A NOVEL DETERMINATION OF THERMODYNAMIC ACTIVITIES OF METALS IN AN AISI 316 STAINLESS-STEEL BY A METASTABLE EMF METHOD

An emf technique was adopted for the first time for direct determination of thermodynamic activities of all major metallic components, namely, Fe, Cr, Ni and Mn in a commercial grade AISI 316 stainless steel in the temperature range of approximately 800 to 1200 K. These activities were used to compute the threshold oxygen levels in Na( IOTA )/AISI 316 stainless steel system for the formation of the corrosion products, viz, NaCrO//2, Na//4FeO//3, MnO and NaMnO//2. These data in conjunction with the carbon, activity in this alloy reported in the literature, and initial composition of the M//2//3C//6 phase, could lead to the estimation of the Gibbs energy change for a typical reaction, 2. 571Cr plus 0. 732Fe plus 0. 303Mo plus 0. 226Ni plus C B ARR LR M//2//3/////6C, as follows: DELTA G degree //f//,//T(M//2//3/////6C, s) (kJ) equals minus 29. 16 minus 0. 0522T(K).