The role that Al3normalFe intermetallic inclusions play during initiation of pitting corrosion on aluminum alloys was investigated in 0.6 M NaCl. In aerated solutions microscopic observations showed the growth of cavities in the host metal adjacent to inclusions. The rest potential of synthetic Al3normalFe was measured in aerated and deaerated NaCl solutions over a range of bulk pH values between 2 and 12 and was found to act as a cathode. In aerated solutions rotating disk electrode experiments on synthetic Al3normalFe electrodes verified that the cathodic reaction corresponded to the reduction of dissolved oxygen. With scanning pH microelectrodes, measurements were carried out near synthetic Al3normalFe electrodes which were coupled in a galvanic cell with Al-6061 in NaCl solution. In addition, the pH measured over Al-6061 at a distance of 25–30 μm from the surface was observed to fluctuate between pH 4 and 8.5 for the first 2 h of immersion in NaCl solutions. Atomic force microscopy images of Al-6061 immersed in buffered aerated 0.6 M NaCl solution (pH 5.5) did not show any evidence of dissolution of the Al around intermetallics. These measurements support the view that Al3normalFe particles in Al-6061 serve as local cathodes, that a high pH develops around the intermetallic particles and creates cavities in the host metal, and that interactions among a cluster of intermetallic particles causes a large number of slowly dissolving alkaline cavities to evolve into a small number of rapidly dissolving acidic pits. © 1999 The Electrochemical Society. All rights reserved.

Copper electrodeposition from three acidic solutions containing (i) no additive, (ii) 100 pM benzotriazole, and (iii) 100 pM thiourea was studied by in. situ atomic force microscopy. The electrodeposited surfaces were analyzed on three levels: (i) qualitatively during in situ monitoring of morphology evolution, (ii) quantitatively by scaling analysis of electrodeposited surface roughness during the course of deposition, and (iii) by modeling the spectral power density of the surface shape evolution. Major differences in deposit morphology were found between the three electrolytic solutions. All three levels of analysis gave consistent interpretations of morphology evolution. Deposition from additive-free solutions leads to rough surface textures due to roughening originating from surface diffusion. Addition of benzotriazole acts to smooth the deposit by diminishing surface diffusion. Deposits grown from thiourea.-containing solutions exhibit formation of three-dimensional islands atop initially flat plates, reflecting a two-stage growth mechanism.Among the more demanding applications of electrodeposition is the manufacture of microelectronic compo-nents2 which require significant decreases in the dimen.sions of patterned Cu films. Essential to obtaining such deposits with satisfactory textural, mechanical, and elec-* Electrochemical Society Student Member.Electrochernical Society Fellow.

Electrochemical dissolution of MnS in deaerated 0.1N NaC1 was found to produce thiosulfate ions. In addition, the presence of both chloride and thiosulfate ions above critical levels of concentration was found to cause rapid depassivation of 304 stainless steel in well-stirred deaerated 0.1N NaC1 solutions. Therefore, the hypothesis was explored that initiation of crevice corrosion in this system is caused by entrapped thiosulfate ions produced by electrochemical dissolution of MnS inclusions. The hypothesis differs from the view that breakdown is caused by acidification owing to Cr dissolution and hydrolysis. An artificial crevice cell, constructed from two optically flat surfaces, was used to measure the initiation time under controlled variation of crevice gap, applied potential, and sulfide inclusion density. It was found that crevice corrosion did not initiate for sufficiently large crevice gaps, negative (cathodic) applied potentials, and low inclusion densities. In Part II of this series of papers, data are compared to predictions based on a mathematical model of the proposed "thiosulfate entrapment" mechanism.

The onset of pitting corrosion at MnS inclusions on 304 stainless steel in 1 M NaCl was studied with and without applied mechanical stress with use of microelectrochemical cells. Polarization curves of areas ͑100 m diam͒ without inclusion showed no pitting at potentials below that of oxidation evolution; stress had no effect on the corrosion behavior. Areas containing five round inclusions of about 4 m in size showed stable pitting at about 400 mV; the effect of stress shifted the pitting potential to values that were 150 mV more negative. Polarization curves measured on large deep MnS inclusions showed active pitting. Curves of areas with single large, shallow MnS inclusions showed multiple current transients during dissolution of the inclusion without stress, but the metastable events did not initiate stable pitting. The dissolution of shallow MnS inclusion did not form a deep microcrevice between the MnS and stainless steel matrix. However, under applied stress, cracks were formed within the shallow MnS inclusion and active pitting occurred. To explore whether such cracks might serve to generate locally high concentrations of aggressive species, the pH and chloride concentration inside a crack were simulated using a finite difference model. For experimental conditions where stable pitting was observed, the simulations predicted that the solution composition at the base of a typical 13 m deep crack correspond to a pH of around 2, and a chloride concentration of about 6 M led to stable pitting.

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.