Chemistry Reference
In-Depth Information
corrosion reaction introduced by high concentrations of chloride. This ion
is well known for its penetrating properties in metallic layers and for pro-
moting the formation of Cl
-
deposits and oxide formation. This mainly
results in an increase of the electrode resistance, which becomes important
in the behaviour of the system.
The fact that this reaction is observed only at 1 ¥ 10
-1
mol l
-1
does not mean
that it does not occur at other electrolyte concentrations. The corrosion rate
is probably lower at lower electrolyte concentrations, and this results in an
increase of the resistance by a value smaller than 10 W. At concentrations
lower or equal to 1 ¥ 10
-2
mol l
-1
, the increase in resistance will not be
detected owing to high resistances measured for the electrolyte and error
margins that are much higher than the shift of resistance caused by the cor-
rosion reaction. In addition, the contribution of the corrosion reaction to
the cell resistance at electrolyte concentrations lower than 1 ¥ 10
-2
mol l
-1
decreases with chloride concentration, because the intensity and rate of that
reaction is determined by different parameters, among them the chloride
concentration.
9.3.4
Type of electrolyte used
Initially, sodium chloride was selected as electrolyte for this study, because
it is the main component in human sweat which contributes to the con-
ductivity of this solution. However, an important drawback of sodium chlo-
ride is that chloride ions (Cl
-
) are known for their very strong penetration
properties in metallic and alloy surfaces
2
. Such a penetration promotes cor-
rosion of the material - an effect that should be avoided in this work. There-
fore, it is necessary also to study other types of electrolytes to avoid this
corrosion reaction. Suitable alternatives are potassium nitrate (KNO
3
) and
sodium perchlorate (NaClO
4
) - two types of electrolytes that are commonly
used in electrochemical experiments owing to their inert nature. These elec-
trolytes also dissociates in 1+ and 1- charged ions (K
+
/NO
3
-
,Na
+
/ClO
4
-
);
the only different contribution to the conductivity of a solution is their rel-
ative ability to diffuse/migrate in the solution. However, these differences
will be reasonably small and unimportant compared with the problem of
having an important corrosion reaction when using Cl
-
-containing
electrolytes.
The availability of an inert electrolyte is of the utmost importance for the
development of a quality-control system for textile electrodes. In such a
system, it is the aim to test the quality of textile electrodes; therefore the
condition and properties of the electrode should not be changed or influ-
enced during this quality-control experiment. This condition cannot be ful-
filled by using NaCl as electrolyte solution, because during the quality
testing, the chloride will affect the properties of the textile electrode tested.
