Kinetic Study of the Passive Film on 304 Stainless Steel Using a Scanning Tunneling Microscope
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INERT
2
-2
-4- 75WK
-4500
-15W0
150
46W
75
voltae (my)
Figure 1: Typical IN curve of 304 stainless steel measured in air with the STM in a spectroscopic mode. Labeled are the active and inert regions along with the width of the inert region, w, which has been defined as the voltage range over which the tunneling current is less then +.3 nA. 199 Mat. Res. Soc. Symp. Proc. Vol. 404 01996 Materials Research Society
model which incorporates these observations into the treatment of the passivity of stainless steel is given below. This model, based on the measurements presented here and previous observations, provides a framework to relate the corrosion process to the electronic structure and thus the defect structure of the material investigated. Dissolution of oxide films entails the movement of point defects in an electrostatic field, the principal defects being cation vacancies (VM'), anion vacancies (Vo+ ), electrons (e-), and holes (e') [21,17,18]. Using the measurements obtained from the present study, a model is proposed which incorporates the influence of these structural aspects with the process of the dissolution of the metal and film. For example, point defects, a product of corrosion, are charged and interact with charged dislocations and grain boundaries in the oxide film. Further, these dislocations and grain boundaries offer short circuit paths for such defects to diffuse into the bulk metal or into the solution accelerating the dissolution of the metal surface; resulting in the formation of pits. Also, dislocations and grain boundaries within the oxide film affect the electrical makeup of the oxide film because of the charge associated with dislocations in semiconductors [22] affects the electric field over a characteristic length, perhaps the thickness of the film. The presence of charged dislocations and charged grain boundaries within the film may, therefore, lead to nonuniformity of the electrical character of the film. Such non-uniformity results in locally higher fields, making the material susceptible to localized corrosion due to the accumulation of charged species in specific regions. An additional consideration of the charged character of the defects is that they may substantially reduce the nucleation barriers for the formation of a pit. These defects will result in alterations in the electronic surface states of the film, which are measured in this study. This model, like the point defect model [17], assumes that the film contains a large number of defects, and that growth and dissolution is a result of the creation and movement of these defects. The present model, unlike the point defect model, establishes a basis for the mechanisms involved in the nucleation and growth of pits, and, thus the corrosion processes. A somewhat more complicated scenario would be required for so called amorphous films, but the idea of the interaction of charged entities may still apply. An amorphous film may contain some short range order, which would certainly contain defects. The pertinent experimen
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