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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.
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.
Geothermal Energy is currently engineered as an “always on” baseload supply, due to the limited flexibility to throttle the well without scaling and fatigue issues, and it is engineered for maximal efficiency at this output level. Scaling is a major problem in geothermal plants, particularly in cases where the geothermal fluid composition and plant operation make it difficult to control scaling. In such areas, particularly where scale inhibitors cannot be employed, the formation of scales can make the process less efficient and in extreme cases can lead to unexpected shutdown.
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Measuring the severity of corrosion on a specific alloy is often accomplished via mass loss using ASTM G-1. These processes work well and provide high fidelity data for many materials, especially steels. However, recent internal findings and disclosures from other research groups have highlighted a potential issue with using mass loss techniques to measure the damage on some aluminum alloy surfaces.
UNS S209101, also known as XM-19 by ASTM A2762, is a nitrogen-strengthened austenitic stainless steel with high strength and excellent corrosion resistance. Besides nitrogen (N) it also contains higher amounts of chromium (Cr), nickel (Ni), manganese (Mn), and a similar molybdenum (Mo) content compared with UNS S31603, as well as small additions of niobium (Nb) and vanadium (V). High contents of Cr, Mo and N confer this stainless steel high localized corrosion resistance. Mo, Mn and Cr increase the nitrogen solubility in iron alloys.