Potentiostatic polarization studies in fused carbonates—II. Stainless steel

Abstract

Electrochemical studies at 600–700°C are reported for niobium-stabilized (347) and (304) chromium—nickel stainless steels in the ternary Li2CO3-Na2CO3-K2CO3 eutectic mixture by a potentiostatic polarization technique. The results leave little doubt that these stainless steels passivate in fused carbonates. The corrosion potentials for both steels, referred to a Ag/Ag+ reference half-cell, are −525 ± 30 mV (Au and Pt, on the same scale, are −470 and −475 mV), and this value is shown to vary qualitatively with environmental effects in accord with the mass-law controlled equilibrium CO32− CO2 + O2− in fused carbonates. Values of the primary passive potentials at 600 and 700°C are reported. It is clear that the passivity conferred by anodic electrolysis for these steels makes possible a performance as electrodes quite comparable to that of Au—20% Pd alloy or Pt in the electrolyses of fused carbonates. These properties for stainless steels are discussed relative to current viewpoints on metal passivity, and relative to structural changes found in the steels.