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Contu F.,University of Texas Health Science Center at Houston | Taylor S.R.,University of Texas Health Science Center at Houston | Taylor S.R.,National Corrosion Center
Electrochimica Acta | Year: 2012

The mechanism of rhenium-nickel electrodeposition on copper substrates from concentrated citrate baths was studied by examining the cathodic behavior of the bath components independently. It was shown that citric acid strongly adsorbed on the copper substrate inhibiting electron exchange involving foreign redox systems. Additionally, it was proposed that the rate determining step of nickel reduction was the dissociation of the nickel-citrate complexes. Such dissociation reactions were catalyzed by the hydrogen evolution reaction. A mechanism was proposed to explain the observation that the co-deposition of nickel increases the Faradic efficiency of rhenium electroplating. © 2012 Elsevier Ltd. All rights reserved. Source


Contu F.,University of Texas Health Science Center at Houston | Fenzy L.,Techspace Aerospace SA | Taylor S.R.,University of Texas Health Science Center at Houston | Taylor S.R.,National Corrosion Center
Progress in Organic Coatings | Year: 2012

FT-IR spectra of polyamide cured epoxy films were collected after exposure to electrolytes containing the ions found in blister solutions on AA2024-T3 substrates, and oxidants in order to study the stability of the resin materials over time. The epoxy films displayed excellent chemical stability to all environments tested. However, analysis of the change in the OH and NH stretching bands (3200-3500 cm -1) as a function of the electrolyte composition suggested that the epoxy coating behaved as a cation-exchange membrane with specific selectivity for different cations. It was proposed that the uptake of solvated protons is less favorable in the presence of competing cations such as Cu(II), Zn(II) or Al(III). Additionally, the interaction between the polymer and the cations is explained by the presence of chelating ligands constituted by nitrogen and oxygen moieties separated by two carbon atoms within the structure of the cured epoxy resin. © 2012 Elsevier B.V. All rights reserved. Source


Doostdar N.,National Corrosion Center | Doostdar N.,Rice University | Manrique C.J.,National Corrosion Center | Manrique C.J.,Rice University | And 4 more authors.
Journal of Biomedical Materials Research - Part A | Year: 2011

Calcific band keratopathy (CBK) is a degenerative condition resulting in the deposition of calcium salts in the superficial layers of the cornea and causing significant visual disturbance and pain of the affected eye. Unfortunately, the amount of CBK precipitates recovered from the affected eye is very small therefore; it would be beneficial to prepare a synthetic material mimicking CBK material to further the development of therapeutics. Analyses of biological samples recovered from patients show the presence of silicon in addition to calcium, as well as a distinctive fused spherical morphology. This prompted us to study the reaction of various sources of silicon (fumed silica, silicic acid, and silicone oil) with CaCO 3 under a range of reaction conditions to gain an understanding of the formation of CBK. A silicon source alone was not found to be responsible for the fused spherical morphology, and a third component, a polar surfactant-like molecule such as sodium dodecyl sulfate or tetradecylphosphonic acid, was also required. The effects of silicon:calcium ratio and reaction time have been studied. The reaction of fumed silica with CaCO 3 in presence of sodium dodecyl sulfate results in the formation of spherical shapes resembling the structures and chemical composition observed in the eye samples, while no such structures were observed in the absence of silicon. Samples closely resembling human samples were also formed from the reaction of silicone oil with CaCO 3 in the presence of tetradecylphosphonic acid. Samples were characterized by SEM, XRD, and XPS and Raman spectroscopy. Copyright © 2011 Wiley Periodicals, Inc. Source


Taylor S.R.,National Corrosion Center | Contu F.,National Corrosion Center | Calle L.M.,NASA | Curran J.P.,ASRC Aerospace Corporation | Li W.,ASRC Aerospace Corporation
Corrosion | Year: 2012

The pace of coatings development is limited by the time required to assess their corrosion protection properties. This study takes a step forward from past studies and correlates the corrosion performance of protective coatings assessed by a series of short-term electrochemical measurements with 18 month and 60 month beachside atmospheric exposure results of coated panels. A series of 11 coating systems on A36 steel (UNS K02600) substrates were tested in a blind study using the damage tolerance test (DTT). In the DTT, a through-film pinhole defect is created, and the electrochemical characteristics of the defect are then monitored over the next 1 to 7 days while immersed in 0.5 M sodium chloride (NaCl). The results from open-circuit potential, anodic potentiostatic polarization, and electrochemical impedance spectroscopy tests were used to characterize the corrosion behavior of the coating systems. The beachside exposure tests were conducted at the Kennedy Space Center (Kennedy Space Center, Florida) according to ASTM D610-01. It was found that the short-term changes in the open-circuit potential and in the charge delivered during a sequence of potentiostatic pulses showed high correlation and even higher correlation to 60 month beachside atmospheric exposure with correlations approaching 100%. © 2012, NACE International. Source


Taylor S.R.,National Corrosion Center | Contu F.,National Corrosion Center | Santhanam R.,Maxwell Technologies | Suwanna P.,Khon Kaen University
Progress in Organic Coatings | Year: 2012

The spatial distribution of pathways by which ions enter and move through organic coatings was examined with the use of molecular fluoroprobes. A bis-phenol-A epoxy coating (10-15 μm thick) on titanium was anodically polarized in an electrolyte containing an activated fluoroprobe. Confocal scanning laser microscopy (CSLM) was used at an excitation/emission of 580/602 nm to characterize the penetration pattern of the Ca 2+-X-Rhod-1 fluoroprobe. Both discrete fluorescence sites and regional fluoroprobe uptake were observed within a single epoxy coating. The chemical and/or physical reasons for the increased susceptibility of ionic species at these sites and regions are not yet known, but now provide focus to the local material sources for loss of barrier properties. This proof-of-concept experiment provides a means to attain unambiguous evidence for the mode of ion entry and passage through organic coatings. These experiments open a new avenue for the characterization of organic coatings. © 2011 Elsevier B.V. All rights reserved. Source

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