Workshop On Federal Programs Involving Supercritical Water Oxidation PDF Download

Laboratory and pilot-scale supercritical water oxidation systems are operating today and that field experience indicates that corrosion in the pre-heater, reactor and cool-down sections of SCWO systems does, as expected, occur: accelerated general corrosion, pitting and stress corrosion cracking have been identified. This provides an imperfect and incomplete if, however, important part of the data base that must, ultimately, be considered in producing large scale commercial SCWO systems. Recognizing the gaps which exist in this base of field experience, this workshop created a first cut at a corrosion research agenda that should provide guidance to designers and equipment manufacturers in screening and qualifying materials of construction for SCWO hardware and monitoring devices. The consensus of those present is that wastage (accelerated general corrosion) is less of a concern than stress corrosion cracking which may lead to premature failure of systems components. A test matrix (see Section 4(a)) was established with the view that this matrix should be pursued in a dedicated SCWO system, the goal being to develop the engineering data base necessary to complement field experience which, in turn, would be used in constructing scaled-up systems. The test matrix takes into account the recognition that stress corrosion cracking is intimately a function of the microstructure of candidate materials (which is dependent on the processing history), the chemistry of the service environment, and the state of mechanical stress of the component in question. Stress corrosion cracking will only occur if a susceptible microstructure is exposed to a hostile environment in the presence of tensile stress. If any one of these elements is absent, SCC will not occur. In essence, what is required is an understanding of the range of conditions in SCWO systems where the coincidence of solid state-chemical-mechanical factors converge leading to SCC. The proposed dedicated SCWO test system would be instrumented to monitor pH, Cl-, 02, H2 and electrode potentials as well as crack initiation / propagation rates, the latter likely achieved by the potential drop method. Additionally, given the sometimes very high rate of corrosion (wastage) that has been observed in the field, provision should be made to include test coupons to allow monitoring of both general and localized (pitting) corrosion. As is often true in the development of engineering systems, engineering is likely to outpace fundamental (mechanistic) understanding of failure phenomena. It is likely that as the data base of both field and laboratory experience expands (including required inspection and maintenance tools and approaches) improved systems will be developed. In the interim it is essential to carefully manage the technological, economic and safety-related issues that are attendant with the development of engineering systems of all kinds..