Establishing Long-Term Corrosion Protection of Copper-Based Alloys in Modern Transmissions 04-13-03-0016

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.