Results of a literature and an experience survey on materials selection in subcritical and supercritical water oxidation processes are presented in this paper. Some hydrothermal process environments with moderate pH levels and low levels of reactive halide ions were mild enough to allow the use of AISI 316 and similar stainless alloys. The engineering alloys most commonly utilized in hydrothermal oxidation processes were found to be alloys C-276 and 625. However, the corrosivity in some environments appeared to be beyond the limits of alloys C-276 and 625 and required materials with higher corrosion resistance. Titanium alloys such as T1-Gr2, Ti-Gr9 and Ti-Gr12 had value in terms of their corrosion resistance under some cases of highly oxidizing conditions. For severe applications, noble metals such as Pt, Pt-Ir and Pt-Rh were used as liners and overlays to minimize corrosive attack, Newer developed alloys such as alloyC-4, C-22, 59,686 and C-2000 haven?t yet been evaluated in hydrothermal oxidation process environments, In addition to materials selection, design innovations were used to minimize conditions for corrosive attack in process equipment.
Near complete destruction of hazardous waste is achieved by hydrothermal oxidation processes. Basically, the aqueous waste is heated and pressurized in the presence of air or oxygen. During hydrothermal oxidation of hazardous waste, the materials of construction of the preheating parts, reactor, heat exchanger, and cooldown parts are subjected to gases, liquids, and solids of various compositions at high temperatures and pressures. Both strength and corrosion resistance are important because chemical and mechanical effects at these conditions interact. High temperatures accelerate corrosion processes. Some gases and liquids which are harmless at room temperature, become aggressive to materials when hot ?l].Chemical composition of the hazardous waste being oxidized plays a large role in the corrosion process. Corrosion rate of most engineering materials increases as the pH decreases. Halide ions such as chloride, fluoride, etc. can cause stress corrosion cracking and breakdown of passive films. In addition to the direct attack to the film, under oxidizing environments, chloride can combine with iron to form ferric chloride with has a melting point of about 280 C. This contributes to the formation of a liquid salt on the alloy surface which removes protective oxide films ?2-31.The sulfate ion is also a primary aggressive ion. Sulfate deposits are known to induce hot corrosion. This type of corrosion occurs when salts or ashes accumulate on the surface of alloys and accelerate the corrosion process under these deposits ?4].Fluoride and bromide can also interact with specific alloys at high temperature and decrease their corrosion resistance.
A literature review and an experience survey on materials selection in subcritical and supercritical water oxidation processes are presented herein. The sources of the published literature included technical, trade and news publications, The experience survey information was obtained from individuals in academic, research and commercial organizations working on hydrothermal oxidation processes.
