274
8 Porous Nanostructured Materials
for the rest, hydrated salts were used). Although solutes with water in the crystalline
form were often used, the enhanced water content of the solution hence applied
did not seem to be an obstacle of the salt formation. The dissolution of the counter
electrode has never been mentioned even if an aggressive chloride salt was applied
and the counter electrode was made of the same material as the cathode (copper or
stainless steel).
Another solution component is a fatty acid (H 3 C–(CH 2 ) n –COOH) with a long
apolar chain (10 < n < 16) whose concentration is typically a few times larger than
that of the metal salt. The sufficient solubility of the fatty acids in ethanol verifies the
choice of the solvent as opposed to water and so does the prevention of the formation
of metal hydroxides instead of the fatty acid salts. For achieving a hydrophobic
surface that is oleophobic at the same time, the application of fluorinated organic
acids is required [123]. When the metal is quite prone to be deposited (like Ni), a
mixed metal/metal salt film can be obtained [120].
Supporting electrolyte is not applied in most of the studies published. When the
metal to be protected with the hydrophobic salt film is connected as cathode in the
electrochemical cell, the hydrogen evolution is the dominant reaction at this electrode,
which causes the alkalization of the cathode vicinity. This leads to the release of the
protons from the acidic groups and hence, an electrochemically induced precipitation
takes place at the cathode without any change in the oxidation number of the cation.
The overall process time varies between 1 and 60 min. Due to the fairly inert solution,
coatings can be produced on metals that are easily corroded in aqueous solution like
Mg [116, 124, 125, 127] and Al [119, 128] alloys. The great advantage of the process
is that it takes place in one single step. It was revealed in several studies that the contact
angle of the water droplet at the hydrophobic surface showed a maximum at some
deposition time [116, 118, 120, 121], and the maximum of the contact angle could
be sufficiently correlated to the finest pore structure in the SEM images. However,
when a nanogranular coating was obtained, the achievement of the full coverage led
to saturation in the contact angle [125, 127, 130, 133], occasionally with a slight
decline only for thick deposit coatings.
Although the major processes in the salt film formation can be understood, the role
of a number of factors is unclear. The salt film formation process is usually driven
with a large cell voltage (5–60 V) in a two-electrode cell; therefore, we cannot speak
about electrode potential since the potential distribution within the cell is not known.
In many cases, a voltage optimum was found. Based on the few reports where the
current density values were given, we can say that it can vary from about 0.5 mA cm
–2
[131] to a few mA cm
–2 [118]. The resulting current density may be a function of
the substrate applied, too [118]. There is no data in the literature about the Faradaic
efficiency of the cathode process. The salt formation can be effectively induced by
a constant voltage, too, but the application of voltage pulsing often leads to deposits
with finer dendritic structures [117, 129].
It is specified in the majority of the studies that the electrode separation was
fixed, and a distance of 2 cm is given in the majority of the studies. This raises the
concern that the reaction product of one electrode can reach the other electrode,
which is a huge factor of uncertainty when cations of varying oxidation state are
8 Porous Nanostructured Materials
for the rest, hydrated salts were used). Although solutes with water in the crystalline
form were often used, the enhanced water content of the solution hence applied
did not seem to be an obstacle of the salt formation. The dissolution of the counter
electrode has never been mentioned even if an aggressive chloride salt was applied
and the counter electrode was made of the same material as the cathode (copper or
stainless steel).
Another solution component is a fatty acid (H 3 C–(CH 2 ) n –COOH) with a long
apolar chain (10 < n < 16) whose concentration is typically a few times larger than
that of the metal salt. The sufficient solubility of the fatty acids in ethanol verifies the
choice of the solvent as opposed to water and so does the prevention of the formation
of metal hydroxides instead of the fatty acid salts. For achieving a hydrophobic
surface that is oleophobic at the same time, the application of fluorinated organic
acids is required [123]. When the metal is quite prone to be deposited (like Ni), a
mixed metal/metal salt film can be obtained [120].
Supporting electrolyte is not applied in most of the studies published. When the
metal to be protected with the hydrophobic salt film is connected as cathode in the
electrochemical cell, the hydrogen evolution is the dominant reaction at this electrode,
which causes the alkalization of the cathode vicinity. This leads to the release of the
protons from the acidic groups and hence, an electrochemically induced precipitation
takes place at the cathode without any change in the oxidation number of the cation.
The overall process time varies between 1 and 60 min. Due to the fairly inert solution,
coatings can be produced on metals that are easily corroded in aqueous solution like
Mg [116, 124, 125, 127] and Al [119, 128] alloys. The great advantage of the process
is that it takes place in one single step. It was revealed in several studies that the contact
angle of the water droplet at the hydrophobic surface showed a maximum at some
deposition time [116, 118, 120, 121], and the maximum of the contact angle could
be sufficiently correlated to the finest pore structure in the SEM images. However,
when a nanogranular coating was obtained, the achievement of the full coverage led
to saturation in the contact angle [125, 127, 130, 133], occasionally with a slight
decline only for thick deposit coatings.
Although the major processes in the salt film formation can be understood, the role
of a number of factors is unclear. The salt film formation process is usually driven
with a large cell voltage (5–60 V) in a two-electrode cell; therefore, we cannot speak
about electrode potential since the potential distribution within the cell is not known.
In many cases, a voltage optimum was found. Based on the few reports where the
current density values were given, we can say that it can vary from about 0.5 mA cm
–2
[131] to a few mA cm
–2 [118]. The resulting current density may be a function of
the substrate applied, too [118]. There is no data in the literature about the Faradaic
efficiency of the cathode process. The salt formation can be effectively induced by
a constant voltage, too, but the application of voltage pulsing often leads to deposits
with finer dendritic structures [117, 129].
It is specified in the majority of the studies that the electrode separation was
fixed, and a distance of 2 cm is given in the majority of the studies. This raises the
concern that the reaction product of one electrode can reach the other electrode,
which is a huge factor of uncertainty when cations of varying oxidation state are