Supercritical water conditions

Electrochemical potentlostat measurements have been performed for the corrosion of iron, carbon steel, and stainless steel alloys in supercritical water. The open circuit potential, the exchange or corrosion current density, and the transfer coefficients were determined for pressures and temperatures from ambient to supercritical water conditions. Corrosion current densities increased exponentially with temperature up to the critical point and then decreased with temperature above the critical point. A semi-empirical model is proposed for describing this phenomenon. Although the current density of iron exceeded that of 304 stainless steel by a factor of three at ambient conditions, the two were comparable at supercritical water conditions. The transfer coefficients did not vary with temperature and pressure while the open circuit potential relative to a silver-silver chloride electrode exhibited complicated behavior. 

A more complex degradation takes place when this process is applied to PVC. The authors propose that PVC depolymerization under supercritical water conditions proceeds in accordance with a mechanism consisting of four different pathways (i) dehydrochlorination and partial oxidation, (ii) dehydrochlorination and chain scission, (iii) dehydrochlorination and total oxidation, and (iv) hydrochlorination. In the reaction products, high yields of vinyl chloride, 1,1-dichloroethane and 1,2-dichloroethane are detected, especially at short reaction times, whereas longer times favour total oxidation products. 

Other possible oxidative treatments for the feedstock recycling of plastic and rubber wastes include partial oxidation with organic peroxides and decomposition by reaction with oxygen by thermooxidation or under supercritical water conditions. However, these latter alternatives have so far not been widely investigated. 

Takahashi T, Tanifuji K., Stafford C. Hashida T. 2003. Permeability enhancement by microfracturing in granite under supercritical water conditions. JSME International Journal, Series A 46(1) pp. 24-29. 

MICROCRACK FORMATION AND FRACTURE CHARACTERISTICS IN GRANITE UNDER SUPERCRITICAL WATER CONDITIONS... 

R. Novotny, P. Hahner, J. Siegl, P. Hausild, S. Ripplinger, S. Penttila, A. Toivonen, Stress corrosion cracking susceptihihty of austenitic stainless steels in supercritical water conditions, J. Nucl. Mater 409 (2) (2011) 117-123. 

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