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Characterization Of The Surface Chemistry At Corroded And Non-Corroded Sites On Aluminum Alloy 7075-T6 Samples Exposed At Coastal Locations

Knowledge of the localized corrosion environment on a metal substrate can provide the critical link between atmospheric data and corrosion morphology and can enable the formation of a framework to predict service life as a function of environment. Over the last few decades the analytical characterization of bare metal surfaces undergoing atmospheric corrosion has improved, resulting in a more complete understanding and consideration of the environmental parameters involved. However, the corrosion processes and the role that the environmental parameters play in what is a multiphase system is rather complex involving chemical reactions and equilibria, ionic transport phenomena, and gaseous, aqueous and solid phases.

Product Number: 51322-17642-SG
Author: Douglas C. Hansen, David Borth, Farrah Cole
Publication Date: 2022
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Replicate samples of bare aluminum alloy AA7075-T6 were exposed at three coastal atmospheric test sites: Kennedy Space Center (KSC) FL, US Naval Research Laboratory in Key West, FL (NRL-KW), and Daytona Beach, FL. The samples were cross sections of rod stock mounted in standard two-part epoxy metallurgical mounts and wet polished with isopropanol to 600 grit finish. The samples were installed on atmospheric exposure racks and retrieved at intervals of 3, 6, 9, and 12 months. Elemental composition of baseline (non-exposed) and exposed samples were measured using a Zeiss EVO-50XP Environmental Scanning Electron microscope equipped with a EDAX Genesis 2000 energy dispersive X-ray spectroscopy (EDS) system. Pitted and non-pitted sites on each sample were analyzed for compositional elements of the alloy as well as non-compositional elements (i.e. environmentally-derived). It was determined that the deposition of elements in pitted locations on the specimens occurred at concentrations of 200% to 800% to that of major ions present in natural seawater. The deposition and concentration of these environmentally derived elements on the metal surface vary as a function of exposure site and length of exposure time.

Replicate samples of bare aluminum alloy AA7075-T6 were exposed at three coastal atmospheric test sites: Kennedy Space Center (KSC) FL, US Naval Research Laboratory in Key West, FL (NRL-KW), and Daytona Beach, FL. The samples were cross sections of rod stock mounted in standard two-part epoxy metallurgical mounts and wet polished with isopropanol to 600 grit finish. The samples were installed on atmospheric exposure racks and retrieved at intervals of 3, 6, 9, and 12 months. Elemental composition of baseline (non-exposed) and exposed samples were measured using a Zeiss EVO-50XP Environmental Scanning Electron microscope equipped with a EDAX Genesis 2000 energy dispersive X-ray spectroscopy (EDS) system. Pitted and non-pitted sites on each sample were analyzed for compositional elements of the alloy as well as non-compositional elements (i.e. environmentally-derived). It was determined that the deposition of elements in pitted locations on the specimens occurred at concentrations of 200% to 800% to that of major ions present in natural seawater. The deposition and concentration of these environmentally derived elements on the metal surface vary as a function of exposure site and length of exposure time.

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