Hydrogen partitioning in pure cast aluminum as determined by dynamic evolution rate measurements

Abstract

A statistical analysis of the porosity in 99.995 wt pct pure commercially available cast aluminum has been correlated with real time hydrogen evolution data obtained in an ultrahigh vacuum furnace in order to estimate the hydrogen partitioning in the aluminum. The dynamic technique employed permitted the detection and separation of hydrogen evolved from solid solution, hydrogen released by the rupture of large pores, and gases desorbed from the aluminum surface. Results of the statistical analysis indicate average pore diameters in pure cast aluminum extend from less than 1 to over 400 μm. Interdendritic pores having diameters greater than 25 μm constitute over 98 pct of the pore volume. The overall volume fraction of pores was determined to be 0.71 pct. Compared to vacuum remelted rolled aluminum, the porosity resulted in a reduction of ultimate tensile strength of 13 pct and a reduction in yield strength of 21 pet. The evolution of hydrogen from the aluminum was observed to occur by large hydrogen pressure pulses due to the rupture of pores near the surface and by a smooth steady desorption from solid solution. The rupturing pores were observed visually and found to occur both in the solid state and after melting. A substantial change in slope of the desorption curve following the pulse train suggests the pores are the primary sources of hydrogen in the bulk. Analysis of the pore and pulse size distributions indicates more than 99 pct of the bulk hydrogen is partitioned in pores greater than 25 μm. Pressures within the larger pores (≈270 μm) were determined to be about 2.4 atm at room temperature. Hydrogen content in the large pores was found to be as high as 2 × 1016 molecules. The total hydrogen content in the pores and in solid solution was determined to be 6.3 × 1017 atoms/cm3 (0.43 cm3/100 g). Measurements on commercially available 99.9995 wt pct cast aluminum indicate the total hydrogen content to be 4.8 × 1017 atoms/cm3 (0.33 cm3/100 g).