| 其他摘要 | In this thesis, the erosion-corrosion processes of AISI316L stainless steel and AISI1020 carbon steel in artificial sea water entrained silica particle were investigated. Firstly, a testing loop for erosion-corrosion in two-phase liquid/solid flow with Laser-Doppler Anemometer was adjusted to conduct the experiment of erosion-corrosion of AISI 304 stainless steel in artificial sea water entrained silica particle. The results showed that the set-up worked well and the collected data were comparable. Hydrodynamics of disturbed flow field in a flow model (25/20/40mm) in single phase flow was measured by 2-D Laser Doppler Anemometer. The axial and tangential mean velocity, its turbulence and turbulent fluctuation were obtained. The results showed that, the reattachment point was at about x/d = 2.5D away from sudden expansion. Circulation took place in the field x/D < 2.5D. Away from x/D = 6D, the flow was established gradually. In the near wall region (about 1.5mm), when the mean velocity was below 1.4m/s, the tangential velocity was very small, and at higher mean velocity, it was prominent. At about x/D = 3D, the maxima of turbulent fluctuation of tangential velocity were reached. When the mean velocity was below 1.4m/s, at about x/D = 2.5d, the maxima of the turbulent fluctuation of axial velocity were reached. At about x/D = 2.5D, the maxima of turbulence of tangential and axial velocity were reached. The erosion-corrosion process of AISI316L stainless steel and SISI1020 carbon steel in artificial sea water entrained 1000ppm silica particle were studied. At x/D = 3D, the Maxima of erosion-corrosion rates of AISI316L stainless steel at different velocities were obtained, which were in accordance with that of tangential velocity and its turbulent fluctuation. As to AISI1020, the maximum erosion-corrosion rate was at about x/D = 2.5, which was coincident with reattachment point. The erosion-corrosion morphologies were observed. For AISI316L stainless steel, in φ20mm pipe, the track of particle impact had a strong direction in accordance with flow. In circulated and established field, the track of particle impact had certain direction, which was in accordance with flow direction. For the position of maximum attack, there were no direction for the track of particle impact. As to AISI 1020 carbon steel, the function of particle impact was mainly to deprive the corrosion products, only a few reached and spalled the matrix. The pitting attack took place in fields of circulation and the position of maximum of attack. Due to the corrosive microcell formed between ferrite and FeC_3, which made the adhesion of pearlite poor, and it was prone to be removed by the impact of particle. The pitting attack could be eliminated by cathodic protection. The influence of particle concentration on the weight loss rate of the maximum attack was studiced. The relation between mass loss rate and velocity were found. as following: For AISI316L stainless steel:
E-C ＝ 0.00123 * V~（2.649） 1000 pmm
＝ 0.00116 * V~（2.947） 2000 ppm
＝ 0.00154 * V~（3.110） 3000 ppm
For AISI 1020 carbon steel:
E-C ＝ 0.17615 * V~（1.105） 1000 ppm
＝ 0.09873 * V~（1.623） 2000 ppm
＝ 0.020432 * V~（1.873） 3000 ppm
According to the exponent of velocity, it was determined that the process of deterioration of AISI316L stainless steel was erosion -corrosion when the particle concentration was smaller than 2000ppm. When the particle concentration was over 3000ppm, the process was erosion dominated. As to AISI316L stainless steel, the critical velocity of erosion-corrosion occurrence was 2m/s, the critical particle concentration between erosion - corrosion and erosion dominated was 3000ppm. On the position of maximum attack the influences of fluid flow on some electrochemical parameters were studied. In terms of potentiodynamic polarization curves, it was observed the anodic and cathodic currents increased with velocity, and there were limited diffusion current. Below 1.4m/s, AISI316L stainless steel could maintain passivation. Over 2.4m/s, the transition from passivation to activation occurred. Critical passive current density I_(max) and passive current density increased with velocity, and the passive regions were shortened sharply. The corrosion potential decreased with velocity. The transfer resistance attained from Nyquist plot decreased with velocity. The effect of oxygen in erosion-corrosion process at the position of the maximum attack was analyzed by limited diffusion current density. The relationships between I_(lim) and V were as follows, V means the mean velocity in φ40 mm pipe. i_1=0.62V~（1.39）（AISI316L）i_1=0.70V~（1.19）（AISI1020）According to the exponent of velocity, it was determined that the erosion-corrosion process of carbon steel was controlled by mass transfer of oxygen. The relations of Sherwood number and Renolds number were as follows,Sh=8.15Re~(0.588) （AISI316L）Sh=9.57Re~(0.63)（AISI2020）The synergistic effects between erosion and corrosion were analyzed. According to the ratio of K_e/K_c, it was determined that, the deterioration process of AISI316L stainless steel was erosion-corrosion, as to AISI 1020 carbon steel, it was mass - transfer controlled corrosion. |
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