Effect of hydrogen-induced plasticity on the stress corrosion cracking of X70 pipeline steel in simulated soil environments

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Abstract

The susceptibility to stress corrosion cracking (SCC) of X70 pipeline steel under cathodic protection in near-neutral pH and acidic solutions was investigated by slow-strain-rate tensile test, circumferential-notch tensile (CNT) test, and three-point-bending (TPB) test. Results confirmed the existence of a hydrogen-induced plasticity (HIP) effect within a particular range of cathodic potentials. HIP effect lowered the SCC risk of X70 steel by releasing stress concentration at crack-initiation spots and then decreasing the stress intensity. Crack-growth behavior examined by CNT and TPB tests proved the existence of an HIP effect.

Introduction

The stress corrosion cracking (SCC) of pipelines in soil environments have posed significant problems to the petroleum industry over the past decades [1], [2], [3]. Two types of SCC, namely, high pH SCC [4], [5] (pH>9.0) and near-neutral pH SCC [6], [7], [8], [9], [10] (at a pH around 6.5), have been identified since the first case of SCC found in the USA. High-pH SCC for pipelines follows the anodic dissolution (AD) mechanism [4], [7], whereas low-pH SCC involves the combined effect of hydrogen and non-steady electrochemical effect on dissolution of the steel [6], [7], [8], [9], [10]. Parkins et al. [10] concluded that near-neutral pH SCC results from the cooperation of anodic dissolution and the effect of hydrogen in the steel. Beavers [11] pointed out that SCC is initiated by anodic dissolution (AD), and its propagation is driven by hydrogen-induced cracking (HIC). Cheng et al. [12], [13] quantified the contributions of stress, hydrogen, and their synergism to dissolution of steel at the crack tip, i.e., hydrogen-induced dissolution, in near-neutral pH solution. Hydrogen evolution is responsible for SCC of pipeline steels. However, Lu et al. [1], [9] reported that hydrogen has limited effect on active dissolution of pipeline steel in near-neutral pH underground water. These results imply that the relationship between the effect of hydrogen and anodic dissolution has not been fully identified. More recently, Liu et al. [14], [15], [16], [17], [18], [19] have extensively studied the SCC mechanism of pipe steels, including X70, X80, and X100 steels, in soil environments. They demonstrated that the different electrochemical statuses between the crack tip and the crack wall give rise to a combined effect of hydrogen. Moreover, both high-pH SCC and near-neutral pH SCC are strongly related to cathodic potential (CP). The SCC of pipeline steels is under the combined effect of AD and HE When the applied CP is about between −730 mVSCE and −920 mVSCE in near neutral solution. The SCC susceptibility increases first and then decreases with the negative shift of CP. However, SCC susceptibility increases again when the applied potential is more negative than −920 mVSCE [14]. This finding introduces a big challenge for the security of pipelines on protecting the steel sufficiently and lowering the risk of SCC at the same time. On one hand, hydrogen evolution is intensified when CP-on, which is believed as one of the main reasons of SCC. On the other hand, hydrogen may have an important effect in metal matrix, i.e., hydrogen-induced plasticity (HIP) [20], [21], [22] other than hydrogen embrittlement, on mechanical and SCC properties. HIP decreases the stress intensity at the crack tip and lowers the SCC risk of pipeline steels by releasing the stress concentration at SCC initiation spots and enlarging deformation zone in front of the crack tip, thereby providing a possibility to optimize the CP condition. However, the detailed mechanism of HIP and its relationship with SCC under CP remain unclear and need further investigation.

In this work, slow-strain-rate tensile (SSRT) tests were performed on API X70 pipeline steel to confirm an effective CP range of HIP with high SCC susceptibility. The mechanism of HIP was investigated by circumferential-notch tensile (CNT) test and three-point-bending (TPB) test.

Section snippets

Experimental

All samples used in this work were made of X70 pipeline steel supplied by Baosteel Co., Ltd., China, with the following chemical composition (wt%): 0.065 C, 0.23 Si, 1.57 Mn, 0.056 Nb, 0.20 Ni,0.18 Cr, 0.22 Cu, 0.027 Al, 0.015Ti, 0.028V, 0.0020 S, and 0.0019 P and Fe balance.

The solutions used include a near-neutral pH solution, i.e., NS4 solution, to simulate the electrolyte trapped under disbanded coating in Canada [18] and an acidic solution with pH≈4 to simulate an acidic soil located in

Results

The stress–strain curves of the X70 steel measured in air and different testing solutions at −850 mVSCE are shown in Fig. 2. The elongation of the steel decreased in both NS4 and acidic solutions compared with that obtained in air, indicating that the X70 steel is susceptible to SCC in these two solutions.

Fig. 3 shows the stress−strain curves of the X70 steel in the NS4 and acidic solutions under various polarizing potentials. The elongation and ultimate tensile strength of the steel exhibited

Analysis of the electrochemical reactions for pipeline steels in simulated soil environments and its relationship with the SCC process.

The anodic reaction of the X70 steel in the testing solutions is the oxidation of iron [26].Fe→Fe2++2e

The cathodic reaction of the steel includes reduction of H+, HCO3−, H2CO3, and H2O:2H++2e↔H22H2CO3+2e↔H2+2HCO3-2HCO3-+2e↔H2+2CO32-2H2O+2e↔H2+2OH-

The corrosion product, a layer of porous Fe(OH)2, is formed on the steel surface with the increase in anodic and cathodic reactions:Fe2++2OH−↔Fe(OH)2

Compared with the cathodic reactions in the NS4 solution, the hydrogen evolution process in the pH 4

Conclusion

A high SCC susceptibility potential range (owing to the non-steady electrochemical effects) and an HIP potential range exist between OCP and −950 mVSCE CPs for pipeline steel under CP. The HIP phenomenon postpones the initiation and the propagation of SCC when the applied potential is within the high SCC susceptibility potential range. This positive effect of HIP would reduce the risk of SCC in the corresponding potential ranges.

Acknowledgements

This work was supported by National Basic Research Program of China (973 Program) (No. 2014CB643300) and the Chinese National Science Foundation (Nos. 51131001 and 51471034).

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