Characterization of Dealloying and Associated Stress Corrosion Cracking: The Effect of Crystallographic Orientation

Selective dissolution (dealloying) of more reactive elements from a solid solution alloy can result in the formation of a nanoporous surface layer enriched in the more noble elements. For example, in Alloy 800 (wt.% Fe-32Ni-21Cr), a remnant nanoporous Ni-rich film is formed following the selective dissolution of Fe and Cr in boiling 50 wt.% NaOH caustic solutions (135-140°C). This surface film has been proposed to act as a precursor to stress corrosion cracking (SCC) by a cleavage mechanism. This study investigates the effect of crystallographic orientation on the characteristics of a dealloyed layer (i.e., thickness and size of ligaments/pores) formed on Alloy 800. Electron backscattered diffraction (EBSD) is used to correlate grain orientation with the geometry and morphology of the dealloyed layer formed in the boiling caustic environment. Results indicate that the dealloyed layer formed on grains with planes close to the {111} family on the sample surface, has a finer geometry (i.e., a thinner layer compared with other grain orientations and consisting of smaller diameter ligaments). Further SCC experiments on Alloy 800 specimens revealed a lower density of cracks initiated from surface planes that were close to {111}, confirming a link between dealloyed layer geometry, texture, and SCC.