Frankel. Individual papers

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    In Situ Studies of Passive Film Chemistry using X-ray Absorption Spectroscopy
    (1993) Davenport, Alison J.; Bardwell, Jennifer A.; Isaacs, Hugh S.; MacDougall, Barry; Frankel, Gerald S.; Schrott, Alex G.
    The passive film on sputter-deposited thin film Fe and Fe-26Cr electrodes has been examined using in situ X-ray Absorption Near Edge Spectroscopy (XANES). During the x-ray spectroscopic measurements, samples were maintained under potentiostatic control in pH 8.4 borate buffer. The passive film on both Fe and Fe-26Cr is formed from the metal without any detectable losses due to dissolution. Cathodic reduction leads to a loss of Fe from the film in both cases, but substantially less Fe is lost in the case of Fe-26Cr. Transpassive dissolution of Cr from Fe-26Cr only occurred on the second potential cycle after the film had been enriched in Cr due to the reductive dissolution of Fe. Simultaneous collection of both the Fe and Cr x-ray absorption edges provides quantitative information on the relative amounts of material lost during passivation and reduction.
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    Electrochemical Quartz Crystal Microbalance Study of Corrosion of Phases in AA2024
    (2003) Baek, Y.; Frankel, G. S.
    The electrochemical quartz crystal microbalance (EQCM) was used to directly measure the dissolution rate at cathodic potentials, and thus the cathodic corrosion rate, of thin-film analogs of phases in AA2024. Thin films of pure Al, Al-4% Cu, and Al2Cu were studied in 0.1 M NaCl containing 0, 10^-4, or 10^-2 M Cr2O7 . A range of cathodic potentials was studied for each material. The true cathodic current density was calculated from the difference of the net current density and the dissolution rate, which was determined by the EQCM. For pure Al and Al-4Cu, the cathodic corrosion rate was large relative to the net current density, so the true cathodic current density was considerably larger than the measured net current density. The cathodic current density was almost identical to the net current density for Al2Cu because the dissolution rate was very small compared to the cathodic reaction rate. Various potentials in the limiting oxygen reduction reaction region were examined, but the effect of the applied potential was small. The presence of dichromate in solution decreased both the cathodic corrosion rate and the cathodic current density on these thin-film analogs. In particular, it decreased more effectively the cathodic reaction rate on Al2Cu, which can support faster cathodic reaction rates.
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    Electrochemical Behavior of Thin Film Analogs of Mg (Zn,Cu,Al)2
    (2001) Ramgopal, T.; Schmutz, P.; Frankel, G. S.
    The electrochemical behavior of the intermetallic phase Mg(Zn, Cu, Al)2 was studied using thin film compositional analogs prepared by flash evaporation. Characterization by scanning electron microscopy, atomic force microscopy, and Auger electron spectroscopy depth profiles indicated that the films were single phase, laterally homogeneous, and homogeneous through the thickness. Experiments in deaerated 0.5 M NaCl showed that the addition of 18 atom % Cu to MgZn2 raised the open circuit potential (OCP) by about 150 mV and the breakdown potential by 170 mV. The attack seemed to be localized dealloying. In high pH solutions where Mg(OH)2 was stable, the films were susceptible to a more classical form of localized breakdown. The breakdown and the repassivation potentials increased with increasing copper concentration. Cyclic polarization curves on these films show the presence of Zn reduction peaks in the cathodic part of the downward potential scan. OCP playback experiments on the intermetallic thin films showed that the Mg(Zn, Cu, Al)2 phase is very active over a wide range of compositions in neutral solutions at the OCP of the 7xxx series alloy.
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    Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings Equilibrium Aspects of CrVI Concentration
    (2000) Xia, Lin; Akiyama, Ejii; Frankel, Gerald; McCreery, Richard
    The release of soluble CrVI species by a chromate conversion coating (CCC) was monitored quantitatively by ultraviolet-visible spectroscopy. By careful selection of measurement wavelength (339 nm), the CrVI concentration could be determined without regard to solution pH or CrVI speciation. The CrVI concentration in solution over a CCC reached an equilibrium value that depended on pH, ionic strength, and the ratio of the CCC surface area to the solution volume (A/V). In separate experiments, the adsorption of CrVI by synthetic CrIII hydroxide to form a CrIII-CrVI mixed oxide was observed, and also led to an equilibrium concentration of CrVI in solution. The equilibrium CrVI concentration was determined for a variety of A/V values on both AA1100 and AA2024-T3 aluminum alloys. The results are inconsistent with release mechanisms based on the solubility of a CrVI salt in the solution or depletion of CrVI from the CCC. However, the observed concentrations are consistent with a mechanism similar to a Langmuirian adsorption-desorption equilibrum of CrVI on a porous, insoluble CrIII hydroxide matrix. The CrIII hydroxide matrix has a finite number of CrVI binding sites and exhibits a nonlinear relationship between solution and solid CrVI concentrations governed by an equation similar to a Langmuir adsorption isotherm. The proposed model incorporates reversible adsorption and de-sorption of CrVI, with adsorption favored at low pH during formation of the CCC, and desorption favored in field conditions. The model quantitatively predicts the observed concentrations after determining the binding constant from fits to the data. The model explains the capacity of a CCC to release active CrVI corrosion inhibitor and provides strong evidence that CrVI storage in a CCC involves reversible formation of a CrVI-O-CrIII mixed oxide.
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    Pit Growth Study in Al Alloys by the Foil Penetration Technique
    (2000) Sehgal, A.; Frankel, G. S.; Zoofan, B.; Rokhlin, S.
    The foil penetration technique was used to study pit growth in AA1100-O and AA2024-T3. Preliminary work on AA1100-O foils of different thicknesses indicated that the pit growth rate increased with increasing applied potential, suggesting that pit growth was not under transport control. Foil penetration experiments were also carried out on AA2024-T3 foils of a given thickness, at open circuit as well as anodic potentials. Dichromate ions and other oxidizing agents were added to some test solutions. Dichromate ions were shown to have little influence on the pit growth rate at controlled anodic potentials, even when added in large concentrations. However, dichromate ions effectively inhibited pitting at open circuit when present in very small amounts. Polarization curves of AA2024-T3 in 1 M NaCl with various additives show a large effect of dichromate ions in the cathodic region and no effect in the anodic region. These observations suggest that chromate (or its reduction product) acts as a cathodic inhibitor. Examination of penetrated samples was performed by optical and scanning electron microscopies, as well as by microradiography.
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    Corrosion Study of AA2024-T3 by Scanning Kelvin Probe Force Microscopy and In Situ Atomic Force Microscopy Scratching
    (1998) Schmutz, P.; Frankel, G. S.
    The localized corrosion of AA2024-T3, and the behavior of intermetallic particles in particular, were studied using different capabilities of the atomic force microscope (AFM). The role of intermetallic particles in determining the locations and rates of localized corrosion was determined using scanning Kelvin probe force microscopy in air after exposure to chloride solutions. Al-Cu-Mg particles, which have a noble Volta potential in air because of an altered surface film, are actively dissolved in chloride solution after a certain induction time. Al-Cu-(Fe, Mn) particles are heterogeneous in nature and exhibit nonuniform dissolution in chloride solution as well as trenching of the matrix around the particles. Light scratching of the surface by rastering with the AFM tip in contact mode in chloride solution results in accelerated dissolution of both pure Al and alloy 2024-T3. The abrasion associated with contact AFM in situ resulted in the immediate dissolution of the Al-Cu-Mg particles because of a destabilization of the surface film.
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    Characterization of AA2024-T3 by Scanning Kelvin Probe Force Microscopy
    (1998) Schmutz, P.; Frankel, G. S.
    Volta potential mapping of AA2024-T3 on surfaces was performed with an atomic force microscope. A linear relation was found between the Volta potential measured in air and the corrosion potential in aqueous solution for a range of pure metal samples, indicating that this potential is a measurement of the practical nobility of the surface. Large differences in the Volta potential of intermetallic particles in AA2024-T3 and the matrix phase resulted in a potential map with high contrast that clearly identifies the location of the particles. All intermetallic particles, including the Mg-containing S-phase particles, had a Volta potential noble to that of the matrix. Surface films on the particles and the matrix were found to have strong effects on the potential, and probably explain the noble nature of the Mg-containing particles, which have been reported to be active to the matrix in solution. The effect of these surface films was examined by refreshing the sample surface using different techniques. Lateral heterogeneities in certain intermetallic particles were also revealed.
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    Corrosion Protection of Untreated AA-2024-T3 in Chloride Solution by a Chromate Conversion Coating Monitored with Raman Spectroscopy
    (1998) Zhao, Jun; McCreery, Richard L.; Frankel, Gerald
    The behavior of chromate conversion coatings (CCCs) on the aluminum aircraft alloy AA 2024-T3 was examined by several types of experiments, using Raman spectroscopy as a primary technique. First, Raman spectra of the CCC film made from a commercial process revealed a Raman feature characteristic of Cr(VI) which was distinct from Raman bands of pure CrO4^−2 or Cr2O7^−2. Second, Raman spectroscopy was used to monitor migration of chromate species from a CCC film to an initially untreated alloy sample. The release of chromate from a CCC was demonstrated, as was redeposition of a chromate film on the fresh alloy surface. Formation of a Raman-observable Cr(VI)-containing deposit was more rapid in or near pits in the untreated alloy sample, and the deposit was spectroscopically very similar to the original CCC film. The initially untreated alloy became much less active toward corrosion after migration of chromate from the nearby CCC film, with the polarization resistance increasing by at least two orders of magnitude and the pitting potential increasing by 60 mV. The results clarify the mechanism of self-healing exhibited by CCC films, in which chromate species released from the CCC migrate to an actively corroding region and stop aluminum dissolution. The migrating chromate is selectively deposited at active corrosion sites, either by forming an insoluble Al/chromate is selectively deposited at active corrosion sites, either by forming an insoluble Al/chromate precipitate or by adsorption by previously formed corrosion.
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    Pitting Corrosion of Metals: A Review of the Critical Factors
    (1998) Frankel, G. S.
    Pitting corrosion is localized accelerated dissolution of metal that occurs as a result of a breakdown of the otherwise protective passive film on the metal surface. This paper provides an overview of the critical factors influencing the pitting corrosion of metals. The phenomenology of pitting corrosion is discussed, including the effects of alloy composition, environment, potential, and temperature. A summary is then given of studies that have focused on various stages of the pitting process, including breakdown of the passive film, metastable pitting, and pit growth.
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    Repassivation of Pits in Aluminum Thin Films
    (1996) Frankel, G. S.; Scully, J. R.; Jahnes, C. V.
    The effect of metal film thickness on repassivation of pits in sputter-deposited Al thin films was investigated in chloride solutions. The repassivation potential and the critical current density, which is the pit current density below which pits stop growing, were determined for pits in Al thin films ranging from 100 Ǻ to 43 μm in thickness. The repassivation potential first decreased as thickness increased from 100 to 4350 Ǻ, and then increased as the film thickness increased further. This behavior was found to be a consequence of the pit current-density/potential relationship. The critical current density, a more informative parameter, decreased for increasing metal film thickness, even when the repassivation potential increased. The critical current density is the minimum current density needed to maintain the critical pit environment and prevent repassivation. The repassivation potential for a given metal film thickness is the potential at which the pit current density drops below the critical value. Mass-transport and ohmic resistance both increase as the metal film thickness increases, but the former enhances pit stability and the latter destabilizes pitting in this system. Pit repassivation, and thus stability, are strongly influenced by mass-transport considerations for pits in very thin pits, even though dissolution at low potentials is not under pure mass-transport control. Ohmic effects become increasingly important as the film thickness increases.
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    Corrosion and Protection of a Conductive Silver Paste
    (1995) Brusic, V.; Frankel, G. S.; Roldan, J.; Saraf, R.
    One of the possible uses for a conductive paste is as an adhesive in interconnect technology that could replace PbSn solder. The interconnections are expected to perform under a variety of environmental conditions, and with an applied voltage. Thus knowledge of their corrosion and dissolution resistance is of utmost importance. This is a study of the dissolution and protection of polymer/metal composite films, prepared with a high loading of silver or gold particles. Electrochemical tests were conducted in a droplet of triple-distilled water with or without benzotriazole (BTA) and BTA derivatives. Results indicate that, in spite of some protection obtained by the polymer, silver paste dissolution at high anodic potentials is rapid, reaching values of 10^-1 A/cm2, which corresponds to a catastrophic silver removal rate of at least 35.6 nm/s. With a reservoir of azole in the corrosive environment, this rate can be reduced by up to five orders of magnitude. This azole effect greatly reduces the probability of electrolytic silver migration, but the Ag dissolution rate is still higher than the anodic activity shown by Au paste under the same conditions.
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    Repassivation Transients Measured with the Breaking-Electrode Technique on Aluminum Thin-Film Samples
    (1995) Frankel, G. S.; Jahnes, C. V.; Brusic, V.; Davenport, A. J.
    The breaking-electrode technique was used to study repassivation transients of small, fresh metal areas of Al exposed to a conductive electrolyte at a range of potentials. The peak current density measured within the first few microseconds after breaking was found to be ohmically limited, with an ohmic resistance that is substantially higher than that measured minutes after breaking. The current decay during the first 3 s was recorded and found to be exponential in nature. By plotting the data as log i vs. (it)^-1/2, it is concluded that oxide growth was better.represented by the direct logarithmic law than by high-field kinetics. Cathodic current transients having a complex shape were observed at low potentials. It is suggested that oxide growth at potentials slightly above the reversible potential for oxide formation retards the water reduction reaction. The advantages and limitations of the breaking-electrode technique are discussed.
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    Copper Corrosion With and Without Inhibitors
    (1991) Brusic, V.; Frisch, M. A.; Eldridge, B. N.; Novak, F. P.; Kaufman, F. B.; Rush, B. M.; Frankel, G. S.
    The utility of copper interconnects may ultimately depend on the ability to protect copper from corrosion. We have studied the capacity of lH-benzotriazole (1H-BTA) to provide a protective and stable surface film able to withstand harsh chemical and thermal environments. The film was characterized with electrochemical techniques, in situ ellipsometry, ex situ time-of-flight static secondary ion mass spectrometry, high-temperature mass spectrometry, and accelerated temperature and humidity tests. Several important passivating film properties (thickness, degree of polymerization, thermal stability, corrosion resistance) depend critically on the details of the film preparation conditions. The best corrosion protection is offered by the thin film formed on an oxidized Cu surface. This film has also shown the slowest growth kinetics and the highest degree of polymerization in the Cu-BTA structure.
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    Potential control under thin aqueous layers using a Kelvin Probe
    (2007) Frankel, G. S.; Stratmann, M.; Rohwerder, M.; Michalik, A.; Maier, B.; Dora, J.; Wicinski, M.
    Kelvin Probes can be modified to control as well as monitor potential. The design and operation of two different Kelvin Probe Potentiostats (KPPs) are described in this paper. One approach uses a permanent magnet and double coil to oscillate the needle at a fixed frequency, an AC backing potential, and software analysis and control schemes. This technique can also control the distance between the tip and sample, thereby tracking the topography of the sample. Both KPPs were used to make measurements on Type 304L stainless steel under thin layers of electrolyte. Cathodic polarization curves exhibited a limiting current density associated with oxygen reduction. The limiting current density varied with solution layer thickness over a finite range of thickness. Anodic polarization curves on 304L in a thin layer of chloride solution resulted in pitting corrosion. The breakdown potential did not vary with solution layer thickness. However, the thin layer was observed to increase in volume remarkably during pit growth owing to the absorption of water from the high humidity environment into the layer with ionic strength increased by the pit dissolution. The open circuit potential (OCP) and solution layer thickness were monitored during drying out of a thin electrolyte layer. Pitting corrosion initiated, as indicated by a sharp drop in the OCP, as the solution thinned and increased in concentration.
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    Statistical modeling and computer simulation of intergranular corrosion growth in AA2024-T3 aluminum alloy
    (2004) Ruan, Shiling; Wolfe, Douglas A.; Frankel, Gerald S.
    An extension of a brick wall model developed in a previous paper (Technometrics, to appear) was used to describe intergranular corrosion in AA2024-T3 aluminum alloys. The extended model simulates the behavior of corrosion paths at intersections of grain boundaries within the metal sample. Situations considered include the cases where a corrosion path might assume an upward turn, skip an intersection (not turn) or split into branches. The splitting of a corrosion path results in a smaller median of the minimum order statistic while the other factors increase the median of the minimum order statistic. Moreover, a larger number of grain layers increases the minimum path length for a sample with given thickness. With a proper combination of these factors, the extended model is able to provide a good fit to the experimental data developed by the foil penetration technique.
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    Effect of Noble Element Alloying on Passivity and Passivity Breakdown of Ni
    (2007) Kim, Yeong Ho; Frankel, G. S.
    Welding of stainless steels can generate welding fumes containing carcinogenic hexavalent chromium (Cr^+6). To mitigate this problem, a new Ni-Cu-Pd welding consumable alloy has been developed. The addition of a small amount of Cu and Pd in Ni enhances the galvanic compatibility of Ni with stainless steels and improves its localized corrosion behavior. In this paper, the artificial pit electrode technique and X-ray photoelectron spectroscopy were used to study the benefits of Cu and Pd alloying. The passive film on Ni-10Cu-1Pd alloy mainly consisted of outer Ni-hydroxide and inner oxide, and the noble elements Pd and Cu apparently did not contribute to the formation of the passive film. However, Pd catalyzed the reduction of Cu at the bottom of artificial pits, which enhanced the cathodic reaction and thus ennobled the protection potential, making stable pit growth more difficult. This catalytic effect of Pd also prevented the propagation of deep pits in a test using a thin foil crevice sample.
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    Aluminum Alloy Corrosion Inhibition by Vanadates
    (2006) Iannuzzi, M.; Young, T.; Frankel, G. S.
    The inhibition of Al alloy corrosion by vanadates was studied in this work. Vanadium speciation is very complicated and vital to the inhibition efficacy. Critical conditions for decavanadate polymerization from clear metavanadate solutions were investigated. Decavanadate only formed when metavanadate was added to solutions of pH 3 or less. It was not possible to change the pH of a metavanadate solution without forming decavanadates, creating an orange-colored solution. According to ^51 V nuclear magnetic resonance, monovanadates were present only in clear metavanadate solutions; orange solutions always contained decavanadates and never contained monovanadates. Orange decavanadate solutions containing 0.5 M NaCl at pH 8.71 exhibited no significant inhibition of the oxygen reduction reaction and increasing decavanadate concentration was detrimental. In contrast, clear metavanadate solutions containing monovanadate exhibited strong inhibition of the oxygen reduction reaction, to a level similar to chromate. At a fixed pH, increased NaVO3 concentration in clear metavanadate solutions increased inhibition efficiency.
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    Effect of Cu Content on Corrosion Behavior of 7xxx Series Aluminum Alloys
    (2004) Meng, Qingjiang; Frankel, G. S.
    The corrosion behavior of 7xxx aluminum alloys with various Cu content was investigated using polarization and electrochemical impedance spectroscopy (EIS) techniques. Two breakdown potentials were found in Cu-containing alloys in deaerated chloride solution, and the values increased logarithmically with increasing Cu content. However, in aerated chloride solution, polarization resistance as determined by EIS decreased with increasing Cu content. The first breakdown potential corresponded to transient dissolution associated with attack of the fine hardening particles and the surrounding solid solution in a thin surface layer. The second breakdown potential was associated with combined intergranular and selective grain attack. The correlation between the microstructure, especially Cu content, and corrosion behavior was made by composition analysis of the grain boundary regions including precipitate free zone and grain boundary precipitates as well as the matrix by scanning transmission electron microscopy. The corrosion potential increased with increasing alloy Cu content in aerated chloride solutions because of enhanced rates of oxygen reduction. This degraded the corrosion resistance despite the increase in the breakdown potentials with Cu content.
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    Investigation of Filiform Corrosion of Epoxy-Coated 1045 Carbon Steel by Scanning Kelvin Probe Force Microscopy
    (2004) Leblanc, Patrick P.; Frankel, G. S.
    The mechanism for filiform corrosion (FFC) is thought to involve oxygen diffusion through the tail to the active head. The primary cathodic region is near the back of the head (at the head/tail boundary), where oxygen concentration is higher, and the primary anodic region is at the front edge of the head of the filament. Although there is experimental support for this mechanism, a high-resolution description of the FFC process has not been presented. The aim of this study was to provide detailed information about the mechanism of FFC on coated steel using the high spatial resolution of scanning Kelvin probe force microscopy. Segments of active filaments were successfully investigated through 150 and 300 nm thin epoxy coatings in air of 93% relative humidity. Volta potential and topographic maps showed separation of active anodes and cathodes in the head and revealed the presence of voids associated with delamination of the coating along the edge of the tail. The morphology of filaments and Volta potential distributions were strongly dependent on the film thickness. Differences in growth characteristics were explained by mass transport considerations.
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    Effects of compressive stress on localized corrosion in AA2024-T3
    (2006) Liu, Xiaodong; Frankel, G. S.
    The effect of compressive stress on intergranular corrosion (IGC) of AA2024-T3 was studied using a constant load and simultaneous electrochemical measurement. A specially designed electrochemical cell was used to compress a pillar-shaped sample and control the potential at a value that promoted IGC. The extent of IGC was assessed by metallurgical cross-sectional images. The effect of the compressive stress depended on the orientation of the stress relative to the elongated microstructure. Application of a compressive stress halfway to yield in the S or through-thickness direction significantly reduced the growth kinetics of IGC in the longitudinal direction, but did not eliminate it totally. The strain change during exposure also was used to quantify the change in radius of the cylindrical sample as a function of time during IGC growth. The effect of compression on reducing IGC was also assessed by the current density measured during potentiodynamic and potentiostatic polarization. The effects of residual compressive stress on IGC were studied using samples treated by low plastic burnishing (LPB), which produces a surface layer with high residual compressive stress. The results depended on the plane of the LPB treatment. A micro-capillary cell was used to measure corrosion behavior at different zones of the section of an LPB-treated sample. The breakdown potential was significantly higher in the zone with residual compressive stress than in the interior of the sample.