沃斯田鐵系不銹鋼於CO-H2-H2O氣氛中之金屬塵化現象研究

Abstract

Metal dusting is a high temperature corrosion phenomenon which mainly occurs in high carbon-activity atmospheres in the temperature ranging of 400 to 800 ℃. Metal dusting process would disintegrate metal materials into a mixture of powdery metal, oxide, carbide and carbon deposits. The metal dusting behavior of Type 304L, 310, 321 and 347 stainless steels (SSs) in a flowing CO/H2/H2O mixed gas stream at 600 ℃ was investigated. After a long-term exposure(500-hr) in a 35 % CO + 60 % H2 + 5 % H2O gas, large pits were formed on steel surfaces. The microstructures and chemical compositions of the reaction products and the substrates under the pits were examined using a scanning electron microscope (SEM) and a transmission electron microscope (TEM), each combined with an energy dispersive spectrometer (EDS). At the bottom of the pits, a thick layer of coke consisting of carbon and disintegrated Fe/Ni particles was found. A thin layer of oxide was observed below this outer layer, and a Cr-depleted precipitate-free zone contained voids just beneath this layer which resulted from the selective oxidation of stainless steels. Massive matrix carbide precipitation occurred below the Cr-depleted zone. Moving toward the interior of the substrate, Massive carbide and intergranular carbides were formed in the inner most portions of the scales beneath the substrate. The aspect ratio of the pits formed in 304L SS was higher than that formed in 347 SS. The experimental results showed that niobium (Nb) could delay the ingress of carbon and retard the metal dusting reaction. According to the results, the edges and corners of 321 SS were the most sensitive sites for pitting attack. The relatively high tendency for oxide passive film to breakdown at the edges and corners was responsible for the initiation of metal dusting. The composition of the metal particles dislodged from 321 SS surface was basically the Fe/Ni metallic phase, which might be either enriched with Ni or Fe. The carbide precipitation zone under the pit increased with increasing the exposure time in the high carbon-activity gas-environment. Matrix carbide in the form of Cr7C3 was observed in the outer zone close to the pit bottom, while grain boundary carbide in the form Cr23C6 was observed in the area far below the pit bottom. After a long-term exposure (500-hr) of 310 SS, the coalescence of small pits would cause the formation of pit with a petal-like feature. The cracks at the substrate under the pit were the preferential sites for carbon inward diffusion and led to metal dusting attack. Secondary pit nucleation at the bottom of the primary pit would lead to the development of irregular appearance of the bottom surface. The relatively high tendency for oxide passive film to breakdown at the Cr/Mn/Fe oxide covered area was responsible for the enlargement of pit size.