Deoxidation of Metals

the removal of dissolved oxygen from molten metals, chiefly steel and other iron alloys. Oxygen is an impurity and is detrimental to the mechanical properties of metals. Metals are deoxidized by reaction with elements that have a greater affinity for oxygen than the metal being treated and by alloys, for example, ferroalloys, of such elements. Thus steel can be deoxidized with aluminum, which forms A1₂O₃, an extremely stable oxide that separates from the molten metal as a distinct solid phase.

The degree of deoxidation, that is, the final concentration of oxygen in the metal, is determined by the deoxidizer’s concentration, the temperature, and the stability of the oxide product. For example, in the reaction D + O = DO, D is the deoxidizer, O is the oxygen, and DO is the oxide; according to the law of mass action, the equilibrium constant for this reaction has the form K = 1/([D][O]), where [O] designates the concentration of oxygen and [D] designates the concentration of the deoxidizer. The numerical value of the constant increases with the stability of the oxide, that is, with the decline in free energy incurred during the formation of the oxide from its elements. A highly stable oxide corresponds to a low concentration of oxygen at a given concentration of deoxidizer and given temperature.

For metals to be deoxidized efficiently, it is essential that the products of the deoxidation reaction not be allowed to remain as nonmetallic inclusions in the steel. The rate at which these products float to the surface of the pool of metal depends on the temperature and viscosity of the metal, the density of the inclusions, and the strength of the currents within the melt. The removal of inclusions is facilitated by the presence of liquid slag, which assimilates the oxides. Deoxidation is used in various processes of nonferrous metallurgy, for example, in the deoxidation of copper with the help of reducing agents containing carbon.