Secondary ion mass spectrometry (SIMS) has been used in conjunction with isotopic labeling to determine the extent and rate of passive film hydration on aluminum. The rates at which oxygen- and hydrogen-containing species migrate through the film have been determined as a function of temperature and applied potential (cathodic and anodic polarization). The results suggest that defects such as hydroxide ions are prevalent and mobile in the oxide film, influencing the kinetics and mechanisms of corrosion and pitting processes.

The use of advanced graphite fiber polymeric matrix composites in multimaterial assemblies which must maintain mechanical integrity in aqueous environments has initiated interest in the galvanic degradation of these materials. Of recent concern is the galvanic degradation of bismaleimide (BMI)/graphite fiber (GE) composites. This study has used electrochemical impedance spectroscopy to monitor material interfacial changes in an 8 ply, 00, unidirectional BMI-GF composite subjected to cathodic and anodic polarization in NaC1 solution, exposure to caustic solutions, and galvanic coupling with various metals in NaC1 solution (Al, Fe, Cu, Ti). Cathodic polarization was found to produce porous electrode behavior which was attributed to breakdown of the fiber/matrix interface and subsequent moisture ingress. This is in contrast to caustic exposure which did not show a porous electrode response, presumably due to general BMI solvation. Results indicate that reaction intermediates generated during the oxygen reduction reaction, e.g., peroxide and superoxide radicals, and not OH-ions are the key damaging species. This will have important implications on the development of more damage resistant polymer chemistries. Couples to titanium, previously believed to be a benign couple, caused changes in the impedance spectra similar to those for low cathodic overpotentials. The long-term implications of these changes on composite durability are not known at this time, and warrant further evaluation. InfroductionThe high specific strength, specific modulus, and toughness of graphite fiber-polymer matrix composites (GFPMC) have made this material an important, if not optimum, candidate for certain structural applications in the aerospace, marine, and automotive industries. The increased use of PMCs as critical structural components

High-purity aluminum samples were implanted with 35 keV Cl ϩ then polarized in both Cl Ϫ -containing and Cl Ϫ -free electrolytes in order to ascertain corrosion behavior as a function of Cl Ϫ content in the oxide. Implant fluence between 5 ϫ 10 15 and 2 ϫ 10 16 Cl ϩ cm Ϫ2 resulted in little or no localized attack. Implant fluences of 3 ϫ 10 16 and 5 ϫ 10 16 Cl ϩ cm Ϫ2 resulted in significant pitting in a Cl Ϫ -free electrolyte with the severity scaling as a function of implant fluence. The low variability in the pitting behavior of the 5 ϫ 10 16 Cl ϩ cm Ϫ2 sample suggests that this implant dosage results in a critical Cl Ϫ concentration in the oxide for pit nucleation. The passive current density (i pass ) decreased with increasing implant fluence. A space-charge effect is proposed to account for this phenomenon, although effects from defect interactions and possible oxide thickening are still under consideration.

A statistical description of pit initiation in high-purity aluminum ͑99.99͒ is developed based on the statistical frameworks established by Baroux ͓Corros. Sci., 28, 969 ͑1988͔͒ and Shibata and Takeyama ͓Corrosion, 33, 243 ͑1977͒ and 52, 813 ͑1996͔͒. The statistical treatment is used to examine the dependency of pitting behavior on electrode area, scan rate, and chloride concentration. It is shown that the average pitting potential is proportional to the log of the scan rate and the log of one over electrode area, implying an exponential dependence of pit germination rate on applied potential. The constants that describe pit germination rate are independent of area for electrodes of diameter 125 m and smaller. However, the constants are area-dependent for larger electrodes. Based on these results a homogeneous pit nucleation process is proposed to account for pit initiation on 125 m diam and smaller electrodes, while a heterogeneous process is proposed to control the behavior of 500 m diam electrodes. The results from these studies also demonstrate the importance of correctly choosing experimental conditions when studying pitting behavior. The dependencies of pitting potential on scan rate and area are found to be sensitive to electrode size, scan rate, and aggressiveness of the electrolyte.

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