Self-Organization Process in Active-State Pitting-Instability of Active-State Pit Formation

Abstract

From the viewpoint of unstable growth of nonequilibrium fluctuations, the self-organization process of the pit in an active state was examined. At less-noble potentials, anodic dissolution of passive metal initially generates a specific pit, called the active-state pit, having a size similar to that of the conventional polishing-state pit formed at more-noble potentials. The inside of the active-state pit has a random irregular surface, sometimes as a result of crystalline micrograins. It was first concluded that the diameter of the pit in the active state as well as in the polishing state is determined by the same fluctuations arising from metal-ion formation due to the presence of specifically adsorbed anions on the metal surface. Then, the irregularity of the inside of the pit was attributed to the fluctuations arising from metal-complex formation following metal-ion formation; the aggressive anions autocatalytically bind with the metal ions, so that the fluctuations, unlike the fluctuations due to metal-ion formation, are randomly distributed. In active dissolution without passive film at much less-noble potentials, because of the high growth rates of the fluctuations due to metal-ion formation, the fluctuations of complex formation regulate the total reaction as the rate-determining step, so that the random irregularity characteristic of such fluctuations appears in the surface morphology. As the potential shifts toward the more-noble side, however, owing to the development of the concentration overpotential, the fluctuations of metal-ion formation are suppressed from growing, so that instead of the fluctuations of complex formation, the fluctuations of metal-ion formation control the total reaction of the formation of the active-state pit. Finally, after computer simulation of the self-organization process, the morphology of the active-state pit was compared with that of the polishing-state pit, and the morphological dependence on the passive film coverage and pitting time was examined.

Long-Term Electrodeposition under a Uniform Parallel Magnetic Field. 1. Instability of Two-Dimensional Nucleation in an Electric Double Layer

“…Namely, all the conditions discussed above are reversed, so that anionic specific adsorption makes it unstable, forming etch pits, sometimes leading to pitting corrosion. In the case of cationic specific adsorption, weak adsorption will give rise to pitting formation. − …”

Self-Organization Process of Secondary Crystal Nodules from Individual Nuclei in Electrodeposition

“…In previous studies, the instability in pitting corrosion was also examined. [51][52][53][54][55][56][57][58] It was then concluded that two different types of pit characteristic of pitting corrosion, i.e., round uniform pit (polishing-state pit) 56) and random irregular pit (active-state pit), 57) appear with their intrinsic fluctuations. Then, it became clear that the instability of nucleation has a 5 m µ quite a different condition from that of the instability of corrosion, i.e., that of the instability of anodic dissolution.…”

“…Then, it became clear that the instability of nucleation has a 5 m µ quite a different condition from that of the instability of corrosion, i.e., that of the instability of anodic dissolution. 24,51,57,58) As discussed below, the unstable condition essential to nucleation requires the absence of the intense specific adsorption of the anion, 24) whereas in anodic dissolution, the intense specific adsorption of the anion is indispensable. 51) These opposite conditions for cathodic deposition and anodic dissolution are ascribed to the difference in the conditions for the specific adsorption of anions between anodic and cathodic polarizations.…”

“…In anodic dissolution, this type of fluctuations is always stable. 51,57,58) On the contrary to anodic dissolution, fluctuations of this type are predominant in electrodeposition; they induce the morphological change in 3D nuclei. In electrodeposition, the fluctuations always become unstable, so that the influence of symmetrical fluctuations is marked for 3D nucleation.…”

Self-organization of Copper Secondary Nodules by the Second Micro-MHD Effect

“…These two types of nucleation come from different nonequilibrium fluctuations. At the early stage of electrodeposition, there are two different kinds of the unstable processes of the fluctuations (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). The first unstable process takes place in the electrical double layer.…”