RATE DETERMINING STEP IN SODIUM CORROSION OF TYPE 316 STAINLESS-STEEL

The steady-state corrosion rates of a Type 316 stainless steel have previously been obtained in sodium at 600-700°C, 1-2 ppm in oxygen level and 4.0 m/s in velocity, which is contained in an isothermal, maximum temperature region in a non-isothermal sodium loop constructed of a commercial Type 316 stainless steel. The corrosion rate equation obtained by Arrhenius plots of the experimental data was log10 S(mg/cm2⋅h) = 2.939-1.123⋅105/2.3037RGT, where RG is the gas constant (J/mol-K) and T is the absolute temperature. In the equation, the time exponent is 1.0 and the activation energy is about 112 kJ/mol. These values have been compared with the corresponding values which have theoretically been obtained for tentatively assumed, four corrosion steps; ‘diffusion in the solid′, ‘dissolution’, ‘diffusion in the laminar sublayer’, and ‘chemical reaction at the solid-liquid interface’. As the sodium velocity was in a range where the corrosion rate was empirically independent of the velocity, the corrosion would be a surface controlled reaction, indicating that the ‘diffusion in the solid’ is not a rate determining step. Also, as the sodium velocity was very rapid, the ‘diffusion in the laminar sublayer’ would be very rapid. Both of the ‘dissolution’ and the ‘chemical reaction’ with 1.0 in time exponent would probably be rate controlling steps in the sodium corrosion, although the activation energy of the dissolution differed from that in the experimental rate equation and that of the chemical reaction could not be obtained. © 1990, The Japan Institute of Metals. All rights reserved.