Electrochemical determination of the oxygen activity in tin melts by means of the solid electrolyte method

Abstract

The measurements of oxygen activity in the tin melt were carried out under inert (light oxidizing) as well as under reducing (N2 with 2 vol.% H2 and 10 vol% H2, respectively) conditions. As solid electrolyte (SE) material Y2O3-, CaO- and MgO-stabilized zirconia was used in the form of a long tube closed at one end. Α short SE tube closed at one end within an alumina tube was applied as industrial probe. The galvanic cells worked with a Pt/air and Me/MeO reference electrode and with electrical leads of steel, tantalum as well as rhenium wires. Under inert conditions (argon with 30 vol.ppm O2) a saturation of the tin with oxygen is always obtained and SnO2 is formed. Under reducing conditions the saturation of the melt with oxygen does not take place and no SnO2 is formed. An equilibrium is adjusted between oxygen in the atmosphere and solute oxygen in the melt. At extremely low oxygen contents no equilibrium will be achieved. The lower operating temperatures of the probes were between 500 and 700 °C. The data taken from the literature were confirmed by EMF measurements (solubility of oxygen in liquid tin, molar free solution enthalpy of oxygen in tin, molar free standard enthalpy of formation of SnO2). This kind of investigations is new as to the behaviour of the tin melt under forming gas and the defined addition of elements and compounds to the tin melt (Na2SnO3, Na2O, Na, Fe2O3, Fe, MgO, Mg). Under reducing conditions and the addition of sodium or magnesium to the tin melt a quick decline of the oxygen activity can be recognized at the moment of addition. The melt becomes strongly deoxidized by the addition of these metals. The subsequent processes of deoxidation, which are different from metal to metal, have still to be investigated. No change of the oxygen activity in the melt was observed by addition of iron in the ppm range to the tin melt. For the measurements alumina and a fireclay brick were used as crucible materials. The tin surface was either in immediate contact with the gas phase or was covered with a float glass melt. The oxygen activity of the tin melt was influenced by the crucible material and the float glass melt, too.