Belinda Hurley

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 2 O, 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.

Characterization of chromate conversion coating formation and breakdown using electrode arrays

concentrations in the coating continued to increase during this electrochemically quiescent period, suggesting continued CCC evolution. Conversion-coated arrays were subject to anodic potentiodynamic polarization in 0.5 M NaCl until all elements on the array exhibited coating breakdown and substrate pitting. Breakdown potentials were found to increase with coating time up to 120 s, indicating anodic inhibition in CCC corrosion protection. Breakdown was found to be more difficult on electrodes that were net cathodes during coating formation. Results also showed that the NaF and K3Fe~CN!6 in commercial CCC bath formulations strongly contributed to coating corrosion resistance. Without Fv, the Al surface passivated quickly during coating formation, and a nonprotective film formed. Without Fe~CN!3 62 , CCCs exhibited lower breakdown potentials.

Raman Spectroscopy Characterization of Aqueous Vanadate Species Interaction with Aluminum Alloy 2024-T3 Surfaces

Raman spectroscopy and electrochemical techniques were used to characterize the interactions of aqueous NaVO 3 /NaCl and NH 4 VO 3 /oxalic acid with AA 2024-T3. The interaction of aqueous NaVO 3 with Cu 0 and Cu 2 O was characterized. At potential values similar to the OCP of AA 2024-T3 in dilute NaCl, aqueous NaVO 3 formed a polyvanadate film on Cu 2 O and formed little or no vanadate film on Cu°. Treatment of AA 2024-T3 with basic, aqueous NaVO 3 /NaCl resulted in a polyvanadate film on copper-rich intermetallic particles and the formation of monovanadates on the matrix. Treatment of AA 2024-T3 with acidic, aqueous NH 4 VO 3 /oxalic resulted in the formation of monovanadates on the matrix and provided no evidence of vanadate species on copper-rich particles. AA 2024-T3 samples pretreated with either aqueous vanadate salt solution displayed modest cathodic inhibition soon after treatment but inhibition degraded with aging. The formation of polymerized vanadates species on copper-rich particles supports the cathodic inhibition mechanism. The presence of vanadate species on copper-rich particles pretreated with aqueous NaVO 3 /NaCl containing predominantly tetrahedral vanadates versus the lack of evidence for similar species on particles treated with aqueous NH 4 VO 3 /oxalic acid containing predominantly octahedral vanadates supports the importance of tetrahedrally coordinated vanadate species for corrosion inhibition.

Encapsulation of NaVO3 as Corrosion Inhibitor into Microparticles and its Active Corrosion Protection for AA2024 Based Upon Inhibitor Control Release

The present work provides a one-step method of encapsulating a corrosion inhibitor, NaVO3, relevant to protection of AA2024-T3, into hollow microparticles. By dispersing these microparticles into a PVB coating, the encapsulated NaVO3 may be continuously released into the electrolyte and heal the corroded sites spontaneously. The improvement of corrosion inhibition by NaVO3 microencapsulation described herein was validated by electrochemical methods and salt spray/immersion test.