Oxide skin strength on molten aluminum

Abstract

The oxide skin strength on molten aluminum has been measured as a function of temperature for pure aluminum and some aluminum alloys. The measured values fit well to previous published data of surface tension of liquid aluminum, σ _lg , combined with the wetting angle, ϑ, and the mechanical strength of the oxide. It is assumed that the work per area needed to stretch and rupture the oxide skin is a sum of the interfacial tensions and tensile strength of oxide, times oxide thickness:

$$\begin{gathered} W = \sigma _{sg} + \sigma _{lg} - \sigma _{sl} + \tau \cdot \delta \hfill \\ = \sigma _{lg} (1 + \cos \theta ) + \tau \cdot \delta \hfill \\ \end{gathered} $$

The work per area calculated from previous published data fits reasonably well with the oxide skin strength measurements presented here. It is found that above about 750 °C, the oxide skin strength increases with increasing temperature. The effect of sodium addition to industrially pure aluminum has also been investigated. Addition of 68 ppm sodium increases the oxide skin strength by 52 pct. Addition of 0.3 pct magnesium to A1-11Si reduces the oxide skin strength by 30 to 35 pct below 620 °C.