Electrochemical measurements at pH

All further studies were conducted at ambient temperature with a view to finding an electrochemical procedure which would identify failure mechanisms 

For both alloy films, the Ret values are high, even for short-term exposures. Further, the R t values initially increase with exposure time, which implies the development with time of a more passive film on both alloys. However, after a particular exposure time (24 h for A and 7 h for B ). Ra no longer increases but begins to decrea.se, implying. some loss of passivity at that point, perhaps due to a change in structure of the film. Values for Cdi did not provide any information about film thickness they all fell in the capacitance range in which most or all of the contribution comes from the electrical double layer at the solution interface rather than from the film it-self. 

EIS measurements carried out at the more oxidizing potential (+.300 mV) were more definitive. For A, Ra values rose rapidly as a function of time and remained high even after a 12 h exposure, indicating the formation of a passive 

The observed film structures may, in fact, have some correspondence to the measured values at the corrosion potential. Initial increases in these values seem to follow an increase in thickness of the films, as measured by AFM. After longer exposures, then decreases at about the same time as when the film structures are seen by AFM to collapse. 

What is the thickness of these oxide films which produce ct values as high as lO Q-cm - The be.st estimate seems possible for the even and integral layer of crystallites on A, seen in Fig. 16c. Assuming that only a single layer of these is present, the layer thickness would then be ca. 30 nm. 

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