| 316L stainless steel contains elements such as Cr, Mo, Ni, N. With its excellentcorrosion resistance,316L stainless steel is widely applied in various industries,especially in the oil industry. Usually,316L stainless steel is fabricated into linings toimprove corrosion resistance.Our project is supported by an oil field in western China. To control severe CO2corrosion along the pipeline, the oil field decided to apply316L stainless steel liningsto the pipes. However, the oil field locates in a place with high water salinity whichmeans a high concentration of Cl-. And in terms of pitting corrosion resistance, therestill lack enough evidence to show that316L stainless steel and its weld work as goodas duplex stainless steel. Failures resulting from pitting corrosion have been observedin weld and heat affected zone within the linings of composite pipes during practicalfabrication process.In order to solve this problem, we investigated the corrosion behavior of316Lstainless steel in solution with different Cl-concentration. Total immersion test wasadopted to conduct the experiment. Optical microscope, scanning electron microscopeand microhardness tester are used to observe and analyze morphology after etching.Moreover, we also construct kinetic models to describe the development of pittingcorrosion with the help of Gumbel distribution and response surface methodology.According to our results, the circumferential weld and longitudinal weld mainlyconsist of delta ferrite and austenite. For the circumferential weld, corrosion initiallyhappens in seal weld with high chromium concentration. Composition segregation ofCr, Ni and C, along with the martensite, increase the hardness of seal weld. For thelongitudinal weld, corrosion is firstly observed in heat affected zone and then spreadsto the whole weld. Based on Gumbel distribution, models are created to describe thedevelopment of pitting corrosion within the linings of composite pipes. From thekinetic model, relation between the max pitting depth and time can be explained by Y=a·Xb..Besides, we also use response surface methodology to predict the max pittingdepth under different working conditions.In our research, we mainly focus on the corrosion morphology and pittingcorrosion resistance of316L stainless steel. The obtained results provide statisticaland theoretical support for the practical application of316L composite pipes. |
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