Characterization of internal oxidation and permeability of oxygen in Pd/Al alloys using hydrogen isotherms

• Published in: Scripta materialia 1996-03, Vol.34 (6), p.863-868
• Main Authors: Noh, H., Flanagan, Ted B., Balasubramaniam, R., Eastman, J.A.
• Format: Article
• Language: English
• Subjects: Applied sciences; Corrosion; Corrosion mechanisms; Exact sciences and technology; Metals. Metallurgy
• ISSN: 1359-6462; 1872-8456
• DOI: 10.1016/1359-6462(95)00614-1

The kinetics of hydrogen absorption and desorption are much faster for samples containing Al sub 2 O sub 3 ceramic precipitates than for pure palladium or the unoxidized Pd sub 0.97 Al sub 0.03 alloy; this is attributed to a fine microcrack network which accompanies hydrogen absorption into the oxidized alloys. After incomplete oxidation, the isotherms measured at 323 K correspond to a sample consisting of one fraction of the alloy depleted of Al, i.e., Pd, and the other to the unoxidized Pd sub 0.97 Al sub 0.03 alloy. After oxidation at 1273 K for 24 h, the latter disappears and the resulting isotherm for the metal matrix containing Al sub 2 O sub 3 precipitates corresponds very closely to that of pure Pd. The hydrogen absorption isotherms clearly show that internal oxidation proceeds by the progressive inward movement of an internal oxidation zone and this is directly confirmed from optical photomicrographs of the interfacial penetration. The measured isotherms and the optical measurements of the interface penetration permit the degree of interface penetration to be calculated. The progression of the internal oxidation zone obeys parabolic kinetics. The permeabilities of oxygen are found to be much higher in Pd-Al sub 2 O sub 3 than in pure Pd although the activation energies appear to be comparable. From penetration rates at four different temperatures, the activation energy for the permeability of oxygen has been determined as 210 kJ/mol O.