The High-Temperature Oxidation Behavior Of TP347H And TP304H Steel Under Steam

18-8 series of improved coarse-grained austenitic stainless steel is popular with countries around the world for its excellent high temperature strength, oxidation flue gas corrosion resistance and relatively low cost.it has been widely used as heating surface pipe row in high temperature (including the final stage superheater and reheater). However, sometimes these kind of austenitic stainless steel will suffer from the problems of severely steam oxidation and the skin of oxidation peeling off the matrix massively in high-temperature steam environment, resulting the overheating tube blockage accident caused by accumulation of oxide in superheater and reheater tubes or abnormal wear and tear in nozzles, blades, impeller partition and so on caused by the steam oxidation particles carried in parts of the steam chamber erosion. Therefore, this article mainly study the oxidation behavior of TP347H and TP304H steel in the high temperature steam environment in hopes of giving help to the safe operation of power stations with TP347H and TP304H steel.First the high-temperature steam oxidation kinetics of TP347H and TP304H steel were studied through weighing continuous by using self-developed thermal steam oxidation laboratory equipment manufactured by Xi'an thermal power research institute. The results show that temperature increased, TP347H and TP304H steel oxidation gradually increased. At 560℃,590℃, and 620℃temperature TP347H and TP304H steel oxidation kinetics equation follows the parabolic law. Am= ktz. And in these temperatures the oxidation of the two kinds of steel did not peel off the layer. TP347H and TP304H steel has the fastest-growing weight gain per unit area at 590℃to 620℃.Then the microstructure and formation mechanism of oxidation corrosion product (oxidation layer) of TP347H and TP304H steel were studied by means of SEM and EDS.The surface oxidation growth pattern of TP347H and TP304H steel was studied,first the oxidation is granular at the surface of matrix, then crystal object formed at the surface of the oxidation, the oxidation crystals grow laterally to form a continuous oxide film, crystal oxidation further grown up and started to grow vertically, then some small holes emerged, as time increased, crystal oxidation continue to grow up and the holes disappear, generated a lot of whiskers on the crystal oxidation. And with the oxidation temperature increased, the growing process gets faster. This shows that as the temperature increased, the antioxidant capacity of the metal reduced.The oxidation on the inner wall of TP347H and TP304H consists of two layers oxides, the outer oxidation layer is relatively loose, it is mainly consist of Fe2O3, the inner oxidation layer is dense, they are (Fe, Cr) spinel oxidation and NiO composition; the interface between inner and outer oxidation is unanimous to the original metal surface, internal and external layers is connected by holes of different sizes at the most situation, with the extension of oxidation time, oxidation layer thickness increased.The growth mode of the inner oxidation layer of TP347H and TP304H steel is pit-like, the main process of pit-like oxidation is as follows:oxidation and metal interface is hilly and the thickness of inner oxidation is heterogeneous, the development of oxidation is from pit-like to the outer wall. At the development stage there is no suppression films in front of the oxidation pit, inhibit films are on both sides of the pit. Oxidation rate in the front of the pit is significantly greater than oxidation rate on both sides of the pit, the depth of the oxidation pit is much greater than the width of the pit, oxidation pit morphology is open. When the oxidation pit at the front developed to the grain boundaries, due to Cr atoms have faster speed at the grain boundary and inhibiting films formed at the front of oxidation, oxidation pit is stagnant and its shape is closed type. Membrane has a high rejection of Cr-containing, in front of the inhibit membrane formed affluent Cr and depleted Ni zone there. Inhibiting film thickness,Cr-containing,Ni-containing,the thickness and extent of poor and rich Ni metal band in the front metal is bigger than those in front of Oxidation pit. The distribution of chemical elements in the inner oxidation layer are asymmetrical, some broken inhibited membrane can be detected in the inner layer of the oxidation, indicating that oxidation is oxide film formation and being overtaken process.