Study On Corrosion Of Materials In High Temperature High Pressure Aqueous Solution

Supercritical water oxidation (SCWO) is a new environmental protection technique, which can efficiently destroy almost any organic in water in less than one minute. However, the severe corrosion problem of the equipment materials in this environment hindered the wide use of the supercritieal water oxidation (SCWO) in industry. For better understanding the corrosion behaviors of several important corrosion resistant alloys, we set up a continuous SCWO experimental system facilities which is special for corrosion researches. The corrosion phenomena of several stainless steels Ni-based alloy and commercially pure titanium in high temperature high pressure water and organic waste solution has been studied using optical microscope, gravimetric, scanning electron microscope (SEM) and energy dispersion analyzer of X-ray (EDAX). Three types of reactors which is the main part of a SCWO system have been built and tested. To solve the high pressure leakage problem of the reactor a special sealing device has been successfully use in our system. Our SCWO system can continuously run for long period under supercritical water oxidation conditions (25MPa and 4500C). The experiment results showed that the corrosion performance of the materials in SCWO conditions is depended on the types of feeding organic. All samples have no obvious weight losses in phenol solution and the general corrosion rate in this medium is lower than 0.05mm/a. Pitting was observed only in 1 Cr1 8Ni9Ti and 316 stainless steel. But almost all alloy samples were attacked heavily in the SCWO system that destroy organic containing chlorine~ nitrogen or sulfur, especially in chlorine contained solution. The general corrosion rate of 316 is higher than 1 00mm/a. Pitting penetration rate is more than 1 5j~tm/h. The experiment results also showed that the corrosion behaviors of the alloys are affected by temperature. The area at temperature lower than 1 500C can be name as 11 corrosion immunity region. All alloys have no obvious weight losses in all tested circumstance in this region. At temperature higher than about 1 50擟 pitting occurs especially in the solution contains chlorine. The penetration rate increase with the increase of temperature. Solutions contained S or N or Cl lead to severe general corrosion at the temperature high than 300 慍. The 316. Ni-825 sanicro28 etc. are completely attacked. The highest corrosion rate is at the temperature about 350 揅. At temperature higher than the critical temperature (?74 揅), the corrosion rate slow down for the drastic changes of the properties of water when increasing the temperature. One exception is that the corrosion rate is very high (the rate for 31 6is higher than 124mm/a) when decomposing organic which contains Cl, P, S and N in SCWO system. From the results of our test, Ni-base alloy and Sanicro28 can be used in pure and phenol solution. 316 and U2 can only be safely used in pure water. The weight loss of pure titanium is negligible in all tests and pitting is very slight also. So, titanium maybe the most suitable candidate for SCWO system.