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I am wondering which materials might be suitable to contain SCW (supercritical water). Here, we are assuming that the pressure is almost equal on the outside of the container than the inside. I have already considered plastics and some metals but these seem likely to corrode due to how reactive SCW is. Also, it needs to remain sufficiently strong at temperatures around 400°C but not much higher. I've tried to Google it but there doesn't seem to be much literature on the subject

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    $\begingroup$ What specific metals and plastics have you considered? How strong is strong enough? $\endgroup$ Mar 12 at 20:47

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Unlikely any organic compound could be used as a container. Even polytetrafluoroethylene (PTFE) has melting point of 327 °C. Polyimides can be extruded at 400 °C, so would be too soft.

Some materials for supercritical water include Inconel and Hastelloy, nickel-chromium steels.

BTW, if you need an observation port or a non-conductive feed-through, silica-based glasses won't do, since they dissolve in supercritical water. Perhaps sapphire ($\ce{Al2O3}$, corundum) might survive for a time, or not -- supercritical water can etch it under certain conditions.

You might research the Benson boiler.

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