| Popis: |
With a view to improving the prediction of primary water stress corrosion cracking in austenitic stainless steels, this investigation measured the oxide growth kinetics of 316L stainless steel when exposed to a simulated primary water environment of a pressurised water reactor (PWR). It is generally accepted that intergranular oxidation at the surface of a metal forms a preferential site for stress corrosion crack (SCC) initiation; therefore the kinetics of both surface and intergranular oxidation were measured. The influence of temperature, within the range of PWR primary water (290°C, 320°C and 360°C), as well as the influence of starting condition (annealed, 20% elongated, 30% elongated and 20% cold rolled) was investigated. Samples were prepared with the various starting conditions and exposed to simulated primary water, at the specified temperatures, for various durations from 1 hour through to several thousand hours to plot the oxide growth on a log scale time axis. Subsequent to the exposure tests, the Cr rich inner oxide depth was measured locally at selected locations. The surface and intergranular oxide depth was directly measured from cross-sections either with a transmission electron microscope for short duration exposures or, for longer exposures with deeper oxides, within a scanning electron microscope. No significant difference was noted on the oxide kinetics between the various starting conditions evaluated. Temperature, however, had a significant influence with oxide growth kinetics decreasing, rather counter-intuitively, as temperature increased through the measured range. In addition a strong dependency on grain orientation was observed. A modification to the Point Defect Model was proposed to arrive at a quantitative expression to describe surface and intergranular inner oxide growth as a function of temperature in 316L stainless steel, which accommodated the deviation from Arrhenius behaviour through the measured temperature range. Functions for both the rate constant, ��3 0, and the transfer coefficient, α3, associated with the metal/oxide interface reactions were developed. The resultant model was able to predict, with reasonable accuracy, the growth of the Cr-rich inner oxide over time. The most consistent explanation for the deviation from Arrhenius behaviour was that the coherency across the metal/oxide interface degraded as the temperature increased through the tested temperature range. This would reduce the potential for ionic transfer across the interface necessary for the interface to migrate and increase the oxide depth. Since a similar temperature dependence on the growth of intergranular stress corrosion cracking (IGSCC) in the primary water environment has been observed within the same temperature range, it is proposed that the above explanation, observed in the absence of applied stress, extends to explain the behaviour of IGSCC kinetics in austenitic stainless steel. |
