New Perspectives on the Temperature Dependence of Lubricant Additives on Copper Corrosion

Modern automotive transmissions contain copper and copper alloys in the form of washers, bushings, brazes and electrical components. Corrosion that occurs with any of these components especially with electrical contacts can result in a malfunction of the vehicle control systems and loss of vehicle drivability. The compatibility of transmission lubricants with copper and copper alloys is an increasingly important consideration in the design of new additive technology. Traditional methods for monitoring corrosion processes and mechanisms in real time can be both time consuming and challenging to interpret, especially when evaluations at multiple temperatures are required. This work challenges some of the industry-held beliefs around lubricant additive corrosion processes, especially at elevated temperature (>130 °C). These new insights on the kinetics and mechanisms of copper corrosion in the presence of lubricant additives over a range of operating temperatures using a new wire resistance test that enables real-time corrosion monitoring will be discussed. The corrosion processes observed here are highly dependent upon temperature and accelerating tests based on increasing temperature presumes the dominant kinetic mechanism remains constant between real world and accelerated conditions. In the past, with end point tests, there was no easy way to assess whether this was the case. The new test does allow an assessment of corrosion mechanism, and demonstrates that certain lubricants do not maintain the same mechanism between real world and typical test temperatures.