ABSTRACT
This paper examines the effect of magnesium additions (0–5 wt%) on the morphology of surface oxides formed on metals at 750°C, and compares it to scale formation on pure aluminium. It is shown that on pure aluminium dross formation proceeds by the rapid oxidation associated with the development of nodular growths. These are produced by the exudation of liquid metal from sites at which local breakdown of the thin first-formed protective oxide occurs. The number and growth of exudations increases with time, with nodules consisting of oxide envelopes containing entrapped metal.
In aluminium melts containing magnesium this process is considerably enhanced. The rate of oxide production increases with the magnesium content of the alloy. The nodules have a similar morphology to those produced on pure aluminium, although more rapidly form a continuous layer of mixed oxides and entrapped metal. It is the formation of these nodular oxide morphologies containing entrapped metal that is responsible for the high metal losses reported when recycling thin section material and accounts for the major increase in metal loss when magnesium additions are present.
