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.
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.1 Various corrosion products, specific to the metallic substrates in the system, and the corrosive species present (anions, cations, acidic and basic salts, particulates, etc.) which interact with each other all vary in amounts and residence time. For the purposes of atmospheric corrosion, the electrochemical nature of the corrosion process requires the presence of an electrolyte, provided by the atmospheric precipitation or adsorption of water molecules on the surface of the metal. Adsorbed water layers, which can range from 15 to 90 atomic layers thick depending upon the relative humidity (RH), play a central role in supporting the electrochemical process.2