Conclusions

(1) Tunneling on passivated metal surfaces with small tunneling voltages (C/t<500mV) is only possible when localized electronic states within the passive layer exist. They act as resonance centers for indirect tunneling processes. Therefore, the topographic images of such surfaces represent the interface metal/oxide and not the passive layer itself.

(2) The passive layer on DIN 1.4301 has significantly more localized states than the one on DIN 1.45429. This fits well with the higher amount of iron in the film.

(3) Ionic inclusions in the bulk material can be detected down to a size of 5 nm in diameter using dl/ds and dF/ds spectroscopy.

(4) A new microelectrochemical technique with a current detection limit of £ 10 fA and the use of an electrochemical microcell allow reduction of the exposed surface area. Such microelectrochemical measurements have several advantages:

- By reducing the sample area, the electrochemical current noise is strongly reduced. Therefore processes can be detected which in large-scale experiments would be hidden by the noise.

- Different aspects of pitting corrosion can be studied by varying the diameter of the microcell.

- The easy positioning of the microcell at different locations enables several measurements on the same sample.

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