Covalent Bonding of Organic Molecules to Cu and Al Alloy 2024 T3 Surfaces via Diazonium Ion Reduction

Cu surfaces and polished aluminum alloy 2024 T3 substrates were derivatized at open-circuit potential with aryl diazonium salts in both aprotic and aqueous media. Raman spectroscopy confirmed the presence of a derivatized film on the substrates before and after exposure to boiling water and sonication in acetone. Two different Cu substrate surfaces were prepared and used for X-ray photoelectron spectroscopy (XPS) analysis of the derivatization results. One surface was native oxide Cu, predominantly in the form of Cu_2O, and one surface was predominantly Cu^0. Results of the XPS analysis indicate the presence of both a Cu-O-C linkage and a Cu-C covalent bond between the aryl ring and the Cu substrate, and a high coverage of the organic layer. XPS results also indicate the formation multilayers on both types of Cu surfaces with different percentages of azo coupling within the multilayers on the two surfaces. Applications of a covalently bonded organic film on copper and alloy surfaces include adhesion promotion, corrosion protection, and possibly inhibition of oxygen reduction

Raman Spectroscopy of Monolayers Formed from Chromate Corrosion Inhibitor on Copper Surfaces

Surface enhanced Raman scattering (SERS) was used to observe interactions of dilute Cr^VI solutions with silver and copper surfaces in situ. Using silver as a model surface which supports strong SERS with a 514.5 nm laser, it was possible to observe Cr^III at the near monolayer level, and the spectra were compared to those from Cr^III oxyhydroxide species and Cr^III/Cr^VI mixed oxide. Similar experiments were conducted with Cu surfaces and 785 nm excitation. Upon exposure to Cr^VI solution, the characteristic Cu oxide Raman bands disappeared, and a Cr^III band increased in intensity over a period of ~20 h. The intensity of the Cr^III band on Cu became self-limiting after the formation of several Cr^III monolayers, as supported by chronoamperometry experiments. This Cr^III spectrum was stable after Cr^VI was removed from the solution provided the potential remained negative of –200 mV vs. Ag/AgCl. The results support the conclusion that Cr^VI is reductively adsorbed to Cu at the near neutral pH and open circuit potentials expected for Cu/Al alloys in field applications. The Cr^III film is stable and is a strong inhibitor of electron transfer in general and oxygen reduction in particular. An important mechanistic feature of Cr^III formation is the substitution lability of Cr^VI compared to Cr^III. The Cr^VI-O bond can be broken much more rapidly than the substitution inert Cr^III-O bond, making formation of Cr^III/Cr^VI mixed oxide kinetically favorable. Once reduced to Cr^III, however, the substitution inert oxyhydroxide film is much less labile. An important and central feature of Cr^VI as a corrosion inhibitor is its transformation via reductive adsorption from a mobile, substitution labile Cr^VI form to an insoluble, substitution inert Cr^III oxyhydroxide. Furthermore, Cr^VI reduction is likely to occur at cathodic sites previously responsible for oxygen reduction, which are then permanently blocked by a stable Cr^III film with a thickness of a few monolayers

Raman Spectroscopy for Chemical Analysis

Owing to its unique combination of high information content and ease of use, Raman spectroscopy, which uses different vibrational energy levels to excite molecules (as opposed to light spectra), has attracted much attention over the past fifteen years. This book covers all aspects of modern Raman spectroscopy, including its growing use in both the laboratory and industrial analysis.Includes bibliographical references and index.Owing to its unique combination of high information content and ease of use, Raman spectroscopy, which uses different vibrational energy levels to excite molecules (as opposed to light spectra), has attracted much attention over the past fifteen years. This book covers all aspects of modern Raman spectroscopy, including its growing use in both the laboratory and industrial analysis

Effects of Chromate and Chromate Conversion Coatings on Corrosion of Aluminum Alloy 2024-T3

Various effects of chromate conversion coatings (CCCs) and chromate in solution on the corrosion of AA2024-T3 and pure Al are studied in this work. Raman spectroscopy was used to investigate the nature of chromate in CCCs through a comparison with the spectra of known standards and artificial Cr(III)/Cr(VI) mixed oxides. Chromate was shown to be released from CCCs and to migrate to and protect a nearby, uncoated area in the artificial scratch cell. However, experiments investigating the effect of chromate in solution on anodic dissolution kinetics under potentiostatic control indicated that large chromate concentrations were needed to have an effect.This work was supported by Major H. DeLong at the United States Air Force Office of Scientific Research under contracts F49620-96-1-0479 and F49620-96-0042

Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings on Al Alloys Kinetics of Release

The release of chromate ions from chromate conversion coatings (CCCs) on Al alloys was studied, and the effect of aging of CCCs on the chromate release kinetics was investigated. Chromate release from CCCs into aqueous solutions was monitored by measuring the change in the chromate concentration in solution using UV-visible spectroscopy. Heat-treatment of the CCC greatly reduced the chromate release rate. The chromate release rate also decreased with increasing aging time at room temperature. A diffusion-control model was proposed based on the notion that the CCC in an aqueous solution is a porous, two-phase structure consisting of a solid phase with adsorbed Cr(VI) species that is in local Langmuir-type equilibrium with an interpenetrating solution phase. This model results in a concentration gradient of soluble Cr(VI) in the solution phase of the CCC as chromate is released. The concentration and diffusion coefficients of soluble Cr(VI) in CCC were estimated. The estimated diffusion coefficient tended to decrease with aging time, suggesting that the CCC is modified with aging time.This work was supported under Air Force Office of Scientific Research Multidisciplinary University Research Initiative contract no. F49620-96-1-0479

Effects of Surface Monolayers on the Electron-Transfer Kinetics and Adsorption of Methyl Viologen and Phenothiazine Derivatives on Glassy Carbon Electrodes

Five organic redox systems were examined in aqueous electrolytes on polished and chemically modified glassy carbon (GC), to evaluate the effects of surface structure on the heterogeneous transfer rate constant, k°. Methyl viologen reduction to its cation radical exhibited a voltammetric peak potential difference which was insensitive to surface modification, with k°decreasing by only 50% when a chemisorbed monolayer was present. Methylene blue and three other phenothiazines adsorbed to polished GC, but the adsorption was suppressed by surface modification. For all four phenothiazines, chemisorbed or physisorbed monolayers of electroinactive species had minor effects on k°, with a compact nitrophenyl monolayer decreasing k°by 50%. This minor change in k°was accompanied by a major decrease in adsorption, apparently due to inhibition of dipole-dipole or π-π interactions between the phenothiazine and GC. Chlorpromazine oxidation to its cation radical was studied in more detail, under conditions where adsorption was suppressed. A plot of the natural log of the observed rate constant vs the monolayer thickness for a variety of chemisorbed monolayers was linear, with a slope of -0.22 Å -1 . The observations are consistent with a through-bond electron-tunneling mechanism for electron transfer to all five redox systems studied. The tunneling constant for CPZ of 0.22 Å -1 is between that reported for electron tunneling through conjugated polyene spacers (0.14 Å -1 ) and that reported for phenyl-methylene spacers (0.57 Å -1 ), on the basis of long-range electron transfer in rigid molecules. Through a variety of efforts from many laboratories, significant progress has been made toward understanding the electrochemical behavior of widely used carbon electrodes. 1-7 Since sp 2 carbon surfaces are difficult to prepare reproducibly and are prone to degradation via oxidation and impurity adsorption, our understanding of the behavior of carbon electrodes has lagged that of metal electrodes, particularly mercury ones. This situation improved dramatically after more attention was paid to surface preparation and the number of uncontrolled surface variables was reduced. In particular, several landmarks indicating reproducible performance of sp 2 carbon electrodes, mainly glassy carbon (GC), have been achieved: 1. Determination of the rapid heterogeneous electron-transfer rate constants (k°), for outer-sphere systems (e.g., Ru(NH 3 ) 6 +3/+2 k°> 0.2 cm/s), 8,9 comparable to those observed on Au and Pt. 10 2. Preparation of low-oxide (O/C < 2%) carbon surfaces which retain their low oxide levels for at least one month in air. 11,12 3. Structural characterization of organic monolayers and submonolayers on carbon with Raman spectroscopy. [13][14][15] 4. Correlation of specific surface sites with electrocatalytic activity for various redox systems, including ascorbic acid, NADH, Fe 3+/2+ , etc. 8,[16][17][18][19] 5