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Research on Pricipitated Phrases and Corrosion Resistance of INCOLOY 028

Author: LinYi
Tutor: SongHongWei；DanAiDang
School: Shanghai Jiaotong University
Course: Materials Science
Keywords: Incoloy028 sensitizing heat treatment precipitated phases σphase pitting corrosion intergranular corrosion
CLC: TE980
Type: Master's thesis
Year: 2012
Downloads: 152
Quote: 1
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Abstract

With the development of the oil industry, more and more acid oil fields which contain H2S/CO2 have been exploited. The severe environment call higher request on the corrosion resistance of the materials which is used in the wells, and the Ni-base alloy which has a lot of advantages in anti the acid oil and gas corrosion has been widely used in the manufactory of oil gas industry. Incoloy 028 is a alloy which contains high contents of Cr,Fe,Ni has good capability in anti oxidizing medium corrosion, anti stress-corrosion, anti pitting corrosion and anti crevice corrosion. In a word, it is a suitable material for OCTG.The hot working of Incoloy 028 has a long time of heating process, and the temperature for the heating is between 700℃and 1000℃. It is possible that the microstructure of Incoloy 028 would precipitate the precipitated phases. The different types and different quantity of precipitated phases would have negative effect on the corrosion resistance of the alloy. Consequently, it is meaningful for ensuring the alloy with excellent corrosion resistance performance that research on precipitated phases formation rules and the effect of it on the corrosion behavior. The generation of precipitated phases in the alloy would has a negative effect on the alloy corrosion resistance, and the heat treatment process would affect the microstructure of the alloy directly. In this dissertation, the specific productive conditions of Incoloy 028 tube were considered. The results of experiment and analysis will provide theory basis and reference basis for many applications.For instance, Suitable plot of heat treatment process including heat treatment temperature and time and avoiding the generation of harmful phase.In the dissertation, thermodynamic calculation software JMatPro was employed to implement the thermodynamic calculation and plot several sensitizing heat treatments. Optical microscope, X-rays Diffraction Spectrum, scanning electron microscope and Transmission electron microscope were employed to investigate the precipitated phases of Incoloy 028 under different sensitizing heat treatments. The type, morphology, distribution, structure and composition of the precipitated phases were revealed. On the basis of experiments results above, The precipitated phase formation mechanism of Incoloy 028 was proposed. Meanwhile, Constant potential anode polarization curve, oxalic acid electrolysis and was used to analyse the effect of the precipitated phases of the alloy on the corrosion resistance.Metallographic analysis results shows that Incoloy 028 obviously precipitate phases after the sensitizing heat treatments of 800℃~1000℃.After a long time of sensitizing heat treatments of 900℃, matrix structure of the alloy precipitate second phase. TEM analysis results shows that the alloy matrix precipitated acicularσphase and globular MC, and there are a string of M23C6 when it is on the initial stage of sensitization processing of 900℃. XRD analysis shows that there are obviousσphase after the sensitization processing of 900 5h,AC. SEM analysis shows that the majority of precipitated phases after the sensitization processing of 2h areσphases, and EDS analysis shows that theseσphases contains much higher element contents of Cr, Mo than the matrix’s . However, the element content of Fe, Ni are lower than the matrix’s.The generation of precipitated phrase after sensitizing heat treatments cause the alloying element content such as Cr, Mo to reduce. Because of that, the ability to generate the passivation coating was weakened and the anode potential was reduced in these areas. There are erosion pit and ditch appearing first in these areas.The Aggregation of Cl- Ion in pitting pit cause the medium environment acid and local corrosion was aggravated. Meanwhile, Rich chromium area and poor chromium area generate Activation and passivation batteries which has a large potential difference. Consequently, the micro-bimetallic corrosion effects were aggravated, which can also weaken the intergranular corrosion resistance of the alloy.

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