The ‘first bubble’ technique for the determination of hydrogen in aluminium has been used to study the formation of gas bubbles. Stereoscan observations of alumina particles found inside bubbles and skimmed from the melt surface show similarities; the particles contain pores and cracks. It is concluded that these defects could act as nucleation sites for bubbles and that therefore a heterogeneous nucleation mechanism is operating. A theoretical consideration of the firstbubble technique shows that these nucleation sites are able to provide conditions under which the atmospheric pressure in the chamber can be related to hydrogen content of the melt.
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