It is obvious that the corrosion testing specimens must be electrically isolated from the testing cell body (typically a metallic pressure vessel). A metallic contact between the specimen and the pressure vessel body would lead to galvanic coupling and further to the electrochemical polarization of the specimen.
A corrosion scientist can easily understand the importance of galvanic coupling by imaging how the specimen’s electrode potential would move vertically in the E – pH diagram. The galvanic coupling usually leads to the specimen electrode potential polarization close to the testing cell body electrode potential, because the surface area of the testing cell is often 1 – 2 orders of magnitude larger than the specimen surface area.
Improving and maintaining the isolation resistance is often related to the polymer or ceramic isolation parts. Check that these parts are mechanically sound and clean. Small currents can flow through the contamination layers on the isolating materials. Check that the wire feed-through ceramic beads and tubes prevent any contact between the WE / CE / DCPD wires and the testing cell body. Replace the isolating Teflon tubes if they are broken or strongly deformed.
Pay attention especially to the electric isolation of the CT specimen and the DCPD wires (up to 6 pieces). The isolation resistance of the CT specimen is inversely related to the number of wires.
Fortunately, it is possible to measure the isolation resistance of the specimen, I typically check the isolation resistance when
- the specimen has been connected to the specimen holders,
- the specimen has been connected to the wire feed-through(s),
- the specimen is immersed in water and
- the operation temperature has been reached.
The first two tests can be performed with a regular portable DVM. The resistance between the specimen and the testing cell body should be preferably in MΩ range when the specimen is still dry. It is a good habit to measure the resistance with both the polarities to check any offsets.
Once the specimen is immersed in water, the isolation resistance measurements can show strange results. The values can be high, low or even negative. Apparently, a regular digital voltmeter (DVM) is not that suitable for this kind of measurements.
This phenomenon has a simple explanation. DVM's have been designed for electric and electronic industry applications where both the resistance measurement poles are at the very same potential. A small current feed will create a voltage signal needed for the resistance measurement. If the specimen and the testing cell body are of different materials and if there is water in the testing cell, their electrode potentials will not be identical. The pre-measurement voltage difference spoils the entire measurement principle.
One should perform a real four-point resistance measurement using a current source and a DVM. A current source will be used to feed a low current signal in the wire connecting the specimen and the testing cell body. Operator will monitor the voltage difference and calculate the resistance using equation R = ΔU/ΔI. This approach requires more work and instrumentation but it gives a better peace of mind. The measured resistance should be at least in 10 kΩ range and preferably higher.
Personally, I am quite paranoid with the isolation resistance especially when working with CT or CER (Contact Electric Resistance) specimens. If you see unexpected noise or peaks in your DCPD voltage or electrochemical potential / current signals, check the isolation resistance of the wires. If needed, stop the test and give a service for the wire feed-throughs and check the specimen holder ceramic parts. It is useless to produce data that you cannot trust.
Check for more information of Cormet's DCPD instrument at https://www.cormettestingsystems.com/product-items/direct-current-potential-drop-(dcpd)