Thermal desorption of gases from aluminum alloy Al 6061, their rates and activation energies

Abstract

The use of Al in the construction of the vacuum chamber of intersecting proton storage rings has several advantages over the use of stainless steel. We have, therefore, undertaken UHV studies on long pipes made of Al alloy 6061 to see whether this material could be used in the proposed proton intersecting storage accelerator facility ISABELLE at Brookhaven. The outgassing rates, measured by two different methods, are compared for several surface treatments and three bakeout temperatures. The lowest outgassing rate of less than 1×10−14 Torr l s−1cm−2 was obtained with Ar and O2 discharge cleaning. The residual gas in the pipe contained more than 99% H2 with traces of CO and CO2. By observing the temperature dependence of the desorption of these gases, we have measured the heat of diffusion of H2 (EH2=10.5±1.0 kcal/mole) and the thermal activation energies of CO (ECO=29±2 kcal/mole) and CO2 (ECO2=25±2 kcal/mole). Ar, with a peak desorption at 280°–300 °C, also appears to be embedded in the metal. Since all commercially available UHV equipment uses stainless steel flanges, we have investigated the behavior of the stainless steel to Al transitions up to 350°C.