In this study, the copper corrosion rates of commercially available lubricants used in electrified and conventional transmissions are measured in both vapor and solution phases simultaneously using an updated version of our previously reported wire resistance test [1].
Unlike the commonly used copper strip tests (versions of the ASTM D130) that generally require high temperatures and long times to differentiate the corrosivity of fluids, the wire resistance test is sufficiently sensitive as to allow real time assessment, thus enabling an efficient and cost-effective way to screen lubricant chemistries over a range of potential operating temperatures.
The results of even our small study underscores the importance of understanding both the vapor and solution corrosion across a wide range of temperatures. We found that in solution corrosion did not occur below 110°C, but above 120°C each lubricant exhibited its own characteristic temperature dependence, with each becoming more corrosive as the temperature increased but at different rates. Corrosion in the vapor phase was more complex with corrosion occurring as low as 100°C and three of five lubricants showing less corrosive vapor at higher temperature than intermediate temperatures.
Corrosion measurements such as these, when interpreted correctly provide reliable data on which to base appropriate safety margins during the design phase.