| Based on the analysis of physicochemical properties of 316 stainless steel,the high temperature and high pressure corrosion rate test under simulated working conditions,four-point bending anti-SCC performance test and in-situ electrochemical test,supplemented by standard tests such as crevice corrosion,intergranular corrosion and erosion were performed in order to study the corrosion resistance of 316 stainless steel in high temperature and high pressure and high corrosive environment at the wellhead.The feasibility of using 316stainless steel as a gasket material for the sealing ring of the sealing part of the tree was also discussed.The results show that the physical and chemical properties of 316 stainless steel meet the requirements of wellhead equipment specifications.316 stainless steel own good anti-SCC performance in high temperature CO2+H2S corrosion environment.The uniform corrosion rate of 316 stainless steel does not change significantly with the increase of temperature.At 220°C,it is only 0.0130 mm/a.It implied the good resistance to uniform corrosion.When the partial pressure of H2S is increased to 2.0 MPa?220°C?,the uniform corrosion rate of 316 stainless steel increases to 0.0278 mm/a,shown moderate corrosion,and pitting was observed on the surface of the sample.Moreover,the corrosion reaction of 316stainless steel is controlled by the anode reaction process,thus the corrosion rate is kept at a low level due to the protection of the passivation film.However,as the increase of temperature,H2S partial pressure and Cl-concentration,Metal sulfides appear in the passivation film,not only the passivation film stability and the polarization resistance reduced,but also the corrosion resistance is lowered.Therefore,when the wellhead temperature exceeds 220°C and the H2S concentration exceeds 0.5 MPa,316 stainless steel is not recommended as a surfacing material for sealing parts of the tree on high temperature and high pressure gas wells.The results of standard crevice corrosion and intergranular corrosion test show that 316stainless steel has poor crevice corrosion resistance in high Cl-concentration and oxidizing metal cation environment,and crevice corrosion becomes more serious with increasing temperature.In addition,316 stainless steel undergoes significant intergranular corrosion in the method B?sulfuric acid-iron sulfate corrosion test?,the method C?65%nitric acid corrosion test?and the method E?sulfuric acid-copper sulfate corrosion test?.The results of the erosion test show that the weight loss rate of the sample is largest when the impact angle is 30°,and then the weight loss rate decreases with the increase of the angle.When the impact angle reaches to 90°,the weight loss rate is smallest.Higher weight loss rate results in higher solid particle flow rates.316 stainless steel as the high temperature and high pressure gas well wellhead sealing part of the surfacing material applicable temperature should not exceed 220°C,hydrogen sulfide partial pressure is not higher than 0.5 MPa.At the same time,reasonable structural design and protective measures should be taken at the sealing part of the tree to reduce the crevice corrosion sensitivity of 316 stainless steel,and optimize the flow field of the sealing part of the tree to improve the flow rate of the gas at the throttle valve,thereby reducing the weight loss rate of 316 stainless steel. |
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