The Oxidation Mechanism Of Power Plant Using TP304H And TP347H Under High-temperature Steam

TP304H and TP347H are 18-8 type austenitic stainless steels, widely used in high-temperature and high-pressure components on power plant because of their good mechanical properties at high temperature and good weldability. But these two kinds of materials can produce oxide layer and Oxide shedding problem in long-term operation process because of their poor corrosion resistance to high temperature steam, which has greatly influenced the safe operation of power plant. In this paper, the oxidation behavior of the TP304H and TP347H steels under high-temperature water-vapor condition was studied and the oxidation mechanism of the two stainless steels was analyzed, which can provide a reference for the study of such stainless steels.In this paper, the oxidation behaviours of TP304H and TP347H under water-vapour at 590℃,620℃and 650℃were studied. The oxidation kinetics of the TP304H and TP347H were studied by discontinuous-weighing method. And the morphology and composition of oxidation layer were observed by optical microscope, SEM and EDS. The oxidation mechanism of the two kinds of materials was analysised.The results indicate that the oxidation kinetics of the TP304H and TP347H steel under high-temperature water-vapor condition follows parabolic equations (Δm=ktz) at the 590℃, 620℃,650℃. Oxidation rate is accelerated with increasing temperature, and between 590℃and 620℃oxidation rate increased rapidly. And the oxidation resistance of TP347H is higher than that of TP304H.The construction of the oxidizing layer of the TP304H and TP347H are two-layer, which include inner layer and outer layer. The inner layer is dense, the outer layer is loose. It is found that some continuous holes appear at the interface of the inner layer and outer layer. The growth mechanism of the outer layer was analysised. First granular oxide generates on the surface of steel and then it grows as continuous crystalloid oxide layer, then the crystal makes a longitudinal growth. After the appearing of some pores, the crystal makes a transverse growth. Then the pores disappear, the oxidation film constantly thicken. The growth mechanism of the inner layer was analysised. Due to the existence of Cr, the chromium rich belt will appear in the growth process of the oxide layer. And the chromium rich belt can effectively slow down the growth rate of oxide layer. However, because Cr makes a slow diffusion in the grain, it is difficult to form continuous chromium-rich belt, oxidation layer can through the chromium-rich belt and then grow up. So black spots can be found in the inner layer. And Cr atom diffuses faster in grain boundary than in grain interior, so it is easy to form continuous and dense chromium-rich belt near the grain boundary, so the grain boundary has good inoxidizable properties. Finally the long and white unoxidized notch or closed nickel-rich area can be found near the grain boundary.