A lifetime-prediction approach to understanding corrosion the corrosion-fatigue and corrosion behavior of a nickel-based superalloy and a nanocrystalline alloy /

by Steward, Rejanah V.

Lifetime-prediction models are useful for simulating a material’s in-service behavior or outcome. Perhaps the greatest advantage of these models is the ability to use the predicted results to help optimize engineering designs and reduce costs. The Hastelloy® C-2000® superalloy is a single-phase material and face-centered cubic in structure at all temperatures. The C-2000® alloy is a commercially designed alloy manufactured to function in both reducing and oxidizing solutions. C-2000 vi ® is used as a fabrication material for heat exchangers, piping for chemical refineries, and storage repositories. The corrosion properties of C-2000® are excellent, and the ductility and fatigue properties are good. In this study, C-2000® is used to verify the theoretical basis of an electrochemical-micromechanical crack-initiation corrosion-fatigue model for materials under passive electrochemical conditions. The results from electrochemical and mechanical experiments, along with the findings from the conventional electron microscopy and a laser interferometer will be presented. A nanocrystalline Ni-18 weight percent (wt.%) Fe alloy is examined to investigate its electrochemical behavior in a 3.5 wt.% NaCl solution. Three Ni-18 wt.% Fe samples were annealed at 400ºC for 3, 8, and 24 hours (hrs.) to study the effects of grain sizes on the electrochemical properties of bulk Ni-18 wt.% Fe. The electrochemical results from the annealed samples are compared with those measured for the as-received Ni-18 wt.% Fe material consisting of an average grain size of 23 nanometers (nm). The samples annealed for times longer than 8 hrs. appear to have undergone an abnormal grain growth, where nanometer and micrometer (?m) grain sizes are present. Unlike the electrochemical results for the as-received material, the annealed nanocrystalline materials appear to be susceptible to localized corrosion. Consequently, these larger grains within the nanoncrystalline-grain matrix are preferentially attacked during electrochemical corrosion. Of the four materials studied, the as-received nanocrystalline alloy possesses the best corrosion properties, and the nanocrystalline material coarsened to an average grain size of 23 ?m has the poorest electrochemical properties. vii