Anodic dissolution of aluminum in hot chloride solutions produces a high density of fine etch tunnels that extend along [100] directions. Tunnels evolve from cubic etch pits when all but one of the pit wall surfaces become passivated; dissolution then occurs at the one active face at a rate that may initially be as high as 20 A/cm2. Tunnels have square cross sections with sides ∼1 μm and aspect ratios as high as 100:1. Tunnel growth may be considered a unique form of pitting corrosion in which dissolution and passivation occur simultaneously with a sustained balance between the two processes.

Pure iron in the form of a shielded electrode facing upward was anodically polarized in hydrochloric, perchloric, and sulfuric acid solutions. Ferrous salt films formed in all three electrolytes at potentials and current densities above threshold values determined by mass transport. In perchloric and sulfuric acids, oxide passivation occurred underneath the salt film at potentials above the passivation potential. Analysis of kinetic and transport conditions under and within these salt films indicated that salt films are necessary precursors to oxide passivation in perchloric and sulfuric acid solutions. Oscillatory phenomena during passivation of iron can be explained by formation and dissolution of salt films coupled to pH changes under the salt film due to electrolytic migration.

Filiform corrosion occurs on painted metals and cannot be prevented by conventional corrosion inhibitors. In order to better understand the corrosion phenomena, the literature was reviewed and the characteristics exhibited by filiform corrosion on different metals were cataloged. Calculations were performed to select corrosion mechanisms that were compatible with these characteristics. The preferred mechanism, compatible with all the primary characteristics, is that oxygen and water reach the corrosion site by diffusing through the porous filiform tail. Experiments were conducted which confirmed this mechanism. The mass transfer occurring ,inside the active corrosion cell, and unresolved questions concerning the speed and width of filiform growth are also discussed.

A mathematical model is developed to describe the processes by which transient appearance of salt films occurs during initial stages of corrosion of oxide-free metal surfaces. The model describes transport-controlled processes which lead to supersaturation by anodic dissolution, to precipitation of a salt film, and eventually to dissolulion of the salt film after oxide passivation of the substrate material. Experimental observations on iron repassivation in ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 155.69.4.4 Downloaded on 2015-06-05 to IP Vol. I25, No. 9

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The purpose of this investigation is to employ simple transport laws to gain a perspective of events which occur during early stages of pit initiation and growth. The initiation of pits on the surface of passive metals undoubtedly occurs at flaws in the passive surface film. At the moment of pit initiation, the ~laws would be expected to be very small, on the order of the passive film thickneses. Calculations indicate that the current density to such small regions would tend .to be exceedingly high and that a salt film is therefore likely to form at very early stages of initiation of pit growth.The presence of a high resistance layer at the surface of corrosion pits growing on passive metals has been recognized for some time. For example, Franck (1) summarized his research on iron in 1961 and directed attention to electropolishing layers which were thought to be responsible for producing polished hemispherical corrosion pits. More recently, Vetter and Strehblow (2) reported studies on micrometer-size pits in iron and made a persuasive case for the presence of a thin, high resistance salt layer along the surface of the pits. These authors also claimed that the salt layer was not in equilibrium with a saturated electrolyte, that the thickness of the layer is controlled by high field conduction, and that the corrosion rate is controlled by the dissolution rate of the salt.A recent survey by Engell (3) provided a compilation of previous observations, as well as a critical evaluation of previous interpretations of experimental data. In particular, Engell demonstrated that sufficient changes in concentration can occur near an active region to cause precipitation of a metal salt of considerable solubility, in contrast to the interpretation of Vetter and Strehblow. Popovet al. (4) and Rosenfeld et al. (5) have also recently contributed views in support of salt passivation of metals.The purpose of this investigation is to employ transport laws to model certain aspects of pit initiation and growth in order to clarify conditions under which salt films may form during pitting corrosion. In the first part which follows, the growth of hemispherical pits is discussed in order to develop a hypothesis of pit growth processes. Mass and energy balances, coupled with reaction kinetics and ohmic resistance considerations, are used. The mathematical model is tested against experimental data available from the literature. In the subsequent section, the very early stages of pit growth are discussed with use of the mathematical model. Various considerations are evaluated in order to determine what processes control behavior during the very early stages of pitting. Pit GrowthObservations of polished hemispherical pits in various metals (6) support the suggestion that an electropolishing layer exists near the metal surface during pitting corrosion (1). In order to examine the implications of this hypothesis it will be assumed, following Ref.(2), that the electropolishing layer includes properties of a poreless barrier salt film contiguous ...

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