Stable isotopes used in the definition of corrosion mechanisms
Résumé
The ability of secondary-ion mass spectrometry (SIMS) to separate isotopes and to analyze thin layers by sputtering provides unique tools for studying corrosion mechanisms. The methodology with oxygen 18 has been tested with success at the beginning of the 70s for the oxidation of tantalum (Ta) by water in two steps: first oxidation by H216O and subsequent one by H218O [1]. Nevertheless, very few corrosion studies have taken the advantage of isotope substitution for the investigation of corrosion mechanisms. The objective of the paper is to show that the use of stable isotopes has been a major step in the understanding and the modeling of several corrosion phenomena. The first illustrations will be linked to the localization of the anodic and cathodic reactions on archeological analogues. Then the use of isotopic tracers will be shown in more complex environments.
In liquid lead-bismuth, sequential experiments with dissolved 18O and 16O have been performed to determine the mechanisms of growth of the duplex structure oxide layer: It was found that the magnetite layer grows at the Pb–Bi/oxide interface whereas the Fe–Cr spinel layer grows at the metal/oxide interface. The modeling of the growth mechanisms of the duplex layer lead to the evaluation of corrosion damages, in accordance with available data. It should be underlined that the same type of growth of duplex layer has been observed in supercritical water with the subsequent used of H216O followed by an exposure to H218O.
Stress corrosion cracking of Alloy 600 in water at 300-350°C has been investigated with Alloy 600 (nickel base alloy with 15% Cr and more than 72% Ni) samples exposed to heavy water with dissolved hydrogen (D2O / H2, diss) or to natural water with dissolved deuterium (H2O / D2, diss). SIMS analysis of D (2H) and 16O were done to determine the deuterium concentration profiles together with the oxide film thickness on the alloy surface. Almost no deuterium is observed for samples exposed in the H2O/D2 environment and only in the oxide layer, whereas the intensity of the deuterium profile is much larger in D2O/H2 with deuterium observed not only in the oxide layer but also in the alloy. Clearly, the main source of hydrogen is the cathodic reaction (water dissociation). Two mechanisms may be proposed for modelling the hydrogen transport associated with the oxide growth during alloy passivation: (i) diffusion of hydrogen as an interstitial proton through the oxide lattice, or (ii) diffusion as a hydroxide ion towards the oxide in the anionic sub-lattice. The latter hypothesis implies the oxygen and hydrogen diffusivities through the oxide layer to be the same. To check which hypothesis is correct, Alloy 600 specimens have been exposed in PWR primary conditions using 2H and 18O as markers. The values obtained for diffusion coefficient of 2H and 18O are very close (around 5 10-17 cm2/s) which supports the idea of a hydrogen transport mechanism through the oxide layer as hydroxide ions.
The strong correlation between hydrogen absorption and oxidation occurs not only for the formation of the oxide layer on the surface of the alloy, but also during intergranular oxidation of grain boundaries. The question here is to assess whether the oxide grown at the grain boundaries in the case of intergranular corrosion would act as a barrier to hydrogen arrival to the oxide/crack tip or not. After a primary oxidation in nominal primary water followed by a short period under the same conditions but with D and 18O isotopes, deuterium and oxygen 18 are found at the tip of the intergranular oxidation, even for short exposure times. The results lead to the conclusion that oxygen and hydrogen transport in the oxidized grain boundary are not the rate-controlling step for SCC initiation in PWR nominal conditions. To check if chromium diffusion is the limiting step, diffusion experiments with 54Cr in Ni-Cr alloys have been performed.
The conclusive remarks will include some recommendations and some interests for the use of radioactive tracers to determine corrosion kinetics.