, 2017, 53(8): 975-982

Investigation on AC-Induced Corrosion Behavior and Product Film of X70 Steel in Aqueous Environment with Various Ions

ZHANG Hui, DU Yanxia, LI Wei, LU Minxu
Corrosion and Protection Center, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
ZHANG Hui, DU Yanxia, LI Wei, LU Minxu. Investigation on AC-Induced Corrosion Behavior and Product Film of X70 Steel in Aqueous Environment with Various Ions. Acta Metallurgica Sinica[J], 2017, 53(8): 975-982 doi:10.11900/0412.1961.2016.00566

Abstract:

During the past decades, more buried oil or gas pipeline failures have been attributed to alternating current (AC) interference, and finally, those corrosion failures were investigated and the AC current density was identified as the critical influence parameter. There is general agreement on AC current density as a chief factor in determining metal wastage condition or assessing the AC corrosion risk for any type of soil in the presence of AC corrosion. Different degrees of AC corrosion may occurr if long distance pipelines pass through different kinds of soil environments, however, the effect of soil ions on pipeline steel without cathodic protection is still not well understood. Therefore, it is imperative to study the AC corrosion behaviors of pipeline steel in different soil environments. In the AC corrosion simulation experiment, by investigating the AC corrosion behaviors of X70 steel in 4 kinds of environmental media, the corrosion rates (Kd) were obtained, whose order was $Kd (NaCl)$>$Kd Na2SO4)$>$Kd (CaCl2)$>$Kd (MgCl2)$. SEM, EDS and XRD were used to characterize and analyse the microscope morphologies, elements and phase compositions of the corrosion product films surfaces/cross-sections. Under the same AC current density, the reasons of differences of X70 steel's corrosion rate in different media were discussed. The presence of corrosive ions SO42- and Cl- would accelerate the corrosion rate, Na+ would not affect the formation of corrosion product film in NaCl and Na2SO4 solution media, however, the presence of Ca2+ and Mg2+ were helpful to form protective corrosion product films in MgCl2 and CaCl2 solution media.

Key words: X70 steel ; AC corrosion ; environmental media ; corrosion rate ; corrosion product film

1 实验方法

Fig.1

Diagram of experiment setup equivalent circuit (RE—reference electrode, WE—working electrode, CE—counter electrode)

X70钢试样实验前处理流程：首先,将试样的6个面使用水磨砂纸逐级打磨至800号,直至表面光滑、无明显划痕。然后依次使用去离子水、无水乙醇等清洗并吹干。其次,采用FA2004B电子天平(精度为0.1 mg)称重并记录,用绝缘胶将工作面密封,将试样的非工作面用导电铜胶与导线粘在一起。最后,将试样用石蜡封入PVC管中,保留1个4 cm2的工作面,确保封装后工作面表面没有气泡和凹槽。实验所用4种模拟环境介质参数如表1所示。

Table 1

Parameters of simulation corrosion media

$K d = 8.76 × W 0 - W Stρ × 10 4$ (1)

2 实验结果
2.1 腐蚀速率

300 A/m2交流干扰下X70钢在4种介质中腐蚀168 h后的腐蚀速率

Fig.2

Corrosion rates of X70 steel under alternating current (AC) interference density of 300 A/m2 in 4 kinds of simulated media exposed for 168 h

2.2 腐蚀产物膜成分

300 A/m2交流干扰下X70钢在4种介质中腐蚀产物的XRD谱

Fig.3

XRD spectra of the corrosion product films on X70 under AC interference density of 300 A/m2 in simulated media of Na2SO4 (a), MgCl2 (b), CaCl2 (c) and NaCl (d)

2.3 腐蚀产物膜微观形貌

X70钢腐蚀产物膜表面微观形貌的SEM像

Fig.4

Low (a, c, e, g) and high (b, d, f, h) magnified surface SEM images of corrosion product films on X70 steel in simulated media of Na2SO4 (a, b), MgCl2 (c, d), CaCl2 (e, f) and NaCl (g, h)

X70钢腐蚀产物膜截面微观形貌的SEM像

Fig.5

Low (a, c, e, g) and high (b, d, f, h) magnified cross-sectional SEM images of corrosion product films on X70 steel in simulated media of Na2SO4 (a, b), MgCl2 (c, d), CaCl2 (e, f) and NaCl (g, h)

2.4 腐蚀产物膜截面EDS分析

X70钢在4种介质中腐蚀产物膜截面成分的EDS分析

Fig.6

EDS analyses of inner corrosion product films (a, c, e, g) and outer corrosion produt films (b, d, f, h) on X70 steel in simulated media of Na2SO4 (a, b), MgCl2 (c, d), CaCl2 (e, f) and NaCl (g, h)

3 分析讨论

$Fe + S O 4 2 - → FeS O 4 + 2 e -$ (2)

$2 FeS O 4 + 2 NaOH + O 2 → F e 2 O 3 · H 2 O + N a 2 S O 4$ (3)

4 结论

(1) 在300 A/m2的交流干扰下,X70钢在4种不同介质中腐蚀168 h后的腐蚀速率差异较大,腐蚀速率 $K d ( NaCl )$ > $K d N a 2 S O 4 )$ > $K d ( CaC l 2 )$ > $K d ( MgC l 2 )$ ,表明交流腐蚀速率不仅与交流电流密度有关,还与环境相关。

(2) X70钢在Na2SO4和NaCl介质中生成的腐蚀产物膜比较疏松,对基体无保护作用;Na+不会影响腐蚀产物膜的形成,侵蚀性离子SO42-和Cl-的存在会加速X70钢的腐蚀。X70钢在MgCl2和CaCl2介质中生成的腐蚀产物膜比较致密,对基体有保护作用;Mg2+和Ca2+的存在影响腐蚀产物膜的形成,有利于在X70钢表面形成致密的保护膜,降低腐蚀速率。

The authors have declared that no competing interests exist.

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