Thermodynamic estimation of hydride formation during hydrogen plasma arc melting

The purification effect by hydrogen plasma arc melting (HPAM) and the respective probabilities of hydride formation for several metals were investigated. Commercially pure Fe, Cr, Ti, V, Zr, Nb, and Ta metals were melted for 60 min using Ar-30 vol% H₂ plasma gas at 5.3 kPa (Ta at 101.3 kPa). Results showed that the degree of removal of each impurity in the metals increases concomitant with the increased vapor pressure difference between the impurity and the metal. This study further established that addition of hydrogen to the plasma gas increases the metal surface temperature, which is a main reason for the superior purification provided by Ar-H₂ plasma arc melting. Thermodynamic calculations showed that the main part of impurities transfers to the gas phase as atomic species, whereas Sb can predominantly form hydrides over other corresponding atomic species.