ABSTRACT
First, we need to understand the transformation of the contact surface with exposure. Clearly, chemical reactions with gas molecules and water vapor in the atmosphere are very difficult to avoid. Thus, galvanic corrosion is the major film growth mechanism. In recent years, the extensive investigation done at Battelle (19) (see also Chap. 3) on corrosion have provided not only understanding of the film growth mechanisms, but also the characterization of the environment and methods of simulating the corrosion in the laboratory. The most challenging environment to characterize is the automotive environment. The large fluctuations in temperature and humidity during the course of each day, the seasonal changes, and the reactive gaseous compositions can change drastically. The other uncontrollable factor is the location of the vehicle. Exposures to the polluted air of Los Angeles are quite different from exposures to the hot humid air with salt mist at Miami Beach. Even within the vehicle itself, the region inside the engine compartment has an entirely different environment from that inside the trunk, for example. Figure 5.19 shows examples of metal coupons exposed to the atmosphere for nine months in different parts of a vehicle. The photograph clearly show a large range of film types and film thickness as reflected by the color and microstructure of the films. These films were analyzed by cathodic reduction method (see Sect. 2.6.3) to determine the types and to estimate the film thicknesses. Figure 5.19 gives representative data showing the films on the metal coupons from different location within the vehicle. We are able to establish the range of film thickness and composition at different locations within the vehicle. By varying the exposure time, the growth dynamics of these films can also be characterized (see Fig. 5.20).
