Research On Low-temperature Pack Aluminizing Of N80Oil Casing Steel And Its Corrosion Resistance

In order to improve the corrosion resistance of N80oil casing steel by pack aluminizingand maintain its good natural mechanical properties, a rapid low-temperature pack aluminizingprocess under560"C was carired out on N80oil casing steel with self-nanocrystallizationsurface fuliflled by surface mechanical treatment (SMAT). The phase composition andmicrostructure of as-prepared aluminizing layer were characterized by means of XRD、SEM、EDS respectively. Furthermore, the effects of aluminizing process parameters including thecontent of Al and NH4CI in the pack powder, pack aluminizing temperature and duration onthe thickness of aluminizing layer were studied by Li6(45) orthogonal expeirments. Based onthe data rfom orthogonal expeirments, a BP net model was built to predict the thickness ofpack aluminizing layer. Moreover, the corrosion resistance and failure mechanism of N80oilcasing steel with rapid low-temperature aluminizing layer were also analyzed. Some mainconclusions are listed as follows:A dense aluminizing layer with uniform thickness of72pm can be obtained under530X)only packing for4hours on the surface of N80oil casing steel pre-treated by SMAT as thepack powder consisting of95%aluminium powder and5%ammonium chloirde powder whichis mainly consist of FeAb, Three kinds of morphologies can be found in the as-obtainedaluminizing layer, the inner layer near to the substrate is composed by some fine grains, longcolumnar crystals existing in the middle layer, and the outmost layer consisting of someequiaxed crystals. With the increasing thickness of aluminizing layer, the content of diffusedAl atoms tested in the cross-sectional aluminizing layer gradiently decreased whilemicrohardness of aluminizing layer is higher than the substrate.Based011the results of Li6(45) orthogonal expeirments mentioned above, it can beconcluded that the influence order of processing parameters on the thickness of aluminizinglayer is as follows: the content of NH4Cl>packing temperature>thc content of Al>packingduration. Consideirng the thickness of the aluminizing layer as the index, the premium packingprocessing should be packed at540^0for10hours by using the pack powder containing 95.24%aluminum and4.76%ammonium chloride. On the other hand, an improved BP neuralnetwork model was built to predict the thickness of aluminizing layer which can give a veryreliable output atfer a good training. The correlation coefficient R is equal to0.993whichshows that there is a good accordance between the net prediction and experimental result.Namely, the improved BP neural network model can be used to perdict the thickness ofaluminizing layer.Compairng the electrochemical properties of original N80oil casing steel, aluminizedN80oil casing steel shows an obvious corrosion resistance and a lower corrosion rate. Underthe simulation corrosion environment of real service condition of oil casing steel, thealuminized N80oil casing steel exhibits a better general and pitting corrosion resistance whenthe partial pressure of C02is IMPa, the partial pressure of H2S is0,5MPa, corrosiontemperature is100’C and salinity is70000mg/L. In addition, the wear resistance of N80oilcasing steel can also be improved by the same low-temperature pack aluminizing.