The codoping of a heavy-metal fluoride glass by two rare-earth ions, erbium and ytterbium, permits the semiconductor laser infrared excitation of two visible neighboring fluorescence lines of erbium by upconversion, the intensity ratio of which is a single variable function of temperature.

The chemical constitution of the passive oxide films formed on aluminum in chromate and chromate/chloride solutions under simple immersion (at open-circuit potential) and under anodic polarization has been investigated by means of x-ray photoelectron spectroscopy. It has been shown that these films, whose thickness is comparable with the mean escape depth of photoelectrons, are composed of a mixture of hydrated chromium (lII) oxide, aluminum (III) oxide, and chromium (VI) specms. A considerable amount of chloride species has been detected in the films formed in chromate/chloride solutions, independently whether chlorides were initially present in the solution or were added only after 18 hr immersion of the sample in chromates. The extent of chloride incorporation within the oxide film from CRO42-/C1 -solution was not modified by the anodic polarization of aluminum from open-circuit potential up to critical pitting potential. This situation is quite different from that observed in chloride solutions, in the absence of chromates, where the amount of chlorides in ~he oxide film increases appreciably between Est and Ecr. The nature of the inhibiting effect of chromate ions on the pitting corrosion of aluminum is discussed on the basis of these results.Although numerous sttidies have been devoted to clarifying the mechanism by which chromate ions inhibit the corrosion of aluminum, the exact nature of this phenomenon is still under discussion. Edeleanu and Evans (1) first attributed the inhibiting effect of chromate solutions to a redox reaction occurring between CrO42-ions and aluminum metal to form aluminum oxide and Cr ~+ ions. Their suggestion was further examined by Pryor (2), and by Heine and Pryor (3), who found that 7-A12Oz films formed on aluminum by passivating chromate solutions exhibited lower ionic resistance and higher electronic resistance than amorphous films of comparable thickness produced by thermal oxidation. According to these authors, the inhibiting effect of chromate ions on the pitting corrosion of aluminum in chloride solutions can be explained by strong specific adsorption of divalent CrO42-ions on the film-solution interface, preventing simultaneous adsorption of monovalent C1-ions and their entry into the 7-A1~O3 lattice. This latter theory has since received indirect support in the investigation of BShni and Uhlig (4), who showed the existence of a linear relationship between the logarithm of the activity of C1-ions, and the logarithm of the activity of CrO4 uions required to shift the critical pitting potential of aluminum to an arbitrary noble value of 0.8V vs. NHE. Recently, Abd Rabbo et aI. (5) have studied the changes in the composition of anodically formed aluminum oxide films after immersion in chromate/dichromate solutions, using secondary ion mass spectrometry (SIMS). A layer of uniformly distributed Cr + ions 1 was detected in the outer regions of the films on immersed specimens, followed by localized Cr+-rich areas down to the metal-oxide interface. On the basis of these results ...

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