An Experimental Study on the Heterometallic Corrosion Mechanism of 30CrMo Steel/625 Alloy for Underwater X-Trees
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摘要:
针对水下采油树在海水和内部流体长期作用下极易发生腐蚀的问题，借助高温高压反应釜，对流花11–1油田水下采油树的30CrMo钢/625合金的电偶腐蚀和堆焊金属腐蚀分别进行了腐蚀浸泡模拟试验，采用扫描电子显微镜和能谱分析仪分析了腐蚀产物膜的微观形貌和化学成分，并分析了30CrMo钢和625合金的异金属腐蚀机理。研究发现，水下采油树的30CrMo钢与625合金接触时，30CrMo钢易发生较严重的均匀腐蚀，且接触位置及堆焊焊缝处30CrMo钢存在较严重的沟槽腐蚀。水下采油树不同金属接触腐蚀的原因包括电偶腐蚀和缝隙腐蚀，堆焊修复后热影响区金相组织的不均匀性也会加剧缝隙腐蚀。研究结果表明，水下采油树应尽量避免不同金属接触形成异金属电偶腐蚀体系，修复已发生局部腐蚀失效的水下采油树时应合理选择堆焊材料并进行全覆盖堆焊，避免异金属腐蚀风险。
Abstract:Underwater X-trees are prone to ongoing corrosion under the action of seawater and internal fluids. To address the problem, simulation experiments for the galvanic corrosion and surfacing metal corrosion of 30CrMo steel/625 alloy for the underwater X (Christmas) tree of the Liuhua 11–1 Oilfield were conducted by means of high temperature high pressure reaction kettle; the micromorphology and chemical composition of corrosion product film were analyzed by scanning electron microscope and energy spectrum analyzer, respectively, and analyzed the heterometallic corrosion mechanism of 30CrMo steel/625 alloy. The studies suggested that 30CrMo steel side of the heterometallic contact surface of the underwater X-tree was more likely to suffer a more severe uniform corrosion, and the 30CrMo steel at the contact position and the surfacing weld had more serious groove corrosion. The causes of heterometallic corrosion in the X-tree include galvanic corrosion and crevice corrosion. After surfacing repair, the unevenness of the metallographic structure in the heat-affected zone also exacerbates the crevice corrosion. The research results showed that the underwater X-tree should try to avoid the heterometallic corrosion caused by the contact of different metals. Best practices now suggest that when repairing a underwater X-tree with local corrosion failure, the surfacing material should be properly selected and adopted fully covered surfacing to avoid the risk of heterometallic corrosion.
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图 1 流花11–1油田水下采油树腐蚀产物XRD图谱
Figure 1. XRD pattern of corrosion products of underwater X-tree in Liuhua 11-1 Oilfield
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图 2 电偶腐蚀模拟试验的试样夹具示意
Figure 2. Schematic diagram of sample fixture for galvanic corrosion simulation experiment
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图 3 偶接金属试样中的30CrMo钢浸泡7 d后的腐蚀情况
Figure 3. Corrosion of 30CrMo steel in a coupled metal specimen after soaking for 7 days
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图 4 焊接金属试样中的30CrMo钢浸泡7 d后的腐蚀情况
Figure 4. Corrosion of 30CrMo steel in welded metal samples after soaking for 7 days
表 1 实际腐蚀环境与腐蚀试验模拟环境参数
Table 1 Environment parameters of actual corrosion and experimentally simulated corrosion
参数 实际范围 试验参数确定 H2S分压/MPa 0.106～0.124 随着H2S含量增加，钢腐蚀速率增加，呈现明显的局部腐蚀特征，故确定H2S分压为0.124 MPa CO2分压/MPa 0.190～0.240 在较低温度下（60 ℃以下），随着CO2分压增大，腐蚀速率增加，故确定CO2分压为0.240 MPa 总压/MPa 0.380 温度/℃ 65（油层）
50（井口）运行工况温度选用井口温度50 ℃；流花11–1油田水下300 m处水温约为12 ℃，确定停产工况温度为12 ℃ 流速/（m∙s–1） 1.7 偶接试样：运行工况及停产工况均为静止状态
堆焊试样：运行工况流速为1.7 m/s，停产工况流速为0 m/s氯离子含量/（mg∙L–1） 18 969 配制的试验溶液与之基本一致 
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