The Interaction of Automatic Transmission Fluid Additives with Copper

In recent years there has been an increase in the number of cars using automatic transmissions and in these systems the control units are often placed in contact with the Automatic Transmission Fluid (ATF). It is therefore important that the additives present in the ATF do not adversely interact with the transmission components, particularly those which are copper-based, such as the solenoids. There is currently very little literature on copper corrosion in oil based systems particularly when looking specifically at the interaction between ATFs and copper. This study looks at the interaction of some common ATF additives with copper surfaces. This is achieved through the combination of simple immersion tests, conducted on coupons, and resistance tests, carried out on wires. The coupons allow detailed analysis of the surface by FTIR, SEM and XPS. The wire tests monitor the resistance of a thin copper wire which can be directly related to the radius and indicate when there is loss of material. This allows the corrosion of the wire to be monitored in situ which is not achievable through immersion tests. The study looked at the standard ASTM D130 rating method as a basis for attributing corrosion to coupons after they had been immersed in a test fluid. It was found that this rating did not really correlate with any other measures of corrosion such as weight change, amount of copper in the used test fluid or radius change measured using the copper wire resistance test. It was concluded that whilst this test may be suitable for screening large numbers of fluids it provides little information on the actual corrosion taking place on the surface and in some instances film formation can be identified as corrosion. The study showed that increasing the temperature at which testing took place did not always speed up the corrosion process. In one instance increasing the temperature slowed the rate of reaction but in most cases increasing the temperature changed the mechanism of corrosion. The most extreme case of this was with the thiadiazole-based corrosion inhibitor which showed good inhibition until 150 °C when it broke down and caused pitting on the copper surface. The wire tests were able to show which additives protected the copper from corrosion, which gave constant corrosion, and the effect of combining additives. An antagonism was seen at 120 °C between dispersant 1 and a mix of antiwear, antioxidant and friction modifier. Detergent 2 combined with a mix of antiwear, antioxidant and friction modifier at 150 °C was also antagonistic. SEM images of the surfaces of coupons tested in full formulation fluids were able to be compared with those tested in single additives and simple mixtures to give information on which of the additives were causing the observed surface structure. This worked well for formulations where the additives were at similar levels to those tested individually but not for those where the concentration of additives was much lower. For each of the individual additives tested a mechanism of interaction with the copper surface has also been proposed.