Thermodynamics of Refractory Powder Passivation Surface Oxide Reduction
Controlling oxygen partial pressure and furnace dew point during thermal reduction strips surface oxides without triggering particle necking or tap density loss.
Calcium Vapor Deoxidation
Meaning
Pyrometallurgical purification technique for the removal of oxygen from titanium or zirconium alloys using gaseous calcium. High purity metals benefit from calcium vapor deoxidation because the process avoids direct contact between the molten salt and the solid metal. This gaseous interaction allows for deep removal of oxygen without introducing unwanted contaminants into the metal matrix.
Vapor Phase
Pressure control within the vacuum chamber determines the rate of evaporation. Since calcium vapor deoxidation relies on the gas phase, the reaction occurs at all exposed surfaces of the metal powder or sponge. Gaseous calcium reacts with dissolved oxygen to form calcium oxide, which remains as a solid byproduct on the surface.
Chemical Driving
Chemical potential of oxygen in the metal must stay higher than the potential in the resulting oxide for the reaction to proceed. Successful treatment yields oxygen concentrations below 100 parts per million while maintaining the structural integrity of the refractory metal. This thermodynamic window limits the choice of reducing agents to those with a high affinity for oxygen.
Slag Removal
Final processing involves leaching the byproduct with a dilute acid solution. This step ensures that calcium vapor deoxidation results in a clean, metallic surface ready for melting or compaction. Leaching removes the calcium oxide layer without damaging the underlying metal structure.