Automatic transmissions utilize solenoids to manage the flow of transmission
fluid throughout the transmission and engage the appropriate clutches during a
gear change. Because of the small clearances between sliding interfaces in a
solenoid, compatibility between materials and fluids is essential to long-term
functionality. The accumulation of films formed from corrosive species on these
components can lead to premature failure. Copper (Cu) corrosion strip tests are
found in almost all lubricant specifications; however, they do not necessarily
provide assurances in the field. Long-duration, powered solenoid soak tests are
undertaken to evaluate the long-term functionality of the transmission. The
complexity of oil-based corrosion mechanisms, including the temperature
dependence of these processes, can be difficult to evaluate even with this
advanced level of testing.
In this study, the corrosion rates of two Cu-based alloys relevant to solenoid
components were evaluated while immersed in two typical commercial fluids at
multiple temperatures. Results from powered solenoid soak tests in heated fluids
were compared against a wire corrosion test method.
The wire test method allows for real-time monitoring of corrosion rates through
resistance measurements of a thin wire immersed in a fluid. Surfaces of solenoid
bearings and wires were analyzed using a scanning electron microscope (SEM) and
by energy-dispersive X-ray spectroscopy (EDS). The wire test method, which
showed good reproducibility between two labs, offers an efficient and
cost-effective way to screen fluid chemistries over a range of temperatures for
potential corrosion issues with transmission component alloys.
