Thomas M. Devine

A surface enhanced Raman spectroscopic study of the passive films formed in borate buffer on iron, nickel, chromium and stainless steel

Abstract Surface enhanced Raman spectroscopy (SERS) was used to investigate the passive films formed on iron, nickel, chromium and 308 stainless steel in borate buffer solution (pH = 8.4) at low potentials in the passive region. The decay of these passive films, as the potential was scanned in the cathodic direction, was observed via SERS. The passive film on iron at −100 mV(SCE) consisted of an amorphous Fe(OH) 2 -like species and amorphous Fe 3 O 4 or γ-Fe 2 O 3 , which have similar Raman spectra. The passive film on nickel at −100 mV consisted primarily of amorphous β-Ni(OH) 2 and some NiO. The passive film on chromium at −100 mV in borate buffer consisted of Cr(OH) 3 and another substance that may be similar to Cr(OH) 2 . The passive film on 308 stainless steel consisted of amorphous Fe(OH) 2 and Fe 3 O 4 or γ-Fe 2 O 3 , Ni(OH) 2 , NiO, Cr(OH) 3 and the Cr(OH) 2 -like species. For all metals, as the electrode was cathodically polarized the various constituents of the passive film were concurrently reduced, a result that is inconsistent with passive films composed of discrete layers. In all SERS experiments carbon, a ubiquitous surface contaminant, was detected. The identity of the carbonaceous species changed with potential from predominantly amorphous graphite and CO 2 at high potentials to saturated hydrocarbons at low potentials.

Aluminum Corrosion in Lithium Batteries An Investigation Using the Electrochemical Quartz Crystal Microbalance

An electrochemical quartz crystal microbalance is used in an investigation of the corrosion of aluminum in electrolytes appropriate for lithium batteries. The electrolytes are solutions of LiN(CF 3 SO 2 ) 2 , LiC(CF 3 SO 2 ) 3 , LiCF 3 SO 3 , LiPF 6 , LiBF 4 , and LiClO 4 , singly or in a limited number of combinations, in propylene carbonate (PC) and polyethylene glycol dimethyl ether. Aluminum that is scratched or abraded in an inert atmosphere (so that a protective surface film does not reform) undergoes significant corrosion in PC containing LiN(CF 3 SO 2 ) 2 , LiC(CF 3 SO 2 ) 3 , LiCF 3 SO 3 , and LiClO 4 , but forms a protective film in PC containing LiPF 6 , or LiBF 4 A mechanism of corrosion of aluminum in LiN(CF 3 SO 2 ) 2 /PC is proposed.

Analysis of Electrodes Displaying Frequency Dispersion in Mott-Schottky Tests

A procedure is proposed for using the Mott-Schottky approximation to obtain the electronic properties (flatband potential and charge carrier density) of passive films that are modeled by equivalent circuits that contain a constant phase element (CPE) rather than a capacitor. Capacitances are calculated from the equivalent circuit containing a CPE using expressions developed by Hsu and Mansfeld [Corrosion, 57, 747 (2001)], and Brug et al. [J. Electroanal. Chem., 176, 275 (1984)]. Both expressions calculate similar flatband potentials for Ti and Ni, but the charge carrier densities are significantly different. The approach is applied to the analyses of passive films formed on titanium in 0.1 M H 2 SO 4 and nickel in 1 M NaOH. The passive film formed on Ti was found to be n-type, have a flatband potential of -330 mV vs a sat. Ag/AgCl reference electrode, and a charge carrier density of 1.4 X 10 21 cm -3 . The film formed on Ni was p-type, had a flatband potential of 100 mV, and a charge carrier density of 3 × 10 21 cm -3 .

Surface-enhanced raman scattering magnified by photochemical activation of the silver electrode in aqueous halide electrolytes

Abstract Laser illumination of a silver electrode during an oxidation-reduction treatment in halide solutions results in a further magnification of the SLRS for pyridine, thiourea and water. This additional enhancement factor appears to be the result of a structural modification of the silver produced by the photoassisted decomposition of the silver halide film formed during the oxidation treatment.

In situ vibrational spectra of the passive film on iron in buffered borate solution

The structures of films formed on iron in borate buffer (pH = 8.4) were investigated by in situ surface enhanced Raman spectroscopy (SERS). The identities of the films were shown to be a function of potential and history. When the films were removed from solution their identities changed. All films were amorphous. In the active range, the film consisted of species resembling FeO and Fe(OH)2. One constituent was common to all films, regardless of the potential at which they were formed. This species had a vibrational spectrum that was similar to that of Fe(OH)2 and yet it appeared to consist of a mixture of Fe(II) and Fe(III). This component is referred to as Fe(II)(III)(OH)∗x. In addition, films formed in the active region also contained a species that resembled FeO. In the potential range between the passivation potential and the Flade poential, the film consisted of a mixture of Fe(II)(III)(OH)∗x and a species that resembled Fe3O4. Films formed above the Flade potential were composed of an inner region of Fe(II)(III)(OH)∗x and Fe3O∗4, and an outer, porous layer that resembled γ-FeOOH.

Relation Between the Semiconducting Properties of a Passive Film and Reduction Reaction Rates

The films forming on pure chromium and Alloy C22 in aqueous borate buffer were investigated with potential polarization scans, electrochemical impedance spectroscopy, and Mott-Schottky analysis. The capacitances of both films were frequency dependent, and a constant phase element (CPE) best described the frequency dispersion. Cathodic polarization scans were used to assess the accuracy of flatband potential values calculated from Mott-Schottky tests using different models to extract an effective capacitance from a CPE. It was found that using effective capacitances calculated with an expression developed by Brug et al. [J. Electroanal. Chem., 176, 275 (1984)] led to flatband potentials that closely agreed with cathodic polarization scans. Both films were found to be n-type, and flatband potentials of -1.217 and -0.617 V vs standard hydrogen electrode were found for Cr and Alloy C22, respectively.

The influence of sulfate ions on the surface enhanced raman spectra of passive films formed on iron

Abstract Surface enhanced Raman (SER) spectra were obtained from the passive films formed on iron in aqueous solutions of sulfate (pH 5 and pH 10) andborate + sulfate. The results indicate that the identities of the passive films are dependent on the pH of the solution, the identity of the anions present and the potential of iron. Either Fe(OH) 2 , or a species similar to Fe(OH) 2 and which is produced by the oxidation of Fe(OH) 2 , was present in all the films including those formed in the active region of the sulfate solution at pH 5. In the passive state additional species were present but could not always be unambiguously identified. The passive films formed in the sulfate solutions of pH 5 and pH 10 appeared to contain a mixture of Fe 3 O 4 and γ-Fe 2 O 3 . In thesulfate + borate solution the passive film contained α-Fe 2 O 3 . The widths of the peaks in the SER spectra suggested that the constituents in the films were either microcrystalline or amorphous. In addition, in all solutions examined sulfate ions and, when present in the bulk solution, borate ions, appear to adsorb on the surface of the passive films. In the case of the films formed in the sulfate solution of pH 5, sulfate ions were covalently bonded in the films in a bidentate configuration.

A SERS investigation of the passive films formed on iron in mildly alkaline solutions of carbonate/bicarbonate and nitrate

Abstract In-situ surface enhanced Raman spectra were obtained for the passive films formed on iron in two different aqueous solutions: 0.3 M NaNO 3 (unbuffered pH = 10) and 0.15 M Na 2 CO 3 + 0.15 M NaHCO 3 (buffered pH = 10). The results indicate that the films formed in the nitrate and carbonate solutions are somewhat alike, independent of potential (over the range of −400 mV- + 100 mV), and consist of a mixture of species similar to microcrystalline/amorphous Fe(OH) 2 + Fe 3 O 4 /γ-Fe 2 O 3 . The results of the present and earlier studies indicate that the oxidation rate of iron is a strong function of the identity of the anion but that the passive film formed on iron in a mildly alkaline solution is nearly independent of the anion.

Obtaining Surface‐Enhanced Raman Spectra from the Passive Film on Iron

This paper reports on surface-enhanced Raman spectra (SERS) obtained from an iron electrode that contains on its surface a discontinuous layer of silver. The mechanism of Raman enhancement of this system has been explored through a systematic study of the time stability and the potential reversability of SERS spectra as well as the relation between the amount of silver deposited and the intensity of SERS. It is concluded that the local electromagnetic field mechanism is the dominant contributor to enhancement in this system. It is also shown that the anodic polarization characteristics of the ion electrode are unaffected by the presence of the discontinuous silver layer. Comparisons between in situ and ex situ, as well as in situ and semi in situ Raman spectra have indicated the importance of characterizing the anodic passive film by in situ techniques. The limitation and possible extensions of the SERS technique in studies of passivity are also discussed.

Corrosion of Aluminum Current Collectors in Lithium-Ion Batteries with Electrolytes Containing LiPF6

Aluminum current collectors for cathodes in 61 life-tested lithium-ion batteries were microscopically examined for evidence of corrosion. In addition, galvanostatic anodic polarization tests were conducted in LiPF 6 -containing battery electrolytes on samples of bare aluminum, of aluminum coated with a LiFePO 4 cathode, and of aluminum affixed with polyethylene crevices. The results indicate that aluminum current collectors are susceptible to corrosion, which most likely occurs as underdeposit corrosion at pores in the cathode.