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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).

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. Reported at SAE world congress 2017 by this author, a new method for measuring the change in resistance of a thin copper wire was applied to provide a way to monitor the corrosion of copper in situ. In this work, a copper alloy in thin wire form has been used to compare the corrosion rates to pure copper. New insights on the kinetics and mechanisms of corrosion in the presence of lubricant additives over a range of operating temperatures using the wire resistance test will be discussed. The corrosion processes observed here are highly dependent upon temperature. Making assessments of corrosion performance through elevated temperature differentiation testing can provide less optimal corrosion protection at the actual operating temperature condition.

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.