采用浸泡失重法、动电位极化曲线和电化学阻抗技术研究了L245钢在不同温度下的油气田模拟水中的腐蚀行为,采用SEM分析了腐蚀产物的表面形貌。结果表明：在饱和CO2条件的溶液中,当温度从30 ℃升高至90 ℃时L245钢的腐蚀电流从66.1 μAcm-2升高到177 μAcm-2,说明L245钢在高温下反应比较剧烈。当温度从30 ℃升高至90 ℃时L245钢的电荷转移电阻从5155 Ωcm2减小到1182 Ωcm2,说明温度的升高减小了腐蚀反应的阻力。以上结果表明,在30~90 ℃时L245钢在油气田模拟水中的腐蚀速率随着温度的升高而加快,当温度升高至60 ℃时开始发生局部腐蚀并随着温度升高而加剧。
The corrosion behavior of L245 steel in simulated oilfield produced water was investigated by means of weight loss measurement, polarization curve measurement, electrochemical impedance spectra (EIS) and SEM. The results showed that the corrosion reaction of L245 steel is more intensive at higher temperature. When the temperature of the solution is elevated from 30 ℃ to 90 ℃, the corrosion current density of L245 steel in the solution with saturated CO2 increases from 66.1 μAcm-2 to 177 μAcm-2 while the charge transfer resistance of L245 steel decreased from 5155 Ωcm2 to 1182 Ωcm2. The above results indicated that the corrosivity of oilfield produced water increased with the increasing temperature within the range of 30~90 ℃, localized corrosion of the steel occurred at 60 ℃, then the localized corrosion would be intensified when the temperature is elevated to 90 ℃.
碳钢是油气田采输过程中使用最广的管材。在含CO2的环境中,碳钢非常容易发生腐蚀。目前,CO2腐蚀已经成为石油天然气工业中的一种重要的腐蚀形式,导致了管线和设备腐蚀失效事故的频发,从而造成巨大的经济损失、人员伤亡和环境破坏。干燥的CO2对钢铁没有腐蚀作用,但当CO2溶于水后,在相同的浓度下其总酸度高于盐酸,因此其对钢铁的腐蚀性极强。国内外学者普遍认可的均匀腐蚀机理认为钢铁的CO2腐蚀过程可分为化学反应和电化学反应：化学反应过程相对简单,包括 CO2溶于水形成H2CO3的过程、H2CO3一级和二级电离过程以及碳钢溶解形成活化粒子和碳酸盐生成的过程;电化学反应过程包括阳极失去电子被氧化的过程、阴极得到电子被还原的过程和电子传输的过程,但由于CO2在水中会发生水合反应和两级电离,因此腐蚀的中间产物多种多样 (如FeOH,Fe(OH)2,Fe(HCO3)2和Fe(HCO3)OH),而阴极反应过程也涉及到H+,H2O,H2CO3和HCO3-的还原。而对于局部腐蚀,反应更为复杂,它的产生和发展与钢材的表面状态、腐蚀产物膜的性质以及环境因素有关,反应过程也可能与均匀腐蚀有所不同[7-9]。由于各油田所处的地理位置和环境不同,因此不同的油田采出水的特点也不尽相同。但是不同的采出水之间也存在着一些共同特性。总体上存在以下特点,水温基本保持在20~90 ℃,温度相对比较高;油田采出水中的矿化度非常高,高的矿化度容易导致管线腐蚀破裂,给工作处理带来困难甚至发生严重事故。影响管线钢腐蚀的因素有很多,其中首要的因素是温度的影响,温度对管线腐蚀有十分重要的影响,通常来说温度主要是影响材料表面的腐蚀产物膜,进而影响腐蚀速率。众所周知,温度的升高,可以加快腐蚀的化学反应、电化学反应以及传质过程,从而加速腐蚀的发生。但是也不是说腐蚀速率会随着温度升高而一味的升高,在温度升高过程中存在很多转折点。研究人员对此进行了大量的工作,Ikeda等测试了不同温度下碳钢在溶液中的极化曲线,结果表明,温度升高使得阴、阳极电流密度增大。Zhang等对饱和CO2环境中的腐蚀速率进行了研究发现腐蚀速率与温度有很密切的关系。本文通过在不同温度下进行浸泡实验和电化学实验研究了L245钢在模拟油气田采出水环境中的腐蚀行为,并为L245钢在油气田采出水中的合理应用提供参考依据。
实验材料采用L245钢,其化学成分 (质量分数,%) 为：C 0.135,Si 0.350,Mn 1.350,S 0.007,P 0.015,Fe余量。
浸泡实验采用试片规格为45 mm×10 mm×3 mm (含
将浸泡240 h后的试样从模拟溶液中取出后用水冲洗,放入酸清洗液中超声辅助清洗5 min。酸清洗液的基本要求为：(1) 应能全部除去试片上的腐蚀产物沉积物;(2) 原则上是既能迅速、顺利地去除试片上的沉积物,又能基本上不侵蚀金属本体。酸清洗液的配比：盐酸100 mL,六次甲基四胺10 g,加水至1 L,取出后用无水乙醇超声脱水,吹干后放入干燥器中,8 h后用Sartorius TE124S天平 (精度0.1 mg) 进行称重,每个条件下对3个平行样分别称重,结果取平均值。
采用三电极体系,在PGSTAT100N型电化学工作站上进行电化学实验。工作电极为L245钢试样,参比电极为饱和甘汞电极 (SCE),辅助电极为Pt片,溶液体积为1000 mL。实验溶液用CO2除氧2 h至饱和。实验温度分别为30,60和90 ℃,压力为常压。动电位极化曲线的扫描速率为0.33 mV/s,扫描电位范围为±250 mV (相对于开路电位),并对曲线进行拟合。电化学阻抗谱的测试频率为105~5×10-3 Hz,交流激励信号为幅值为±5 mV的正弦波。
采用FEI-2000型扫描电子显微镜 (SEM) 对浸泡实验后的试样进行腐蚀形貌观察。
(1) 温度对L245钢的CO2腐蚀有显著影响,在低温阶段 (≤90 ℃),腐蚀速率随着温度的升高而加快。
(2) 常温下 (30 ℃) L245钢的CO2腐蚀主要为均匀腐蚀,当温度升高至60 ℃开始发生局部腐蚀,局部腐蚀反应随着温度升高而加剧。
The authors have declared that no competing interests exist.
Abstract The prediction of the corrosivity of CO//2 dissolved in water is very important in connection with the transport of CO//2-containing wet natural gas. In this study, the relation between the corrosion rate of steel in carbonic acid and the CO//2 partial pressure was determined by means of weight loss and polarization resistance measurements. This relation is compared with that expected for completely dissociated acids, and appears to be significantly different. A theoretical analysis of this effect shows that it can be quantitatively explained.
This paper describes the effects of dissolved carbon dioxide on the electrochemical reactions of two pipeline steels: a low alloy steel and a 13% Cr stainless steel. The dissolved gas reacts with the iron component of these steels in such a way as to contribute to their depassivation. The mechanism proposed is that of complexing with Fe (II) within the hydroxide film to give both ferrous carbonate and a dissolved complex. The effect is negligible for the 13% Cr stainless steel, but contributes to the corrosion of the low alloy steel. The cathodic regime representing evolution of hydrogen is also affected by the presence of dissolved carbon dioxide. At low cathodic overpotentials this appears to arise from a pH buffering action. At high cathodic overpotentials there is some evidence that the carbon component of the dissolved CO 2 is reduced, probably to carbon monoxide, and this reduces the rate of hydrogen evolution by cathode poisoning. The roles of pH buffer and of electrolytic anion are discussed.
Abstract Studies of the susceptibility of carbon steel (line pipe steel) to pitting were performed at room temperature in deaerated, NaCl-saturated CO2-containing solution simulating natural brine. Measurements were made in a wide range of pHs and electrode potentials. The primary corrosion product was identified by X-ray diffraction analysis as Fe(HCO3)2. This compound was found to form a tight, adherent film on the metal surface. Longer exposures caused transformation of Fe(HCO3)2 into FeCO3 in the form of a porous, non-adherent and non-protective layer. Pitting resulted from the action of galvanic couples between different sites on the steel surface, either uncovered or screened by porous layers of various corrosion products.
In this work, corrosion and localized corrosion behavior of X65 pipeline steel were studied in a simulated, CO 2 -saturated oilfield formation water by various electrochemical measurement techniques, including electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curves, galvanic current and localized EIS (LEIS). The morphology and composition of the formed corrosion scale were characterized by scanning electron microscopy and energy-dispersive X-ray analysis. A conceptual model was developed to illustrate the occurrence of localized corrosion of the steel under scale. Both galvanic current and LEIS measurements showed that a galvanic effect existed between the bare steel and the scale-covered region. The scale-covered region served as cathode and the bare steel site as the anode. The big cathode vs. small anode geometry accelerated the local corrosion reaction. At an elevated temperature, a compact, crystalline scale was formed on the steel surface, enhancing the galvanic effect. Moreover, the stability of the scale was increased with time, and localized corrosion of the steel under scale experienced mechanistic changes with time.
The localized electrochemical dissolution behavior at surface irregularities, including scratch, mechanically induced hole and corrosion pit, on pipeline steel was investigated in both near-neutral pH and high pH solutions by scanning vibrating micro-electrode and localized electrochemical impedance spectroscopy measurements. In near-neutral pH solution, the localized dissolution behavior at surface irregularities is dependent of their geometrical depth, which is critical to development of a local electrochemical condition to support the further localized dissolution. Therefore, surface irregularities exceeding a certain depth provide potential sites to initiate stress corrosion cracks in near-neutral pH solution. The strong passivating capability of high pH solution would result in the formation of oxide film over the whole electrode surface to 鈥渆qualize鈥 the electrochemical activity at irregularities to the intact area. Therefore, the irregularities would not result in localized dissolution electrochemistry. Consequently, localized corrosion and crack initiation are not anticipated to initiate from the geometrical irregularities in high pH solution. However, corrosion pits generating due to passive film breakdown could support the high local dissolution kinetics in high pH solution, providing potential sites for crack initiation. The effects of hydrogen-charging on anodic dissolution at regularities depend on the defect geometry and the solution pH.
Electrochemical corrosion behavior of X65 steel in CO 2-containing oilfield formation water in the presence of acetic acid (HAc) was investigated by various electrochemical measurements and analyses as well as thermodynamic calculations of ionic concentrations, reaction rate constants and equilibrium electrode potentials. A conceptual model was developed to illustrate corrosion processes of steel in oilfield formation water system. The anodic reactions of the steel contain a direct dissolution of Fe, Fe 鈫 Fe 2+ + 2e, and the formation of corrosion scale, FeCO 3, by Fe + HCO 3 - 鈫 FeCO 3 + H + + 2e. The cathodic processes contain the reduction of H +, HCO 3 -, H 2O and HAc, where reduction of HAc has the least negative equilibrium potential and thus dominates the cathodic process. With addition of HAc in the solution, both cathodic and anodic reaction rates increase remarkably. It is attributed to the fact that HAc inhibits or degrades the formation of protective scales due to the decrease of solution pH. Upon electrode rotation, the measured impedance decreases with the increase in HAc concentration. The FeCO 3 scale will not form on electrode surface. When HAc concentration is less than 1000 ppm, the adsorbed intermediate product is not significant, resulting in generation of a low-frequency inductive loop in EIS plots. When HAc concentration is more than 3000 ppm, the adsorption of intermediate product is significant, generating overlapped capacitive semicircles in EIS measurements.
The state-of-the-art in modelling of internal corrosion of oil and gas pipelines made from carbon steel is reviewed. The review covers the effects of: electrochemistry, water chemistry, formation of protective scales and scales, temperature, flow, steel, inhibition, water condensation, glycol/methanol and localized attack. Various mathematical modelling strategies are discussed.