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Figure 1. Measurement cell made of copper used to record an impedance spectrum
Heating and cooling the test specimen is performed using a climate chamber where the cell is placed into a faraday cage to optimize signal / noise ratio.
According to ISO TR 16208 the spectrum is analyzed evaluating charge transfer resistance Rct, electrolyte resistance Rs and double layer capacitance Cdl.
An example of a gear oil measurement between +0°C and +100°C is shown Figure 2. The Arrhenius-Plot where the natural logarithm of conductivity ln(σ) is plotted against the reverse of temperature 1000/T is used for in
Figure 3. REM picture and photo taken after aging oil and copper test specimen in a rancimat depicting severe pitting corrosion and surface layer formation.
depth analysis. For liquid electrolytes, as lubricants are, ln(σ) versus 1/T should give a straight line where the slope is associated to the activation energy of charge transfer at the cell electrodes (equivalent to corrosion tendency). Everything different from a straight line can be attributed to a system change such as chemical reactions (e.g. corrosive matter formed) or phase changes (e.g. pourpoint).
In our case at +40°C the plot shows a significant change in slope pinpointing to formation of highly reactive species. At elevated temperatures +80°C and
above surface layers are formed and the slope flattens. After heating up to +100°C reactive species still remain in the oil depicted as significant higher conductivities cooling down again to +0°C.
Figure 3 shows a raster electron microscope picture (left) and a photo (right) after oxidative aging of the gear oil which was performed using rancimat technology where the samples was exposed to heat, increased volumes of air and a copper stripe catalyst over 4 weeks.
Figure 2. Arrhenius-Plot of conductivity calculated from charge transfer resistance (Rct) which is identical to the corrosion tendency of the gear oils in contact with copper metal surfaces.
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LUBE MAGAZINE NO.130 DECEMBER 2015
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