The formation of hydrogen related porosity by double oxide film defects in Al alloys

Liquid aluminium readily forms a surface oxide film, and during melting, casting and metal transfer operations double oxide film defects may be readily entrained into the liquid metal. Dissolved H is thought to diffuse into these defects and re-combine to form diatomic hydrogen molecules, leading to the formation of H porosity in the solid casting. However the diffusion of H into double oxide film defects may be considered unlikely as even thin Al oxide films have been reported to be effective barriers to the diffusion of H. In the experiments reported here, solid samples of commercial purity Al alloy were degassed by repeated use of a LECO hydrogen measurement device, then melted and exposed to an atmosphere of either oxygen or nitrogen. This created a surface containing Al₂O₃, or Al₂O₃ and AlN, respectively. When exposed to a H₂ gas atmosphere, the absorption of H was found to be greater in specimens containing AlN on the surface. The results suggest that AlN can form in cracks in an existing oxide layer and allowed greater diffusion of hydrogen when compared to samples with only alumina on their surface.