A Study of the Intergranular Cracking of U-7.5 wt% Nb-2.5 wt% Zr (Mulberry) Alloy in Aqueous Chloride Solutions

Abstract

A qualitative study was made of the electrochemical processes and chemical species involved in the intergranular stress corrosion cracking (SCC) of U-7.5 wt% Nb-2.5 wt% Zr (Mulberry) alloy in aqueous Cl− environments. Optical microscopy, pH analysis, electron microprobe analysis, and Auger spectroscopy were used to examine the SCC behavior of simple bend specimens. It was found that in the absence of reducible cations, crack initiation is preceded by an incubation period during which a differential aeration cell is established. Crack propagation is believed to proceed by alternation of anodic dissolution and brittle fracture processes at the crack tip. Hydrogen evolution from within the crack was observed during propagation and is believed to indicate that reduction of H+ ions to H2(g) is the rate-controlling cathodic process during crack propagation. Observed enrichment of Nb at the fracture faces is believed to indicate a preferential dissolution of U and Zr, leaving Nb rich oxide, a phenomenon consistent with the Pourbaix diagrams for these elements.