MITIGATING EFFECT OF MAGNETITE BUFFERING ON ALLOY 800 TUBING DEGRADATION IN ACIDIC, SULPHATE-DOMINATED ENVIRONMENTS AT 300 °C

摘要

Sulphate may enter steam generators with feedwater, either as chronic in-leakage (for example, due to sulphate contamination in the condensers) or as an acute ingress (for example, from a loss of ion-exchange resin into the feedwater system). Following either ingress event, sulphate may accumulate in steam-generator crevices; possibly affecting tubing degradation by lowering the local pH_T, creating localized environments that are aggressive but the magnetite that is associated with steam generator crevices may set limits on how low the pH_T can drop. The work described in this paper investigates the effect of these aspects on the degradation of Alloy 800 and comprises two parts: evaluation of pH_T buffering by magnetite and corrosion testing in sulphate-dominated environments. For the corrosion testing, the aggressiveness of sulphate, by itself and in conjunction with chloride, to Alloy 800 has been investigated at 300 °C and at a pH_(300°C) between 3 and 4. Cyclic and potentiostatic polarization techniques were used to quantify the susceptibility of Alloy 800 in these environments and to identify the type of degradation. For the pH_T evaluation, the pH_T was experimentally measured in the same sulphate-bearing environment, in the absence, and in the presence, ofin-situ generated iron corrosion products, to investigate the possibility that pH_T could be buffered by dissolution of magnetite, adsorption of sulphates on the corrosion products that accumulate in the crevice, or other effects.