工程科学与技术   2020, Vol. 52 Issue (5): 257-262
高温高压含O2溴盐完井液中13Cr不锈钢的腐蚀行为研究
朱金阳1, 张玉楠1, 郑子易2, 许立宁2     
1. 北京科技大学 国家材料服役安全科学中心,北京 100083;
2. 北京科技大学 新材料技术研究院,北京 100083
基金项目: 国家自然科学基金项目(51871025)
摘要: 随着越来越复杂开发工艺的应用,油气田现场服役环境也变得更加复杂苛刻,一些现场操作环节(如井下回注CO2、回注采出水、药剂加注、环空注氮、管道试压、维修操作等)不可避免会将O2引入井下环境。O2的混入会导致井下管材发生不可预期的腐蚀风险,尤其对以13Cr不锈钢为代表的耐蚀油套管材,潜在局部腐蚀风险很高。针对溴盐完井液中O2混入导致井下管材腐蚀风险加剧的问题,采用高温高压腐蚀模拟实验,结合扫描电镜、能谱分析等微观表征手段,研究了高温高压含O2环境下不同浓度溴盐完井液中普通13Cr和超级13Cr两种典型不锈钢油套管材的腐蚀行为及机理。结果表明:在高温高压含O2溴盐完井液环境下,两种13Cr不锈钢的腐蚀速率均较高,尤其局部腐蚀速率;参照NACE PR—0775标准,普通13Cr在1.01 g/cm3浓度的溴盐溶液中就已经属于严重腐蚀,并且随着溴盐质量浓度的升高,腐蚀程度不断加剧,在浓度为1.10 g/cm3达到极严重腐蚀;当溴盐质量浓度达到1.40 g/cm3时,两种材料的最大局部腐蚀速率均已超过5 mm/a;微观形貌分析结果表明,溴盐完井液中O2混入对13Cr不锈钢管材的耐蚀性能具有显著影响,这主要是由于O2混入降低了材料表面钝化膜的稳定性,点蚀萌生于材料基体表面致密富Cr钝化膜的破损处,向基体深部发展,蚀坑周边区域有大量腐蚀产物堆积,蚀坑与周边区域存在局部的电偶效应,进一步加速蚀坑的发展。
关键词: 溴盐完井液    氧腐蚀    普通13Cr    超级13Cr    高温高压    点蚀    
Corrosion Behaviors of 13Cr Stainless Steels in O2-contained Bromine Completion Fluids Under High Temperature and High Pressure
ZHU Jinyang1, ZHANG Yunan1, ZHENG Ziyi2, XU Lining2     
1. National Center for Materials Service Safety, Univ. of Sci. & Technol. Beijing, Beijing 100083, China;
2. Inst. for Advanced Materials and Technol., Univ. of Sci. & Technol. Beijing, Beijing 100083, China
Abstract: With the application of more and more complex oil and gas production technologies, the field service environment in oil and gas fields has become harsher. Furthermore, some field operations (water injection, agent injection, annular nitrogen injection, reinjection of CO2, pressure testing, etc.) will lead O2 into the downhole environment. O2 is highly corrosive and it may cause some unexpected and severe corrosion risks of the downhole tubing, particularly for 13Cr and some other stainless steels, O2 interfusing can significantly increase the localized corrosion risk and brings a lot of corrosion problems during oil and gas exploitation. In this study, the corrosion behaviors and localized corrosion mechanisms of plain and super 13Cr stainless steels under high temperature and high pressure in O2-contained bromine completion fluids was investigated by corrosion simulation tests and surface characterizations. The corrosion simulation test was performed by high temperature and high pressure autoclave. The corrosion product film that formed on the steel substrate was analyzed by scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) techniques. The results showed that both plain and super 13Cr steels exhibit bad corrosion resistance in O2-contained bromine completion fluids with various KBr concentrations. Particularly for localized corrosion rate, the corrosion grades of both plain and super 13Cr steels reach to extremely serious. According to NACE PR—0775 standard, the plain 13Cr steel in the solution of 1.01 g/cm3 reaches the severe corrosion degree, and with the increase of bromine concentration, the degree of corrosion increases, reaching extremely severe corrosion at 1.10 g/cm3. With the increase of bromide concentration, the corrosion rate increases evidently. Even if the super 13Cr stainless steel is subjected to a high bromide concentration, it can still experience severe localized corrosion (> 5 mm/a). The microscopic morphology analysis showed that the pitting initiates at the damaged area of the Cr-rich passive layer on substrate surface. The corrosion products accumulate around the pitting. As a result, a local galvanic effect forms between the pitting and the surrounding area, which further accelerates the propagation of the pitting.
Key words: bromine completion fluid    oxygen corrosion    plain 13Cr    super 13Cr    high temperature and high pressure    pitting corrosion    

油气资源是中国重要的能源战略资源。随着国内油气资源的不断深入开发,越来越复杂开发工艺随之应用,油气田现场环境变得更加复杂苛刻,一些现场操作环节(如药剂加注、环空注氮、井下回注CO2、回注采出水、管道试压、维修操作等)不可避免会将O2引入井下环境[1-2]。O2的混入会导致井下管材发生不可预期的腐蚀风险,尤其对于13Cr这种高含Cr不锈钢管材,面临较高的点蚀破裂风险[3-5]

另一方面,在油气田开采过程中,高压深井一般需要采用高密度的入井流体进行完井试油,或者当做环空保护液替入油套环空。目前国内使用较多的为有机盐完井液和溴盐完井液,其中溴盐完井液中含有较多的攻击性阴离子[6],溴离子与氧气共存条件下油套管将面临更为严峻的腐蚀挑战。然而,目前针对13Cr不锈钢在油气田环境下的腐蚀研究主要集中于温度、CO2分压、H2S含量、流速、Cl浓度等因素对腐蚀影响[7-12],而对于完井液环境下13Cr不锈钢腐蚀行为的研究则相对较少[6,13-14],尤其对于在含氧溴盐完井液体系下13Cr不锈钢的腐蚀目前鲜有报道。

作者针对油气田现场复杂工况可能导致井下混入O2和完井试油过程中注入的溴盐完井液共存环境下的腐蚀问题,以普通13Cr和超级13Cr两种典型不锈钢油套管材为实验对象,采用高温高压腐蚀反应釜模拟国内某超深井现场使用环境,结合扫描电镜、能谱分析等测试手段,研究高温高压含O2环境下溴盐完井液中普通13Cr和超级13Cr 两种不锈钢油套管材的腐蚀行为,并初步探讨其局部腐蚀发展机制。

1 实 验 1.1 实验材料

实验对象油气田广泛使用的普通13Cr和超级13Cr不锈钢油套管材,材质成分如表1所示。

表1 实验所用的两种13Cr不锈钢材质成分 Tab. 1 Main chemical composition of two kinds of 13Cr steels used in this work

1.2 腐蚀模拟实验

为了模拟现场井下的高温高压运行工况,采用高温高压磁力驱动反应釜进行腐蚀模拟实验,腐蚀速率通过失重法获取(参照式(1)),失重测试平行试样3个。腐蚀溶液通过配置不同质量浓度的KBr溶液(1.01、1.10、1.40 g/cm3)以模拟溴盐完井液成分,实验温度150 ℃,压力3 MPa,气体采用合成空气(21% O2+79% N2),实验周期7 d。试样采用条形挂片形式,试样尺寸50 mm(长)×10 mm(宽)×3 mm(厚)。

$C{R_i} = \frac{{87\;600({W_{0i}} - {W_{1i}})}}{{t\rho S}},i = 1,2,\cdots$ (1)

式中:CRi为平均腐蚀速率,mm/a;W0i为试样腐蚀前原始质量,g;W1i为试样酸洗后质量,g;t为腐蚀模拟实验周期,h; $\;\rho $ 为材料密度,g/cm3S为试样暴露面积,cm2

试验结束后,取出试样去离子水清洗,酒精、丙酮除油,冷风吹干,拍照记录宏观腐蚀形貌。随后,部分试样(至少3个)进行酸洗去除表面腐蚀产物(参照ASTM G01[15]),称重计算获取平均腐蚀速率,其余试样进行腐蚀产物膜微观形貌、产物膜成分以及局部腐蚀形貌的分析。

1.3 腐蚀产物膜微观形貌及成分分析

采用LEO–1450型扫描电镜(SEM)对试样表面腐蚀产物膜进行微观形貌的观察和分析,并结合Kevex SuperDry型能谱分析(EDS)系统分析腐蚀产物膜的成分。

2 结果与讨论 2.1 腐蚀速率

图1给出了普通13Cr和超级13Cr两种不锈钢油套管材在高温高压含O2条件下不同浓度溴盐溶液中的腐蚀速率对比图。其中,最大局部腐蚀速率是通过点蚀深度仪测量获得。可以看出,随着溴盐浓度的升高,试样的平均/局部腐蚀速率整体均呈上升趋势,相比之下超级13Cr整体耐蚀性更优,腐蚀速率更低,这主要与超级13Cr中Ni、Mo的添加有关[16-18]。参照NACE PR—0775[19]中对材料耐蚀性能的相关评价标准,普通13Cr在1.01 g/cm3浓度的溴盐溶液中的平均腐蚀速率就已经属于严重腐蚀,并且随着溴盐质量浓度的升高,腐蚀程度不断加剧,在浓度为1.10 g/cm3达到极严重腐蚀。超级13Cr在低浓度溴盐完井液中平均腐蚀速率约为0.01 mm/a,属于轻度腐蚀,但其最大局部腐蚀速率达到0.52 mm/a,属于极严重腐蚀,随着溴盐浓度的提高,局部腐蚀不断加剧,在1.40 g/cm3时其最大局部腐蚀速率已超过5 mm/a。可见O2的引入对上述两种13Cr不锈钢油套管材的耐蚀性能都具有显著的影响,在实际使用中应重点关注氧气混入可能带来的潜在腐蚀风险。

图1 高温高压含O2条件下普通13Cr和超级13Cr在不同质量浓度溴盐溶液中的腐蚀速率 Fig. 1 Corrosion rates of plain and super 13Cr steels in O2-contained solutions with different KBr concentrations

2.2 宏观腐蚀形貌

图2给出了普通13Cr在高温高压含O2条件下不同质量浓度溴盐溶液中腐蚀7 d后酸洗前和酸洗后的宏观腐蚀形貌照片。

图2 高温高压含O2条件下普通13Cr在不同质量浓度溴盐溶液中腐蚀7 d后酸洗前和酸洗后的宏观腐蚀形貌 Fig. 2 Macro-photographs of plain 13Cr steel before and after cleaning after seven days exposure in O2-contained solutions with different KBr concentrations

图2可以看出,在低浓度溴盐溶液下(图2(a)(d)),试样表面被疏松的腐蚀产物覆盖,酸洗后无明显点蚀,而当浓度达到1.10 g/cm3,酸洗后试样表面开始出现肉眼可见的蚀坑,数量相对较少,随着浓度的提高,在浓度为1.40 g/cm3时试样表面蚀坑数量明显增多,分布也更为广泛,蚀坑尺寸明显增大,并且在蚀坑周围呈现特殊的白环形貌,后文会对该白环的形成开展进一步的分析。

相比之下,超级13Cr不锈钢材料整体腐蚀较为轻微(图3),在低浓度条件下无明显点蚀(图3(a)(d)),中等浓度条件下试样表面开始出现点蚀倾向,表现为局部的黑色圆形区(图3(b)(e)),但酸洗后并无肉眼可见的蚀坑出现。当浓度达到1.40 g/cm3时发生明显点蚀,酸洗前点蚀区域表现为一个规则的圆形区(图3(c)),圆形区域内部外侧被大量锈蚀产物覆盖,有趣的是在酸洗后,发现被锈蚀产物覆盖的区域底部基体并未发生明显的腐蚀(图3(f)),相反在圆形区域中心位置处则发生了严重的点蚀,蚀坑边缘呈不规则形状。出现这种现象可能与圆形区域内局部的电偶效应有关。

图3 高温高压含O2条件下超级13Cr在不同质量浓度溴盐溶液中腐蚀7 d后酸洗前和酸洗后的宏观腐蚀形貌 Fig. 3 Macro-photographs of super 13Cr steel before and after cleaning after seven days exposure in O2-contained solutions with different KBr concentrations

2.3 普通13Cr表面微观腐蚀形貌和腐蚀产物膜成分分析

对于普通13Cr,如图4所示,在浓度为1.10 g/cm3的溴盐溶液中腐蚀后试样表面为典型的氧还原腐蚀产物形貌,疏松多孔、粗糙度高,保护性差。能谱结果表明该腐蚀产物主要为Fe的氧化物,同时含有少量Ni和Cr的氧化物。酸洗去除腐蚀产物膜后,如图5所示,可以看出试样表面仍然有难以除去的黑色产物膜,对该黑色产物膜的A、B两个典型区域进行放大观察,可以看到A区域有大量的点蚀坑分布,且点蚀的萌生位置均在膜层的破损区域,经能谱分析破损位置底部Cr含量约为15%,O含量则较低,说明该区域为金属基体,上部膜层区域Cr、O含量分别达33%和56%,同时含有少量Fe,说明该膜层主要为富Cr的氧化膜。

图4 普通13Cr在浓度为1.10 g/cm3含O2溴盐溶液中腐蚀7 d后的表面微观腐蚀形貌(酸洗前) Fig. 4 SEM photo of plain 13Cr steel after seven days exposure in a O2-contianed KBr solution with a concentration of 1.10 g/cm3 (before cleaning)

图5 普通13Cr在浓度为1.10 g/cm3含O2溴盐溶液中腐蚀7 d后的腐蚀形貌和能谱分析结果(酸洗后) Fig. 5 SEM photos and EDS results of the corrosion product film formed on super 13Cr steel after seven days exposure in a KBr solution with a concentration of 1.10 g·cm–3 (after cleaning)

当溴盐溶液的质量浓度达到1.40 g/cm3时,试样表面蚀坑数量更多、尺寸更大,并且如前文所述在蚀坑周边出现了特殊的白环形貌,通过对白环区域和其他深色区域进行能谱分析,如图6所示,发现该白环区域主要为Fe和Cr,且Cr含量与基体接近,说明此处为金属基体,而周边深色区域则主要为Cr和O,即为富Cr的氧化物膜层。这与前文图5在较低浓度下试样表面蚀坑周边区域的腐蚀产物特征分布类似。

图6 普通13Cr在浓度为1.40 g/cm3含O2溴盐溶液中腐蚀7 d后表面不同典型区域元素分布对比(酸洗后) Fig. 6 Comparisons of Fe/Cr/O contents in different regions of corrosion product film that formed on super 13Cr steel after 7 days exposure in a KBr solution with a concentration of 1.40 g/cm3 (after cleaning)

2.4 超级13Cr表面微观腐蚀形貌和腐蚀产物膜成分分析

在1.10 g/cm3溴盐浓度下超级13Cr表面开始出现较为明显的局部腐蚀倾向,宏观表现为黑色的圆形区域,图7给出了该黑色圆形区域在酸洗前后的微观腐蚀形貌照片,可以看出,在去除腐蚀产物前该圆形区域呈现火山口形貌特征,外侧周边凸起,内侧较为平整,能谱结果表面该外侧凸起区域主要为Fe的氧化物,含有少量的Cr、Ni。酸洗后该圆形区域仍清晰可见,且内部局部区域(虚线圆圈所示)出现裂纹和破损,表现出一定的局部腐蚀倾向。当盐溶液浓度进一步升高,如图8所示,在1.40 g/cm3溴盐浓度下超级13Cr出现明显的局部腐蚀,酸洗去除腐蚀产物前,试样表面的局部腐蚀区域同1.10 g/cm3溴盐浓度下的类似,也表现为一个规则的圆形区,且在圆形区域内分为两个明显不同的A和B区域,内侧A区域形状不规则,外侧B区域为典型含氧条件下的疏松锈蚀产物,能谱结果也验证了这一结论。酸洗后如图8(b)所示,可以看出真正蚀坑的形成和发展并不是在整个圆形区域内进行,而是在A区域的底部进行,B区域虽然有大量的锈蚀产物覆盖,但其底部基体并没有局部腐蚀行为的发生。这与图7(b)较低浓度溴盐溶液中试样在深色圆形内局部区域所表现出的点蚀倾向相吻合。对C区域进行能谱分析结果表明该区域主要为富Cr的氧化物膜层,这与前文普通13Cr蚀坑周边的产物膜特征类似。

图7 超级13Cr在1.10 g/cm3含O2溴盐溶液中腐蚀7 d后的表面微观腐蚀形貌 Fig. 7 SEM photos of plain 13Cr steel after seven days exposure in a O2-contianed KBr solution with a concentration of 1.10 g/cm3

图8 超级13Cr在浓度为1.40 g/cm3含O2溴盐溶液中腐蚀7 d 后的表面 Fig. 8 SEM photos of super 13Cr steel after seven days exposure in a O2-contianed KBr solution with a concentration of 1.40 g/cm3

3 讨 论

通过前述对不同浓度下普通13Cr和超级13Cr表面微观腐蚀形貌和产物膜特性的分析,可以发现,不论是普通13Cr还是超级13Cr,其蚀坑周边均具有相似的产物膜分布特征,即近蚀坑区周边圆形区域内有较多腐蚀产物覆盖,宏观表现为的凸起区域,能谱分析表明该腐蚀产物主要为Fe和Cr的氧化物,而远离蚀坑区则主要为平整的富Cr氧化膜,起到显著的钝化作用,宏观表现为基本不腐蚀。基于这种特殊的局部腐蚀产物膜形貌特征分布,如图9所示,蚀坑内的基体作为阳极,蚀坑周边区域则作为阴极,蚀坑内部Fe、Cr失电子,铁离子和铬离子进入溶液并在蚀坑周边区域沉积形成较为致密的腐蚀产物膜,该腐蚀产物膜具有一定的保护性,导致蚀坑周边阴极区电位进一步正移,蚀坑周边与蚀坑内部电偶腐蚀效应加剧,从而进一步加速了蚀坑内部金属的溶解,促进蚀坑进一步发展。

图9 13Cr不锈钢在含O2溴盐溶液中蚀坑发展示意图 Fig. 9 Schematic diagram of the pitting propagation on 13Cr steel in a O2-contianed KBr solution

4 结 论

1)在高温高压含O2溴盐完井液环境中,普通13Cr和超级13Cr不锈钢的腐蚀速率均较高,尤其点蚀风险较高。

2)随着溴盐浓度的提高,13Cr不锈钢的腐蚀速率增大。

3)点蚀均是在材料表面富Cr层破损处开始萌生,且蚀坑内部与蚀坑周边产物膜覆盖区域存在局部的电偶效应,加速了点蚀的进一步发展。

4)O2的引入会显著提高13Cr不锈钢在实际使用中潜在的点蚀穿孔风险,因此,在现场实际使用中应最大限度地避免井下环境中O2的混入,这对井下油井管的服役安全至关重要。

参考文献
[1]
Martin R L.Corrosion consequences of oxygen entry into oilfield brines[C]//SPE Permian Basin Oil and Gas Recovery onference,Midland:NACE,2002,02270.
[2]
Wang S.Effect of oxygen on CO2 corrosion of mild steel[D].Ohio:Ohio University,2009.
[3]
Luo B W,Zhou J,Bai P P,et al. Comparative study on the corrosion behavior of X52,3Cr,and 13Cr steel in an O2–H2O–CO2 system:Products,reaction kinetics,and pitting sensitivity [J]. International Journal of Minerals,Metallurgy and Materials, 2017, 24(6): 646-656. DOI:10.1007/s12613-017-1447-9
[4]
Yue X Q,Zhang L,Li D P,et al. Effect of traces of dissolved oxygen on the passivation stability of super 13Cr stainless steel under high CO2/H2S conditions [J]. International Journal of Electrochemical Science, 2017, 12(8): 7853-7868. DOI:10.20964/2017.08.33
[5]
Hua Y,Jonnalagadda R,Zhang L,et al. Assessment of general and localized corrosion behavior of X65 and 13Cr steels in water-saturated supercritical CO2 environments with SO2/O2[J]. International Journal of Greenhouse Gas Control, 2017, 64: 126-136. DOI:10.1016/j.ijggc.2017.07.012
[6]
Liu Y,Xu L N,Zhu J Y,et al. Pitting corrosion of 13Cr steel in aerated brine completion fluids[J]. Materials and Corrosion, 2014, 65(11): 1096-1102. DOI:10.1002/maco.201307489
[7]
Zhang Guochao,Lin Guanfa,Sun Yulu,et al. Research on corrosion resistance of 13Cr stainless steel[J]. Total Corrosion Control, 2011, 25(4): 16-20. [张国超,林冠发,孙育禄,等. 13Cr不锈钢腐蚀性能的研究现状与进展[J]. 全面腐蚀控制, 2011, 25(4): 16-20. DOI:10.13726/j.cnki.11-2706/tq.2011.04.009]
[8]
Yin Z F,Wang X Z,Liu L. Characterization of corrosion product layers from CO2 corrosion of 13Cr stainless steel in simulated oilfield solution [J]. Journal of Materials Engineering & Performance, 2011, 20(7): 1330-1335. DOI:10.1007/s11665-010-9769-z
[9]
Lu Xianghong, Zhao Guoxian, Fan Zhihai, et al. Effects of CI- concentration and CO2 partial pressure on pitting behavior of 13Cr stainless steel under high temperature and high pressure [J]. Materials Protection, 2004, 37(6): 34-36. [吕祥鸿,赵国仙,樊治海,等. 高温高压下Cl-浓度、CO2分压对13Cr不锈钢点蚀的影响 [J]. 材料保护, 2004, 37(6): 34-36. DOI:10.16577/j.cnki.42-1215/tb.2004.06.015]
[10]
Hou Zan,Zhou Qingjun,Wang Qijiang, et al. Investigation on carbon dioxide corrosion performance of various 13Cr steels in simulated stratum water[J]. Journal of Chinese Society for Corrosion and Protection, 2012, 32(4): 300-305. [侯赞,周庆军,王起江,等. 13Cr系列不锈钢在模拟井下介质中的CO2腐蚀研究 [J]. 中国腐蚀与防护学报, 2012, 32(4): 300-305. DOI:CNKI:SUN:ZGFF.0.2012-04-006]
[11]
Zhao Y,Li X P,Zhang C,et al. Investigation of the rotation speed on corrosion behavior of HP-13Cr stainless steel in the extremely aggressive oilfield environment by using the rotating cage test[J]. Corrosion Science, 2018, 145: 307-319. DOI:10.1016/j.corsci.2018.10.011
[12]
Lei X W,Wang H Y,Mao F X,et al. Electrochemical behaviour of martensitic stainless steel after immersion in a H2S-saturated solution [J]. Corrosion Science, 2018, 131: 164-173. DOI:10.1016/j.corsci.2017.10.015
[13]
Xu L N,Meng Y,Shi Y G,et al. Pitting corrosion of 13Cr steel in oxygen-free completion fluids of organic salt[J]. Acta Metallurgica Sinica-English Letters, 2013, 26(3): 271-276. DOI:10.1007/s40195-012-0283-1
[14]
Zhen C,Li L,Guo Z,et al. Inhibition effect of propargyl alcohol on the stress corrosion cracking of super 13Cr steel in a completion fluid[J]. Corrosion Science, 2013, 69: 205-210. DOI:10.1016/j.corsci.2012.12.004
[15]
ASTM International.Practice for preparing,cleaning,and evaluating corrosion test specimens:ASTM G1—03[S].Houston:ASTM International,2011.
[16]
Moon J,Ha H Y,Park S J,et al. Effect of Mo and Cr additions on the microstructure,mechanical properties and pitting corrosion resistance of austenitic Fe–30Mn–10.5Al–1.1C lightweight steels[J]. Journal of Alloys and Compounds, 2019, 775: 1136-1146. DOI:10.1016/j.jallcom.2018.10.253
[17]
Si J,Wu Y,Wang T,et al. Composition-controlled active-passive transition and corrosion behavior of Fe–Cr(Mo)–Zr–B bulk amorphous steels[J]. Applied Surface Science, 2018, 445: 496-504. DOI:10.1016/j.apsusc.2018.03.186
[18]
Guo F F,Dong G N,Dong L S. High temperature passive film on the surface of Co–Cr–Mo alloy and its tribological properties[J]. Applied Surface Science, 2014, 314: 777-785. DOI:10.1016/j.apsusc.2014.07.086
[19]
NACE International.Preparation,installation,analysis,and interpretation of corrosion coupons in oilfield operations:NACE PR0775—05[S].Houston:ASTM International,2005.