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Since the Scanning Reference Electrode Technique (SRET) relies on measuring potential variations existing close to the surface of an electrochemically active specimen in an electrolyte, a need exists to relate these potential variations to the current flowing at the surface and hence to measure current densities and so corrosion rates.
Given that we may know the conductivity of the electrolyte, the probe seperation and probe to sample distance, a direct calculation will give the current density for a known current injection and area of activity, however this is a fairly cumbersome procedure. A much simpler calibration of the system can be readily acheived by using the Point-In-Space (PIS) approach described here.
The PIS approach allows the calibration of the SRET system to read data directly in current density which will then be accurately and unambiguously calibrated for a specific electrolyte and probe to specimen distance.
The basis of the PIS approach is to find a direct conversion factor between the current density occuring at the end of a PIS specimen, and the measured SRET signal for that configuration.
In order to achieve this, a PIS specimen is inserted into the SRET sample holder as shown in figure 1.
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Figure 1. Experimental configuration of PIS current density calibration procedure.
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If we then inject a known current into the PIS specimen, for example, by using a potentiostat in galvanostat mode, and we know the exposed area of the PIS specimen, we then directly know the current density, J, at the end of the PIS specimen. We can then measure directly the SRET signal occuring for a given probe to sample separation. This will give us a multiplication factor to then input directly into the SRET software, and enable us to then make measurements with the system fully pre-calibrated to read in current density.
An example of this is shown in figure 2, which shows a SRET area map of a PIS sample calibrated for current density measurement. In this case, a current of 3.75 x 10-7 Amps was injected into a PIS specimen with an exposed area of 0.03 mm2 which equals 1.25 mA/cm2. The corresponding SRET signal was then measured, and a multiplication factor deduced, which was then input to the software. The SRET area map will then display the data calibrated to read in current density, as shown in figure 2.
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Figure 2. SRET area map showing PIS specimen calibrated to measure current density.
[click here for larger image]
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