Effect of alternating current on stress corrosion cracking behavior and mechanism of X80 pipeline steel in near-neutral solution

https://doi.org/10.1016/j.jngse.2017.01.008Get rights and content

Highlights

  • The SCC mechanism of X80 pipeline steel was investigated in near-neutral solution under the AC effect.

  • The threshold value of AC current density to improve the SCC susceptibility for hydrogen evolution reaction is found.

  • In this work, only 1.5%–2% of AC was involved in the steel and acted as faradaic current.

Abstract

The influence of alternating current (AC) density on stress corrosion cracking (SCC)behavior and the mechanism of X80 pipeline steel was investigated in NS4 near-neutral solution by slow strain rate tensile tests (SSRT), surface analysis techniques, data acquisition technique, and electrochemical measurements. Results showed that the SCC susceptibility of X80 pipeline steel was improved with the increasing of AC density, and the SCC mechanism was collectively controlled by anodic dissolution (AD) and hydrogen embrittlement (HE). When the AC density was below 10 A m−2, the corrosion enhanced because of a vibrating effect. However, when the AC density was no less than 30 A m−2, SCC susceptibility enhanced because of the hydrogen evolution reaction. With regard to potential acquisition, only a very small percentage about 1.5%–2% of AC was involved acting as faradaic current, which resulted in hydrogen evolution reaction and then improved the SCC susceptibility.

Introduction

With the rapid growth of energy demand and the adjustment of energy structure, X80 pipeline steel is widely used in west-east natural gas project, the length of the pipeline steel is about 350000 km. However, for the limitation of geographical spatial position, the pipeline steel would be set near high-voltage wire or electrified railway system when transport natural gas (Wen et al., 2015). The buried pipeline steel near the high voltage transmission line and electrified railway system would occur AC corrosion at lesions coating by effect of the resistance coupling, the capacitance coupling and inductance coupling, especially when the pipeline in parallel to the long distance (Bortels et al., 2010). Previous studies have acknowledged (Jiang et al., 2014, Muralidharan et al., 2007, Vasudevan and Lakshmi, 2011, Zhang et al., 2008) that AC interference could accelerate corrosion of most metal. In particular, reports have increased regarding AC-induced pipeline corrosion, AC may break down insulation layer of pipeline, destroy the cathodic protection system, and threaten the personal safety. What's more, the AC would also accelerate the natural gas pipeline corrosion, thus, it was vital significant to research AC corrosion mechanism on natural gas transmission pipeline steel, which attracted widespread attention in recent years (Gummow et al., 1998, Ibrahim et al., 2007, Ragault, 1998, Wakelin and Sheldon, 2004). Much research has investigated the AC corrosion of material under different conditions. Xu (Xu et al., 2012) demonstrated that applied AC can enhance the 16Mn pipeline steel corrosion. Lalvani (Lalvani and Lin, 1994, Lalvani and Lin, 1996) put forward a mathematical model to predict the AC effect on the corrosion behavior of steel. Fu (Fu and Cheng, 2010a) reported that uniform corrosion would occur on steel at a low AC current density, whereas pitting corrosion was observed at a high AC current density. Linhardt and Ball (2006) also found that the AC interference would induce localized corrosion, for example, pitting corrosion, on pipeline steel. Liu et al. (Zhiyong et al., 2014) studied the effect of dynamic direct current (DC) on SCC in X80 steel using square-wave polarization. The group demonstrated that non-steady state conditions increase the rates of anodic dissolution and cathodic hydrogen evolution, which resulted in increased SCC susceptibility. Zhu (Zhu et al., 2014a, Zhu et al., 2014b) found that the applied AC interference greatly enhanced the SCC susceptibility of the steel and the mechanism was transgranular mechanism without AC which changed into intergranular mechanism after applying AC in high pH solution, AC can cause local corrosion easily and local corrosion is the source of stress corrosion. As we all know, SCC is a vital threat to the safety of pipeline service, and the pipelines failures in natural gas transportation have occurred worldwide (Liu et al., 2008, Liu et al., 2016, Van Boven et al., 2007, Zhang et al., 2011). And pipelines in near-neutral environment are susceptible to SCC (Javidi and Horeh, 2014, Jia et al., 2011, Liu et al., 2012, Maocheng et al., 2016, Tang and Cheng, 2011, Yu et al., 2016). However, few studies have been studied on the AC corrosion of X80 pipeline steel in near-neutral solutions; the SCC behavior and mechanism remained unknown. Hence, studying the AC effect on the SCC behavior and mechanism of X80 pipeline steel in near-neutral solution is important.

In this article, the influence of different AC densities on the SCC behavior of X80 pipeline steel in NS4 near-neutral solution was studied by SSRT tests, surface analysis techniques, data acquisition technique (DAQ), and electrochemical measurements to study the mechanism of SCC.

Section snippets

Material and solution

The specimens were derived from a type of X80 pipeline steel produced in China. The chemical compositions of the steel in mass% are listed in Table 1. The electrochemical and SSRT samples were cut from the pipeline along the direction of parallel to the rolling. The tensile strength and the yield strength of the steel were measured to be 635 MPa and 560 MPa, respectively. Fig. 1 shows the microstructure of the steel which is mainly composed of polygonal ferrite, acicular ferrite and a number of

SSRT

Fig. 4 shows the stress–strain curves of X80 pipeline steel at different AC densities. When the X80 pipeline steel was tensile tested in air, exhibited the highest tensile strength and largest elongation. However, when different AC densities were applied, the elongation values were all less than that without the applied AC. The elongation displayed a declining trend with AC density, indicating that AC disturbance increased the SCC susceptibility of the X80 pipeline steel in near-neutral

Discussion

The results indicated that the elongation-loss rate (Iδ) and reduction in area (Iψ) showed increase tendency with the increasing of AC density (Fig. 5). Moreover, the surface fracture presented brittle characteristics (Fig. 6), and the secondary cracks became wider and deeper (Fig. 7). The crack propagation mechanism of steels all exhibited transgranular (TGSCC) fracture features (Fig. 8). The SCC behavior and mechanism of X80 pipeline steel under different AC densities comprised a mixture of

Conclusions

The effect of AC on the SCC behavior and mechanism of X80 steel was investigated in NS4 near-neutral solution by SSRT, surface analysis techniques, data acquisition technique, and electrochemical measurements. The results indicated that the SCC susceptibility of X80 steel is improved with AC density, and the SCC mechanism is collectively controlled by AD and HE. When the AC density is below 10 A m−2, the corrosion enhances because of a vibrating effect. However, when the current density is no

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Nos. 51371036 51131001 and 51471034) and the Beijing Higher Education Young Elite Teacher Project.

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