Resistance to Forms of Corrosive Attack

Galvanic Corrosion

A measure of the electrode potentials of a few uranium alloys was obtained in both seawater and 0.1 N HCL. These values are used to determine the potential for galvanic corrosion in similar environments, when in contact with dissimilar metals as covered in Section 2.2. Table 42 gives the values measured in the two environments.

Stress Corrosion Cracking

Stress corrosion cracking has been found to be problematic with uranium alloys. The U-0.75Ti alloy has the highest susceptibility, with SCC also occurring for the U-Mo and U-Nb alloys. The study of U-0.75Ti alloy in varying environments showed water to be the primary variable responsible for SCC with oxygen deterring SCC. The U-Mo alloys revealed susceptibility for Mo concentrations of 0.6-12%. From 0.6 to ~5%, metastable materials were produced containing the α-phase, showing greater susceptibility to SCC. Above 5%, oxygen is the primary variable responsible for SCC, just the opposite as for the U-0.75Ti alloy. Increasing carbon content in the U-Mo alloys also produces increased susceptibility. Heat treating quenched alloys produced a more equilibrium microstructure, proving to be less susceptible. Uranium-niobium alloys showed water induced susceptibility for the lower content Nb alloys (2.3 and 4.5%) and oxygen induced susceptibility for the higher content alloys (6 and 8%). Water vapor further increases the rate of attack for the U-6Nb and U-9Nb materials in an oxygen environment. The U-7.5Nb-2.5Zr alloy has been observed to form intergranular cracking which is easily propagated in the presence of oxygen, water, and chloride. Transgranular cracking has also been seen for U-7.5Nb-2.5Zr in oxygen environments, but propagates slowly. The standard aging temperature for this alloy is 150ºC, showing the slowest crack propagation rates.