The present work deals with localized dissolution processes (pit and pore initiation and growth) of p-and n-type (100) GaAs. Pit and pore growth can be electrochemically initiated on both conduction types in chloride-containing solutions and leads after extended periods of time to the formation of a porous GaAs structure. In the case of p-type material, localized dissolution is only observed if a passivating film is present on the surface, otherwise, e.g., in acidic solutions, the material suffers from a uniform attack (electropolishing) which is independent of the anion present. In contrast, localized dissolution (pitting corrosion) and pore formation on n-type material can be triggered independent of the presence of an oxide film. This is explained in terms of the different current limiting factor for the differently doped materials (oxide film in the case of the p-and a space charge layer in the case of the n-GaAs). The porous structure was characterized by scanning electron microscopy, energy dispersive x-ray analysis, and Auger electron spectroscopy, and consists mainly of GaAs. From scratch experiments it is clear that the pit initiation process is strongly influenced by surface defects. For ntype material, atomic force microscopy investigations show that light induced roughening of the order of several hundred nanometers occurs under nonpassivating conditions. This nanometer-scale roughening however does not affect the pitting process.

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An in situ scanning tunneling microscopy study of morphological changes during formation and reduction of an oxide monolayer on Au(111) in 0.1M HC1Q is presented. During oxidation of a freshly prepared surface, the (1 × 1) arrangement of metal atoms persisted through the hydroxide peak at 1.35 V vs. reversible hydrogen electrode (RHE). At the peak potential for AuO formation (1.55 V vs. RHE), a slightly roughened layer propagated across terraces starting from edges. An ordered oxidized surface structure was not resolved and the surface roughness is approximately 0.08 -0.02 nm. This is ascribed tentatively to the formation of a monolayer of oxide by place exchange. During a slow cathodic sweep, the oxidized surface restructured into wormlike islands of monatomic height at the peak potential for oxide reduction at 1.15 V vs. RHE. These islands coalesced to form flat terraces with monatomic pits in the surface at potentials within the doublelayer region. Pits eventually fused with terrace ledges to restore the original terrace morphology.

The kinetics and mechanism of transpassive dissolution of thin, sputter‐deposited Cr2O3 films and passivated Cr were studied with in situ x‐ray near‐edge spectroscopy in 1 M H2SO4 , borate buffer (pH 8.4), and 1 M NaOH. The onset potentials of the transpassive dissolution and the plateau potentials during galvanostatic oxidation and their pH dependence are very similar for passive Cr and sputter‐deposited Cr2O3 films and indicate that the mechanism of transpassive dissolution of Cr can be experimentally modeled with Cr2O3 . X‐ray near‐edge spectroscopy spectra acquired during anodic potential steps reveal that, prior to transpassive dissolution, Cr(VI) is trapped in the Cr2O3 film. There is no evidence of formation of intermediate Cr(IV); it appears that Cr2O3 is directly oxidized to CrO42− (or Cr2O72− in acidic solutions). X‐ray near‐edge spectroscopy measurements made during galvanostatic oxidation/dissolution show that the reaction Cr2O3→CrO42− (or Cr2O72− ) takes place with a 100% current efficiency over the whole pH range (1 to 13). The results suggest that the transpassive dissolution of metallic chromium is a two‐stage process normalCr→Cr2O3→CrO42− false(Cr2O72−false) , with a faster kinetics of the first step; hence, the thermodynamics and kinetics of the transpassive dissolution of Cr are completely determined by the surface oxide. The significance of present findings for other experimental techniques and possible consequences for the corrosion resistance of stainless steels are discussed.