Activities of metals in an AISI 316LN stainless steel by emf measurements

For a comparison the activity of the metal M (M=Cr, Mn or Fe) in the austenitic phase of type 316LN SS was measured by a metastable CaF2-emf technique using M/MF2 as the reference electrode. These activities were found to be: log aCr (±0.009)=-0.613+54.95/T (769-1046 K), log aMn (±0.013)=0.688-2628.53/T (806-1085 K), log aFe (±0.003)=0.094-132.77/T (802-1023 K) and log aNi (±0.007)=-0.481+268.37/T (784-956 K). The Mo activity was measured using an oxide cell and MoO2 as coexisting phase: log aMo (±0.06)=-6.711+4474.92/T (810-990 K). Using these data and the reported composition of M23C6 formed in 316LN SS, the ΔGf0 of M23C6 was calculated to be: ΔGf0(M23/6C)(kJ)=-17.75-0.07846T(K). These results were used to compare the sensitization behaviour of 316LN with that of 304 and 316 steels. (orig./MM)