Research On Corrosion Behavior Of Power Plant Boiler Tube Materials In Supercritical Water

Ultra supercritical technology needs higher steam temperature and pressure, so it will enhance the energy conversion efficiency. However, the property of the working fluid in supercritical condition would substantially change, so the material properties of the key components in power plant put forward higher requirements and the frequent occurrence of tube rupture also seriously affect the safe operation of ultra-supercritical power plant. Therefore, research for the typical boiler piping materials corrosion in supercritical water has a significant meaning.Investigating the high temperature corrosion experiments through dynamic water cycle on Ferritic-Martensitic steel P92, Austenitic steel TP347HFG at550,600℃and25MPa supercritical water environment, corrosion behavior were studied for experimental period from200to1000hours, the weight gain, surface morphology and microstructure, XRD and cross-sectional EDS, and element distribution of two materials were analyzed. Weight gain of F/M steel P92is much larger than Austenitic steel TP347HFG. Temperature have a very significant influence on two steels, weight gain at600℃is much higher than at550℃. Dissolved oxygen also have an obvious effect on P92, but the effect on TP347HFG is not so clear. Weight gain and the oxide structures increased with oxidation time. Both materials have a typical dual oxide layer structure, the outer layer contains loose and porous Fe3O4oxides, inner layer of P92is denser and composed of Fe3O4and FeCr2O4spinel mixture. The outer layer of TP347HFG are mostly modular structure (Fe,Cr,Ni)3O4oxide, and insufficient to cover the alloy surface. TP347HFG corrosion resistance at550,600℃and25MPa supercritical water environments is better than P92, P92could only be applied at550℃with low dissolved oxygen content. TP347HFG has an excellent oxidation resistance under the test condition.Weight gain of F/M steel P92in supercritical water are1.5-2times higher than in team, and has a higher corrosion rate exponent, which shows the environment of supercritical water could enhance corrosion. The oxide layer formed in steam is much denser, there are a few pores, and mainly located in the outer-inner layer interface. The oxide layer formed in supercritical water is much thicker than in the steam and has more pores. The differences of two environments is the pressure, supercritical water has much higher density of the medium, resulting in the increase of iron oxidation rate on the outer layer surface, thus increasing the Fe2+/Fe3+ratio difference between SCW-oxide interface and the oxide-metal interface. This leads to a higher driving force for diffusion and corrosion of iron. The corrosion rate difference between steam and supercritical water suggests that the oxidation mechanism of rate limiting step for F/M steel P92may be iron diffusion to the outer layer.