Local Corrosion Behavior of 825 Alloy in Elemental Sulfur-Containing High Temperature and High Acid Environment
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摘要:
在132 ℃、H2S分压4.8 MPa、CO2分压1.6 MPa、氯离子质量浓度42 750 mg·L−1的气田模拟地层水中,通过腐蚀挂片试验研究了825合金在含单质硫(每升地层水中包裹合金的单质硫质量为10 g)与不含单质硫条件下的局部腐蚀行为。结果表明:在不含单质硫条件下825合金几乎不发生腐蚀,腐蚀6 d时的腐蚀速率仅为0.007 7 mm·a−1;添加单质硫腐蚀3 d时腐蚀速率达到0.055 2 mm·a−1,合金发生了严重的局部腐蚀,随腐蚀时间的延长,合金表面局部腐蚀坑面积、腐蚀坑最大深度和腐蚀速率增大。在不含单质硫条件下合金表面腐蚀产物极少,主要为FeCO3,在含单质硫条件下,随着腐蚀时间的延长,表面腐蚀产物增多,主要为FeCO3与FeS;单质硫在高温下发生歧化反应产生H+和S2−,使825合金表面腐蚀产物膜破裂,从而加剧了局部腐蚀。
Abstract:The local corrosion behavior of 825 alloy in simulated formation water of gas field at 132 ℃ with H2S partial pressure of 4.8 MPa, CO2 partial pressure of 1.6 MPa, and chloride ion mass concentration of 42 750 mg · L−1 under elemental sulfur-containing condition (mass of elemental sulfur coated alloy in per liter formation water was 10 g)and under elemental sulfur-free condition was investigated by corrosion coupon tests. The results show that under the elemental sulfur-free condition, 825 alloy almost did not corrode, and the corrosion rate was only 0.007 7 mm · a−1 during corrosion for 6 d. The corrosion rate reached 0.055 2 mm · a−1 duing corrosion for 3 d under elemental sulfur-containing condition, showing the serious local corrosion of the alloy. With the increase of the corrosion time, the area of local corrosion pits on the alloy surface, the maximum depth of corrosion pits and the corrosion rate increased. The corrosion products of the alloy under the elemental sulfur-free condition were very few and were mainly composed of FeCO3. Under the elemental sulfur-containing condition, the surface corrosion products increased in amount with the corrosion time and were mainly composed of FeCO3 and FeS. The elemental sulfur underwent disproportionation reaction at high temperature, producing H+ and S2−, making 825 alloy corrosion product film rupture, therefore promoting the local corrosion.
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0. 引言
酸性气田中存在H2S和CO2等腐蚀介质,在进行开发时管材的腐蚀已成为一个巨大的难题,并且若存在单质硫,腐蚀更加严重[1-9]。在这种高酸性环境中,管道的选材转向抗H2S和单质硫腐蚀能力更强的Ni-Fe-Cr三元合金或Ni-Cr-Fe-Mo-Cu多元合金,如元坝气田采用了028、825、G3等镍基合金管[10],普光气田采用了718等镍基合金管[11]。
镍基合金具有优异的防腐性能,但在高温、H2S、CO2与单质硫的环境中仍可能发生较为严重的腐蚀。方建波等[12]分析了某热采井中825合金的腐蚀原因,发现在高温高压环境中825合金发生了氧腐蚀与硫腐蚀。张瑞等[13]发现在205 ℃,含H2S、CO2、氯离子及单质硫的环境中,718合金发生明显的点蚀与均匀腐蚀,高温下的单质硫直接或间接与金属发生反应导致大面积的均匀腐蚀。根据ISO15156—2020,在温度高于132 ℃时,825等镍基合金在含单质硫环境下的适用性并不明确,而目前对高含硫环境中825合金的耐局部腐蚀能力的研究较少。为此,作者根据某高酸性气田生产工况,利用高温高压反应釜模拟出高温,含H2S、CO2、氯离子的气田模拟地层水环境,并对825合金进行腐蚀挂片试验,对比分析了825合金在含单质硫与不含单质硫条件下的局部腐蚀行为,以期为在含单质硫酸性气田中管道的选材提供一定指导。
1. 试样制备与试验方法
试验材料选用新的825合金无缝管,由江苏武进不锈股份有限公司提供,外径为168.3 mm,壁厚为14.3 mm,化学成分如表1所示。在825合金管上截取尺寸为30 mm×15 mm×3 mm的腐蚀挂片试样,将试样用300#,600#,800#,1200#砂纸逐级打磨,经石油醚、无水乙醇清洗并风干后,用精度为0.1 mg的电子天平称取质量并置于干燥皿中待用。
元素 C Cr Ni Mo Ti Si Cu P Mn Fe 质量分数/% 0.014 21.26 39.54 2.89 0.81 0.18 1.82 0.01 0.44 33.04 采用西南石油大学自研的高温高压反应釜进行腐蚀挂片试验,试验溶液为某高酸性气田模拟地层水,组成见表2,采用NaCl(分析纯)、Na2SO4(分析纯)、NaHCO3(分析纯)、CaCl2(分析纯)、MgCl2·6H2O(分析纯)、KCl(分析纯)及去离子水(一级水)配制。在高温高压反应釜中加入2 L试验溶液,将试样挂在试样架上并放入反应釜中,然后关闭反应釜并密封;向高温高压反应釜中持续通入低流量高纯氮除氧2 h,之后将反应釜温度升至132 ℃,待温度稳定后,先向釜内通入H2S使压力达到4.8 MPa,然后通入CO2达到总压6.4 MPa。试验设置5种工况:工况1为不添加单质硫腐蚀6 d;工况2~5均以熔覆的方式添加单质硫(试验开始前将单质硫与试样紧密包裹,放入高温高压釜中,待温度上升至132 ℃后,单质硫即处于熔覆态,与试样充分接触),添加量为每升试验溶液中添加10 g单质硫,腐蚀时间分别为3,6,9,12 d。不同试验条件下均设置5个平行试样,其中3个试样用于计算腐蚀速率,其余2个试样用于腐蚀形貌观察及微区成分分析。
组成 Na++K+ Ca2+ Mg2+ Cl− 质量浓度/(mg·L−1) 31 702 372 73 42 750 8 816 824 用去膜液(10 g六次甲基四胺+100 mL浓盐酸+去离子水定容至1 L)去除试样表面腐蚀产物,再用去离子水清洗,无水乙醇脱水后风干;用精度为0.1 mg的电子天平称取腐蚀后的试样质量,取3个试样的平均值计算腐蚀速率,计算公式为
(1) 式中:v为腐蚀速率,mm·a−1;Δm为腐蚀前后试样的质量差,g;ρ为试样密度,g·cm−3;A为试样表面积,cm2;t为腐蚀时间,h。
采用FRI Quanta 650 FEG型扫描电镜(SEM)观察试样的腐蚀形貌,用附带的能谱仪(EDS)分析腐蚀产物的元素组成。采用VHX-7000型景深三维显微镜观察试样的局部腐蚀形貌。
2. 试验结果与讨论
2.1 宏观腐蚀形貌
由图1可见:在未添加单质硫腐蚀6 d(工况1)条件下,825合金表面仍有金属光泽,几乎未见腐蚀现象;在添加单质硫腐蚀3 d(工况2)条件下,合金表面存在肉眼可见的点蚀,当腐蚀时间延长至6,9,12 d(工况3,4,5)时,表面存在明显的局部腐蚀坑,并且局部腐蚀坑的面积随腐蚀时间的延长而增大,说明局部腐蚀随时间延长越发严重。
2.2 局部腐蚀形貌
由图2可知:在未添加单质硫腐蚀6 d时,825合金表面较平整,腐蚀痕迹轻微;添加单质硫后,随着腐蚀时间由3 d延长至12 d,合金表面的局部腐蚀坑最大深度由7.95 μm增大到48.29 μm,并且局部腐蚀坑的面积也增大。与未添加单质硫腐蚀6 d条件下(局部腐蚀最大深度2.63 μm)相比,添加单质硫腐蚀6 d时合金的局部腐蚀坑最大深度增加到15.41 μm,局部腐蚀程度加重。
2.3 腐蚀速率
工况1、工况2、工况3、工况4、工况5下合金的腐蚀速率分别为0.007 7,0.055 2,0.073 6,0.088 0,0.114 5 mm·a−1。对比可知,添加单质硫腐蚀6 d时,825合金的腐蚀速率相比于未添加单质硫腐蚀6 d时增大了8.56倍,这可能是因为单质硫使825合金表面形成的钝化膜结构与成分发生了变化[14],对基体的保护作用有所减弱。添加单质硫条件下825合金的腐蚀速率随着腐蚀时间的延长逐渐增大。
2.4 微观腐蚀形貌与腐蚀产物组成
由图3和表3可以看出:在未添加单质硫腐蚀6 d条件下,825合金表面光滑,仍可见加工痕迹,几乎未被腐蚀,腐蚀产物极少,腐蚀产物中的硫含量极低,碳、氧含量也较低,判断表面形成FeCO3产物膜[15-16];在添加单质硫条件下,随着腐蚀时间的延长,合金表面腐蚀产物增多,腐蚀产物中碳、氧、硫含量均增加,铁、铬、镍含量均降低,判断腐蚀产物主要为FeCO3与FeS[17-18]。与未添加单质硫腐蚀6 d时相比,添加单质硫腐蚀6 d时合金表面的腐蚀产物明显增多。
工况 质量分数/% C O S Cr Fe Ni Ti Cu 1 5.71 0.91 0.28 21.72 29.93 38.66 0.99 1.80 2 6.52 0.83 0.92 21.09 30.33 37.66 0.83 1.82 3 7.63 1.36 1.99 20.74 29.78 36.15 0.80 1.55 4 8.02 3.28 3.40 20.69 27.85 34.46 0.87 1.43 5 9.57 4.67 4.52 20.09 24.75 34.32 0.68 1.40 由图4可见,在添加单质硫腐蚀12 d条件下,825合金表面呈现出2种腐蚀形貌,一部分表面较平整,另一部分表面破裂且存在腐蚀产物的堆积。碳元素和氧元素的分布规律一致,结合铁元素的分布判断腐蚀产物为FeCO3。结合铁元素与硫元素的分布可知,表面破裂区域的FeS腐蚀产物膜发生剥落,导致硫元素继续向内部沉积并富集,从而加剧了腐蚀[19-20]。当温度高于单质硫熔点(120 ℃)时,合金表面的单质硫极易发生歧化反应生成H2S与H2SO4,产生大量H+造成局部酸化,使钝化膜溶解;生成的S2−可与Cl−及OH−竞争吸附于部分氧空位处,并逐渐在钝化膜表层形成金属硫化物FeS,同时S2−借助空位迁移扩散到钝化膜内层,降低钝化膜的完整性,并形成点蚀核。随着腐蚀时间的延长,基体与腐蚀介质接触的时间延长,导致腐蚀区域扩大,从而形成局部腐蚀坑[21-22]。综上,在含单质硫条件下,825合金的局部腐蚀严重,因此在实际应用中,建议使用溶硫剂等措施来增加825合金的安全服役寿命。
3. 结论
(1)在132 ℃、H2S分压4.8 MPa、CO2分压1.6 MPa、氯离子质量浓度42 750 mg·L−1的模拟地层水环境中,825合金几乎不发生腐蚀,但添加单质硫后,825合金发生了严重的局部腐蚀,且随腐蚀时间由3 d延长至12 d,腐蚀程度加剧,局部腐蚀坑面积增大,腐蚀坑最大深度由7.95 μm增大到48.29 μm。
(2)添加单质硫腐蚀6 d时825合金的腐蚀速率相比未添加单质硫腐蚀6 d时增大8.56倍,且随着腐蚀时间的延长,腐蚀速率逐渐增大。在不含单质硫条件下合金表面腐蚀产物极少,主要为FeCO3,在含单质硫条件下,随着腐蚀时间的延长,表面腐蚀产物增多,主要为FeCO3与FeS。在高于单质硫熔点(120 ℃)的高温条件下,单质硫发生歧化反应产生H+和S2−,使825合金表面腐蚀产物膜破裂,从而加剧局部腐蚀,降低了825合金的耐腐蚀性能。
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表 1 825合金的化学成分
Table 1 Chemical composition of 825 alloy
元素 C Cr Ni Mo Ti Si Cu P Mn Fe 质量分数/% 0.014 21.26 39.54 2.89 0.81 0.18 1.82 0.01 0.44 33.04 表 2 气田模拟地层水的组成
Table 2 Composition of simulated formation water in gasfield
组成 Na++K+ Ca2+ Mg2+ Cl− 质量浓度/(mg·L−1) 31 702 372 73 42 750 8 816 824 Table 3 EDS analysis results of corrosion product (box area in Fig. 3) on surface of 825 alloy under different conditions
工况 质量分数/% C O S Cr Fe Ni Ti Cu 1 5.71 0.91 0.28 21.72 29.93 38.66 0.99 1.80 2 6.52 0.83 0.92 21.09 30.33 37.66 0.83 1.82 3 7.63 1.36 1.99 20.74 29.78 36.15 0.80 1.55 4 8.02 3.28 3.40 20.69 27.85 34.46 0.87 1.43 5 9.57 4.67 4.52 20.09 24.75 34.32 0.68 1.40 -
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