Correlation between chromium physicochemical properties in silicate melts and the corrosion behavior of chromia-forming alloy

Corrosion and depassivation of a chromia-forming Ni-based alloy in the 0.75Na2O-B2O3-2.75SiO2 melt

“…The low sodium concentration limits the volatilization of that element from the melt and ensures the stability of the composition during long time exposure at high temperature [14]. Composition was controlled by EPMA, results are available in ref [13]. Acid-base properties of the silica-based liquids are expressed in the well-known concept of optical basicity [15].…”

“…The working, counter and reference electrodes were immersed at the end of the pre-oxidation time. Corrosion potential of the working electrode was measured during immersion; temperature was increased at a constant rate of 50°C/h until the measured potential fell down to active state (< -1000 mV/Ref above 1000 °C) [13]. Temperature was measured in the liquid with an S-type thermocouple (Pt/Pt-10Rh) during the experiment to be sure of the thermal equilibrium in the liquid.…”

“…• During the first moments of immersion, the formation of the oxide is limited by its own thickness [17] while the absence of chromium in the liquid generates a high concentration gradient of chromium. Schmucker et al assumed that the dissolution is limited by the diffusion of chromium in the liquid [13]. In this case the high chromium gradient involves a fast dissolution rate overcoming the formation rate.…”

“…Two hypotheses can be proposed to explain the last point: either the dissolution of the chromia layer, or the formation rate become respectively greater and lower than expected from the extrapolation of lower temperature values. For the first hypothesis, data on solubility and diffusion of chromium in molten silicates, so as silica-based liquids viscosity are pretty well known [10,11,13,21,22]. In each case, an Arrhenius type law describes the evolution of these parameters with temperature.…”

“…In the present work the depassivation of Ni-30Cr alloy in a ternary Na2O-B2O3-SiO2 was studied. The selected glass composition was 0.75Na2O-B2O3-2.75SiO2 (0.75NB2.75S) which was first formulated as a model nuclear glass [13]. Various values of the temperature increase rate, the pre-oxide thickness and the starting immersion temperature were tested in order to optimize the experimental procedure.…”