Hi I am about to fabricate a part exposed to super critical water (600C & 250bar). In the past people have been using titanium or convoluted methods to prevent excessive corrosion induced by dissolved solids (principally Cl-), which under supercritical conditions precipitate, coming back into solution in high pressure/high temperature but sub-supercritical water. FYI, the water will also have a high to near-saturated dissolve oxygen level. Also some shockwaves will travel throughout the water when the arc ignites the introduced oxidizer to flash heat the water up to the required temperatures, ergo ceramics have also not performed well. Now, even titanium or titanium coated steel didn't last particularly long in the days when people were working with supercritical oxidation. I would hope that by now a new material have been discovered, which can withstand these conditions for 5000hrs. The material will be supported by the pressure vessel, so it only needs to be able to withstand temperatures up to 600-700C and the corrosion.