
metal surface. It was generally assumed, although not evidenced directly, that the
immersion and spontaneous adsorption method leads to a monomolecular coverage.
In order to enhance the hydrophobic character of the organic layer, highly fluorinated
organic acids were also tested as adsorbates and yielded good results [89, 96–98].
The acid chemisorption method was shown to work for a variety of electroplated
porous layers composed of either metallic elements such as Zn [98–101], Ni [87,
89, 90, 102], Co [88], Cr [103] and Cu [104–106], or alloys such as Cu–Zn [96,
97]. An aluminium nanoparticle layer attached to steel surface by electrophoretic
deposition was also suitable for the same treatment [107]. The surface modification
of metals with organic acids works in the same manner in the case when the coating
contains codeposited non-metallic particles [108, 109]. The immersion time applied
to accomplish the formation of the hydrophobic coatings varies from minutes to a few
days. Electrochemical tests of the thus obtained specimens in laboratory environment
indicate a reduction of the corrosion rate by a factor between 2 and 30. The stability
of the self-assembled fatty acid layer is sufficient at pH > 3, but acidic solutions lead
to a loss of the hydrophobic character, indicating the degradation of the chemical
bond between the adlayer and the surface. The removal of the organic acids by heat
treatment also leads to the loss of hydrophobicity, which can be recovered by a
repeated immersion treatment into the organic acid solution [78, 99].
Attachment of thiols to porous metal surfaces can take place under similar conditions than the chemisorption of organic acids. However, thiols are known to form
self-assembled monolayers and the bond between the thiol group and the metal
surface is rather covalent. The surface modification with thiols was elaborated for
Zn [110] and Cu [111–113] porous layers. When alkyl trialkoxy silane compounds
with a long alkyl chain are used for achieving the hydrophobic character [114, 115],
the bond to the surface is assumed to form with the condensation of the surface –OH
groups of the metal. The chemical bond to the surface with siloxanes is assumed