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.

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.

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.

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