The dissolution rate and Cu surface enrichment of sputter-deposited Al 2 CuMg thin films exposed to 0.5 M NaCl solutions and chromate conversion coating solutions has been characterized. Dissolution rates were measured using an electrochemical quartz crystal microbalance, and Cu surface enrichment was characterized by energy-dispersive spectroscopy ͑EDS͒. Surface morphology and Volta potential were characterized by scanning probe microscopy. In 0.5 M NaCl solutions, Al 2 CuMg dissolves rapidly by dealloying, forming a passivating Cu-rich surface layer. Upon layer breakdown a dissolution rate of about 0.08 g/cm 2 s was observed. In chromate conversion coating baths, Al 2 CuMg demonstrated a net mass loss rate, which included mass loss from dissolution and mass gain from film formation. Cu surface enrichment was not detected after conversion coating. By comparison, pure Al thin films exhibited a net mass gain under similar conditions. Al 2 CuMg samples that were first chromate conversion coated and then exposed to 0.5 M NaCl solution exhibited low dissolution rates, 0.001 g/cm 2 s, suggesting that the film formed in the conversion bath was somewhat protective. EDS showed that Cu enrichment occurred on conversion-coated Al 2 CuMg during exposure to 0.5 M NaCl, indicating that conversion coatings on Al 2 CuMg cannot completely suppress Cu surface enrichment, Cu redistribution, and deposition corrosion phenomena.

The bioceramic shell of the Eastern oyster was investigated to determine its physical and chemical compositions. Resistance and electrolyte conductivity measurements through the shell using a four-electrode cell configuration were made. The scanning electron microscopy/energy-dispersive spectroscopy analysis of oyster shell cross sections revealed that this multilayered biocomposite material is composed of numerous incongruent “chalky” and “nonchalky” layers, each containing similar chemical elements but having different overall chemical compositions and porosities, with the chalky layers being quite porous and voluminous, whereas the nonchalky layers appear to be nonporous, more dense, and compact. The electrolyte conductivity within the pores of the shell was dependent on the ionic concentration of the electrolyte, suggesting that the tortuosity across the shell material via the various chalky and nonchalky layers has an effect on the transport of ions. Rather, consideration of the data in terms of the space charge model indicates that the surface charges within the porous biocomposite facilitate the conduction of ions, much like a ceramic membrane. These findings suggest that the oyster shell may function not only as a protective physical structure, but also as a ceramic membrane allowing for the exchange of ions from the external environment.

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Thin film compositional analogs of the intermetallic compound , Mg͑Zn,xCu͒ 2 ͑0 ഛ x ഛ 35 atom %͒, found in high strength Al-Zn-Mg-Cu alloys were fabricated using a flash evaporation technique for characterization of electrochemical behavior and for chromate conversion coating formation experiments. In potentiodynamic polarization measurements in deaerated 0.1 M NaCl, the open-circuit and breakdown potentials increased and dissolution rates decreased with increasing Cu content of the analogs. When the analogs were chromate conversion coated from a ferricyanide-accelerated coating bath, Raman spectra showed that coating thicknesses decreased with increasing Cu content. Other coating formation experiments showed that ferricyanide additions to the coating bath acted as a coating formation accelerator for MgZn 2 . However, coating formation was so strongly inhibited for the Cu-bearing analogs, that the effect of ferricyanide on coating formation was indiscernible.MgZn 2 is an important strengthening phase in Al-Zn-Mg-Cu alloys. This phase forms by nucleation and growth processes during artificial aging of a supersaturated solid solution ͑SSSS͒ on grain boundaries and within grains resulting in a uniform, high number density dispersion of fine precipitates. The precipitation sequence is 1-11where GP denotes Guinier-Preston zones. This precipitation process contributes greatly to alloy strengthening. Larger precipitates form on grain boundaries than in the matrix and attempts to measure grain boundary precipitate chemistry show that the phase contains Cu and perhaps Al as aging progresses from peak to overaged tempers. While precise chemistries have been difficult to obtain, the solubility for Cu and Al is so extensive, that MgZn 2 and AlCuMg are isostructural. 2,12 It is observed that localized corrosion susceptibility of Al-ZnMg-Cu alloys changes significantly from peakaged to overaged tempers. The overaged temper exhibits lower pitting potentials, but is less susceptible to intergranular corrosion. Distinctive differences in localized corrosion morphologies mainly related to the occurrence of intergranular corrosion have also been noted. [13][14][15][16] The electrochemistry of compositional analogs of compounds ranging in chemistry from MgZn 2 to near AlCuMg has been characterized using polarization techniques. 17 Results show that while the phase remains electrochemically active with respect to polarization behavior of Al-Zn-Mg-Cu alloys of commercial composition, increasing Cu content in the phase increases the corrosion and pitting potentials and decreases the passive current density. These effects are most pronounced when the Cu content exceeds 20 atom %. Additions of Al up to 10 atom % have a comparatively minor effect on the polarization response. While the change in intergranular corrosion susceptibility of Al-Zn-Mg-Cu alloys and ennoblement of -phase dissolution kinetics with increase aging has been noted, there is no broad agreement on a causal relationship. [13][14][15]18 Alloy temper also affects the form...