Oxidation of Al-containing austenitic stainless steels as related to the formation of strong glass-ceramic to metal seals

Abstract

In glass-ceramic to metal seals used in pyrotechnic actuators and ignitors, Ni-based alloys and Al-containing austenitic stainless steels are used. Metal attack by the glass is severe if Ni based alloys are used, less so for the Al-containing alloys. In this paper, lithia-alumina-silica glass-ceramic was sealed to Al-containing alloys that were first oxidized prior to sealing (preoxidation). Results show that this preoxidation substantially reduces the probability of glass/metal reactions during seal formation, thus improving the overall quality of the interface without loss of seal bond strength. Mechanism of surface oxide formation of these Al- containing steels is discussed. Auger data show the composition of the resulting oxides to be a function of oxidation temperature. There are two theories on the oxidation mechanism: (1) oxidation occurring at the air/oxide interface (Abderrazik et al, 1987), and (2) oxidation taking place at the oxide/metal interface (Hindam and Smeltzer, 1980). To test the theories, oxidation of the Al-containing alloys was carried out, first in pure oxygen-16, and then followed by pure oxygen-18. SIMS showed no layered structure, but did show a mixture of oxides. Thus, the oxidation mechanism is not simple and must be allowing oxygen to have access at all stages of the oxidation process.