g ABSTRACTEllipsometry was used to investigate the effects of temperature and potential on the growth kinetics of passive films on iron exposed to a pH 8.6 borate buffer solution. It was found that over the temperature range of 0~176 the growth kinetics could be described with an equal degree of confidence by either logarithmic, inverse logarithmic, or a modified form of inverse logarithmic kinetics. None of the existing models for film growth were found to be completely consistent with the temperature and potential dependencies of the growth constants.
The corrosion properties of friction-stir-welded (FSW) AA7050-T7651 have been investigated. Immersion in a modified exfoliation corrosion (EXCO) solution showed that the grain boundaries in the nugget, partially recrystallized zone (PRZ), and heat-affected zone (HAZ) were sensitized. The most heavily sensitized region was the nugget/PRZ interface. Pitting potentials were determined potentiodynamically in 0.6 M sodium chloride (NaCl). The nugget had the lowest pitting potential, which was 75 mV less than the parent material. The pitting potential of the HAZ was 50 mV less than the parent material. In results from the slow strain rate test, the percent elongation in 0.6 M NaCl for the slowest strain rate used was ~20% that in air, indicating susceptibility to stress corrosion cracking. Metallography showed that the fracture was intergranular and that the crack path was on the nugget side of the nugget/PRZ interface. Analytical electron transmission microscopy (ATEM) does not conclusively identify a grain boundary, precipitate-free zone, or precipitate chemistry, which correlates with sensitization. These chemistries are complex and vary with distance from the center line of the weld.
Auger electron spectroscopy has been used to investigate the influences of pH, Cl− addition, and potential on the composition of passive films on Fe‐Cr‐Mo ferritic stainless steels. It was found that the improved corrosion resistance properties obtained by the addition of molybdenum cannot be explained by its enrichment of the protective film. Molybdenum was not found in the film even at those potential and pH values at which its oxide is the stable species. However, the relative iron and chromium content in the film followed predictions based on their solubilities in aqueous solutions. These results also indicate changes in bonding in the film, as a function of distance from the substrate.
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