Carlos A. Melendres

Composition and structure of the anodic films on titanium in aqueous solutions.

Surface-enhanced Raman spectra of the amorphous film on titanium have been obtained for the first time. The anodic corrosion film at low potentials is identified to be highly disordered TiO{sub 2} with some Ti{sub 2}O{sub 3} also present. The presence of the anatase and rutile modifications of TiO{sub 2} at high voltages ({ge}80 V) is confirmed

Surface enhanced Raman spectroelectrochemical studies of the corrosion films on iron and chromium in aqueous solution environments

“In situ” laser Raman spectra of the corrosion films on iron have been observed in aerated 5 M KOH and 0.15 M NaCl solutions via surface enhancement by the electrodeposition of a silver overlayer. Essentially the same spectra are observed in the two solutions as a function of applied potential in spite of a breakdown of passivity on iron in the chloride solution. Fe(OH)2 and Fe3O4 are found in the prepassive potential region while FeOOH is present in the passive region. A film which is very difficult to reduce appears to be always present on the electrode surface even at hydrogen evolution potentials; this film is believed to be α-FeOOH. Surface enhanced Raman spectra of the corrosion films on chromium have also been obtained in NaCl solution for the first time. The passive film has a composition that corresponds most closely to an amorphous form of Cr2O3, with some Cr(OH)3 also present. The film is converted in the transpassive region to a higher oxide form, presumably CrO2−4. Reversible reduction of this species to Cr2O3 is indicated.

An In Situ X‐Ray Absorption Spectroscopic Study of Charged Li ( 1 − z ) Ni ( 1 + z ) O 2 Cathode Material

We have measured in situ the Ni K-edge X-ray absorption spectra of Li (1-z) Ni (1+z) O 2 cathode material charged in a nonaqueous cell. The material was charged to various states of charge (i.e., Li content) which corresponded to x = 0.0, 0.12, 0.24, 0.37, 0.49, and 0.86 in Li (1-x-z) Ni (1+z) O 2 . We have determined variations in the Ni-O and Ni-Ni coordination numbers, bond lengths, and local disorders as well as the Ni K-edge energies as a function of Li content. We show that in the pristine state, the composition of the material can be described by the formula Li 0,86 Ni 1.14 O 2 (i.e., x = 0 and z = 0.14). That is, the material consists of Ni 2+ (25%) and Ni 3 + (75%) with half the Ni 2+ atoms residing in Li sites and the other half in the NiO 2 slabs. Upon charging, initially Ni 2+ is oxidized to Ni 3+ up to a state of charge which corresponds to x = 2z. Upon further charging to states corresponding to 2z < x ≤ 1 - z, Ni 3+ is oxidized to Ni 4+ with fractions being dependent on the values of x and z. Analysis of the edge energies for NiO, stoichiometric LiNiO 2 , and KNiIO 6 as reference compounds for Ni 2+ , Ni 3+ , and Ni 4+ , respectively, shows a quadratic dependence for edge energy vs. oxidation state. This type of correlation is consistent with variations observed in earlier studies for some Mn reference compounds in the same range of oxidation states. Oxidation-state determination of Ni in Li (1-x-z) Ni (1+z) O 2 as a function of state of charge (i.e., Li content or x) on the basis of edge energies yielded results which are in excellent agreement with oxidation state determinations made on the basis of the mole fractions for Ni 2+ , Ni 3+ , and Ni 4+ extracted from extended X-ray absorption fine structure spectra.

An in situ X-ray absorption spectroscopic study of charged Li{sub (1{minus}z)}Ni{sub (1+z)}O{sub 2} cathode material

We have measured in situ the Ni K-edge X-ray absorption spectra of Li (1-z) Ni (1+z) O 2 cathode material charged in a nonaqueous cell. The material was charged to various states of charge (i.e., Li content) which corresponded to x = 0.0, 0.12, 0.24, 0.37, 0.49, and 0.86 in Li (1-x-z) Ni (1+z) O 2 . We have determined variations in the Ni-O and Ni-Ni coordination numbers, bond lengths, and local disorders as well as the Ni K-edge energies as a function of Li content. We show that in the pristine state, the composition of the material can be described by the formula Li 0,86 Ni 1.14 O 2 (i.e., x = 0 and z = 0.14). That is, the material consists of Ni 2+ (25%) and Ni 3 + (75%) with half the Ni 2+ atoms residing in Li sites and the other half in the NiO 2 slabs. Upon charging, initially Ni 2+ is oxidized to Ni 3+ up to a state of charge which corresponds to x = 2z. Upon further charging to states corresponding to 2z < x ≤ 1 - z, Ni 3+ is oxidized to Ni 4+ with fractions being dependent on the values of x and z. Analysis of the edge energies for NiO, stoichiometric LiNiO 2 , and KNiIO 6 as reference compounds for Ni 2+ , Ni 3+ , and Ni 4+ , respectively, shows a quadratic dependence for edge energy vs. oxidation state. This type of correlation is consistent with variations observed in earlier studies for some Mn reference compounds in the same range of oxidation states. Oxidation-state determination of Ni in Li (1-x-z) Ni (1+z) O 2 as a function of state of charge (i.e., Li content or x) on the basis of edge energies yielded results which are in excellent agreement with oxidation state determinations made on the basis of the mole fractions for Ni 2+ , Ni 3+ , and Ni 4+ extracted from extended X-ray absorption fine structure spectra.

Kinetics of Electrochemical Incorporation of Lithium into Aluminum

The electrochemical incorporation of lithium into aluminum was investigated by chronocoulometric, galvanostatic, and potentiostatic techniques. Results of the chronocoulometric measurements were interpreted in terms of the theory developed by Astakhov. Incorporation of lithium to form the solid solution ..cap alpha..-phase is limited by its rate of diffusion into aluminum, with a measured diffusion coefficient of about 4 x 10/sup -10/ cm/sup 2//sec at 450/sup 0/C. Implantation at potentials where a layer of intermetallic compound forms on the electrode surface was found to be limited initially by the reaction to form the compound LiAl (..beta..-phase) and subsequently by the rate of lithium diffusion into the growing ..beta..-phase layer. The results of galvanostatic transient measurements indicate a diffusivity of lithium in ..beta..-LiAl of the order of 10/sup -8/ cm/sup 2//sec and a fast charge transfer step. The initial steps in the formation of the intermetallic compound appear to involve typical electrocrystallization (i.e., nucleation and growth) processes.

Surface‐Enhanced Raman Spectroelectrochemical Studies of Corrosion Films on Iron in Aqueous Carbonate Solution

The corrosion films on iron in aqueous carbonate/bicarbonate solutions were studied as a function of concentration, pH, and temperature by using the technique of surface-enhanced Raman spectroscopy with electrodeposited silver. Both Fe(OH){sub 2} and Fe{sub 3}O{sub 4} were detected in the surface oxide film at prepassivation potentials. Iron carbonate (siderite) was observed in the corrosion film in a limited pH, temperature, and applied potential range. At lower carbonate/bicarbonate concentration (e.g., 0.01 M), passivation was lost, and continuous anodic dissolution of the iron occurred. The cathodic reduction of the corrosion film was enhanced at 75 C.

Characterization of Slightly Hydrated Ni(OH)2 by XPS

We report x‐ray photoemission spectra (XPS) of slightly hydrated Ni(OH)2. XPS spectra were measured with the Physical Electronics Model 5400 x‐ray photoelectron spectrometer using unmonochromatized Mg Kα x rays at two pass energy settings corresponding to analyzer energy resolutions of 1.34 and 0.54 eV. We present the survey spectrum (binding energy range of 0–1100 eV) measured at an analyzer energy resolution of 1.34 eV. Multiplexes of the C, O, and Ni photoemission lines, valence band region, as well as Ni LVV Auger lines were measured at an analzyer energy resolution of 0.54 eV. The slightly hydrated Ni(OH)2 sample was prepared by reacting stoichiometric Ni(NO3)2 with KOH in CO2 free H2O at 35 °C.

Electrochemical and Laser Raman Spectroscopy Studies of Stainless Steel in 0.15M NaCl Solution

This paper reports on potentiodynamic polarization curves measured by Type AISI 304 and 316 stainless steels in 0.15M NaCl solution at 4, 20, and 40[degrees]C. The pitting potentials decreased with increasing temperature. A positive effect on the inhibition of passivity breakdown was found in the presence of molybdenum. Surface enhanced Raman spectroscopy (SERS) was carried out on AISI 316 stainless steel to identify the species present on the electrode surface as a function of potential. Results indicate the corrosion films to be highly disordered and most likely to consist of a mixture of the oxides and hydroxides of the component elements of the stainless steel. The potential dependence of the spectra may reflect the behavior of iron, which is the most abundant component in the alloy and most probably in the film.

On the Breakdown of Passivity of Iron by Thiocyanate

In this paper, the authors report that small amounts (0.001 to 0.02M) of SCN{sup {minus}} significantly increased the rate of anodic dissolution of copper and iron; SCN{sup {minus}} causes a breakdown of the passivity of iron but not of copper. The authors communicate results of our studies using the surface enhanced Raman (SER) effect in order to further elucidate the passivity of iron and its breakdown by thiocyanate.

X-ray off-specular reflectivity studies of electrochemical pitting of Cu surfaces in sodium bicarbonate solution

Abstract We have studied the electrochemically-induced pitting process on a Cu electrode in NaHCO3 solution using in-situ X-ray off-specular reflectivity measurements. The morphology and growth dynamics of the localized corrosion sites or pits were studied as the applied potential was varied from the cathodic region where the Cu surface is relatively free of oxide films to the anodic region where surface roughening occurs by general corrosion with concomitant formation of an oxide film. Quantitative analysis of the experimental results indicates that early pitting proceeds in favor of nucleation of pit clusters over individual pit growth. It was found that the lateral distribution of the pits is not random but exhibits a short-range order as evidenced by the appearance of a side peak in the transverse off-specular reflectivity. The position, height, and width of the peak was modeled to yield the average size, nearest-neighbor distance (within any one of the clusters), and over-all density of the pits averaged over the entire illuminated surface. In addition, measurements of the longitudinal off-specular reflectivity indicate a bimodal depth distribution for the pits, suggesting a “film breaking” type of pitting mechanism.