| Summary: | The material selection for Supercritical Water-Cooled Reactor is quite challenging as the operating temperature and pressure is fairly high and the assessment of the interaction between material and the environment is not well known. One of the potential alloys to be used in the cooling system of the reactor is Alloy 800 and this work is focused on the evaluation of the electrochemical corrosion behavior of this material.
The results at low temperature showed that the corrosion is controlled by the reduction of hydrogen peroxide on the surface of the alloy. The comparison between the corrosion in buffered and unbuffered solutions showed that the overall behavior is the same, although the corrosion rate was lower in the buffered solution. The study of kinetics of growth of passive film showed that the extrapolation of short term electrochemical corrosion rate measurements underestimate the actual long term corrosion rate.
A Solution-filled External Pressure Balanced Reference Electrode was constructed to make the measurements in high temperature high pressure solutions. The reference electrode potential reading was characterized and validated using the potential of Cu²⁺/Cu redox.
The corrosion rate of Alloy 800 increased by one order of magnitude by changing the temperature from 100 to 200°C. The corrosion rates by immersion tests at 200°C over a 14-day period show an increase from 9.3 to 81μm/year, which are lower than the equivalent electrochemical measurements. This likely refers to the long term growth of the passive layer which reduces the corrosion rate. It was concluded that Alloy 800 is a safe choice in terms of general corrosion in temperatures up to 200°C.
An innovative presentation of electrochemical stability diagrams, as electrochemical potential-temperature diagrams, were developed for aqueous corrosion of iron, nickel, and chromium in sulfate solutions up to 200°C. The viability and usefulness of these diagrams were demonstrated by open circuit potential (OCP) and polarization resistance measurements of the elements in sulfate solutions. This type of diagrams may be a more convenient tool for assessing the stability regions of different species in the cooling system subjected to temperature variation.
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