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Corrosion Of Alloys In Supercritical Water Oxidation System

Posted on:2003-05-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:J S LuFull Text:PDF
GTID:1101360062975892Subject:Metal material and heat treatment
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Supercritical water oxidation (SCWO) is a relatively new technology in environment protection. It utilizes the special properties of supercritical water (T>374.2℃, P>22.1MPa) that all organic and most gases can be dissolved in it. Nearly complete destruction of hazardous wastes in a few minutes can be achieved by SCWO. However, corrosion of materials of the equipment in SCWO system is a major problem that hinders the widespread application of the technology.To study the corrosion of materials in SCWO conditions, a continuous SCWO system which can be operated for long time at supercritical conditions (usually at 25MPa and in less than 450 "C) has been built. Six metallic materials (four stainless steels :!Crl8Ni9Ti, 316, U2, and Sanicro28; one nickel base alloy N1825, and one commercially pure titanium)were exposed in six types of SCWO mediums (pure water, and SCWO destroying phenol, organic containing S, organic containing N, organic containing Cl and P, organic containing S> N, CK and P) for as short as a few hours to as long as one week. Metal samples were positioned in the reactor of the system which is specially designed for frequent open and close.Corrosion rates of the materials were measured by gravimetric method. Corrosion morphology and localized corrosion were revealed by optical microscope and SEM. Compositions of corrosion products were analyzed by ED AX and XRD.Corrosion test of the six materials in high temperature pure water showed that !Crl8Ni9Ti and U2 stainless steels were attacked in supercritical (3 92 ℃) water.The results show that corrosion performance of the materials in SCWO depends on the types of organic being treated. In destroying phenol (CeHeO) , the general corrosion rates of all the six materials were less than 0.1 mm/a. Sanicro28 stainless steel and Ni825 nickel base alloy were only slightly corroded on superficial level while !Crl8Ni9Ti and 316 stainless steels showed pit corrosion. Therefore, Sanicro28 and Ni825 can be used as corrosion resistant materials in such SCWO environment. However, in SCWO destroying organic containing S (sodium dodecyl sulphonate, CpHasCeHsSOaNa) and organic containing N (n-amine phenol, Ce^N^OH), all the materials except Ti suffered heavy general corrosion and pit corrosion in near critical temperature region. Corrosion rate as high as 17 u m/h and 5 u m/h was found respectively for !Crl8Ni9Ti and Sanicro28. In SCWO destroying trichlorphon (C4H8O4PCl3) and chlorpyrifos (CgHnOSPNCla), serious pitting corrosion (as high as 1.5 mm/h of penetration rate) and general corrosion were experienced for the stainless steels and nickel base alloy. Ti also showed being corroded in the SCWO destroying chlorpyrifos.The results also show that corrosion of the materials are highly temperature dependent. In SCWO destroying sodium dodecyl sulphonate and n-amine phenol, general corrosion of stainless steels were closely related with temperature, being heavy corrosion in near critical temperature, moderate corrosion in lower and upper temperature, and light corrosion in less than 300℃ and supercritical temperature. However, in SCWO medium destroying chlorpyrifos, the general corrosion rates of 316 stainless steel and Ni825 nickel base alloy go higher as temperature rise from 150to 300, and even higher rates at supercritical temperature (450) were observed. Pit corrosions of the two alloys also show temperature related: below 150℃, no pit was initiated; at 150~250℃, pitting is the main form of the corrosion and deep pits were developed; at temperature higher than 250℃, shallow pits were formed due to increasing higher general dissolution.At temperature higher than 300 in SCWO destroying chlorpyrifos, especially around critical temperature, titanium samples were covered with corrosion product with microscopically rough surface. X ray diffraction analysis showed that the corrosion product consists of titanium oxy- phosphates and titanium oxide in which TisO4(PO4)4 is the main phase. This means that the corrosion is caused by phosphoric acid. Pit co...
Keywords/Search Tags:corrosion, stainless steel, nickel base alloy, titanium, supercritical water oxidation
PDF Full Text Request
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