| The influence of surface conditions on coking behaviors on the inner surface of cracking tube was very important. In this paper,25Cr35Ni alloy surface oxidizing behavior at air, steam and H2/H2O atmosphere were studied. The compound oxide films with excellent anti-coking performance containing MnCr2O4spinel were in-situ prepared on the surface of25Cr35Ni alloy.25Cr35Ni alloy samples were oxidized by steam and air at different temperatures, and the different oxidation layers, acicular Fe rich and granular Cr rich oxide, were formed on the surface of the samples respectively. The results of the subsequent coking tests based on light naphtha showed that these different surface conditions had little effect on the morphology and structure of cokes deposited on the oxide layers. The cokes on the surface were mainly formed by catalytic coking.The multilayer oxide films with different function were in-situ prepared on the surface of25Cr35Ni alloy at the low oxygen partial pressure formed by H2/steam atmosphere from900to1200℃. The total thickness of oxide films was about7μm, which could be characterized as three layers, i.e., the outer MnCr2O4spinel film, the middle Cr2O3rich film, and the most inner SiO2film, respectively. The weight of MnCr2O4spinel film was increased with the increasing heating temperature, while the morphologies of the film change form acicular granulate. The size of spinel granules formed at1200℃was about1μm. The micro-hardness, thermal shock resistance ability and coking inhibition of the multilayer oxidation layer were investigated, and the results showed that the average micro-hardness of multilayer oxidation layer was991HV. It could bear at least20times thermal shock cycles in water quenching at900℃. The results coking tests with naptha feedback showed that the catalytic coking and total coking inhibition rate of spiel were86.5%and67.92%respectively. The coking inhibition rate after3times coking and decoking test, was54.34%at least. |
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