| At present,waste incineration is one of the most effective and promising means to deal with garbage.The generated heat energy can be used to generate electricity and heating to achieve economic and environmental goals.However,there are serious high temperature corrosion problems in the waste incineration system,resulting in the reduction of metal tube wall and even the detonation of heat exchanger equipment.These problems cause serious safety accidents and increase maintenance costs.For the moment,low grade of stainless steels or nickel base alloys are generally used in the severe corroded parts in waste incineration system.However,the corrosion resistance of low grade of stainless steels is not so good that it must be changed frequently,and the nickel base alloys are so expensive that the cost of equipment has been greatly increased.Super austenitic stainless steel 254SMO is one type of high alloy stainless steels.It has excellent corrosion resistance and good comprehensive mechanical properties.Compared to nickel base alloy,its cost is relatively cheap.So it is expected to partially replace nickel base alloys to be the preferred material of heat exchange equipment.However,the study of high temperature corrosion behavior in the waste incineration environment is limited to low grade stainless steels and nickel base alloys,and there is little information about 254SMO on this issue.Therefore,the research on the high temperature corrosion behavior and corrosion mechanism of 254SMO is very important.It can also provide theoretical guidance for the application of this steel in the waste incineration system.In our study,thermodynamic calculations were performed using HSC 6.0 and Thermo-Calc software to determine the likelihood of the reactions and the stability of corrosion products.The corrosion behavior of the steel in high temperature mixed salt and high temperature mixed gases was studied using X-ray diffraction(XRD),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS)and transmission electron microscopy(TEM),respectively.The corrosion kinetics curves,macroscopic morphology of samples after corrosion,micro-morphology and composition of the corrosion product layer were analyzed.The related mechanism is proposed.HSC 6.0 software calculation results showed that the alloy elements of 254SMO could react with O2 and Cl2 from 400? to 1000?,and the order of oxide stability was:Cr2O3>Mn2O3>FeO>MoO2>Fe2O3>NiO>MoO3.The stable sequence of chlorides was:CrCl2>FeCl2>NiCl2>CrCl3>FeCl3.As temperature increased,the oxygen partial pressure of the oxides and the chlorine partial pressure of the chlorides gradually increased.The results of Thermo-Calc software showed that the main secondary phases formed in 254SMO were Laves,?,? phase and Cr2N.in the mixed salt environment.When the temperature was 400?,corrosion reactions did not happen and only mixed salts were found on the sample surface.When the temperature increased to 600?,corrosion weight of the samples increased slightly,and the chlorinated reaction was the main corrosion form.The temperature was so low that the corrosion reaction was slow,and the oxide layer was very dense.Autocatalytic effect of Cl2 was so small that the corrosion degree is very light.At the temperature between 800? and 1000?within 12h,the corrosion rate was lighter and corrosion mechanism is same as that of 600?.In the opposite,the corrosion rate of samples was increased significantly after 12h,and the main reaction form was oxidation.The higher the temperature,the more serious the oxidation corrosion was.The growth stress of Cr2O3 led to the cracking or peeling of the corrosion product layer.Meanwhile,the formation of Laves,?,? phase and Cr2N caused Cr-and Mo-depleted zones in surrounding matrix,which promoted the oxidation corrosion reaction.As a result,the catastrophic oxidation happened.The corrosion weight of 254SMO was very small at 400? and 600? in the mixed gases environment.At 400?,temperature was so low that the Fe and Cr atoms of matrix and O2,Cl2 diffused very slowly,the sample almost did not react with HCl gas,the corrosion weight gain was almost equal to zero.When temperature incresed to 600? light corrosion occured in local areas of the sample and a small amount of oxides generated.The corrosion rate of 600? was slightly higher than 400 ?.When temperature reached to 800? and 1000? the corrosion weight was higher.The Fe and Cr atoms of matrix and O2,Cl2 diffused faster,and the corrosion reaction rate increased dramaticly,a complete corrosion product layer formed on the sample surface.At 800?,corrosion reaction followed parabolic law,the corrosion products were denser and good adhesion with matrix and had a certain protective effect on the matrix.At 1000?,corrosion reaction followed linear law,the formation of Cr2O3 destroyed the density and integrity of corrosion product layer.In addition,the formation of precipitates played an accelerating role in the corrosion.Based on the above analysis,the corrosion degree at 400? and 600? is lighter in the mixed salt environment,and the corrosion degree greatly increases at 800-1000? after 12h.In the mixed gases environment,the sample hardly react at 400?,the corrosion degree is slighter at 600?.However,the corrosion weight gain increase significantly at 800? and 1000?.So 254SMO can be used in waste incineration environment under 600?. |
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